阅读说明：本技术 室上性快速性心律失常判别 (Supraventricular tachyarrhythmia discrimination ) 是由 X·张 J·曹 Y·刘 于 2018-04-16 设计创作，主要内容包括：植入式心脏复律除颤器(ICD)执行一种方法,该方法包括确定心脏电信号是否满足用于检测室性快速性心律失常的第一标准。该ICD从一组心脏信号分段中的心脏信号分段中确定特征,并且确定特征的第一部分是否满足单形性波形标准并且确定特征的第二部分是否满足室上性搏动标准。该ICD确定是否满足用于检测室性快速性心律失常的第二标准,并且响应于满足了单形性波形标准以及室上性搏动标准,抑制室性快速性心律失常的检测。(An Implantable Cardioverter Defibrillator (ICD) performs a method that includes determining whether a cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia. The ICD determines a feature from cardiac signal segments in a set of cardiac signal segments and determines whether a first portion of the feature satisfies a monomorphic waveform criterion and determines whether a second portion of the feature satisfies a supraventricular beat criterion. The ICD determines whether a second criterion for detecting the ventricular tachyarrhythmia is satisfied and suppresses detection of the ventricular tachyarrhythmia in response to the monomorphic waveform criterion and the supraventricular beat criterion being satisfied.)

1. A medical device system, comprising:

therapy delivery circuitry configured to generate electrical stimulation therapy for delivery to a heart of a patient;

Sensing circuitry configured to receive at least a first cardiac electrical signal via a sensing electrode vector; and

a control circuit coupled to the sensing circuit and the therapy delivery circuit, and configured to:

Determining whether the first cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia;

Determining a plurality of features from each of a plurality of cardiac signal segments of the first cardiac electrical signal;

In response to the first criterion being satisfied, determining whether a first portion of the plurality of features determined from each of the plurality of cardiac signal segments satisfies a simplex waveform criterion;

Determining whether a second portion of the plurality of features determined from each of the plurality of cardiac signal segments meets supraventricular beat criteria;

Determining whether a second criterion for detecting the ventricular tachyarrhythmia is met;

In response to the monomorphic waveform criterion and the supraventricular beat criterion being met, inhibiting detection of the ventricular tachyarrhythmia; and

In response to the first criteria and the second criteria being met, and at least one of the monomorphic waveform criteria not being met or the supraventricular beat criteria not being met, detecting the ventricular tachyarrhythmia and controlling the therapy delivery circuit to deliver the electrical stimulation therapy.

2. The system of claim 1, wherein the control circuit is configured to determine whether the first criterion is satisfied by:

Determining a morphology matching score between each of a plurality of cardiac signal segments of the first cardiac signal and a morphology template;

determining that the first criterion is satisfied in response to a first threshold number of cardiac signal segments of the plurality of cardiac signal segments having morphology matching scores less than a first matching threshold.

3. The system of claim 2, wherein the control circuit is further configured to:

In response to the first criterion being met, determining whether a supraventricular tachyarrhythmia discrimination criterion is met;

In response to the supraventricular tachyarrhythmia discrimination criteria not being met, refraining from determining the plurality of features; and

Detecting the ventricular tachyarrhythmia in response to the first and second criteria for detecting the ventricular tachyarrhythmia being met and the criteria for detecting the supraventricular tachyarrhythmia discrimination not being met.

4. the system of any one of claims 2-3, wherein the control circuit is configured to:

Determining that the supraventricular tachyarrhythmia discrimination criteria is satisfied by determining that at least a second threshold number of cardiac signal segments of the plurality of cardiac signal segments have a morphology matching score greater than a second matching threshold, the second matching threshold being less than the first matching threshold.

5. the system of any one of claims 2-4, wherein the control circuit is further configured to:

Setting a threshold count value in response to fewer than the threshold number of cardiac signal segments of the plurality of cardiac signal segments having morphology matching scores less than the first matching threshold;

Adjusting a tachyarrhythmia count value in response to the threshold number of cardiac signal segments in a next plurality of cardiac signal segments having morphology matching scores less than the first matching threshold; and

Determining that the first criterion is met when the tachyarrhythmia count value reaches the threshold count value.

6. the system of any of claims 1-5, wherein the control circuit is further configured to:

Determining a sense event interval between successive R-waves sensed by the sensing circuit;

Comparing the sensed event interval to a tachyarrhythmia detection interval;

In response to each of the determined perceived event intervals being less than the tachyarrhythmia detection interval, increasing a count of tachyarrhythmia detection intervals; and

In response to a value of the count of tachyarrhythmia detection intervals being equal to or greater than a tachyheart rate threshold, determining whether the first criterion for detecting the ventricular tachyarrhythmia is met.

7. The system of any one of claims 1-6, wherein the control circuitry is configured to determine whether the supraventricular beat criterion is met by:

Comparing each feature of the first portion of the features determined from each of the plurality of cardiac signal segments to similar features of an supraventricular R-wave template; and

Determining that the supraventricular beat criteria are satisfied in response to a threshold number of the cardiac signal segments having the first portion of the feature that matches the similar feature of the supraventricular R-wave template.

8. The system of any one of claims 1-7, wherein the control circuitry is configured to:

Determining the features of each of the plurality of cardiac signal segments by determining at least a polarity pattern, a peak time interval, and a normalized width for each of the plurality of cardiac signal segments; and is

Determining whether the supraventricular beat criterion is met by:

Determining, for each of the plurality of cardiac signal segments, whether the determined polarity pattern matches a polarity pattern of a supraventricular R-wave template;

For each of the plurality of cardiac signal segments, determining whether the determined peak time interval matches a peak time interval of the supraventricular R-wave template within a peak time interval match threshold range;

Determining, for each of the plurality of cardiac signal segments, whether the determined normalized width matches the normalized width of the supraventricular R-wave within a normalized width matching threshold range; and

Determining that the supraventricular beat criterion is satisfied in response to a threshold number of the cardiac signal segments having the determined features of polarity pattern, maximum peak amplitude, and normalized width that match the similar features of the supraventricular R-wave template.

9. The system of any one of claims 1-8, wherein the control circuitry is configured to determine whether the simplex waveform criteria are satisfied by:

Determining a variability of each of the features of the second portion of the features determined from each of the plurality of cardiac signal segments;

comparing the variability of each of the features of the second portion of the features to a corresponding variability threshold;

Determining that the simplex waveform criteria are satisfied in response to the variability of each of the features of the second portion of the features being less than the corresponding variability threshold.

10. the system of any one of claims 1-9, wherein the control circuit is configured to:

determining the polarity of the maximum peak of the supraventricular sexual R-wave template;

Determining the characteristic of each of the plurality of cardiac signal segments by determining at least an amplitude and a timing of a maximum peak of the cardiac signal segments having a polarity matching the polarity of the maximum peak of the supraventricular R-wave template;

determining an amplitude variability and a timing variability of the maximum peaks of the plurality of cardiac signal segments; and

determining that the monomorphic waveform criterion is satisfied in response to the amplitude variability being less than an amplitude variability threshold and the timing variability being less than a timing variability threshold.

11. The system of any of claims 1-10, wherein the control circuit is further configured to:

Determining an event interval between successive events sensed by the sensing circuit;

Comparing the event intervals to tachyarrhythmia detection intervals;

In response to each of the determined inter-event intervals being less than the tachyarrhythmia detection interval, increasing a count of tachyarrhythmia detection intervals;

Determining that the second criterion for detecting the ventricular tachyarrhythmia is satisfied in response to a value of the count of tachyarrhythmia detection intervals being equal to or greater than a value of a detection threshold.

12. the system according to any one of claims 1 to 11, wherein:

The sensing circuitry comprises a first sensing channel for receiving the first cardiac signal and a second sensing channel for receiving a second cardiac signal, and the sensing circuitry is configured to sense R-waves from the second cardiac signal and to generate R-wave sense event signals in response to each sensed R-wave;

The control circuit is configured to:

Determining a perceptual event interval between each pair of consecutive R-wave perceptual event signals generated by the sensing circuit;

Comparing the sensed event interval to a tachyarrhythmia detection interval;

in response to each of the determined perceived event intervals being less than the tachyarrhythmia detection interval, increasing a count of tachyarrhythmia detection intervals;

In response to a value of the count of tachyarrhythmia detection intervals being equal to or greater than a fast heart rate threshold, buffering the plurality of cardiac signal segments from the first cardiac signal, each of the plurality of cardiac signal segments corresponding to an R-wave perceptual event signal generated by the sensing circuitry; and

Determining that the second criterion for detecting the ventricular tachyarrhythmia is satisfied in response to the count of tachyarrhythmia detection intervals being equal to or greater than a value of a detection threshold.

13. The system of any one of claims 1-12, wherein the control circuit is further configured to adjust the first criterion in response to both the monomorphic waveform criterion and the supraventricular beat criterion being met.

14. The system of any one of claims 1-13, further comprising a housing enclosing the therapy delivery circuitry, the sensing circuitry, and the control circuitry and having a connector block for receiving an extra-cardiovascular lead carrying at least one electrode in the sensing electrode vector.

Technical Field

The present disclosure relates generally to implantable medical devices and methods for distinguishing supraventricular tachyarrhythmias from ventricular tachyarrhythmias.

Background

Medical devices, such as cardiac pacemakers and Implantable Cardioverter Defibrillators (ICDs), provide therapeutic electrical stimulation to a patient's heart via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include signals such as pacing pulses, or cardioversion or defibrillation shocks. In some cases, the medical device may sense cardiac electrical signals that accompany intrinsic depolarizations or pacing-induced depolarizations of the heart and control delivery of stimulation signals to the heart based on the sensed cardiac electrical signals.

Upon detection of an abnormal heart rhythm, such as bradycardia, tachycardia or fibrillation, one or more appropriate electrical stimulation signals may be delivered in order to restore or maintain a more normal heart rhythm. For example, an ICD may deliver pacing pulses to the patient's heart when bradycardia or tachycardia is detected, or deliver cardioversion or defibrillation shocks to the heart when tachycardia or fibrillation is detected. An ICD may sense cardiac electrical signals in a heart chamber using electrodes carried by intravenous medical electrical leads and deliver electrical stimulation therapy to the heart chamber. The cardiac signals sensed within the heart typically have high signal strength and quality for reliably sensing cardiac electrical events, such as R-waves. In other examples, a non-transvenous lead may be coupled to the ICD, in which case cardiac signal sensing presents new challenges in accurately sensing cardiac electrical events and properly detecting and differentiating between different types of cardiac arrhythmias.

proper detection and differentiation of different tachyarrhythmias is important in automatically selecting and delivering effective electrical stimulation therapy by an implantable medical device system and avoiding unnecessary therapy. For example, supraventricular tachyarrhythmias originate in the upper atrial heart chamber and are conducted to the lower ventricular heart chamber. Supraventricular tachyarrhythmias (SVTs) generally cannot be successfully terminated by delivering electrical stimulation therapy to the ventricles because the rhythm originates from the superior heart chamber. On the other hand, ventricular tachyarrhythmias originating from the lower ventricular heart chambers can often be successfully treated by delivering electrical stimulation therapy to the ventricle for terminating abnormal ventricular rhythms. Accordingly, discrimination of supraventricular tachyarrhythmias originating from the superior heart chamber from ventricular tachyarrhythmias originating from the inferior heart chamber allows for appropriate therapy selection and delivery while avoiding unnecessary or potentially ineffective electrical stimulation therapy being delivered to the patient's heart.

Disclosure of Invention

in general, the invention relates to techniques for distinguishing SVT from ventricular tachyarrhythmias (e.g., Ventricular Tachycardia (VT) and Ventricular Fibrillation (VF)) and suppressing VT and VF detection and treatment when SVT is detected. In some examples, an ICD system operating according to techniques disclosed herein may determine features of cardiac signal segments corresponding to a perceived R-wave that occurs at a tachyarrhythmia heart rate and has a morphology indicative of ventricular tachycardia. A first set of the determined features of the cardiac signal segments can be compared to a monomorphic rhythm criterion and a second set of the determined features can be compared to an SVT beat criterion. If both the monomorphic rhythm criteria and the SVT beat criteria are met, the rhythm may be identified as a supraventricular rhythm. In response to detecting a supraventricular arrhythmia, the detection of, and treatment of, a ventricular tachyarrhythmia is delayed or suppressed.

In one example, the present disclosure provides an ICD including therapy delivery circuitry, sensing circuitry, and control circuitry coupled to the sensing circuitry and the therapy delivery circuitry. The therapy delivery circuit is configured to generate electrical stimulation therapy for delivery to a heart of a patient. The sensing circuitry is configured to receive cardiac electrical signals via a sensing electrode vector. The control circuit is configured to determine whether the cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia and determine a characteristic of each of a set of cardiac signal segments of the cardiac electrical signal. In response to the first criterion being met, the control circuitry determines whether a first portion of the feature determined from each of the cardiac signal segments meets a monomorphic waveform criterion and determines whether a second portion of the feature determined from each of the cardiac signal segments meets a supraventricular beat criterion. The control circuit determines whether a second criterion for detecting a ventricular tachyarrhythmia is satisfied and suppresses detection of a ventricular tachyarrhythmia in response to both the monomorphic waveform criterion and the supraventricular beat criterion being satisfied. In response to the first criterion and the second criterion being met, and at least one of the monomorphic waveform criterion not being met and/or the supraventricular beat criterion not being met, the control circuit detects a ventricular tachyarrhythmia and controls the therapy delivery circuit to deliver electrical stimulation therapy.

in another example, the present disclosure provides a method comprising: receiving, by a sensing circuit, a cardiac electrical signal via a sensing electrode vector; determining, by the control circuit, whether the cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia; determining a characteristic of each of a set of cardiac signal segments of the cardiac electrical signal; and in response to the first criterion being met, determining whether a first portion of the feature determined from each of the cardiac signal segments meets a monomorphic waveform criterion and whether a second portion of the feature determined from each of the cardiac signal segments meets a supraventricular beat criterion. The method further comprises the following steps: determining whether a second criterion for detecting ventricular tachyarrhythmia is met; determining whether a first portion of the plurality of features satisfies both a monomorphic waveform criterion and a second portion of the plurality of features satisfies a supraventricular beat criterion; and in response to both the first portion of the plurality of features satisfying the monomorphic waveform criterion and the second portion of the plurality of features satisfying the supraventricular beat criterion, inhibiting detection of the ventricular tachyarrhythmia. The method includes detecting a ventricular tachyarrhythmia and controlling therapy delivery circuitry to deliver electrical stimulation therapy in response to the first criterion and the second criterion being met and the first portion of the plurality of features not meeting the monomorphic waveform criterion and/or the second portion of the plurality of features not meeting at least one of the supraventricular beat criteria.

in another example, the present disclosure provides a non-transitory computer-readable storage medium storing a set of instructions that, when executed by control circuitry of an ICD, cause the ICD to: receiving, by a sensing circuit, a cardiac electrical signal via a sensing electrode vector; determining whether the cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia; determining a characteristic of each of a set of cardiac signal segments of the cardiac electrical signal; in response to the first criterion being met, determining whether a first portion of the feature determined from each of the cardiac signal segments meets a simplex waveform criterion; determining whether a second portion of the features determined from each of the cardiac signal segments meets supraventricular beat criteria; determining whether a second criterion for detecting ventricular tachyarrhythmia is met; in response to the monomorphic waveform criterion and the supraventricular beat criterion being met, inhibiting detection of the ventricular tachyarrhythmia; and in response to at least one of the first criteria and the second criteria being met and the simplex waveform criteria not being met or the supraventricular beat criteria not being met, detecting a ventricular tachyarrhythmia and delivering electrical stimulation therapy through the therapy delivery circuit.

