Method and apparatus for selecting and timing anti-tachyarrhythmia pacing using cardiac cycle length stability

An implantable medical device delivers anti-tachyarrhythmia therapies including anti-tachyarrhythmia pacing (ATP). When a tachyarrhythmia episode is detected, the implantable medical device analyzes cardiac cycle length stability to determine whether and/or when to deliver an ATP. In one embodiment, the cardiac cycle length stability is measured by existence of stable ventricular tachyarrhythmia clusters (SVTCs) during the tachyarrhythmia episode. Each SVTC includes at least a specified minimum number of heart beats over which the cardiac cycle lengths meet a stability criterion.

TECHNICAL FIELD

This document relates generally to cardiac rhythm management (CRM) systems and particularly, but not by way of limitation, to a system that controls the selection and timing of anti-tachyarrhythmia pacing (ATP) therapy based on cardiac cycle length stability.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmias generally include supraventricular tachyarrhythmia (SVT, including atrial tachyarrhythmia, AT) and ventricular tachyarrhythmia (VT). Fibrillation is a form of tachyarrhythmia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. VT occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles or when a natural pacemaker in a ventricle usurps control of the heart rate from the sinoatrial node. When the atria and the ventricles become dissociated during VT, the ventricles may contract before they are properly filed with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.

Cardioversion and defibrillation are used to terminate most tachyarrhythmias, including AT, VT, and VF. An implantable cardioverter/defibrillator (ICD) is a cardiac rhythm management (CRM) device that delivers an electric shock to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory.

Another type of electrical therapy for tachyarrhythmia is anti-tachyarrhythmia pacing (ATP). In ATP, the heart is competitively paced in an effort to interrupt the reentrant loop causing the tachyarrhythmia. An exemplary ICD includes ATP and defibrillation capabilities so that ATP is delivered to the heart when a non-fibrillation VT is detected, while a defibrillation shock is delivered when fibrillation occurs. Although cardioversion and/or defibrillation are effective in terminating tachyarrhythmia, they consume a large amount of power and result in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible.

The efficacy of ATP in terminating tachyarrhythmia depends on the type of the tachyarrhythmia and the timing of ATP delivery. To be effective, an ATP therapy is to be delivered to the heart during an excitable gap in the reentrant loop. Inaccurate timing of an ATP delivery will contribute to the failure in terminating tachyarrhythmia using ATP.

For these and other reasons, there is a need for determining whether and when to deliver an ATP therapy.

SUMMARY

An implantable medical device delivers anti-tachyarrhythmia therapies including anti-tachyarrhythmia pacing (ATP). When a tachyarrhythmia episode is detected, the implantable medical device analyzes cardiac cycle length stability to determine whether and/or when to deliver an ATP. In one embodiment, the cardiac cycle length stability is measured by existence of stable ventricular tachyarrhythmia clusters (SVTCs) during the tachyarrhythmia episode. Each SVTC includes at least a specified minimum number of heart beats over which the cardiac cycle lengths meet a stability criterion.

In one embodiment, an implantable medical device includes a sensing circuit, a pacing output, a tachyarrhythmia detector, a tachyarrhythmia classifier, a primary duration timer, a cycle length stability analyzer, a secondary duration timer, and an ATP controller. The sensing circuit senses a cardiac signal. The pacing output circuit delivers pacing pulses. The tachyarrhythmia detector detects a tachyarrhythmia episode using the cardiac signal. The tachyarrhythmia classifier classifies the detected tachyarrhythmia episode. The primary duration timer initiates a primary duration when the tachyarrhythmia episode is detected. The cycle length stability analyzer analyzes stability of cardiac cycle lengths using the cardiac signal. The secondary duration timer controls an initiation of a secondary duration using the stability of cardiac cycle lengths analyzed during the primary duration and the classification of the detected tachyarrhythmia episode. The ATP controller controls the delivery of the pacing pulses according to an ATP mode using the stability of cardiac cycle lengths analyzed during the primary duration, a stability of cardiac cycle lengths analyzed during the secondary duration, and the classification of the detected tachyarrhythmia episode.

In one embodiment, a method for controlling ATP is provided. A cardiac signal is sensed. A tachyarrhythmia episode is detected using the cardiac signal. The detected tachyarrhythmia episode is classified. A primary duration is initiated when the tachyarrhythmia episode is detected. The stability of cardiac cycle lengths is analyzed during the primary duration. The initiation of a secondary duration is controlled using the stability of cardiac cycle lengths analyzed during the primary duration and the classification of the detected tachyarrhythmia episode. The stability of cardiac cycle lengths is analyzed during the second duration if the secondary duration is initiated. A delivery of pacing pulses is controlled according to an ATP mode using the stability of cardiac cycle lengths analyzed during the primary duration, the stability of cardiac cycle lengths analyzed during the secondary duration if the secondary duration is initiated, and the classification of the detected tachyarrhythmia episode.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their legal equivalents.

It should be noted that references to “an”, “one”, or “various” embodiments in this document are not necessarily to the same embodiment, and such references contemplate more than one embodiment.

The relationship between a heart rate and a cardiac cycle length (also known as cardiac interval), as used in this document, is the relationship between a frequency and its corresponding period. If a heart rate is given in beats per minute (bpm), its corresponding cardiac cycle length in milliseconds is calculated by dividing 60,000 by the heart rate (where 60,000 is the number of milliseconds in a minute). Any process, such as a comparison, using a heart rate is to be modified accordingly when a cardiac cycle length is used instead. For example, if a tachyarrhythmia is detected when the ventricular rate exceeds a tachyarrhythmia threshold rate, an equivalent process is to detect the tachyarrhythmia when the ventricular cycle length (also known as ventricular interval) falls below a tachyarrhythmia threshold interval. The appended claims should be construed to cover such variations.

This document discusses a cardiac rhythm management (CRM) system that delivers anti-tachyarrhythmia therapies including anti-tachyarrhythmia pacing (ATP) and controls the anti-tachyarrhythmia therapies, including selection and delivery time of the ATP, based on cardiac cycle length stability. An ATP therapy is most effective when delivered during an “excitable gap” after ventricular repolarization (T wave) and before ventricular depolarization (QRS complex). The delivery of an ATP therapy is typically initiated at the end of a “coupled interval” (CI), which starts with a ventricular depolarization and has a length intended to end during the excitable gap. When the CI is calculated as a percentage of a cardiac cycle length, such as an average of time intervals between two consecutive ventricular depolarizations, the ATP therapy is most effective when delivered while the cardiac cycle lengths are substantially stable (with small variations). When the cardiac cycle lengths are substantially unstable, the ATP may not be effective in terminating a tachyarrhythmia episode because of the difficulty in ensuring that the ATP therapy is delivered during the excitable gap. One solution is to deliver a more aggressive therapy, such as cardioversion/defibrillation, when the cardiac cycle lengths are substantially unstable at the time of a scheduled ATP delivery. However, it is still possible to use ATP, when it is preferable over the more aggressive therapy, by searching for a relatively short period during which the cardiac cycle lengths are substantially stable. That is, when the cardiac cycle lengths during the tachyarrhythmia episode is generally unstable over a relatively long period, an ATP therapy may still be effective if delivered when the cardiac cycle lengths are substantially stable over a relatively short period (such as a few consecutive heart beats), if such a relatively short period exists and is detectable.