This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the following figures and description. Further details of one or more examples are set forth in the accompanying drawings and the description below.

Drawings

fig. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICD system according to one example.

Fig. 2A-2C are schematic illustrations of a patient implanted with the extravascular ICD system of fig. 1A in different implantation configurations.

Fig. 3 is a schematic diagram of the ICD of fig. 1A-2C according to one example.

FIG. 4 is a diagram of circuitry included in the sensing circuit of FIG. 3 according to one example.

Fig. 5 is a flow diagram of a method performed by an ICD for differentiating between SVT and ventricular tachyarrhythmias according to one example.

FIG. 6 is a flow diagram of a method that may be performed by an ICD for establishing SVT morphology templates.

Fig. 7 is a diagram of one example of cardiac signal segments from which cardiac signal segment characteristics are determined when SVC discrimination is enabled.

fig. 8 is a diagram of an example cardiac signal segment with a monophasic polarity pattern.

Fig. 9 is a schematic diagram of a method for determining features from cardiac signal segments for use in determining whether a simplex waveform criterion is satisfied.

Fig. 10 is a flow diagram of a method for differentiating SVT from VT/VF and for adjusting VT/VF morphology criteria according to another example.

FIG. 11 is a flow diagram for determining whether a simplex waveform criterion is satisfied according to one example.

Fig. 12 is a flow chart for determining whether SVT beat criteria are met.

Fig. 13 is a flow diagram of a method for detecting ventricular tachyarrhythmias in accordance with one example of using SVT discrimination techniques disclosed herein.

Fig. 14 is a flow diagram of a method for detecting ventricular tachyarrhythmias according to another example.

Detailed Description

in summary, the present disclosure describes techniques for differentiating SVT from VT and VF by a cardiac medical device or system and inhibiting detection of ventricular tachyarrhythmia in response to detecting SVT. In the presence of SVT, criteria for detecting ventricular tachyarrhythmia (such as heart rate-based criteria) may be satisfied. Thus, heart rate alone may not be sufficient to reliably distinguish between SVT and VT/VF. The techniques described herein for detecting SVT allow for suppression of detection of tachyarrhythmia when evidence of SVT is identified.

in some examples, the cardiac medical device may be an extra-cardiovascular ICD system. As used herein, the term "extravascular" refers to a location outside of a blood vessel, the heart, and the pericardium surrounding the heart of a patient. Implantable electrodes carried by an extravascular lead may be positioned either outside the thorax (outside the chest cavity and sternum) or within the thorax (below the chest cavity or sternum), but are typically not in intimate contact with myocardial tissue. The techniques disclosed herein for detecting SVT and suppressing VT/VF detection may be applied to cardiac electrical signals acquired using extra-cardiovascular electrodes.

These techniques are presented herein in connection with ICDs and implantable medical leads carrying extra-cardiovascular electrodes, but aspects disclosed herein may be used in connection with other cardiac medical devices or systems. For example, the techniques for detecting SVTs described in connection with the figures may be implemented in any implantable or external medical device enabled for sensing cardiac electrical signals, including an implantable cardiac pacemaker, ICD, or cardiac monitor coupled to a transvenous, pericardial, or epicardial lead carrying sensing electrodes and therapy delivery electrodes; a leadless pacemaker, ICD, or cardiac monitor having a housing-based sensing electrode; and an external or wearable pacemaker, defibrillator, or cardiac monitor coupled to the external, body surface, or skin electrodes.

Fig. 1A and 1B are conceptual diagrams of an extravascular ICD system 10 according to one example. Fig. 1A is a front view of ICD system 10 implanted in patient 12. Fig. 1B is a side view of ICD system 10 implanted in patient 12. ICD system 10 includes ICD14 connected to extra-cardiovascular electrical stimulation and sensing lead 16. Fig. 1A and 1B are described in the context of an ICD system 10 capable of providing defibrillation and/or cardioversion shocks and pacing pulses.

ICD14 includes a housing 15, where housing 15 forms a hermetic seal that protects the internal components of ICD 14. Housing 15 of ICD14 may be formed from a conductive material such as titanium or a titanium alloy. The housing 15 may function as an electrode (sometimes referred to as a "can" electrode). The housing 15 may serve as an active (active) can electrode for use in delivering cardioversion/defibrillation (CV/DF) shocks or other high voltage pulses delivered using the high voltage therapy circuitry. In other examples, housing 15 may be used to deliver unipolar, low voltage cardiac pacing pulses in conjunction with electrodes carried by lead 16 and/or to sense cardiac electrical signals. In other examples, housing 15 of ICD14 may include a plurality of electrodes on an exterior portion of the housing. The outer portion(s) of the housing 15 that serve as the electrode(s) may be coated with a material such as, for example, titanium nitride for reducing post-stimulation polarization artifacts.

ICD14 includes a connector assembly 17 (also referred to as a connector block or header), connector assembly 17 including an electrical feedthrough through housing 15 to provide an electrical connection between conductors extending within lead body 18 of lead 16 and the electronic components included within housing 15 of ICD 14. As will be described in further detail herein, the housing 15 may house one or more processors, memories, transceivers, electrical cardiac signal sensing circuitry, therapy delivery circuitry, power sources, and other components for sensing cardiac electrical signals, detecting heart rhythms, and controlling and delivering electrical stimulation pulses to treat abnormal heart rhythms.

The elongate lead body 18 has a proximal end 27 and a distal portion 25, the proximal end 27 including a lead connector (not shown) configured to connect to the ICD connector assembly 17, the distal portion 25 including one or more electrodes. In the example shown in fig. 1A and 1B, distal portion 25 of lead body 18 includes defibrillation electrodes 24 and 26 and pace/sense electrodes 28 and 30. In some cases, defibrillation electrodes 24 and 26 may together form a defibrillation electrode because they may be configured to activate simultaneously. Alternatively, defibrillation electrodes 24 and 26 may form separate defibrillation electrodes, in which case each of electrodes 24 and 26 may be activated independently.

The electrodes 24 and 26 (and in some examples, the housing 15) are referred to herein as defibrillation electrodes because they are used individually or collectively to deliver high voltage stimulation therapy (e.g., cardioversion or defibrillation shocks). Electrodes 24 and 26 may be elongated coil electrodes and typically have a relatively high surface area for delivering high voltage electrical stimulation pulses as compared to pace and sense electrodes 28 and 30. However, electrodes 24 and 26 and housing 15 may also be used to provide pacing functions, sensing functions, or both pacing and sensing functions in addition to or instead of the high voltage stimulation therapy. In this sense, the use of the term "defibrillation electrode" herein should not be construed as limiting the electrodes 24 and 26 to only use for high voltage cardioversion/defibrillation shock therapy applications. For example, electrodes 24 and 26 may be used in sensing vectors for sensing cardiac electrical signals and detecting and differentiating SVT, VT, and VF.

electrodes 28 and 30 are relatively small surface area electrodes that may be used in sensing electrode vectors for sensing cardiac electrical signals, and in some configurations may be used to deliver relatively low voltage pacing pulses. Electrodes 28 and 30 are referred to as pace/sense electrodes because they are typically configured for low voltage applications, e.g., to serve as a cathode or anode for delivering pacing pulses and/or sensing cardiac electrical signals, as opposed to delivering a high voltage cardioversion defibrillation shock. In some examples, electrodes 28 and 30 may provide only pacing functions, only sensing functions, or both.

ICD14 may obtain cardiac electrical signals corresponding to electrical activity of heart 8 via a combination of sensing vectors that includes a combination of electrodes 24, 26, 28, and/or 30. In some examples, housing 15 of ICD14 is used in combination with one or more of electrodes 24, 26, 28, and/or 30 in a sensing electrode vector. Various sensing electrode vectors utilizing combinations of electrodes 24, 26, 28, and 30 and housing 15 are described below for acquiring first and second cardiac electrical signals, respectively, using respective first and second sensing electrode vectors selectable by sensing circuitry included in ICD 14.

In the example shown in fig. 1A and 1B, electrode 28 is located proximal to defibrillation electrode 24, and electrode 30 is located between defibrillation electrodes 24 and 26. The lead body 18 may carry one, two, or more pace/sense electrodes. For example, in some examples, a third pace/sense electrode may be positioned distal to defibrillation electrode 26. The electrodes 28 and 30 are shown as ring electrodes, however, the electrodes 28 and 30 may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, and the like. The electrodes 28 and 30 may be positioned at any location along the lead body 18 and are not limited to the locations shown. In other examples, lead 16 may not include any electrodes or include one or more pace/sense electrodes and/or one or more defibrillation electrodes.

In the example shown, lead 16 extends centrally from connector assembly 27 of ICD14 over thoracic cavity 32, subcutaneously or submuscularly toward the center of the torso of patient 12 (e.g., toward xiphoid process 20 of patient 12). At a location near xiphoid process 20, lead 16 bends or turns and extends subcutaneously or submuscularly superior over the chest cavity and/or sternum, substantially parallel to sternum 22. Although shown in fig. 1A as being laterally offset from sternum 22 and extending substantially parallel to sternum 22, distal portion 25 of lead 16 may be implanted at other locations, such as above sternum 22, offset to the right or left of sternum 22, angled from the sternum toward the left or right, etc. Alternatively, lead 16 may be placed along other subcutaneous or submuscular paths. The path of an extra-cardiovascular lead 16 may depend on the location of ICD14, the arrangement and location of electrodes carried by lead body 18, and/or other factors.

Electrical conductors (not shown) extend from the lead connector at the proximal lead end 27 through one or more lumens of the elongate lead body 18 of the lead 16 to electrodes 25, 26, 28 and 30 located along the distal portion 24 of the lead body 18. The elongated electrical conductors contained within lead body 18 are each electrically coupled to respective defibrillation electrodes 24 and 26 and pace/sense electrodes 28 and 30, which may be separate respective insulated conductors within lead body 18. The respective conductors electrically couple electrodes 24, 26, 28, and 30 to circuitry of ICD14, such as therapy delivery circuitry and/or sensing circuitry, via connections in connector assembly 17, including associated electrical feedthroughs through housing 15. Electrical conductors transmit therapy from therapy delivery circuitry within ICD14 to one or more of defibrillation electrodes 24 and 26 and/or pace/sense electrodes 28 and 30 and transmit sensed electrical signals from one or more of defibrillation electrodes 24 and 26 and/or pace/sense electrodes 28 and 30 to sensing circuitry within ICD 14.

Lead body 18 of lead 16 may be formed of a non-conductive material (including silicone, polyurethane, fluoropolymers, mixtures thereof, and other suitable materials) and shaped to form one or more lumens within which one or more conductors extend. The lead body 18 may have a tubular or cylindrical shape. In other examples, the distal portion 25 (or all portions) of the elongate lead body 18 may have a flat, ribbon, or paddle-like shape. The lead body 18 may be formed with a preformed distal portion 25, the preformed distal portion 25 being generally straight, curved, serpentine, undulating, or serrated.

In the example shown, the lead body 18 can include a curvilinear distal portion 25, the curvilinear distal portion 25 including two "C" shaped curves, which together can be similar to the greek letter epsilon ". The defibrillation electrodes 24 and 26 are each carried by one of two respective C-shaped portions of the lead body distal portion 25. The two C-shaped curves extend or curve in the same direction away from the central axis of lead body 18 along which pace/sense electrodes 28 and 30 are positioned. In some examples, pace/sense electrodes 28 and 30 may be approximately aligned with a central axis of a straight proximal portion of lead body 18, thereby laterally offsetting a midpoint of defibrillation electrodes 24 and 26 from pace/sense electrodes 28 and 30.

Other examples of extravascular leads including one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by a curved, serpentine, wavy, or saw-tooth shaped distal portion of lead body 18, which may be implemented using the techniques described herein, are generally disclosed in pending U.S. patent publication No.2016/0158567(Marshall et al). However, the techniques disclosed herein are not limited to any particular lead body design. In other examples, the lead body 18 is a flexible elongate lead body that does not have any preformed shape, bend, or curve. Various example configurations of extra-cardiovascular leads and electrodes and dimensions that may be implemented in connection with the SVT discrimination techniques disclosed herein are described in pending U.S. publication No.2015/0306375(Marshall et al) and pending U.S. publication No.2015/0306410(Marshall et al).

ICD14 analyzes cardiac electrical signals received from one or more sensing electrode vectors to monitor abnormal rhythms, such as bradycardia, SVT, VT, or vf. ICD14 may analyze heart rate and morphology of cardiac electrical signals to monitor tachyarrhythmias according to any of a variety of tachyarrhythmia detection techniques. One example technique for detecting tachyarrhythmias is described in U.S. patent No.7,761,150(Ghanem et al). Example techniques for detecting VT and VF are described below in connection with the figures. The techniques disclosed herein for differentiating SVTs from VT or VF for suppressing VT or VF detection may be incorporated into various VT/VF detection algorithms. Examples of devices and tachyarrhythmia detection algorithms that may be adapted to utilize the techniques of SVT discrimination described herein are generally disclosed in U.S. patent No.5,354,316(Keimel), U.S. patent No.5,545,186(Olson et al), U.S. patent No.6,393,316(Gillberg et al), U.S. patent No.7,031,771(Brown et al), U.S. patent No.8,160,684(Ghanem et al), and U.S. patent No.8,437,842(Zhang et al).

In response to detecting a tachyarrhythmia (e.g., VT or VF), ICD14 generates and delivers electrical stimulation therapy using a therapy delivery electrode vector (which may be selected from any of the available electrodes 24, 26, 28, 30 and/or housing 15). ICD14 may deliver ATP in response to VT detection and, in some cases, may deliver ATP prior to the CV/DF shock or during high voltage capacitor charging in an attempt to avoid the need to deliver a CV/DF shock. ICD14 may deliver one or more CV/DF shocks via one or both of defibrillation electrodes 24 and 26 and/or housing 15 if ATP did not successfully terminate VT or when VF is detected. ICD14 may deliver a CV/DF shock using electrodes 24 and 26 alone, or using electrodes 24 and 26 together as a cathode (or anode) and housing 15 as an anode (or cathode). ICD14 may generate and deliver other types of electrical stimulation pulses, such as post-shock pacing pulses or bradycardia pacing pulses, using a pacing electrode vector that includes one or more of electrodes 24, 26, 28, and 30 and housing 15 of ICD 14.

Fig. 1A and 1B are exemplary in nature and should not be considered limiting of the practice of the techniques disclosed herein. ICD14 is shown implanted subcutaneously on the left side of patient 12 along the thoracic cavity 32. In some examples, ICD14 may be implanted between the left posterior axillary line and the left anterior axillary line of patient 12. ICD14 may, however, be implanted at other subcutaneous or submuscular locations within patient 12. For example, ICD14 may be implanted in a subcutaneous pocket (pocket) in the pectoral region. In this case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn from the manubrium and extend subcutaneously or submuscularly to a desired location. In yet another example, ICD14 may be placed on the abdomen. Lead 16 may also be implanted in other extra-cardiovascular locations. For example, as described with respect to fig. 2A-2C, distal portion 25 of lead 16 may be implanted under the sternum/thorax in the substernal space.

external device 40 is shown in telemetry communication with ICD14 via communication link 42. External device 40 may include a processor, a display, a user interface, a telemetry unit, and other components for communicating with ICD14 for transmitting and receiving data via communication link 42. A Radio Frequency (RF) link may be used (such as,Wi-Fi or Medical Implant Communication Service (MICS) or other RF or communication band) establishes a communication link 42 between ICD14 and external device 40.

external device 40 may be implemented as a programmer for use in a hospital, clinic, or physician's office to retrieve data from ICD14 and to program operating parameters and algorithms in ICD14 for controlling ICD functions. External device 40 may be used to program cardiac event sensing parameters (e.g., R-wave sensing parameters), rhythm detection parameters (e.g., VT and VF detection parameters and SVT discrimination parameters), and therapy control parameters used by ICD 14. Data stored or acquired by ICD14, including physiological signals or associated data derived therefrom, results of device diagnostics, and history of detected rhythm episodes and delivered therapy, may be retrieved from ICD14 by external device 40 after an interrogation command. The external device 40 may alternatively be implemented as a home monitor or a handheld device.

fig. 2A-2C are conceptual views of a patient 12 implanted with an extravascular ICD system 10 in an implantation configuration that is different from the arrangement shown in fig. 1A-1B. Fig. 2A is an elevation view of patient 12 implanted with ICD system 10. Fig. 2B is a side view of patient 12 implanted with ICD system 10. Fig. 2C is a lateral view of patient 12 implanted with ICD system 10. In this arrangement, the extra-cardiovascular lead 16 of the system 10 is implanted at least partially below the sternum 22 of the patient 12. Lead 16 extends subcutaneously or submuscularly from ICD14 toward xiphoid process 20 and curves or turns at a location near xiphoid process 20 and extends superiorly within anterior mediastinum 36 in a substernal location.

Anterior mediastinum 36 may be considered to be bounded laterally by pleura 39, posteriorly by pericardium 38, and anteriorly by sternum 22 (see fig. 2C). Distal portion 25 of lead 16 extends substantially within the loose connective tissue of anterior mediastinum 36 and/or the substernal muscles along the posterior side of sternum 22. A lead implanted such that distal portion 25 is substantially within anterior mediastinum 36 may be referred to as a "substernal lead".

In the example shown in fig. 2A-2C, lead 16 is positioned substantially centrally below sternum 22. However, in other examples, lead 16 may be implanted such that it is laterally offset from the center of sternum 22. In some examples, lead 16 may extend laterally such that distal portion 25 of lead 16 is below chest cavity 32 in addition to sternum 22 or in place of sternum 22. In other examples, the distal portion 25 of the lead 16 may be implanted in other extra-cardiovascular intrathoracic locations (including the pleural cavity), or around and near but generally not within the perimeter of the pericardium 38 of the heart 8. Other implant locations and lead and electrode arrangements that may be used in conjunction with the SVT discrimination techniques described herein are generally disclosed in the references incorporated above.

fig. 3 is a schematic diagram of ICD14 according to one example. The electronic circuitry enclosed within housing 15 (shown schematically as electrodes in fig. 3) includes software, firmware, and hardware that cooperatively monitor cardiac electrical signals, determine when electrical stimulation therapy is needed, and deliver therapy as needed according to programmed therapy delivery algorithms and control parameters. The software, firmware, and hardware are configured to detect tachyarrhythmias and deliver anti-tachyarrhythmia therapy, e.g., detect ventricular tachyarrhythmias, and in some cases distinguish VT from VF to determine when ATP or CV/DF shocks are needed. ICD14 is coupled to an extra-cardiovascular lead, such as lead 16 carrying extra-cardiovascular electrodes 24, 26, 28 and 30, for delivering electrical stimulation pulses to the patient's heart and for sensing cardiac electrical signals.

ICD14 includes control circuitry 80, memory 82, therapy delivery circuitry 84, sensing circuitry 86, and telemetry circuitry 88. Power supply 98 provides power to the circuitry of ICD14 (including each of components 80, 82, 84, 86 and 88) as needed. Power supply 98 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between the power supply 98 and each of the other components 80, 82, 84, 86 and 88 will be understood from the general block diagram of fig. 3, but are not shown for clarity. For example, the power supply 98 may be coupled to one or more charging circuits included in the therapy delivery circuitry 84 for charging a holding capacitor included in the delivery circuitry 84 that discharges at appropriate times under the control of the control circuitry 80 for generating electrical pulses according to a therapy regimen, such as for bradycardia pacing, post-shock pacing, ATP and/or CV/DF shock pulses. The power supply 98 is also coupled to components of the sensing circuitry 86, such as sense amplifiers, analog-to-digital converters, switching circuitry, and the like, as desired.

The functional blocks shown in fig. 3 represent functions included in ICD14 and may include any discrete and/or integrated electronic circuit components that implement analog circuitry and/or digital circuitry capable of producing the functions attributed to ICD14 herein. The various components may include Application Specific Integrated Circuits (ASICs), electronic circuits, processors (shared, dedicated, or group) and memory that execute one or more software or firmware programs, combinational logic circuits, state machines, or other suitable components or combinations of components that provide the described functionality. The particular form of software, hardware, and/or firmware employed to implement the functionality disclosed herein will be determined primarily by the particular system architecture employed in the ICD and the particular detection and therapy delivery methods employed by the ICD. It is within the ability of those skilled in the art, given the disclosure herein, to provide software, hardware, and/or firmware to implement the described functionality in the context of any modern ICD system.

The memory 82 may include any volatile, non-volatile, magnetic, or electrical non-transitory computer-readable storage medium, such as Random Access Memory (RAM), Read Only Memory (ROM), non-volatile RAM (nvram), electrically erasable programmable ROM (eeprom), flash memory, or any other memory device. Further, memory 82 may include a non-transitory computer-readable medium storing instructions that, when executed by one or more processing circuits, cause control circuit 80 and/or other ICD components to perform various functions attributed to ICD14 or those ICD components. The non-transitory computer readable medium storing the instructions may include any of the media listed above.

The functionality attributed to ICD14 herein may be implemented as one or more integrated circuits. Depiction of different features as circuits is intended to highlight different functional aspects and does not necessarily imply that these circuits must be realized by separate hardware or software components. Rather, functionality associated with one or more circuits may be performed by separate hardware, firmware, or software components, or integrated within a common hardware, firmware, or software component. For example, cardiac event sensing operations and tachyarrhythmia detection operations may be performed by the sensing circuitry 86 and the control circuitry 80 in cooperation, and may include operations implemented in a processor or other signal processing circuitry included in the control circuitry 80 that executes instructions stored in the memory 82 and control signals sent from the control circuitry 80 to the sensing circuitry 86, such as blanking and timing intervals and sensing threshold amplitude signals.

the control circuitry 80 communicates with therapy delivery circuitry 84 and sensing circuitry 86, e.g., via a data bus, for sensing cardiac electrical activity, detecting heart rhythm, and controlling delivery of cardiac electrical stimulation therapy in response to sensed cardiac signals. Therapy delivery circuitry 84 and sensing circuitry 86 are electrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 and housing 15, housing 15 may function as a common or ground electrode or as an active can electrode for delivering CV/DF shock or cardiac pacing pulses.

Sensing circuitry 86 may be selectively coupled to electrodes 28, 30 and/or housing 15 in order to monitor electrical activity of the patient's heart. Sensing circuitry 86 may additionally be selectively coupled to defibrillation electrodes 24 and/or 26 for use in a sensing electrode vector or in combination with one or more of electrodes 28, 30 and/or housing 15. Sensing circuitry 86 may be enabled to selectively receive cardiac electrical signals from at least two sensing electrode vectors from available electrodes 24, 26, 28, 30 and housing 15. At least two cardiac electrical signals from two different sensing electrode vectors may be received by sensing circuitry 86 simultaneously. The sensing circuitry 86 may monitor one or both of the cardiac electrical signals at a time to sense cardiac electrical events (e.g., P-waves attendant to atrial myocardial depolarization and/or R-waves attendant to ventricular myocardial depolarization) and provide digitized cardiac signal waveforms for analysis by the control circuitry 80. For example, the sensing circuitry 86 may include switching circuitry (not shown) for selecting which of the electrodes 24, 26, 28, 30 and the housing 15 are coupled to the first sensing channel 83 and which are coupled to the second channel 85 of the sensing circuitry. The switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable for selectively coupling components of sensing circuitry 86 to selected electrodes.

Each sensing channel 83 and 85 may be configured to amplify, filter, and digitize cardiac electrical signals received from selected electrodes coupled to the respective sensing channel to improve signal quality for detecting cardiac electrical events (such as R-waves) or performing other signal analysis. The cardiac event detection circuitry within sensing circuitry 86 may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), timers, or other analog or digital components as further described in connection with fig. 4. The cardiac event sensing threshold may be automatically adjusted by sensing circuitry 86 under the control of control circuitry 80 based on the timing interval and sensing threshold determined by control circuitry 80, stored in memory 82, and/or controlled by hardware, firmware, and/or software of control circuitry 80 and/or sensing circuitry 86.

Upon detection of a cardiac electrical event based on the crossing of the sensing threshold, the sensing circuitry 86 may generate a sense event signal, such as an R-wave sense event signal, that is communicated to the control circuitry 80. In some examples, the sensing event signals may be used by the control circuitry 80 to trigger storage of segments of cardiac electrical signals for analysis to confirm the R-wave sensing event signals and differentiate SVTs as described below.

The R-wave sense event signal is also used by the control circuit 80 to determine RR intervals (RRIs) to detect tachyarrhythmias and determine a need for therapy. RRI is the time interval between two consecutive sensed R-waves and may be determined between two consecutive R-wave sensed event signals received from the sensing circuit 86. For example, the control circuitry 80 may include timing circuitry 90 for determining RRIs between successive R-wave sensed event signals received from the sensing circuitry 86 and for controlling various timers and/or counters used to time therapy delivery by the therapy delivery circuitry 84. The timing circuit 90 may additionally set time windows such as morphology template windows, morphology analysis windows, or perform other timing related functions of ICD14, including synchronizing CV/DF shocks or other therapies delivered by therapy delivery circuit 84 with sensed cardiac events.

Tachyarrhythmia detector 92 is configured to analyze signals received from sensing circuitry 86 for detecting tachyarrhythmia episodes. Tachyarrhythmia detector 92 may be implemented in control circuitry 80 as software, hardware, and/or firmware that processes and analyzes signals received from sensing circuitry 86 for detecting VT and/or VF. In some examples, tachyarrhythmia detector 92 may include comparators and counters to count RRIs determined by timing circuit 92 that fall into various frequency detection regions in order to determine ventricular rates or perform other frequency-based or interval-based evaluations for detecting and distinguishing VT and VF. For example, tachyarrhythmia detector 92 may compare the RRIs determined by timing circuit 90 to one or more tachyarrhythmia detection interval regions, such as tachycardia detection interval regions and fibrillation detection interval regions. RRIs falling in the detection interval region are counted by the respective VT interval counter or VF interval counter, and in some cases in a combined VT/VF interval counter included in tachyarrhythmia detector 92.

When the VT or VF interval counter reaches a threshold count value (commonly referred to as "number of intervals to detect" or "NID"), a ventricular tachyarrhythmia may be detected by the control circuit 80. However, tachyarrhythmia detector 92 may be configured to perform other signal analysis for determining whether other detection conditions are met before VT or VF is detected when NID has been reached. For example, cardiac signal analysis may be performed for determining whether R-wave morphology criteria, onset criteria, and oversensing rejection criteria are met in order to determine whether VT/VF detection should be conducted or suppressed. As disclosed herein, if analysis of the cardiac signal waveform characteristics indicates that the rhythm is an SVT rhythm, tachyarrhythmia detector 92 may suppress VT or VF detection when the NID has been reached.

To support additional cardiac signal analysis performed by tachyarrhythmia detector 92, sensing circuitry 86 may deliver digitized cardiac electrical signals to control circuitry 80. Cardiac electrical signals from selected sensing channels, e.g., from the first sensing channel 83 and/or the second sensing channel 85, may be passed through a filter and amplifier, provided to a multiplexer, and thereafter converted to a multi-bit digital signal by an analog-to-digital converter for storage in the memory 82, all included in the sensing circuitry 86.

The memory 82 may include a Read Only Memory (ROM) in which stored programs that control the operation of the control circuit 80 reside. Memory 82 may also include Random Access Memory (RAM) or other memory device configured as a plurality of recirculation buffers capable of holding a series of measured RRIs, counts, or other data for analysis by tachyarrhythmia detector 92. The memory 82 may be configured to store a predetermined number of cardiac electrical signal segments in a circular buffer under control of the control circuit 80. For example, up to eight cardiac electrical signal segments, each of which corresponds to an R-wave perceptual event signal, may be stored in the memory 82. Additionally or alternatively, features derived from each of up to eight cardiac signal segments each corresponding to an R-wave perceptual event signal may be buffered in memory 82 for use in SVT discrimination as described below.

therapy delivery circuitry 84 includes charging circuitry, one or more charge storage devices (such as one or more high voltage capacitors and/or low voltage capacitors), and switching circuitry that controls when the capacitor(s) discharge across a selected pacing electrode vector or CV/DF shock vector. Charging the capacitor to the programmed pulse amplitude and discharging the capacitor to the programmed pulse width may be performed by the therapy delivery circuit 84 in accordance with control signals received from the control circuit 80. The timing circuit 90 of the control circuit 80 may include various timers or counters that control when ATP or other cardiac pacing pulses are delivered. For example, timing circuit 90 may include a programmable digital counter that is set by the microprocessor of control circuit 80 for controlling the basic pacing time intervals associated with the various pacing modes or ATP sequences delivered by ICD 14. The microprocessor of the control circuit 80 may also set the amplitude, pulse width, polarity, or other characteristic of the cardiac pacing pulses, which may be based on programmed values stored in the memory 82.

In response to detecting VT or VF, the control circuitry 80 may control the therapy delivery circuitry 84 to deliver therapy such as ATP and/or CV/DF therapy. The therapy may be delivered by initiating the charging of a high voltage capacitor via a charging circuit, both of which are included in therapy delivery circuit 84. The charging is controlled by a control circuit 80, the control circuit 80 monitoring the voltage on the high voltage capacitor, which voltage is transmitted to the control circuit 80 via a charging control line. When the voltage reaches a predetermined value set by the control circuit 80, a logic signal is generated on the capacitor all-line and passed to the therapy delivery circuit 84 to terminate charging. The CV/DF pulses are delivered to the heart by the output circuitry of therapy delivery circuit 84 via a control bus under the control of timing circuit 90. The output circuit may include an output capacitor through which the charged high voltage capacitor is discharged via a switching circuit (e.g., an H-bridge) that determines the electrodes and pulse waveform used to deliver the cardioversion or defibrillation pulse. In some examples, high voltage therapy circuitry configured to deliver CV/DF shock pulses may be controlled by control circuitry 80 to deliver pacing pulses (e.g., for delivering ATP or post-shock pacing pulses). In other examples, therapy delivery circuitry 84 may include low voltage therapy circuitry for generating and delivering relatively low voltage pacing pulses for various pacing needs. Therapy delivery and control circuitry generally disclosed in any of the patents incorporated above may be implemented in ICD 14.

It is recognized that the methods disclosed herein may be implemented in an implantable medical device for monitoring cardiac electrical signals by the sensing circuitry 86 and the control circuitry 80 without therapy delivery capability, or in an implantable medical device for monitoring cardiac electrical signals by the therapy delivery circuitry 84 and delivering cardiac pacing therapy without high voltage therapy capability (such as cardioversion/defibrillation shock capability or vice versa).