The present CRM system detects stable ventricular tachyarrhythmia clusters (SVTCs), each including at least a specified minimum number of consecutive heart beats over which the cardiac cycle lengths meet a stability criterion. An ATP therapy is selected when at least one SVTC is detected and delivered upon the detection of an SVTC. In response to the detection of a tachyarrhythmia episode based on heart rate, a primary duration is initiated. The detected tachyarrhythmia is classified, such as by its origin, and the SVTCs are detected, during the primary duration. If the detected tachyarrhythmia is classified as ventricular tachyarrhythmia (VT), and the primary duration expires with an SVTC, an ATP therapy is delivered when the primary duration expires. If the detected tachyarrhythmia is classified as VT, but no SVTC is detected during the primary duration, a more aggressive therapy, such as a cardioversion/defibrillation therapy, is delivered. If the detected tachyarrhythmia is classified as VT, and at least one SVTC is detected during the primary duration, but the primary duration does not expire with an SVTC, a secondary (extended) duration is initiated. If the detected tachyarrhythmia is classified as supraventricular tachyarrhythmia (SVT), and at least one SVTC is detected during the primary duration, the secondary duration is initiated, after the primary duration expires, if and when the detected tachyarrhythmia becomes classified as VT. The ATP therapy is initiated when an SVTC is detected during the secondary duration. If no SVTC is detected during the secondary duration, a more aggressive therapy, such as a cardioversion/defibrillation therapy, is delivered when the secondary duration expires.

FIG. 1is an illustration of one embodiment of a CRM system100and portions of the environment in which CRM system100operates. CRM system100includes an implantable medical device101that is electrically coupled to a heart199through one or more electrodes, such as on leads105and110. An external system102communicates with implantable medical device101via a telemetry link103.

Implantable medical device101delivers anti-tachyarrhythmia therapies including ATP and cardioversion/defibrillation therapies. In one embodiment, implantable medical device101is an implantable cardioverter/defibrillator (ICD) with cardiac pacing capabilities. In another embodiment, in addition to a pacemaker and a cardioverter/defibrillator, implantable medical device101further includes one or more of other monitoring and/or therapeutic devices such as a neural stimulator, a drug delivery device, and a biological therapy device. Implantable medical device101includes a hermetically sealed can housing an electronic circuit that senses physiological signals and delivers therapeutic electrical pulses. The hermetically sealed can also functions as an electrode for sensing and/or pulse delivery purposes. In one embodiment, as illustrated inFIG. 1, the electronic circuit senses at least an atrial electrogram and a ventricular electrogram from heart199and delivers pacing and cardioversion/defibrillation pulses to heart199. Lead105is typically a pacing lead that includes a proximal end106connected to implantable medical device101and a distal end107placed in the right atrium (RA) of heart199. A pacing-sensing electrode108is located at distal end107. Another pacing-sensing electrode109is located near distal end107. Electrodes108and109are electronically connected to implantable medical device101via separate conductors in lead105to allow sensing of the atrial electrogram and/or delivery of atrial pacing pulses. Lead110is typically a defibrillation lead that includes a proximal end111connected to implantable medical device101and a distal end112placed in the right ventricle (RV) of heart199. A pacing-sensing electrode113is located at distal end112. A defibrillation electrode114is located near distal end112but electrically separated from pacing-sensing electrode113. Another defibrillation electrode115is located at a distance from distal end112for supraventricular placement. Electrodes113,114, and115are electrically connected to implantable medical device101via separate conductors in lead110. Electrode113allows sensing of the ventricular electrogram and/or delivery of ventricular pacing pulses. Electrodes114and115allow delivery of ventricular cardioversion/defibrillation pulses.

Implantable medical device101includes an anti-tachyarrhythmia therapy circuit120that selects and times anti-tachyarrhythmia therapies using cardiac cycle length stability. In various embodiments, anti-tachyarrhythmia therapy circuit120provides implantable medical device101with ATP and cardioversion/defibrillation therapy modes. An ATP therapy mode is selected when the cardiac cycle length is determined to be substantially stable for at least a short period that last over a plurality of heart beats. The delivery of the selected ATP therapy is initiated while the cardiac cycle length is substantially stable. In one embodiment, the cardiac cycle stability is measured by the detection of SVTC. Various embodiments of system120are discussed below, with reference toFIGS. 2 and 3. In various embodiments, implantable medical device101also includes one or more of other cardiac electrical therapy circuits such as an anti-bradyarrhythmia circuit, a cardiac resynchronization therapy (CRT) circuit, a cardiac remodeling control therapy (RCT) circuit.