Control parameters used by the control circuit 80 to detect cardiac arrhythmias and control therapy delivery may be programmed into the memory 82 via the telemetry circuit 88. Telemetry circuitry 88 includes a transceiver and antenna for communicating with external device 40 (shown in fig. 1A) using RF communication or other communication protocols as described above. Under the control of control circuitry 80, telemetry circuitry 88 may receive downlink telemetry from external device 40 and transmit uplink telemetry to external device 40. In some cases, the telemetry circuitry 88 may be used to transmit and receive communication signals to and from another medical device implanted within the patient 12.

Fig. 4 is a diagram of circuitry included in first sensing channel 83 and second sensing channel 85 of sensing circuitry 86 according to one example. The first sense channel 83 may be selectively coupled via the switching circuitry 61 to a first sense electrode vector comprising electrodes carried by the extravascular lead 16 as shown in fig. 1A-2C for receiving a first cardiac electrical signal. The first sense channel 83 may be coupled to a sense electrode vector that is a short bipolar (bipole) having a relatively shorter inter-electrode distance or spacing than a second electrode vector coupled to the second sense channel 85. For example, a first sensing electrode vector may include pace/sense electrodes 28 and 30. In other examples, the first sensing electrode vector coupled to sensing channel 83 may include defibrillation electrodes 24 and/or 26, e.g., a sensing electrode vector between pace/sense electrode 28 and defibrillation electrode 24 or between pace/sense electrode 30 and either of defibrillation electrodes 24 or 26. In still other examples, the first sensing electrode vector may be between defibrillation electrodes 24 and 26.

In some patients, bipolarity between the electrodes carried by the lead 16 may result in patient body posture-dependent changes in cardiac electrical signals, as the bipolar sensing vector relative to the cardiac axis changes with changes in patient body posture or body motion. Accordingly, the sensing electrode vector coupled to first sensing channel 83 may include housing 15 and any of electrodes 24, 26, 28, and 30 carried by lead 16. A relatively long bipolar electrode comprising the housing 15 and lead-based electrodes may be less sensitive to posture changes. Cardiac electrical signals received via extra-cardiovascular electrodes may be affected by changes in the patient's posture compared to electrodes carried by venous leads. For example, as the patient's posture changes, the amplitude, polarity, and waveform of the R-wave may change. Thus, the R-wave morphology analysis performed to distinguish between SVT and VT/VF may result in false VT/VF detections when the amplitude and/or morphology of the R-wave has changed due to patient posture changes. The techniques disclosed herein can be used to detect and differentiate SVTs even when changes in patient posture result in QRS amplitude and morphology changes for avoiding false detections of VT and VF and unnecessary electrical stimulation treatment.

Sensing circuitry 86 includes a second sensing channel 85, which second sensing channel 85 receives a second cardiac electrical signal from a second sensing vector, such as a vector including pace/sense electrodes 28 or 30 paired with housing 15. In some examples, second sensing channel 85 may be selectively coupled to other sensing electrode vectors, which may form relatively long dipoles with larger inter-electrode distances or spacings than the sensing electrode vectors coupled to first sensing channel 83. As described below, the second cardiac electrical signal received by the second sensing channel 85 via the long dipole may be used by the control circuit 80 for morphology analysis and for determining cardiac signal segmentation characteristics for SVT discrimination. In other examples, any vector selected from available electrodes (e.g., electrodes 24, 26, 28, 30 and/or housing 15) may be included in the sensing electrode vector coupled to second sensing channel 85. The sense electrode vectors coupled to first sense channel 83 and second sense channel 85 are typically different sense electrode vectors that may have no common electrodes or only one common electrode instead of two.

in the illustrative example shown in FIG. 4, the electrical signals generated across the first sensing electrode vector are received by sensing channel 83, and the electrical signals generated across the second sensing electrode vector are received by sensing channel 85. The cardiac electrical signals are provided as differential input signals to the pre-filter and pre-amplifier 62 and the pre-filter and pre-amplifier 72 of the first sensing channel 83 and the second sensing channel 85, respectively. The non-physiological high frequency and DC signals may be filtered by low pass or band pass filters included in each of the pre-filter and pre-amplifier 62 and the pre-filter and pre-amplifier 72, and the high voltage signals may be removed by protection diodes included in the pre-filter and pre-amplifier 62 and the pre-filter and pre-amplifier 72. The pre-filter and pre-amplifier 62 and 72 may amplify the pre-filtered signals by a gain between 10 and 100, and in one example by a gain of 17.5, and may convert the differential signals to single-ended output signals that are transmitted to an analog-to-digital converter (ADC)63 in the first sensing channel 83 and an ADC 73 in the second sensing channel 85. The pre-filter and amplifier 62 and the pre-filter and amplifier 72 may provide anti-aliasing filtering and noise reduction prior to digitization.

The ADC63 and the ADC 73 convert the first cardiac electrical signal from an analog signal to a first digital bit stream and the second cardiac electrical signal to a second digital bit stream, respectively. In one example, the ADCs 63 and 73 may be Sigma Delta Converters (SDC), although other types of ADCs may be used. In some examples, the outputs of ADC63 and ADC 73 may be provided to decimators (not shown) that act as digital low pass filters that increase resolution and reduce the sampling rate of the respective first and second cardiac electrical signals.

In the first sensing channel 83, the digital output of the ADC63 is passed to a filter 64, which filter 64 may be a digital band pass filter having a band pass of about 10Hz to 30Hz for passing cardiac electrical signals such as R-waves that typically occur in this frequency range. The band-pass filtered signal is passed from the filter 64 to a rectifier 65 and then to an R-wave detector 66. In some examples, the filtered, digitized cardiac electrical signal from sensing channel 83 (e.g., the output of filter 64 or rectifier 65) may be stored in memory 82 for signal processing by control circuit 80 for detecting and distinguishing tachyarrhythmia episodes.

the R-wave detector 66 may include an automatically adjusting sense amplifier, comparator, and/or other detection circuitry that compares, in real time, the filtered and rectified first cardiac electrical signal to an R-wave sensing threshold and generates an R-wave perceptual event signal 68 when the cardiac electrical signal crosses the R-wave sensing threshold outside of a post-sensing blanking period.

the R-wave sensing threshold controlled by sensing circuit 86 and/or control circuit 80 may be a multi-level sensing threshold as disclosed in pending U.S. patent application No.15/142,171(Cao et al, filed 4/29 2016). Briefly, the multi-level sensing threshold may have a starting sensing threshold that is held for a time interval (which may be equal to a tachycardia detection interval or an expected R-wave to T-wave interval) and then falls to a second sensing threshold that is held until expiration of a falling time interval (which may be a 1 to 2 second field). After the fall time interval, the sensing threshold is lowered to a minimum sensing threshold, which may correspond to a programmed sensitivity. In other examples, the R-wave sensing threshold used by R-wave detector 66 may be set to a starting value based on the most recently perceived amplitude of the R-wave peak and decay linearly or exponentially over time until a minimum sensing threshold is reached. The techniques described herein are not limited to a particular behavior of the sensing threshold. Rather, other attenuated, step-wise adjusted, or other automatically adjusted sensing thresholds may be used.

The second cardiac electrical signal digitized by the ADC 73 of the sensing channel 85 may be passed to the filter 74 for bandpass filtering. In some examples, filter 74 is a broadband filter for passing frequencies from 1Hz to 30Hz or higher. In some examples, sense channel 85 includes notch filter 76. The notch filter 76 may be implemented in firmware or hardware and is used to attenuate 50Hz or 60Hz electrical noise in the second cardiac electrical signal. Cardiac electrical signals acquired using extra-cardiovascular electrodes are more susceptible to 50Hz to 60Hz electrical noise, muscle noise, and other EMI, electrical noise, or artifacts than cardiac electrical signals acquired using intravenous or intracardiac electrodes. Accordingly, a notch filter 76 may be provided for significantly attenuating the amplitude of signals in the range of 50Hz to 60Hz, with minimal attenuation of signals in the range of approximately 1Hz to 30Hz corresponding to typical cardiac electrical signal frequencies.

The output signal 78 of the notch filter 76 may be transferred from the sensing circuit 86 to the memory 82 under the control of the control circuit 80 in order to store the segment of the second cardiac electrical signal 78 in a temporary buffer of the memory 82. For example, the timing circuit 90 of the control circuit 80 may set a time interval or a plurality of sampling points over which the second cardiac electrical signal 78 is stored in the memory 82 relative to the R-wave perceptual event signal 68 received from the first sensing channel 83. The buffered second cardiac electrical signal segment may be analyzed by the control circuitry 80 on a triggered, as needed basis, e.g., as described in connection with fig. 13, for determining cardiac signal segment characteristics for differentiating SVT and suppressing interval-based VT or VF detection even if other R-wave morphology analyses meet VT/VF detection criteria.

Notch filter 76 may be implemented as a digital filter for real-time filtering performed by firmware as part of sense channel 85 or by control circuit 80 to filter the buffered digital output of filter 74. In some examples, the output of filter 74 of sense channel 85 may be stored in memory 82 in time segments defined relative to R-wave perceptual event signal 68 prior to filtering by notch filter 76. When the control circuit 80 is triggered to buffer and analyze the segment of the second cardiac electrical signal, for example as described in connection with fig. 13, the notch filter 76 may be applied to the second cardiac electrical signal prior to the morphology analysis and determination of the cardiac signal segment characteristics for SVT discrimination.

the configuration of sensing channels 83 and 85 shown in fig. 4 is exemplary in nature and should not be considered limiting of the techniques described herein. Sense channels 83 and 85 of sensing circuitry 86 may include more or fewer components than shown and described in fig. 4. The first sensing channel 83 may be configured for detecting R-waves from the first cardiac electrical signal in real-time based on crossing an R-wave sensing threshold by the first cardiac electrical signal, for example in a hardware-implemented component, and the second sensing channel 85 may be configured for providing the second cardiac electrical signal for storage in the memory 82 for processing and analysis by the control circuitry 80 for determining whether a signal waveform morphology corresponding to R-waves perceived in the first sensing channel is indicative of VT or VF, or whether a signal waveform characteristic supports SVT detection and suppression of VT or VF detection. In other examples, the two sensing channels 83 and 85 may be capable of sensing R-waves in real-time and/or the two channels 83 and 85 may provide digitized cardiac signals for buffering in the memory 82 for morphology signal analysis during the VT/VF detection algorithm.

Fig. 5 is a flowchart 100 of a method performed by ICD14 for distinguishing SVT from ventricular tachyarrhythmias, according to one example. The flow diagram 100 provides techniques for: SVT is differentiated from VT/VF even when certain VT/VF morphology criteria are met to account for QRS morphology changes that may occur due to patient posture changes in an extravascular ICD system. Techniques for SVT discrimination may be usefully practiced in other ICD or other medical device systems and are not necessarily limited to implantable extravascular systems. The techniques of flowchart 100 may be implemented in connection with various VT/VF detection algorithms for causing VT/VF detection to be suppressed when SVT discrimination criteria are met even though other VT/VF detection criteria may be met (such as RRI-based detection criteria and/or R-wave morphology-based VT/VF detection criteria).

At block 102, the control circuit 80 builds an SVT morphology template. The morphology template may be established according to the techniques disclosed in U.S. patent No.6,393,316(Gillberg et al) incorporated above, as well as generally described below in connection with fig. 6. The SVT morphology template represents the expected R waveform morphology during supraventricular rhythms, which may be sinus rhythm or atrial tachyarrhythmia conducted to the ventricles. Although referred to herein as an "SVT" template, the template may be acquired during a slow non-paced ventricular rhythm representing a normal QRS waveform resulting from the sinoatrial node, and not necessarily during supraventricular tachycardia. In other examples, the SVT templates may be acquired during sinus tachycardia, such as during patient exercise.

At block 104, the control circuitry 80 compares the SVT template to the morphology of the waveform of the cardiac electrical signal corresponding to the perceived R-wave received from the sensing circuitry 86. The comparison may be made on a continuous beat-to-beat basis or only when other conditions are met, such as when the heart rate rises. The morphological comparison may be performed using the wavelet transform techniques generally disclosed in the above-incorporated' 316 patent. The comparison determines a morphology matching score, which is a measure of the correlation between the SVT morphology template and the unknown cardiac electrical signal waveform that may be stored in memory in response to the sensed R-wave. For example, a Haar transform or other wavelet transform technique may generate a set of wavelet coefficients of a signal waveform. The wavelet coefficients may have predetermined weights representing the amplitudes of the frequency components of the signal waveform. These wavelet coefficients may be compared to wavelet coefficients determined from the SVT template, and the morphology matching score may represent a correlation between the wavelet coefficients of the SVT template and the unknown signal waveform. A match score threshold may be defined below which unknown cardiac signal waveforms are not considered R-waves corresponding to supraventricular rhythms and above which the waveforms are considered R-waves of supraventricular rhythms. In one example, the morphology matching score may be from 1 to 100, and the morphology matching score may be set to 60, 70, or another predetermined value.

At block 104, the control circuit 80 determines whether morphology criteria for detecting VT/VF are met based on a morphology comparison between the SVT template and an unknown cardiac signal waveform received by the control circuit 80 from the sensing circuit 86 and corresponding to the R-wave perceptual event signals. VT/VF morphology criteria may be defined for determining whether a cardiac signal is likely to represent a heart rhythm originating in a ventricle. In one example, VT/VF morphology criteria may require that a predetermined percentage or ratio of R-wave sensed event signals be classified as non-SVT beats based on a morphological analysis of the cardiac signal waveform corresponding to each R-wave sensed event signal. The R-wave perceptual event signals may be classified as SVT beats or non-SVT beats based on the most recent Y-shape matching score of the unknown cardiac signal waveform.

For example, if at least 6 of the 8 of the most recently acquired cardiac signal waveforms (each corresponding to an R-wave perceptual event signal) do not match the SVT morphology template based on the morphology matching score being below the matching threshold, the latest one of the R-wave perceptual event signals in the group of 8 perceptual event signals is classified as a potential VT/VF beat. When a threshold number of R-wave perceptual event signals are classified as potential VT/VF beats, VT/VF morphology criteria may be satisfied at block 104. The threshold number of R-wave perceptual events classified as potential VT/VF beats required to meet the VT/VF morphology criteria may be one or more and may be dynamically adjusted by the control circuit 80, for example, as described in connection with fig. 10.

Each of the eight (or other predetermined number) cardiac signal waveforms used to classify a given R-wave perceptual event signal as a potential VT/VF beat may be acquired from a cardiac electrical signal segment that is derived from a second cardiac electrical signal received from a second sensing channel 85 of the sensing circuit 86 and buffered in the memory 82. These cardiac signal segments may be acquired over a time interval set based on the timing of the R-wave perceptual event signals produced by the first sensing channel 83. As described below, when the control circuit 80 determines that a particular condition is satisfied, e.g., when evidence of a fast ventricular rate has been detected based on counting a predetermined number of VT or VF intervals, buffering of these cardiac signal segments, each corresponding to an R-wave perceptual event signal generated by the first sensing channel 83, may be triggered at block 104.

if the VT/VF morphology criteria are not met at block 104, the control circuitry 80 continues to monitor the cardiac electrical signal by analyzing the buffered cardiac signal segment corresponding to the R-wave sensed event signal for evidence of VT/VF morphology. If the VT/VF morphology criteria are met at block 104, the control circuit 80 determines whether the criteria for enabling SVT discrimination are met at block 106. In one example, the criteria for enabling SVT discrimination require that the VT/VF morphology criteria be satisfied at block 104 and that at least X of the Y most recent cardiac signal waveforms match the SVT morphology template with a match score greater than a second match threshold. This second match threshold is less than the first morphology match threshold applied at block 104 for classifying an R-wave perceptual event as a potential VT/VF. In one example, if the first match threshold applied at block 104 is 60, then the second match threshold applied at block 106 is 20.

For purposes of illustration, if the cardiac signal waveforms are required to have a matching threshold 60 less than according to the VT/VF morphology criteria at block 104, SVT discrimination may be enabled at block 106 when the VT/VF morphology criteria are met and at least 6 of the 8 most recent cardiac signal waveforms corresponding to R-wave perceptual event signals match the SVT morphology template with a matching score of at least 20. SVT discrimination may be enabled when the waveform match score of the most recent set of cardiac signal waveforms is not highly correlated with the SVT template (below a first match threshold) (suggesting the presence of ventricular tachyarrhythmia) but is greater than a second, lower match threshold. The QRS morphology during the supraventricular rhythm compared to the SVT template may change (e.g., due to a change in posture of the patient), resulting in a morphology match score below the first match threshold. To avoid false VT/VF detections due to changes in QRS morphology during the SVT rhythm, SVT discrimination is enabled at block 106 if the morphology match score that results in satisfying the VT/VF morphology criteria is at least greater than a second match threshold.

On the other hand, a very low morphology match score, less than the second match threshold, indicates a very low correlation with the SVT template. If less than 6 (or other predetermined percentage) of the 8 of the most recent cardiac signal segments have morphology matching scores below a second threshold, then there is a likelihood of a VT/VF rhythm. In this case, SVT discrimination is not enabled. If the criteria for enabling SVT discrimination are not met at block 106, the VT/VF morphology criteria met at block 104 may be determined to be evidence of ventricular-derived tachyarrhythmia and no further discrimination is required. The control circuit 80 proceeds toward VT/VF detection at block 120 without performing additional morphological analysis of the cardiac signal to differentiate SVTs.

If the criteria for enabling SVT discrimination are met at block 106, for example, if at least X of the Y morphology matching scores are greater than a second, lower morphology matching threshold (but less than a first, higher matching threshold), the control circuit 80 begins determining a plurality of features for distinguishing SVT from each of the buffered cardiac signal segments of VT/VF at block 108. The determination of the cardiac signal segmentation characteristics is described below in connection with fig. 7-9. At block 110, a first portion of cardiac signal segment characteristics determined from each of a set of Y cardiac signal segments is compared to a simplex waveform standard. At block 112, a second portion of the cardiac signal segment characteristics determined from each of the Y buffered signal segments is compared to the SVT beat criteria.

If the monomorphic waveform criteria and the SVT beat criteria are met at blocks 110 and 112, the rhythm may be identified as an SVT rhythm at block 114. VT/VF detection, indicated by the satisfaction of the VT/VF morphology criteria at block 104, is suppressed at block 116 and no VT/VF therapy is subsequently delivered. At block 118, if the cardiac signal segment continues to fail to match the SVT template based on the first match threshold at block 104, the control circuit 80 may adjust the VT/VF morphology detection criteria to delay VT/VF detection. For example, the number of R-wave perceptual event signals required to be classified as potential VT/VF beats in order to satisfy the VT/VF morphological criteria at block 104 may be reset or adjusted at block 118, as described below in connection with fig. 10.

in some examples, both the monomorphic waveform criteria and the SVT beat criteria are required to be satisfied at blocks 110 and 112 in order to detect SVT and inhibit VT/VF detection and treatment. If either of the simplex waveform criteria or the SVT beat criteria is not met, then no SVT is detected. The control circuit 80 proceeds to block 120 and if the VT or VF detection criteria are met at block 120, VT or VF is detected at block 122. For example, if the VT/VF morphology criteria are met at block 104 and SVT detection is not performed ("no" branch of block 112), the control circuitry 80 may detect a VT or VF at block 122 if the corresponding VT or VF NID is reached at block 120. If the interval-based VT/VF detection criteria are not met at block 120, the control circuitry continues to monitor the cardiac electrical signal by returning to block 104.

If the VT/VF morphology criteria and VT/VF interval criteria are met at respective blocks 104 and 120, respectively, and the SVT beat criteria are not met at block 112 (or the SVT arbiter is not enabled at block 106), VT or VF is detected at block 122. Electrical stimulation therapy may be scheduled and delivered at block 122. The electrical stimulation therapy may include ATP and/or CV/DF shocks delivered by therapy delivery circuitry 84 according to a programmed therapy protocol for a detected VT or VF rhythm.

The blocks shown in fig. 5 may be performed in a different order than the specific order shown in fig. 5. In other examples, in response to a threshold number of RRIs falling within a VT or VF interval region (indicating that a fast heart rate is likely to occur), morphology matching scores and cardiac signal segment characteristics may be determined at blocks 104 and 108. The control circuit 80 may determine whether the simplex waveform criteria and the SVT beat criteria are met at blocks 110 and 112, respectively, in response to the NID having been reached for determining whether VT/VF detection should be performed or suppressed based on the SVT discrimination criteria being met. In yet other examples, the morphology matching score and/or cardiac signal segment characteristics may be determined on a beat-by-beat basis, such that VT or VF detection may be conducted once NID is reached as long as at least a recent set of Y cardiac signal segments does not satisfy the simplex waveform and SVT beat criteria.

Fig. 6 is a flow chart 150 of a method that may be performed by the control circuit 80 for establishing SVT morphology templates at block 102 of fig. 5. At block 152, the control circuit 80 may acquire a predetermined number of R-wave signals (or QRS complexes) from the cardiac electrical signals received from the sensing circuit 86 during a known supraventricular rhythm. For example, a sinus rhythm may be confirmed manually by a user using external device 40, or automatically by detecting a normal heart rate (e.g., below a tachyarrhythmia rate associated with a VT/VF detection interval) and/or a regular, stable R-wave signal. For example, three or more R-wave signals may be acquired at block 152. These R-wave signals may be notch filtered signals received from the second sensing channel 85, each of which corresponds to an R-wave perceptual event signal received from the first sensing channel 83. The notch filtered R-wave signal segments may be time aligned with respect to a corresponding R-wave perceptual event signal. In other examples, different reference time points or sample numbers may be used to align R-wave signal segments, such as maximum peaks or other reference points. The notch filtered R-wave signal may then be ensemble averaged to obtain an average R-wave signal for use in building an SVT morphology template for use in determining whether VT/VF morphology criteria are met at block 104 of fig. 5. In other examples, a template may be generated from a single R-wave signal acquired during a sinus rhythm.

At block 154, wavelet transform coefficients are determined from the average R-wave signal. The determination of wavelet transform coefficients may be performed according to the' 316 patent (Gillberg et al) incorporated above. The digitized average R-wave signal and/or wavelet transform coefficients may be stored in the memory 82 as an SVT morphology template.

At block 156, template features are determined from the averaged R-wave signal. These template features are used when performing SVT discrimination (e.g., whether SVT discrimination criteria are met at block 106 of fig. 5). In some examples, the template features determined at block 156 are used at block 112 of fig. 5 for comparison with a portion of the signal waveform features determined from each cardiac signal segment in a set of Y cardiac signal segments. When SVT discrimination has been enabled for determining whether SVT beat criteria are satisfied at block 112, the template features may be compared to signal waveform features determined from cardiac signal segments. The template features determined at block 156 may be used or not used in determining whether the simplex waveform criteria are satisfied at block 110 of fig. 5. As described below in connection with fig. 11, a portion of the signal waveform characteristics determined from each of a set of Y cardiac signal segments may be compared to one another for use in determining whether the simplex waveform criteria are satisfied without comparing the signal waveform characteristics to SVT template characteristics.

the template features determined at block 156 may include a polarity pattern, a peak time interval, and an average signal width. Example techniques for determining these characteristics are described in conjunction with fig. 7 and 8. When SVT discrimination has been enabled by the control circuit 80, the template features are stored in the memory 82 for use in determining whether SVT beat criteria are met.

Fig. 7 is a diagram of one example of a notch filtered cardiac signal segment 160 from which cardiac signal segment features are determined at block 108 of fig. 5 when SVT discrimination is enabled. Cardiac signal segment 160 can include a predetermined number of sample points before and after an R-wave sense event signal 162 generated by sensing circuitry 86, the R-wave sense event signal 162 corresponding to a time at which an R-wave sensing threshold is crossed by the cardiac electrical signal. In one example, cardiac signal segment 160 includes 48 sample points, with R-wave perceptual event signal 162 occurring at twenty-four sample points.

R-wave sensed event signal 162 may be generated when cardiac signal segment 160 crosses an R-wave sensing threshold, but R-wave sensed event signal 162 may be generated when a different cardiac electrical signal (e.g., from a different one of sensing channels 83 and 85) crosses an R-wave sensing threshold. For example, in response to a first cardiac electrical signal received by the first sensing channel 83 crossing an R-wave sensing threshold, the first sensing channel 83 may generate an R-wave perceptual event signal 162. Cardiac signal segment 160 may be buffered from a second cardiac electrical signal received by control loop 80 from second sensing channel 85. The R-wave perceptual event signal 162 from the first sensing channel 83 is used as a timing marker for selecting a starting sample point and an ending sample point stored from the second cardiac electrical signal for buffering the cardiac signal segment 160. In this way, the first sensing channel 83 may be used to sense R-waves, and the second sensing channel 85 may be used to acquire cardiac signal segments from different sensing vectors. Each cardiac signal segment corresponds to an R-wave perceptual event signal 162. Cardiac signal segment features are determined from each cardiac signal segment, such as segment 160, for SVT discrimination.

One characteristic determined from the cardiac signal segment 160 may be its polarity pattern. The R-wave signal may have a biphasic polarity with a distinct positive peak and a distinct negative peak. At other times, the R-wave signal may have a single phase polarity (positive or negative) characterized by a single main peak. The control circuit 80 may be configured to identify and distinguish four polarity patterns: a biphasic polarity pattern with a positive peak followed by a negative peak; a biphasic polarity pattern with a negative peak followed by a positive peak; single phase polarity pattern with positive main peaks or single phase polarity pattern with negative main peaks. For a predetermined number of cardiac signal segments analyzed during SVT discrimination, a polarity pattern value may be assigned to each of the possible polarity patterns for buffering in memory 82. For example, the four polarity patterns listed above may be assigned values of 1 through 4, respectively. In other examples, the polarity pattern identified by the control circuit 80 may not be limited to only the four patterns listed above; the control circuit 80 may be configured to identify fewer, additional, or different polarity patterns than the four polarity patterns listed above. The identified polarity pattern may be customized for the individual patient or based on the implant location of the sensing electrode vector. For example, the R-wave signal may include more than two distinct peaks in a three-phase signal or may include a signal with distinct separate positive peaks and distinct negative peaks (or vice versa).

The control circuit 80 may determine the polarity pattern of the cardiac signal segment 160 by determining the maximum positive amplitude 164 of the maximum peak 163 and the maximum negative amplitude 166 of the minimum peak 165. Maximum positive amplitude 164 and maximum negative amplitude 166 maximum absolute values are identified and may be used by the control circuit 80 for setting the polarity pattern amplitude threshold. If the absolute values of both the maximum positive amplitude 164 and the maximum negative amplitude 166 are greater than the polarity pattern amplitude threshold, then the cardiac signal segment 160 is determined to have a biphasic polarity pattern. If only one of the maximum amplitudes 164 or 166 is greater than the polarity pattern amplitude threshold, the cardiac signal segment is determined to have a monophasic polarity pattern.

In the illustrative example, the polarity pattern amplitude threshold is set to 25% of the largest of the maximum positive amplitude 164 and the maximum negative amplitude 166. In the particular example shown in fig. 7, the absolute value of the maximum positive amplitude 164 is greater than the absolute value of the maximum negative amplitude 166. The control circuit 80 thus uses the maximum positive amplitude 164 to set the polarity pattern amplitude threshold to 25% of the maximum positive amplitude 164. The absolute value of the maximum negative amplitude 166 is compared to a polarity pattern amplitude threshold. The cardiac signal segment 160 is determined to have a biphasic polarity pattern because it is greater than the polarity pattern amplitude threshold, i.e., greater than 25% of the maximum positive amplitude 164 in this example.

the control circuit 80 may further determine that the positive peak 163 occurs earlier in time than the negative peak 165, resulting in a bi-phase polarity pattern with the positive peak first. The number of sample points for the maximum peak 163 and the minimum peak 165 may be compared for determining whether the biphasic mode is with the positive peak first or the negative peak first. The sample points in the cardiac signal segment 160 may be numbered consecutively from beginning to end, e.g., from 1 to 48 when 48 sample points are included in the cardiac signal segment 160. The lower number of sample points for the maximum peak 163 and the higher number of sample points for the minimum peak 165 indicate the polarity pattern of the first positive peak. The control circuit 80 may store a value in the memory 82 indicating that the polarity pattern of the cardiac signal segment 160 is the biphasic first positive peak.

The second SVT discrimination feature of the cardiac signal segment 160 may be determined as the peak time interval 168. In the example of a bi-phase polarity pattern, the peak time interval 168 may be determined as the time interval between the maximum peak 163 and the minimum peak 165. For cardiac signal segments 160, a peak time interval 168 may be determined as the difference between the corresponding number of sample points for the largest positive peak 163 and the smallest negative peak 165 and the peak event interval 168 stored in memory.

Fig. 8 is a diagram of an example cardiac signal segment 170 having a monophasic polarity pattern. The maximum positive amplitude 174 is used by the control circuit 80 to set the polarity pattern amplitude threshold because the maximum positive amplitude 174 is greater than the maximum negative amplitude 176. The absolute value of the maximum negative amplitude 176 of the minimum peak 175 is less than the polarity pattern threshold, which may be set to one-quarter of the maximum positive amplitude 174. The maximum peak 173 is therefore the only major peak. The control circuit 80 identifies the cardiac signal segment 170 as having a single-phase positive polarity pattern and stores a polarity pattern value in the memory 82 that indicates the polarity pattern of the cardiac signal segment 170.

When the polarity pattern is determined to be single-phase, the control circuit 80 may determine the peak time interval 178 using a different method than the peak time interval 168 used to determine the bi-phase polarity pattern signal 160 as shown in fig. 7. The peak time interval 178 of the monophasic signal may be determined as the time interval or sample point number difference between the R-wave perceived event signal 172 and the main peak (maximum peak 173 in this example).

In addition to the peak time intervals and polarity patterns, the third SVT discrimination features that may be determined from the cardiac signal segments 160 and 170 of fig. 7 and 8 may be normalized signal widths. The cardiac signal segment 160 or 170 may be rectified and all sample point amplitudes may be summed to obtain the region defined by the signal segment 160 or 170. The region of the signal 160 or 170 may be divided by the largest absolute value of the maximum peak amplitude 164 or 174 or the minimum peak amplitude 166 or 176 of the respective signal segment for obtaining the normalized signal width. Each of these three features (i.e., polarity pattern, peak time interval, and normalized signal width) is stored for each buffered cardiac signal segment during SVT discrimination. These three cardiac signal segment features may be used to determine whether the perceived R-wave satisfies the SVT beat criteria at block 112 of fig. 5 and as further described below in connection with fig. 12.