External system102allows for programming of implantable medical device101and receives signals acquired by implantable medical device101. In one embodiment, external system102includes a programmer. In another embodiment, external system102is a patient management system including an external device in proximity of implantable medical device101, a remote device in a relatively distant location, and a telecommunication network linking the external device and the remote device. The patient management system allows access to implantable medical device101from a remote location, such as for monitoring patient status and adjusting therapies. Telemetry link103is a wireless communication link providing for bidirectional data transmission between implantable medical device101and external system102. In one embodiment, telemetry link103is an inductive telemetry link. In an alternative embodiment, telemetry link103is a far-field radio-frequency telemetry link. Telemetry link103provides for data transmission from implantable medical device101to external system102. This may include, for example, transmitting real-time physiological data acquired by implantable medical device101, extracting physiological data acquired by and stored in implantable medical device101, extracting therapy history data stored in implantable medical device101, and extracting data indicating an operational status of implantable medical device101(e.g., battery status and lead impedance). Telemetry link103also provides for data transmission from external system102to implantable medical device101. This may include, for example, programming implantable medical device101to acquire physiological data, programming implantable medical device101to perform at least one self-diagnostic test (such as for a device operational status), programming implantable medical device101to enable an available monitoring or therapeutic function (such as ATP), and programming implantable medical device101to adjust therapeutic parameters such as pacing and/or cardioversion/defibrillation parameters.

FIG. 2is a block diagram illustrating an embodiment of an anti-tachyarrhythmia therapy circuit220, which is a specific embodiment of anti-tachyarrhythmia therapy circuit120. Anti-tachyarrhythmia therapy circuit220includes a sensing circuit222, a pacing output circuit224, a tachyarrhythmia detector226, a tachyarrhythmia classifier228, a cycle length stability analyzer230, a primary duration timer232, a secondary duration timer234, and an ATP controller236.

Sensing circuit222senses one or more cardiac signals through one or more leads such as leads105and110. Pacing output circuit224delivers pacing pulses to the heart through one or more leads such as leads105and110. Tachyarrhythmia detector226detects a tachyarrhythmia episode using the one or more cardiac signals. Tachyarrhythmia classifier228classifies the detected tachyarrhythmia episode as one of predetermined types of tachyarrhythmia. Cycle length stability analyzer230analyzes the stability of cardiac cycle lengths indicated by the one or more cardiac signals.

In response to the detection of a tachyarrhythmia episode by tachyarrhythmia detector226, primary duration timer232initiates a primary duration. Cycle length stability analyzer230monitors the stability of cardiac cycle lengths during the primary duration. The detected tachyarrhythmia episode is classified by tachyarrhythmia classifier228at the end of the primary duration. Secondary duration timer234controls the initiation of a secondary duration using the stability of the cardiac cycle lengths during the primary duration and the classification of the detected tachyarrhythmia episode. If the secondary duration is initiated, cycle length stability analyzer230monitors the stability of cardiac cycle lengths during the secondary duration. ATP controller236controls the delivery of the pacing pulses according to an ATP mode using the stability of the cardiac cycle lengths during the primary duration, the stability of the cardiac cycle lengths during the secondary duration, and the classification of the detected tachyarrhythmia episode.

FIG. 3is a block diagram illustrating an embodiment of an anti-tachyarrhythmia therapy circuit320, which is a specific embodiment of anti-tachyarrhythmia therapy circuit220. Anti-tachyarrhythmia therapy circuit220includes a sensing circuit322, a beat detector340, a cycle length detector342, a tachyarrhythmia detector326, a tachyarrhythmia classifier328, a cycle length stability analyzer330, a pacing output circuit324, a cardioversion/defibrillation output circuit350, a primary duration timer332, a secondary duration timer334, a duration generator346, an ATP controller336, and a cardioversion/defibrillation controller348.

Sensing circuit322senses at least one cardiac signal. Beat detector340detects heart beats from the cardiac signal. Cycle length detector342detects cardiac cycle lengths each being a time interval between two consecutively detected heart beats. Tachyarrhythmia detector328detects a tachyarrhythmia episode using the cardiac cycle lengths. In one embodiment, tachyarrhythmia detector326declares a detection of the tachyarrhythmia episode using the cardiac cycle lengths and one or more tachyarrhythmia detection thresholds. Tachyarrhythmia classifier328classifies the detected tachyarrhythmia episode as one of predetermined types of tachyarrhythmia. In one embodiment, the predetermined types of tachyarrhythmia include VT and SVT. In a specific embodiment, sensing circuit322senses a ventricular electrogram through lead110. Beat detector340detects ventricular depolarizations as the heart beats from the ventricular electrogram. Cycle length detector342detects ventricular cycle lengths (also referred to as ventricular intervals) each being a time interval between two consecutively detected ventricular depolarizations. Tachyarrhythmia detector326declares a detection of a VT episode when the ventricular cycle lengths fall within a VT detection zone defined by at least one tachyarrhythmia detection threshold. Tachyarrhythmia classifier328confirms the detection of the VT episode by continuously or periodically classifying the detected VT episode as one of VT and SVT by examining the ventricular cycle lengths and ventricular electrogram morphologies.

Pacing output circuit324deliver pacing pulses. To allow ventricular ATP, pacing output circuit324includes a ventricular pacing output to deliver ventricular pacing pulses through lead110. Cardioversion/defibrillation output circuit348delivers cardioversion/defibrillation pulses. To terminate ventricular tachyarrhythmia, cardioversion/defibrillation output circuit348includes a ventricular cardioversion/defibrillation output to deliver ventricular cardioversion/defibrillation pulses through lead110.

Cycle length stability analyzer330includes an SVTC detector344to analyze the stability of the cardiac cycle lengths by detecting SVTCs. SVTC detector344detects SVTCs from the cardiac signal sensed by circuit322. The SVTCs each include at least a specified minimum number of consecutively detected heart beats over which the cardiac cycle lengths meet a specified stability criterion. When the specified minimum number of consecutive heart beats are detected during each of the SVTCs, SVTC detector344declares an SVTC detection. In one embodiment, the minimum number is specified in range of approximately 4 to 8, with approximately 5 being a specific example. In one embodiment, the minimum number is programmable. In one embodiment, SVTC detector344calculates a cycle-to-cycle variation that is the difference between cardiac cycle lengths associated with two consecutively detected heart beats, compares the cycle-to-cycle variation to a predetermined threshold, and declares the SVTC detection when the cycle-to-cycle variation is below the predetermined threshold for the specified minimum number of consecutively detected heart beats. In another embodiment, SVTC detector344calculates a running average of the cardiac cycle lengths, compares the cardiac cycle length associated with each of the consecutively detected heart beats to the running average of the cardiac cycle lengths, and declares the SVTC detection when the difference between the cardiac cycle length associated with each of the consecutively detected heart beats and the running average of the cardiac cycle lengths is within a predetermined margin for the specified minimum number of consecutively detected heart beats. In another embodiment, SVTC detector344analyzes a correlation between the morphology of the cardiac signal over each of the detected heart beats and a predetermined template morphology and declares the SVTC detection when the morphology of the cardiac signal over each of the detected heart beats substantially correlates to the predetermined template morphology for the specified minimum number of consecutively detected heart beats.