The polarity pattern, peak time interval, and normalized signal width may be determined from the SVT templates in a similar manner at block 156 of fig. 6. In this manner, cardiac signal segment features determined from signal segments obtained during unknown cardiac rhythms may be compared to similar (analog) SVT template features when SVT discrimination is enabled for determining whether SVT beat criteria are satisfied.

Fig. 9 is a schematic diagram of a notch filtered cardiac signal segment 180 for use in determining whether the simplex waveform criteria are met at block 110 of fig. 5. In addition to the features determined from the cardiac signal segments for determining whether the SVT beat criteria are met, features for applying the simplex waveform criteria are determined from the buffered cardiac signal segments when SVT discrimination is enabled. One characteristic determined by the control circuit 80 for use in determining whether the simplex waveform criteria are met may be a maximum number of peak samples. As described above, the sample points included in the buffered cardiac signal segment 180 may be numbered consecutively from beginning to end, e.g., from 1 to 48, with the corresponding R-wave perceptual event signal 182 at sample point 24. The polarity of the main peak of the SVT template may be determined at block 156 of fig. 6. The maximum peak sample number for the signal segment 180 is determined as the number of sample points having the largest absolute amplitude and the same polarity as the main peak of the SVT template.

For example, if the SVT template is identified as having a primary peak with a positive polarity, i.e., the absolute maximum amplitude of the SVT template is positive, then the number of sample points for the maximum peak 183 is identified as the maximum peak sample number. However, if the SVT template has the largest absolute amplitude corresponding to the smallest (negative) peak, then the number of sample points for the smallest peak 185 is determined to be the maximum peak sample number. The control circuit 80 may search for the maximum of all positive values of the cardiac signal segment 180 if the dominant peak of the SVT template is positive, or the control circuit 80 may search for the minimum of all negative values of the cardiac signal segment 180 if the dominant peak of the SVT template is negative. The corresponding maximum or minimum number of sample points is stored as the maximum number of peak samples for the corresponding cardiac signal segment 180.

In addition to determining the maximum peak sample number, the control circuit 80 may determine the maximum peak amplitude 184 or 186 of the maximum peak 183 or 185 having a polarity matching the dominant peak of the SVT template. In some cases, the maximum peak sample number and maximum peak amplitude determined from cardiac signal segment 180 as an SVT discrimination feature may or may not correspond to the actual absolute maximum peak amplitude of segment 180 when the actual absolute maximum peak of segment 180 has a polarity different from the polarity of the maximum absolute amplitude of the SVT template.

The third SVT discrimination feature determined from the cardiac signal segment 180 for determining whether the simplex waveform criterion is satisfied may be the RRI from the R-wave perceptual event signal 182 to the nearest preceding R-wave perceptual event signal. The maximum peak sample number and maximum peak amplitude with a polarity matching the maximum absolute amplitude of the SVT template and RRIs may both be stored as characteristics of segment 180 for use in determining whether the simplex waveform criteria are satisfied when SVT discrimination is enabled.

In other examples, the absolute maximum peak of the cardiac signal segment 180 may be identified and its amplitude and number of samples may be stored as a characteristic of the cardiac signal segment 180 regardless of its polarity. The maximum peak 183 has a greater amplitude 184 than the amplitude 186 of the minimum peak 185. The maximum peak amplitude 184 and the number of samples of the maximum peak 183 may be stored as characteristics of the cardiac signal segment 180. If the minimum peak 185 has a negative peak amplitude 186 (absolute value) that is greater than the positive peak amplitude 184, the absolute value of the negative peak amplitude 186 and the number of samples of the minimum peak 185 may be stored as a feature of the cardiac signal segment 180.

Fig. 10 is a flow chart 200 of a method for distinguishing SVTs from VT/VFs and for adjusting VT/VF morphological criteria, according to another example. At block 202, the control circuit 80 sets the VT/VF morphology count to an initial value. The control circuitry 80 may include a counter for counting the number of perceived R-waves classified as potential VT/VF beats based on at least X of the Y most recent cardiac signal segments having morphology matching scores less than a first matching threshold. In the illustrative example presented herein, the counter is initially set to a value of zero, thereby causing a single set of Y cardiac signal segments that result in only the most recent R-wave perceptual event signals to be classified as resulting in potential VT/VF beats that meet VT/VF morphological criteria.

at block 204, morphology matching scores for the Y cardiac signal segments buffered in the memory 82 are determined. The Y morphology match scores are compared to a match threshold at block 206. These Y morphology match scores may be compared to a match threshold on a beat-by-beat basis, thereby enabling each R-wave perceptual event signal to be classified as a potential VT/VF beat if at least X of the most recent Y morphology match scores stored in the rolling buffer of the memory 82 are less than the first match threshold. If fewer than X of the Y morphology match scores are less than the match threshold (i.e., the "no" branch of block 206), the most recently perceived R-wave is not classified as a potential VT/VF beat.

In response to not classifying the most recently sensed R-wave as a potential VT/VF beat, the control circuit adjusts the VT/VF counter for delaying detection of VT or VF. For example, when at least X of the Y match scores are not less than the match threshold at block 206, the counter value initialized to zero at block 202 may be set to a non-zero value, e.g., 10, at block 207. This means that more than Y-X morphology match scores are greater than the match threshold and may represent heartbeats indicative of SVT rhythm originating from the upper heart chamber. This evidence of an SVT rhythm ensures delayed VT or VF detection. Thus, the VT/VF morphology counter is set to a non-zero value at block 207 for delayed VT/VF detection. For example, whenever less than X of the Y cardiac signal segments have morphology match scores less than the first match threshold, the currently perceived R-wave is not classified as a potential VT/VF beat, and the VT/VF counter is set to 10 at block 207. Before VT or VF can be detected, it may be required to count down the VT/VF counter from a non-zero value to zero, thereby delaying VT or VF detection in the presence of morphology-based SVT rhythm evidence. To reach a count of zero, 10 consecutive perceived R-waves may be required to be classified as potential VT/VF beats based on at least X of the Y most recent cardiac signal segments having a morphology matching score greater than a first matching threshold. At block 230, the control circuit 80 advances to the next R-wave signal for determining the next morphology matching score in the moving window of Y R-wave signals.

Using the previous example, if 6 of the 8 cardiac signal segments have morphology match scores less than the first match threshold (i.e., 60), the control circuit 80 classifies the most recently sensed R-wave as a potential VT/VF beat at block 208. At block 209, the VT/VF morphology counter value is decreased by one. If the counter value is at zero, e.g., still at an initialized zero value after evaluation of the first set of Y morphology matching scores, no adjustment is necessary at block 209. Otherwise, if the counter value has been previously adjusted at block 207, the counter value is decreased by one. If the counter value is not equal to zero at block 210, the control circuit 80 proceeds to block 230 to determine a next morphology matching score for the next buffered cardiac signal segment corresponding to the next R-wave perceptual event signal. The oldest buffered cardiac signal segment and its matching score may be discarded (drop) such that the moving window of Y cardiac signal segments is advanced one perceived R-wave forward. The process of determining the next morphology match score and whether X of the Y morphology match scores are less than the morphology match threshold (block 206) continues until the VT/VF morphology counter reaches a value of zero at block 210.

If the VT/VF morphology counter value is zero at block 210, the control circuit 80 proceeds to block 212 for determining whether the criteria for enabling SVT discrimination are met. A VT/VF morphology counter value of zero indicates that at least one R-wave perceptual event signal is classified as a potential VT/VF beat at block 208 based on X of the Y match scores being less than the first match threshold. When the VT/VF morphology counter reaches zero, the VT/VF morphology criteria can be met, and if other VT/VF detection criteria are also met, VT or VF detection is supported.

However, changes in posture or other factors may affect the cardiac electrical signals received by the sensing circuitry 86. Thus, a relatively low morphology matching score may occur, resulting in the VT/VF morphology criteria being met at block 210 when the heart rhythm is actually a supraventricular rhythm. Thus, the control circuit 80 determines whether the SVT discrimination criteria are met at block 212 before supporting VT/VF detection based on the VT/VF morphology criteria being met according to the counter value being zero at block 210.

In one example, if at least X of the most recent Y cardiac signal segments have morphology match scores greater than a second match threshold, which may be referred to as an SVT decision match threshold, at block 214, the control circuit 80 enables SVT decision. The SVT discriminant match threshold applied at block 212 may be less than the first match threshold applied to the morphology match score at block 206 for determining whether VT/VF morphology criteria are met. In the example given above, a first match threshold of 60 is applied at block 206. The SVT match threshold applied at block 212 may be 20. The SVT discrimination criteria may require X cardiac signal segments among the Y cardiac signal segments to have a morphology matching score of at least 20 in order to enable SVT discrimination at block 214.

If the requirement is not met (i.e., the "no" branch of block 212), the control circuit 80 may proceed to block 216 for determining whether the VT/VF detection criteria are met without enabling SVT discrimination. If other VT/VF detection criteria are met, for example, if the VT or VF interval counter reaches the corresponding NID and the VT/VF morphology counter value is zero as determined at block 210, the control circuit 80 may detect VT or VF at block 222 without performing SVT discrimination.

However, if the SVT discrimination criteria are satisfied at block 212, the control circuit 80 enables SVT discrimination at block 214. The control circuit 80 enables SVT discrimination at block 214 by determining cardiac signal segment characteristics from each buffered cardiac signal segment as described in connection with fig. 7-9. If the VT/VF detection criteria are met at block 216, e.g., if the NID is reached and the VT/VF morphology counter has reached a count of zero at block 210, the control circuit 80 determines whether the SVT detection criteria are met at block 218.

At block 218, a first portion of the features determined from each of the currently buffered cardiac signal segments is used to determine whether a set of Y cardiac signal segments represents a monomorphic waveform, and a second portion of the features determined from each of the Y cardiac signal segments is used to determine whether the set of Y cardiac signal segments represents an SVT beat. A method for determining whether the simplex waveform criteria and the SVT beat criteria are met is described below in conjunction with fig. 11 and 12. If the Y cardiac signal segments are determined to satisfy the simplex waveform criteria as well as the SVT beat criteria, then the SVT detection criteria are satisfied at block 218. An SVT is detected at block 224. VT/VF detection is suppressed at block 226 and VT/VF therapy is not delivered.

in response to the SVT detection criteria being met at block 218, the control circuit 80 may adjust the VT/VF morphology counter to a non-zero value at block 228. In one example, the VT/VF morphology counter reaching zero at block 210 may be increased to 5 or other predetermined value for delaying VT/VF detection. Since the VT/VF morphology counter is no longer a value of zero, the SVT discrimination may be disabled at block 229. The VT/VF morphology counter may be required to count down to zero from a count value adjusted to 5 before VT or VF can be detected. The control circuit 80 returns to block 230 to advance the moving window of Y R-wave signals to the next perceived R-wave and determine the morphology matching score for the next buffered cardiac signal segment.

If the SVT detection criteria are not met at block 218 and all other VT/VF detection criteria are met, such as VT or VF NID is reached when the value of the VT/VF morphology counter is zero, VT or VF is detected at block 222. Control circuitry 80 may control therapy delivery module 84 to deliver therapy according to a therapy protocol programmed for a detected VT or VF episode.

Fig. 11 is a flow diagram 300 of a method that may be performed at block 110 of fig. 5 or at block 218 of fig. 10 for determining whether a simplex waveform criterion is satisfied according to one example. After SVT discrimination is enabled at block 106 of fig. 5 (or at block 214 of fig. 10), the control circuitry 80 may be configured to determine characteristics from cardiac signal segments buffered in the memory 82 corresponding to R-wave perceptual event signals received from the sensing circuitry 86. As described above, the cardiac signal segment may be acquired from the second cardiac electrical signal received at the second sensing channel 85. The cardiac signal segment may include 48 sample points, with the R-wave perceptual event signal located at sample point 24. The second cardiac electrical signal may be notch filtered by the second sensing channel 85 prior to determining characteristics of the buffered cardiac signal segment.

As described in connection with the examples of fig. 7-9, six different features are determined from each cardiac signal segment. The six different features include three features determined from each of the cardiac signal segments for determining whether a simplex waveform criterion is satisfied. The other three features determined from each of the cardiac signal segments are used to determine whether the SVT beat criteria are met as described below in connection with fig. 12. As each signal segment is acquired and buffered in memory 82, features may be determined from each cardiac signal segment on a beat-by-beat basis. The determined characteristics of a predetermined number of signal segments (e.g., 8 signal segments) corresponding to successive sensed R-waves may be buffered in memory 82 on a first-in-first-out basis. In this manner, if the VT/VF detection criteria become satisfied, the control circuit 80 may use the most recent Y buffered cardiac signal segment characteristics to determine whether an SVT is detected. Based on the SVT detection, VT/VF detection may be suppressed.

The three features determined and stored for each of the cardiac signal segments for determining whether the simplex waveform criteria are met include the maximum peak amplitude, the maximum number of peak samples, and the RRI described in connection with fig. 9 above. At block 302, the control circuit 80 determines a variability of the maximum peak amplitude among the Y maximum peak amplitudes determined and stored for the Y cardiac signal segments. The variability of the maximum peak amplitude may be determined by determining the largest maximum peak amplitude and the smallest maximum peak amplitude among the Y maximum peak amplitudes buffered. The variability of the maximum peak amplitude may be determined at block 302 as the difference between the maximum and minimum maximum peak amplitudes divided by the average of the maximum peak amplitudes of the Y buffers.

At block 306, the maximum peak timing variability is determined using the Y maximum peak sample numbers buffered for the Y cardiac signal segments. The maximum peak timing variability may be determined as the difference between the maximum and minimum maximum peak sample numbers stored for the Y cardiac signal segments.

The control circuit 80 determines RRI variability at block 308. RRI variability may be determined by subtracting the smallest RRI from the largest RRI stored for the Y cardiac signal segments and dividing the difference by the average RRI determined from the Y RRIs. In one example, the average RRI is a trimmed mean (trimmed mean) determined by averaging the buffered RRI values after the largest and smallest RRIs are removed. In some examples, RRI variability may be determined over more than Y cardiac signal segments. For example, the last 12 RRIs may be used at block 308 for determining RRI variability. The trimmed mean may be determined by dropping the largest two RRIs and the smallest two RRIs and averaging the remaining 8 RRIs. RRI variability may be determined at block 308 by dividing the difference between the largest and smallest RRIs of all 12 RRIs by the trimmed mean.

it is recognized that other techniques may be used to determine: variability of maximum peak amplitude; variability in maximum peak timing, which is related to the timing of the R-wave perceptual event signals because the sample point numbering of the signal segments is based on the timing of the R-wave perceptual event signals (as shown in fig. 9); and variability in RRI. Once SVT discrimination is enabled, the cardiac signal segment characteristics required to apply the simplex waveform criteria may be determined, thereby enabling the variability of the cardiac signal segment characteristics of the most recent Y cardiac signal segments to be performed on a beat-by-beat basis.

At block 310, the control circuit 80 compares each of these variability metrics determined at blocks 302, 306, and 308 to a corresponding threshold. If each variability metric is less than its corresponding threshold, indicating relatively low variability in maximum peak amplitude, maximum peak timing, and RRI, then the simplex waveform criteria are satisfied as indicated at block 312. In one example, the simplex waveform criterion is met if the maximum peak amplitude variability is less than 60%, the maximum peak timing variability is less than 8 sample points (for a sampling rate of 256 Hz), and the RRI variability is less than 15%. These examples are intended to be illustrative in nature and not restrictive; other variability thresholds may be used for identifying the simplex waveform.