Primary duration timer332initiates a primary duration (T1) when the tachyarrhythmia episode is detected by tachyarrhythmia detector326. In one embodiment, the primary duration is a programmed constant duration in a range of approximately 1 to 30 seconds, with approximately 5 seconds as a specific example. Secondary duration timer334controls an initiation of a secondary duration (T2) using the stability of the cardiac cycle lengths during the primary duration and the classification of the detected tachyarrhythmia episode. In one embodiment, if the tachyarrhythmia is classified as VT when the primary duration expires and the stable SVTC detector declares at least one SVTC detection during the primary duration, but the primary duration does not expire with a detected SVTC, secondary duration timer334initiates the secondary duration when the primary duration expires. If the tachyarrhythmia is classified as VT after the primary duration expires and the stable SVTC detector declares at least one SVTC detection during the primary duration, secondary duration timer334initiates the secondary duration when the tachyarrhythmia is classified as VT. In one embodiment, the second duration is a programmed constant duration in a range of approximately 1 to 10 seconds, with approximately 2.5 seconds as a specific example. In another embodiment, duration generator346produces the second duration based on at least one of the number of SVTCs detected during the primary duration and the size (number of beats) of each of the SVTCs detected during the primary duration. The length of the secondary duration is increased for each SVTC detected during the primary duration and for each heart beat detected during each SVTC detected during the primary duration.

ATP controller336controls the delivery of the pacing pulses according to an ATP mode using the stability of the cardiac cycle lengths during the primary duration, the stability of the cardiac cycle lengths during the secondary duration, and the classification of the detected tachyarrhythmia episode. The delivery of the pacing pulses according to the ATP mode includes a delivery of at least one burst of pacing pulses. In one embodiment, each burst of pacing pulses includes approximately 3 to 10 pacing pulses, with approximately 5 pacing pulses as a specific example, at a pacing rate of approximately 200 to 250 pulses per minute, with approximately 200 pulses per minute as a specific example. In one embodiment, ATP controller336initiates the ATP therapy when the primary duration expires if the tachyarrhythmia is classified as VT when the primary duration expires and the primary duration expires with an SVTC. If the secondary duration is initiated by secondary duration timer334, ATP controller336initiates the ATP therapy when a detection of SVTC is declared during the secondary duration if the tachyarrhythmia is classified as VT when the SVTC is detected during the secondary duration. In one embodiment, ATP controller336calculates a coupling interval (CI) using a predetermined number of the cardiac cycle lengths detected during the SVTC. The coupling interval is the time interval between the last detected beat of the SVTC and the first pacing pulse of the ATP therapy. In a specific embodiment, ATP controller336calculates the coupling interval as a specified percentage of an average of the predetermined number of the cardiac cycle lengths detected during the SVTC.

If no SVTC is detected during the primary duration, or if no SVTC is detected when the primary duration expires and no SVTC is detected during the secondary duration, a therapy more aggressive than the ATP is selected. Examples of such more aggressive therapies include cardioversion/defibrillation and ATP before charge (ATP-BC), which are discussed below.

Cardioversion/defibrillation controller348controls a cardioversion/defibrillation therapy using the stability of the cardiac cycle lengths during the primary duration, the stability of the cardiac cycle lengths during the secondary duration, and the classification of the detected tachyarrhythmia episode. In one embodiment, cardioversion/defibrillation controller348initiates a delivery of the cardioversion/defibrillation therapy if the tachyarrhythmia is classified as VT when the primary duration expires and no SVTC is detected during the primary duration. If the secondary duration is initiated by secondary duration timer334, cardioversion/defibrillation controller348initiates a delivery of the cardioversion/defibrillation therapy if the tachyarrhythmia is classified as VT when the second duration expires and no SVTC is detected during the second duration. ATP-BC provides for an attempt to terminate the detected tachyarrhythmia episode using ATP immediately before charging a defibrillation capacitor. The defibrillation capacitor stores energy for a cardioversion/defibrillation pulse and is charged for the delivery of each cardioversion/defibrillation pulse. An example of ATP-BC is discussed in U.S. Pat. No. 7,277,752, entitled “METHOD AND APPARATUS FOR ANTI-TACHYARRHYTHMIA PACING AND DEFIBRILLATION,” filed on Apr. 2, 2004, assigned to Cardiac Pacemakers, Inc., which is incorporated herein by reference in its entirety. In one embodiment, ATP controller336initiates the delivery of an ATP-BC therapy if the tachyarrhythmia is classified as VT when the primary duration expires and no SVTC is detected during the primary duration. If the secondary duration is initiated by secondary duration timer334, ATP controller336initiates the delivery of the ATP-BC therapy if the tachyarrhythmia is classified as VT when the second duration expires and no SVTC is detected during the second duration.

FIG. 4is a flow chart illustrating an embodiment of a method400for selecting and timing anti-tachyarrhythmia therapies using cardiac cycle length stability. In one embodiment, the method is performed by anti-tachyarrhythmia therapy circuit120, including its various specific embodiments.

A cardiac signal is sensed at402. A tachyarrhythmia episode is detected from the cardiac signal at404. In one embodiment, a ventricular electrogram is sensed. Ventricular depolarizations are detected from the ventricular electrogram. Ventricular cycle lengths (also referred to as ventricular intervals) are detected, each as a time interval between two consecutively detected ventricular depolarizations. The detection of a VT episode is declared when the ventricular cycle lengths fall within a VT detection zone defined by one or more tachyarrhythmia detection thresholds.

If the tachyarrhythmia episode is detected at406, a primary duration (T1) is initiated and timed at408. In one embodiment, the primary duration is programmed to a value between approximately 1 and 30 seconds, with approximately 5 seconds as a specific example.

SVTCs are detected at410as a measure of stability of the cardiac cycle lengths. In one embodiment, SVTCs are detected between the time when the tachyarrhythmia episode is detected and the time when a therapy is delivered or the tachyarrhythmia episode is no longer detected. The SVTCs each include at least a specified minimum number of consecutively detected heart beats (e.g., ventricular depolarizations) over which the cardiac cycle lengths meet a specified stability criterion. An SVTC detection is declared when the specified minimum number of consecutive heart beats have been detected during each of the SVTCs. In one embodiment, the minimum number is programmed to a number between approximately 4 and 8, with approximately 5 being a specific example. In one embodiment, to detect the SVTCs, a cycle-to-cycle variation is calculated. The cycle-to-cycle variation is the difference between cardiac cycle lengths associated with two consecutively detected heart beats. The cycle-to-cycle variation is compared to a predetermined threshold. An SVTC detection is declared when the cycle-to-cycle variation is below the predetermined threshold for the specified minimum number of consecutively detected heart beats. In another embodiment, to detect the SVTCs, a running average of the cardiac cycle lengths is calculated. The cardiac cycle length associated with each of the consecutively detected heart beats are compared to the running average of the cardiac cycle lengths. An SVTC detection is declared when the difference between the cardiac cycle length associated with the each of the consecutively detected heart beats and the running average of the cardiac cycle lengths is within a predetermined margin for the specified minimum number of consecutively detected heart beats. In another embodiment, to detect the SVTCs, a correlation between a morphology of the cardiac signal over each of the detected heart beats and a predetermined template morphology is analyzed. An SVTC detection is declared if the morphology of the cardiac signal over the each of the detected heart beats substantially correlates to the predetermined template morphology for the specified minimum number of consecutively detected heart beats.

The detected tachyarrhythmia episode is classified at412as one of predetermined tachyarrhythmia types. In one embodiment, the detected tachyarrhythmia episode is classified as one of VT and SVT. In one embodiment, the classification is performed periodically or continuously when the tachyarrhythmia episode is being detected to confirm the detection of the VT episode.