In some examples, all three variability metrics are required to be less than their corresponding thresholds, otherwise the simplex waveform criteria are not met as indicated at block 314. In other examples, it is desirable that at least one or both of the variability metrics be less than their corresponding thresholds in order to satisfy the simplex waveform criteria. In the analysis of fig. 11, the three features of maximum peak amplitude, maximum peak timing, and RRI determined for each of the buffered Y cardiac signal segments are compared to one another for determining a measure of variability and determining whether the corresponding perceived R-wave is monomorphic. In the example method of fig. 11 for determining whether Y cardiac signal segments satisfy the simplex waveform criteria, these features may not be compared to SVT template features.

Fig. 12 is a flowchart 400 of a method for determining whether SVT beat criteria are met at block 112 of fig. 5 or block 218 of fig. 10. In the example illustrated in connection with fig. 7 and 8, the three features determined from the buffered cardiac signal segments to determine whether the SVT beat criteria are met are the polarity pattern, the peak time interval, and the normalized signal width. At block 410, the control circuit 80 compares the three features determined from the most recently buffered cardiac signal segment with the similar three features of the SVT template (determined at block 156 of fig. 6 and stored in the memory 82).

If the three features match the corresponding SVT template features within a predetermined threshold, the most recently sensed R-wave corresponding to the cardiac signal segment is counted as an SVT beat at block 414. If the cardiac signal segmentation features do not match similar SVT template features within a predetermined threshold, the process proceeds to block 416 without counting the most recently sensed R-waves as SVT beats.

In one example, the determined polarity patterns of the cardiac signal segment and the SVT template are required to be the same in order to facilitate matching at block 412. For example, both the cardiac signal segmentation and the SVT template are biphasic, positive peak-first polarity patterns; both are biphasic, negative peak first polarity patterns; all single-phase positive peak polarity patterns or all single-phase negative peak polarity patterns for matching. The peak time interval of the cardiac signal segment may be required to be within 5 sample points (for a sampling rate of 256 Hz) of the peak time interval of the SVT template in order for the peak time interval to match the peak time interval of the SVT template. The normalized signal width may be required to be within 30% or another percentage threshold of the normalized signal width of the SVT template. If each of the comparisons of these three features with similar SVT template features is a match, the most recently perceived R-wave corresponding to the cardiac signal segment from which the feature was derived is counted as an SVT beat at block 414.

The control circuit 80 may include an SVT beat counter for counting each of the most recent Y cardiac signal segments having the three SVT discrimination features that match the SVT template features. If the counter is equal to or greater than the SVT beat count threshold at block 416, then the SVT beat criteria are determined to be met at block 422. If at block 416, the SVT beat counter is not equal to or greater than the SVT beat count threshold (the "NO" branch), the SVT beat criteria for the Y cardiac signal segments are determined to not be met. The SVT count threshold applied at block 416 may require that of the last 8 cardiac signal segments, 3 are counted as SVT beats. In other examples, of the 8 cardiac signal segments, more or less than 3 cardiac signal segments may be required to be counted as SVT beats in order to meet the SVT beat criteria at block 422.

As described above, the SVT detection criteria are met if the SVT discrimination features determined from the most recent Y cardiac signal segments satisfy both the simplex waveform criteria and the SVT beat criteria. In other examples, the control circuitry 80 may require that, for a group of more than one set of Y cardiac signal segments, the SVT discrimination features determined from the most recent Y cardiac signal segments satisfy both the simplex waveform criteria and the SVT beat criteria. For example, the most recent Y cardiac signal segments on three or more consecutive sensed R-waves may be required to satisfy the simplex waveform and SVT beat criteria. VT/VF detection is suppressed if the SVT detection criteria are met when the VT/VF detection criteria are met.

Fig. 13 is a flow chart 500 of a method for detecting ventricular tachyarrhythmias in accordance with one example of using SVT discrimination techniques disclosed herein. At block 502, a sensing electrode vector is selected by sensing circuitry 86 for receiving cardiac electrical signals through first sensing channel 83 for sensing R-waves. The first sensing vector selected at block 502 for acquiring cardiac electrical signals for sensing R-waves may be relatively short bipolar, e.g., between electrodes 28 and 30 or between electrodes 28 and 24 of lead 16 or other electrode combinations as described above. The first sensing vector may be a vertical sensing vector (relative to an upright or standing position of the patient) or substantially aligned with the cardiac axis in order to maximize the amplitude of the R-wave in the cardiac electrical signal for reliable R-wave sensing. In other examples, the first sensing vector may be a vector between one electrode carried along distal portion 25 of lead 16 and ICD housing 15 (shown in fig. 1A).

sensing circuitry 86 may generate an R-sense event signal at block 506 in response to first sensing channel 83 detecting that the cardiac electrical signal crosses the R-wave sensing threshold outside of the blanking period. The R-wave sense event signal may be communicated to the control circuit 80. In response to the R-wave sense event signal, the timing circuit 90 of the control circuit 80 determines an RRI at block 510 that ends with the current R-wave sense event signal and begins with the most recent previous R-wave sense event signal. The timing circuit 90 of the control circuit 80 may communicate RRI timing information to the tachyarrhythmia detection circuit 92, the tachyarrhythmia detection circuit 92 adjusting the tachyarrhythmia interval counter at block 512.

If the RRI is shorter than the Tachycardia Detection Interval (TDI) but longer than the Fibrillation Detection Interval (FDI), i.e., if the RRI is in the tachycardia detection interval region, the VT interval counter is increased at block 512. If the VT interval counter is configured to count consecutive VT intervals for detecting VT, the VT interval counter may be reset to zero if RRI is longer than TDI. If the RRI is shorter than the FDI, the VF counter is incremented. The VF counter may be a probabilistic VF counter that counts VF intervals in X out of Y such that VF is detectable when a threshold number of VF intervals (not necessarily consecutive) are detected. In some examples, if RRI is less than TDI, the combined VT/VF interval counter is increased.

After updating the tachyarrhythmia interval counter at block 512, the tachyarrhythmia detector 92 compares the VT and VF interval counter values to a suspected fast rate threshold at block 514, which is less than the corresponding VT NID and VF NID. If a threshold number of short RRIs are counted, a fast heart rate is suspected to occur. If the VT or VF detection interval counter has reached the fast heart rate threshold, the "yes" branch of block 514, then control circuit 80 enables cardiac signal segment buffering at block 604. In this example, the determination of the morphology matching score between the SVT template and the buffered cardiac signal segments may be performed on an event-by-event basis only after at least one of the VT or VF interval counter values has reached the fast heart rate threshold. In addition to or instead of applying a fast rate threshold to separate VT and VF counters, a fast rate threshold may also be applied to a combined VT/VF interval counter. The fast heart rate threshold may be one or more values. Different fast heart rate thresholds may be applied to the VT interval counter and the VF interval counter. For example, the fast heart rate threshold may be count two on a VT interval counter and count three on a VF interval counter. In other examples, the fast heart rate threshold is a higher number, such as five or higher, but may be less than the number of intervals required to detect VT or VF.

If any of the tachyarrhythmia interval counters have not reached the tachyrhythm threshold at block 514, the control circuit 80 returns to block 506 and waits for the next R-wave sense event signal. A morphological analysis of a cardiac signal segment from the second cardiac electrical signal need not be performed until at least a threshold number of VT or VF intervals are counted as indicative of a suspected fast heart rate and a NID is predicted to be reached. In this manner, the control circuit 80 may be able to make a determination of whether VT/VF morphology criteria are met and acquire data for SVT discrimination when one of the VT or VF interval counters reaches NID.

If the fast heart rate threshold is reached at block 514, the control circuit 80 enables waveform buffering at block 604. In response to each R-wave perceptual event signal generated at block 506 by the first sensing channel 83, the control circuitry 80 buffers the cardiac electrical signal received by the second sensing channel 85. Sensing circuitry 86 selects a second sensing vector for receiving cardiac signals buffered for acquisition of cardiac signal segments for morphology analysis and SVT discrimination at block 602.

The digitized segment of the cardiac electrical signal received by the second sensing channel 85 can be buffered for a time segment defined relative to the crossing of the R-wave sensing threshold and the corresponding sample point time of the R-wave perceptual event signal received from the sensing circuitry 86. For example, the digitized segment may be 100ms to 500ms long. In one example, the buffered segment of the second cardiac electrical signal is at least 48 sample points, or about 188ms, obtained at a sampling rate of 256Hz, 24 of the 48 sample points may precede and include the sample point at which the R-wave perceptual event signal is received, and 24 of the sample points may extend after the sample point at which the R-wave perceptual event signal is received. In other examples, cardiac electrical signal segments may be buffered over a longer time interval at block 604 for use in other cardiac signal analyses performed to detect noise in the cardiac signal, T-wave oversensing, or other sensing issues that may lead to false VT or VF detections.

The buffered cardiac signal segment may be notch filtered at block 605. The notch filter applied at block 605 may correspond to the filter described in provisional U.S. patent application No.62/367,166. The notch filtering performed at block 605 significantly attenuates 50-60Hz electrical noise, muscle noise, other EMI, and other noise/artifacts in the stored cardiac signal segment from the second cardiac signal electrical signal.

In one example, the notch filtering performed at block 605 is implemented in firmware as a digital integer filter the output of the digital notch filter may be determined by firmware implemented in the second sensing channel 85 according to the following equation:

Y(n)＝(x(n)+2x(n-2)+x(n-4))/4

where x (n) is the amplitude of the nth sample point of the digital signal received by notch filter 76 (fig. 4), x (n-2) is the amplitude of the nth-2 sample point, and x (n-4) is the amplitude of the nth-4 sample point (for a sampling rate of 256 Hz). Y (n) is the amplitude of the nth sample point of the notch-filtered, digital second cardiac electrical signal. The attenuation of the amplitude of y (n) is-40 decibels (dB) at a frequency of 60 Hz. At a frequency of 50Hz, the attenuation is-20 dB, and at 23Hz, which may be typical for the R-wave of cardiac electrical signals, the attenuation is limited to-3 dB. Accordingly, the notch filtering at block 605 may provide highly attenuated 50 and 60Hz noise, muscle noise, other EMI, and other electrical noise/artifacts while conveying lower frequency cardiac signals in the cardiac electrical signal output of the second sensing channel 85.

When different sampling rates than 256Hz are used, the sample point numbers indicated in the equations above for determining the notch filtered signal may be modified as needed, and the resulting frequency response may be slightly different from the examples given above. In other examples, other digital filters may be used to perform the 50Hz and 60Hz attenuations. For example, for a sampling rate of 256Hz, the filtered signal y (n) may be determined as y (n) ═ x (n) + x (n-1) + x (n-2) + x (n-3))/4, which may have relatively small attenuation at 50Hz and 60Hz, but acts as a low-pass notch filter with relatively large attenuation at higher frequencies (greater than 60 Hz).

Under the control of the control circuit 80, a predetermined number of cardiac signal segments may be stored in a rolling first-in-first-out buffer in the memory 82. In the illustrative example described herein, eight cardiac signal segments are buffered in memory 82. At block 606, the control circuit 80 may determine a morphology matching score for each of the buffered cardiac signal segments on a beat-by-beat basis as each signal segment is stored. The morphology matching score may be determined by comparing wavelet transform coefficients determined from a given cardiac signal segment with wavelet transform coefficients of a previously established SVT template (e.g., as described in connection with fig. 6). Other techniques for determining the morphology matching score may be used.

SVT discrimination features may also be derived from each cardiac signal segment buffered in memory 82. Six SVT discrimination features described in connection with fig. 7-9 may be determined for each stored signal segment. When a new cardiac signal segment is stored, the oldest signal segment and its morphology matching score and SVT discriminative features may be deleted.

At block 608, the control circuit 80 determines whether the VT/VF morphology criteria are met using the buffered morphology match scores determined at block 606. Once the buffer for storing the eight cardiac signal segments and the corresponding morphology matching scores and SVT discrimination features is full, the control circuit 80 determines whether at least X of the Y morphology matching scores (e.g., whether at least 6 of the 8 morphology matching scores) are less than a first matching threshold. If less than a threshold number (or percentage) of cardiac signal segments have morphology matching scores less than a matching threshold, the VT/VF morphology criteria are not met. The results indicate that at least Y-X cardiac signal segments have relatively high correlation with the SVT template and are evidence that the heart rhythm is supraventricular. If the VT/VF morphology criteria are not met (the "NO" branch of block 608), a morphology rejection rule may be set at block 610. When the requirements for setting the rejection rule are met, VT or VF detection may be suppressed when the VT or VF interval counter reaches the corresponding NID.

If the VT/VF morphology criteria are met at block 608, the control circuit 80 may determine whether the SVT criteria are met at block 612. When the VT/VF morphological criteria are satisfied and at least X of the Y buffered cardiac signal segments are greater than the SVT discrimination match threshold, SVT discrimination may be enabled at block 612. For example, if at block 608, of the 8 match scores, at least 6 match scores are less than a first match threshold, but at least 6 of the 8 are greater than an SVT discriminate match threshold (which is less than the first match threshold), then the SVT discrimination criteria are satisfied and SVT discrimination is enabled at block 612.

If SVT discrimination is enabled at block 612, the control circuit 80 determines whether SVT detection criteria are met at block 614. The SVT discrimination features determined from each notch filtered cardiac signal segment are used to determine whether SVT detection criteria are met. The first portion of the features determined for each cardiac signal segment is used to determine whether the Y cardiac signal segments are monomorphic waveforms. The second portion of the features determined for each cardiac signal segment is used to determine whether the SVT beat criteria are satisfied. If the simplex waveform criteria and the SVT beat criteria are met, then the SVT detection criteria are met at block 614. The SVT rejection rule is set at block 616. The control circuit 80 may set the rejection rule by setting a bit value stored in a register or memory 82 to a high value (e.g., to 1) for indicating that the rejection rule is satisfied and that VT/VF detection should be rejected based on the NID being satisfied (and/or other detection criteria). If a rejection rule is not set, e.g., the corresponding register bit value is low or zero, then VT/VF detection is not suppressed based on the rejection rule.

If the morphology rejection rule is set at block 610, in response to the VT/VF morphology criteria not being met at block 608, or the SVT rejection rule is set at block 616, the control circuitry 80 may adjust the VT/VF morphology criteria at block 618. The VT/VF morphology criteria may be adjusted to increase the time required to detect VT or VF due to evidence of a supraventricular rhythm associated with setting the morphology rejection rule and/or setting the SVT rejection rule. The VT/VF morphology criteria may be adjusted for increasing the number of cardiac signal segments required to have a morphology matching score less than the first matching threshold before VT or VF may be detected again. For example, the VT/VF morphology counter that counts down to zero when a perceived R-wave is classified as a potential VT/VF beat may be adjusted to an increased value, e.g., to a value of five or ten as described in connection with fig. 10. In other examples, the VT/VF morphology counter may start at zero and count up when a perceived R-wave is classified as a potential VT/VF beat. The counter may be reset to zero at block 618 and/or the threshold count value required to meet the VT/VF morphology criteria may be increased, for example, from the initial threshold value of one potential VT/VF beat to the threshold value of five or ten potential VT/VF beats.

If one of the VT, VF, or combined VT/VF interval counters does not reach the NID at block 516, the control circuit 80 returns to block 506 to sense the next R-wave, determine the next RRI to update the interval counter, and if so, buffer the next cardiac signal segment. The morphology rejection rules and SVT rejection rules may be updated on a beat-by-beat basis based on analysis of a new set of buffered cardiac signal segments.

If one of the VT, VF or combined VT/VF interval counters has reached NID at block 516, the control circuit 80 checks at block 518 whether the rejection rule is satisfied. If the morphology rejection rule is set, then the VT or VF is suppressed at block 524 even though the NID has been reached. If the SVT rejection rule is set, then the VT or VF is suppressed at block 524 even though the VT/VF morphology criteria have been met at block 608 and the NID has been reached at block 516. The therapy is not delivered. The control circuit 80 proceeds to the next sensed R-wave to continue updating the VT and VF interval counters and analyzes the next set of buffered cardiac signal segments for updating the morphology and the state of the SVT rejection rules.

If the NID is reached and neither morphology rejection rules nor SVT rejection rules are set, meaning that the VT/VF morphology criteria are met and the SVT detection criteria are not met (or SVT discrimination is not enabled), then VT or VF is detected at block 520 based on the interval counter at which its corresponding NID is reached. The control circuitry 80 controls the therapy delivery circuitry 84 to deliver therapy at block 522, which may include ATP, CV/DF shocks, and/or post-shock pacing in some examples.

fig. 14 is a flow chart 700 of a method for detecting a ventricular tachyarrhythmia according to another example. Like numbered blocks in fig. 14 correspond to like numbered blocks described in connection with fig. 13. As described above, the VT/VF morphology criteria may be satisfied when, among the Y morphology match scores, X morphology match scores are less than the first match threshold and the VT/VF morphology counter is zero. The VT/VF morphology counter may be initialized to a zero value, thereby classifying a single perceived R-wave as a potential VT/VF beat based on a set of Y morphology matching scores initially satisfying the VT/VF morphology criteria.

As shown in fig. 14, after the VT or VF interval counter reaches the fast heart rate threshold, the cardiac signal segments are stored in a rolling buffer in memory 82, as determined at block 514. Morphology matching scores and SVT discriminative features for the buffered cardiac signal segments are determined on a beat-by-beat basis at block 606. If at block 708, of the Y morphology match scores determined for each buffered cardiac signal segment, at least X morphology match scores are less than the first match threshold, then the VT/VF morphology counter is decremented by one (unless already a zero value) at block 710. If less than X of the Y morphology match scores are less than the first match threshold, indicating a relatively high correlation between the signal segment and the SVT template, then the VT/VF morphology criteria is adjusted by increasing the value of the VT/VF counter by one at block 712. In one example, the VT/VF morphology counter is set to a value of 10 or another selected value greater than zero every time less than X of the Y morphology matching scores are less than the first matching threshold.

after reducing the VT/VF counter at block 710 or adjusting the VT/VF counter to a non-zero value at block 712, the control circuit 80 determines whether any of the updated VT, VF, or combined VT/VF interval counters at block 512 has reached the NID. If the NID is reached, the control circuit 80 determines whether the value of the VT/VF morphology counter is zero at block 714. If not, the VT/VF morphology criteria have not been met and the process returns to block 506 to wait for the next R-wave sense event signal.

If the value of the VT/VF morphology counter is zero at block 714, the control circuit 80 determines whether the SVT discrimination criteria is satisfied by determining whether at least X match scores among the Y match scores stored in the memory 82 for the buffered cardiac signal segments are greater than a second match threshold (which is less than the first match threshold) at block 715. If not, then the Y cardiac signal segments have very low correlation with the SVT template. SVT discrimination is not required. Based on the NID being reached at block 516 and the VT/VF morphology criteria being met at block 714, the VT/VF detection criteria are met. VT or VF is detected at block 520.

if at block 715, of the Y morphology matching scores, at least X morphology matching scores are greater than the second, SVT discrimination threshold, the control circuit 80 determines at block 716 whether the SVT detection criteria are met. As previously described, e.g., in conjunction with fig. 11 and 12, the SVT discrimination features determined from the buffered cardiac signal segments at block 606 are analyzed for determining whether the simplex waveform criteria are met and whether the SVT beat criteria are met. If at least one of the monomorphic waveform criteria or the SVT beat criteria is not met, then the SVT detection criteria are not met at block 716 (the "NO" branch). VT or VF is detected at block 520 and the appropriate therapy is delivered at block 522. In other examples, the SVT detection criteria are satisfied when one of the monomorphic waveform criteria or the SVT beat criteria is satisfied, such that the SVT detection criteria are not satisfied only when neither the monomorphic waveform criteria nor the SVT beat criteria are satisfied at block 716.

if the SVT detection criteria are met at block 716, e.g., the simplex waveform criteria and the SVT beat criteria are met, the VT/VF morphology criteria are adjusted at block 718 by setting the VT/VF morphology counter to a non-zero value (e.g., to a value of five). By requiring the VT/VF morphology counter to count down to a zero value before VT or VF can be detected, VT/VF detection is effectively suppressed and delayed. To return to a zero value in the illustrative example presented herein, the VT/VF morphology counter must be decremented by one at block 710 over five consecutive perceived R-waves based on X of the Y most recent morphology matching scores being less than the first matching threshold over each of the five consecutive perceived R-waves. If the SVT detection criteria are met at block 716, VT/VF detection is suppressed at block 524 after adjusting the VT/VF counter and therapy is not delivered even though the NID is reached at block 516 and the VT/VF morphology criteria are met based on the zero value of the VT/VF morphology counter at block 714.

Thus, presented herein are techniques for satisfying SVT detection criteria based on cardiac signal segment characteristics, suppressing VT or VF detection even when both RRI-based and waveform morphology-based VT/VF detection criteria are satisfied. Cardiac signal segment feature analysis for SVT discrimination avoids false VT or VF detection in the event of a changed cardiac signal morphology due to changes in the position of the patient's body or posture or other factors that may affect the cardiac signal waveform such as the R wave (or QRS complex). The techniques disclosed herein may be implemented in conjunction with additional VT/VF rejection rules that may result in the suppression of VT or VF detection based on additional analysis of buffered cardiac signal segments. Various cardiac signal analysis techniques and VT/VF rejection rules that may be implemented in conjunction with the SVT discrimination techniques disclosed herein are generally disclosed in provisional U.S. patent application No.62/367,166 (attorney docket No. c00013307.usp1), U.S. patent application No.62/367,170 (attorney docket No. c00013169.usp1), U.S. patent application No.62/367,221 (attorney docket No. c00013321.usp1), and U.S. patent application No.15/140,802(Zhang et al). Additional analysis for detecting electromagnetic interference or other noise in the cardiac electrical signal, T-wave oversensing, or verifying the perceived R-wave may be performed. These analyses may be used for other rejection rules for suppressing VT or VF detection when NID is reached. As such, the control circuitry may check the status of the plurality of rejection rules at block 518 of fig. 13, as generally disclosed in the above-incorporated patent application.

Thus, ICD systems and methods for distinguishing SVT from ventricular tachyarrhythmias and inhibiting ventricular tachyarrhythmia detection and treatment in response to detecting SVT have been presented in the foregoing description with reference to specific embodiments. In other examples, the various methods described herein may include steps performed in a different order or in a different combination than the illustrative examples shown and described herein. It should be understood that various modifications may be made to the reference embodiments without departing from the scope of the disclosure and the appended claims and examples.

Example 1. a method, comprising: receiving, by a sensing circuit, at least a first cardiac electrical signal via a sensing electrode vector; determining, by the control circuit, whether the first cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia; determining a plurality of features for each of a plurality of cardiac signal segments of a first cardiac electrical signal; in response to the first criterion being satisfied, determining whether a first portion of the plurality of features determined from each of the plurality of cardiac signal segments satisfies a simplex waveform criterion; determining whether a second portion of the plurality of features determined from each of the plurality of cardiac signal segments meets supraventricular beat criteria; determining whether a second criterion for detecting ventricular tachyarrhythmia is met; determining whether a first portion of the plurality of features satisfies a monomorphic waveform criterion and a second portion of the plurality of features satisfies a supraventricular beat criterion; inhibiting detection of a ventricular tachyarrhythmia in response to a first portion of the plurality of features satisfying a monomorphic waveform criterion and a second portion of the plurality of features satisfying a supraventricular beat criterion; and

in response to the first criteria and the second criteria being met, and the first portion of the plurality of features not meeting the monomorphic waveform criteria and/or the second portion of the plurality of features not meeting at least one of the supraventricular beat criteria, a ventricular tachyarrhythmia is detected and the therapy delivery circuit is controlled to deliver electrical stimulation therapy.

Example 2. the method of example 1, wherein determining whether the first criterion is satisfied comprises: determining a morphology matching score between each of a plurality of cardiac signal segments of the first cardiac signal and a morphology template; the first criterion is determined to be satisfied in response to a first threshold number of cardiac signal segments of the plurality of cardiac signal segments having morphology matching scores less than a first threshold.

Example 3. the method according to example 2, further comprising: in response to the first criterion being met, determining whether a criterion for supraventricular tachyarrhythmia discrimination is met; in response to the criteria for supraventricular tachyarrhythmia discrimination not being met, refraining from determining the plurality of features; and detecting a ventricular tachyarrhythmia in response to the first and second criteria for detecting a ventricular tachyarrhythmia being met and the supraventricular tachyarrhythmia discrimination criteria not being met.

Example 4. the method according to example 3, further comprising: determining that the supraventricular tachyarrhythmia discrimination criteria are satisfied by determining that at least a second threshold number of cardiac signal segments of the plurality of cardiac signal segments have a morphology matching score greater than a second matching threshold, the second matching threshold being less than the first matching threshold.

example 5. the method of any of examples 3 or 4, further comprising: setting a threshold count value in response to less than a threshold number of cardiac signal segments of the plurality of cardiac signal segments having morphology matching scores less than a first matching threshold; adjusting a tachyarrhythmia count value in response to a threshold number of cardiac signal segments of the next plurality of cardiac signal segments having morphology matching scores less than a first matching threshold; and determining that the first criterion is met when the tachyarrhythmia count reaches a threshold count.

example 6. the method according to any of examples 1-5, further comprising: determining a sense event interval between successive R-waves sensed by the sensing circuit; comparing the sensed event interval to a tachyarrhythmia detection interval; in response to each of the determined sensed inter-event intervals being less than the tachyarrhythmia detection interval, increasing a count of tachyarrhythmia detection intervals; and in response to the value of the count of tachyarrhythmia detection intervals being equal to or greater than the fast heart rate threshold, determining whether a first criterion for detecting ventricular tachyarrhythmia is met.

Example 7. the method of any of examples 1-6, wherein determining whether supraventricular beat criteria are met comprises: comparing each feature in the first portion of features determined from each of the plurality of cardiac signal segments to similar features of the supraventricular R-wave template; and determining that the supraventricular beat criterion is satisfied in response to a threshold number of cardiac signal segments having a first portion of features that match similar features of the supraventricular R-wave template.

Example 8. the method of any of examples 1-7, wherein: determining features for each of a plurality of cardiac signal segments comprises determining at least a polarity pattern, a peak time interval, and a normalized width for each of a plurality of cardiac signal segments; and determining whether the supraventricular beat criteria are met comprises: determining, for each of a plurality of cardiac signal segments, whether the determined polarity pattern matches a polarity pattern of a supraventricular R-wave template; determining, for each of a plurality of cardiac signal segments, whether the determined peak time interval matches a peak time interval of a supraventricular R-wave template within a peak time interval match threshold range; determining, for each of a plurality of cardiac signal segments, whether the determined normalized width matches a normalized width of a supraventricular R-wave within a normalized width matching threshold; and determining whether a threshold number of cardiac signal segments have the determined characteristics of polarity pattern, maximum peak amplitude, and normalized width that match similar characteristics of the supraventricular R-wave template.

Example 9 the method of any of examples 1-8, wherein determining whether the simplex waveform criteria are met comprises: determining a variability of each of a second portion of the plurality of features determined from each of the plurality of cardiac signal segments; comparing the variability of each feature of the second portion to a corresponding variability threshold; and determining that the simplex waveform criterion is satisfied in response to the variability of each of the second portion of the plurality of features being less than the corresponding variability threshold.

Example 10. the method of any of examples 1-9, further comprising: determining the polarity of the maximum peak of the supraventricular sexual R-wave template; determining a characteristic of each of the plurality of cardiac signal segments by determining at least an amplitude and a timing of a maximum peak of the cardiac signal segment having a polarity matching a polarity of a maximum peak of the supraventricular R-wave template; determining an amplitude variability and a timing variability of a maximum peak of a plurality of cardiac signal segments; and determining that the simplex waveform criterion is satisfied in response to the amplitude variability being less than the amplitude variability threshold and the timing variability being less than the timing variability threshold.

Example 11. the method of any of examples 1-10, further comprising: determining an event interval between successive events sensed by the sensing circuit; comparing the event interval to a tachyarrhythmia detection interval; in response to each of the determined inter-event intervals being less than the tachyarrhythmia detection interval, increasing a count of tachyarrhythmia detection intervals; in response to the value of the count of tachyarrhythmia detection intervals being equal to or greater than the value of the detection threshold, it is determined that a second criterion for detecting ventricular tachyarrhythmia is satisfied.

Example 12. the method of any of examples 1-11, further comprising: receiving a first cardiac signal through a first sensing channel of a sensing circuit; receiving, by a second sensing channel of the sensing circuit, a second cardiac signal; sensing an R-wave from a second cardiac signal; generating an R-wave sense event signal in response to each sensed R-wave; determining a perceptual event interval between each pair of successive R-wave perceptual event signals generated by the sensing circuit; comparing the sensed event interval to a tachyarrhythmia detection interval; in response to each of the determined sensed event intervals being less than the tachyarrhythmia detection interval, increasing a count of tachyarrhythmia detection intervals; in response to a value of the count of tachyarrhythmia detection intervals being equal to or greater than a perceptual event confirmation threshold, buffering a plurality of cardiac signal segments from the first cardiac signal, each of the plurality of cardiac signal segments corresponding to an R-wave perceptual event signal generated by the sensing circuit; and in response to the count of tachyarrhythmia detection intervals being equal to or greater than the value of the detection threshold, determining that a second criterion for detecting ventricular tachyarrhythmia is satisfied.

example 13. the method of any of examples 1-12, further comprising adjusting the first criterion in response to meeting a monomorphic waveform criterion and a supraventricular beat criterion.

Example 14. the method of any of examples 1-13, further comprising receiving the first cardiac electrical signal via a sensing electrode vector comprising at least one electrode carried by an extravascular lead.

Example 15 a non-transitory computer-readable storage medium comprising a set of instructions that, when executed by control circuitry of an Implantable Cardioverter Defibrillator (ICD), cause the ICD to: receiving, by a sensing circuit, a cardiac electrical signal via a sensing electrode vector; determining whether the cardiac electrical signal satisfies a first criterion for detecting ventricular tachyarrhythmia; determining a plurality of features for each of a plurality of cardiac signal segments of the cardiac electrical signal; in response to the first criterion being satisfied, determining whether a first portion of the plurality of features determined from each of the plurality of cardiac signal segments satisfies a simplex waveform criterion; determining whether a second portion of the plurality of features determined from each of the plurality of cardiac signal segments meets supraventricular beat criteria; in response to the monomorphic waveform criterion being satisfied and the supraventricular beat criterion being satisfied, inhibiting detection of the ventricular tachyarrhythmia; and

In response to the first criteria and the second criteria being met and at least one of the monomorphic waveform criteria or the supraventricular beat criteria not being met, a ventricular tachyarrhythmia is detected and electrical stimulation therapy is delivered by the therapy delivery circuit.