If the classification of the detected tachyarrhythmia episode indicates that an anti-tachyarrhythmia therapy is needed at414, at least one SVTC is detected during the primary duration at416, and the primary duration ends with an SVTC at418, an ATP therapy is delivered at the end of the primary duration at424. If the classification of the detected tachyarrhythmia episode indicates that no anti-tachyarrhythmia therapy is needed at414, the classification at412continues as long as the tachyarrhythmia episode is still being detected. If the classification of the detected tachyarrhythmia episode indicates that an anti-tachyarrhythmia therapy is needed at414, but no SVTC is detected during the primary duration at416, a more aggressive therapy, such as the cardioversion/defibrillation therapy or the ATP-BC therapy, is delivered at the end of the primary duration at426. If the classification of the detected tachyarrhythmia episode indicates that an anti-tachyarrhythmia therapy is needed at414, at least one SVTC is detected during the primary duration at416, but the primary duration does not end with an SVTC at418, a secondary duration (T2) is initiated and timed at420. In one embodiment, the second duration is programmed to a value between approximately 1 and 10 seconds, with approximately 2.5 seconds as a specific example. In another embodiment, the second duration is produced as a function of at least one of a number of SVTCs detected during the primary duration and a size (number of heart beats) of each of the SVTCs detected during the primary duration. The second duration is lengthened for each SVTC detected during the primary duration and/or for each beat detected during each SVTC detected during the primary duration.

If an SVTC is detected during the secondary duration at422, the ATP therapy is delivered at424, when the SVTC detection is declared. That is, if one or more SVTCs are detected during the primary duration, the ATP therapy is delivered at the end of the primary duration only if the primary duration ends with an SVTC, but if an SVTC is detected during the secondary duration, the ATP therapy is delivered as soon as the SVTC detection is declared. If no SVTC is detected during the secondary duration at422, the more aggressive therapy is delivered at426.

The ATP therapy includes the delivery of at least one burst of pacing pulses. The burst of pacing pulses includes approximately 3 to 10 pacing pulses, with approximately 5 pacing pulses as a specific example, at a pacing rate of approximately 200 to 250 pulses per minute, with approximately 200 pulses per minute as a specific example. The first pacing pulse of the ATP therapy is delivered at the end of a coupling interval that starts with the last intrinsic heart beat, such as represented by the last ventricular depolarization (R wave). In one embodiment, the coupling interval is calculated by using a predetermined number of the cardiac cycle lengths detected during the SVTC. In a specific embodiment, the coupling interval is calculated as a specified percentage of an average of the predetermined number of the cardiac cycle lengths detected during the SVTC.

FIGS. 5-8illustrate various timing scenarios of how method400is applied to detect and treat a tachyarrhythmia. These timing scenarios are presented to illustrate, but not to restrict, how method400operates. In all the illustrated scenarios, the primary duration is initiated and timed in response to the detection of a tachyarrhythmia episode, and the minimum number of consecutively detected heart beats required for declaring the detection of an SVTC is 4.

InFIG. 5, an SVTC is detected during the primary duration (T1). At the end of the primary duration, the detected tachyarrhythmia episode is classified as VT, but the primary duration does not end with an SVTC. Therefore, the secondary duration (T2) is initiated and timed when the primary duration expires. Another SVTC is detected during the secondary duration. An ATP therapy is delivered as soon as the detection of the SVTC during the secondary duration is declared.

InFIG. 6, an SVTC is detected during the primary duration (T1). At the end of the primary duration, the detected tachyarrhythmia episode is classified as SVT. Therefore, the secondary duration (T2) is initiated and timed if and when the detected tachyarrhythmia episode becomes classified as VT. If the classification of detected tachyarrhythmia episode does not become VT, the secondary duration is not initiated, and no therapy delivery is needed. Another SVTC is detected during the secondary duration. An ATP therapy is delivered as soon as the detection of the SVTC during the secondary duration is declared.

InFIG. 7, no SVTC is detected during the primary duration (T1). At the end of the primary duration, the detected tachyarrhythmia episode is classified as VT. Therefore, a cardioversion/defibrillation therapy is delivered following the end of the primary duration.

InFIG. 8, an SVTC is detected during the primary duration (T1). At the end of the primary duration, the detected tachyarrhythmia episode is classified as VT. The primary duration ends with an SVTC. Therefore, an ATP therapy is delivered at the end of the primary duration.