Patent Description:
A variety of implantable medical devices (IMDs) for delivering a therapy, monitoring a physiological condition of a patient or a combination thereof have been clinically implanted or proposed for clinical implantation in patients. Some IMDs may employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. IMDs may deliver therapy to or monitor conditions of a variety of organs, nerves, muscle or tissue, such as the heart, brain, stomach, spinal cord, pelvic floor, or the like. Implantable medical electrical leads may be configured to position electrodes or other sensors at desired locations for delivery of electrical stimulation or sensing of physiological conditions. For example, electrodes or sensors may be carried along a distal portion of a lead that is extended subcutaneously, submuscularly, or transvenously. A proximal portion of the lead may be coupled to an implantable medical device housing, which contains circuitry such as signal generation circuitry and/or sensing circuitry.

Some IMDs, such as cardiac pacemakers or implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to the heart of the patient via electrodes carried by one or more implantable leads and/or the housing of the pacemaker or ICD. The leads may be transvenous, e.g., advanced into the heart through one or more veins to position endocardial electrodes in intimate contact with the heart tissue. Other leads may be non-transvenous leads implanted outside the heart, e.g., implanted epicardially, pericardially, or subcutaneously. The electrodes are used to deliver electrical pulses to the heart to address abnormal cardiac rhythms.

IMDs capable of delivering electrical pulses for treating abnormal cardiac rhythms typically sense signals representative of intrinsic depolarizations of the heart and analyze the sensed signals to identify the abnormal rhythms. Upon detection of an abnormal rhythm, the device may deliver an appropriate electrical stimulation therapy to restore a more normal rhythm. For example, a pacemaker or ICD may deliver pacing pulses to the heart upon detecting bradycardia or tachycardia. An ICD may deliver high voltage cardioversion or defibrillation shocks to the heart upon detecting fast ventricular tachycardia or fibrillation.

<CIT> relates to systems and methods for treating cardiac arrhythmias. <CIT> relates to synchronization of anti-tachycardia pacing in an extra-cardiovascular implantable system. <CIT> relates to an apparatus and method for treating tachyarrhythmia. <CIT> relates to a method and apparatus for selecting and timing anti-tachyarrhythmia pacing using cardiac signal morphology.

Any methods disclosed hereinafter are presented for illustrative purposes only and do not, by themselves, form part of the invention.

The techniques of this disclosure generally relate to a medical device and method for delivering anti-tachycardia pacing (ATP) in the presence of T-wave alternans (TWA). The ATP pulses are separated by alternating ATP time intervals corresponding to alternating phases of a Q-T or R-T time interval present during TWA. In some examples, the TWA may be detected before and/or after the onset of the ATP pulses for establishing the alternating ATP time intervals. In other examples, sensing of T-waves during ATP delivery enables ATP pulses to be delivered after sensed T-waves resulting in alternating ATP time interval in the presence of TWA.

In one example, the disclosure provides a medical device including a sensing circuit, a therapy delivery circuit and a control circuit. The sensing circuit is configured to receive a cardiac electrical signal from a patient's heart and sense R-waves and T-waves from the cardiac electrical signal. The therapy delivery circuit is configured to generate and deliver ATP pulses to the patient's heart via electrodes coupled to the therapy delivery circuit. The control circuit is coupled to the sensing circuit and to the therapy delivery circuit and is configured to detect a ventricular tachyarrhythmia from the cardiac electrical signal received by the sensing circuit. Responsive to the detected ventricular tachyarrhythmia, the control circuit controls the therapy delivery circuit to deliver a series of ATP pulses including alternating ATP time intervals separating the ATP pulses. The alternating ATP time intervals include at least a first ATP time interval separating a first pair of the ATP pulses and a second ATP time interval separating a second pair of the ATP pulses. The second ATP time interval is different than the first ATP time interval and consecutively follows the first ATP time interval.

In another example, the disclosure provides a method that includes detecting a ventricular tachyarrhythmia from a cardiac electrical signal device of a patient's heart and responsive to detecting the ventricular tachyarrhythmia, delivering a plurality of ATP pulses including alternating time intervals separating the ATP pulses in the series. The alternating time intervals include at least a first ATP time interval separating a first pair of the ATP pulses and a second ATP time interval separating a second pair of the ATP pulses. The second ATP time interval is different than the first ATP time interval and consecutively follows the first ATP time interval.

In yet another example, the disclosure provides a non-transitory computer-readable medium storing a set of instructions which when executed by a control circuit of a medical device, cause the medical device to detect a ventricular tachyarrhythmia from a cardiac electrical signal received by the medical device and, responsive to the detected ventricular tachyarrhythmia, control a therapy delivery circuit of the medical device to deliver a series of ATP pulses including alternating ATP time intervals separating the ATP pulses in the series. The alternating time intervals include at least a first ATP time interval separating a first pair of the ATP pulses and a second ATP time interval separating a second pair of the ATP pulses. The second ATP time interval is different than the first ATP time interval and consecutively follows the first ATP time interval.

In general, this disclosure describes techniques for delivering ATP in the presence of TWA. The T-wave in a cardiac electrical signal, e.g., a surface electrocardiogram, intrathoracic electrogram or intracardiac electrogram signal, is the signal attendant to the repolarization of the ventricular myocardium. TWA is the beat-to-beat variation of the amplitude, shape and/or timing of the T-wave. TWA may therefore represent the temporal and spatial dispersion of repolarization of the myocardium. TWA may be present during a tachyarrhythmia and may be altered or introduced during delivery of ATP to treat the tachyarrhythmia.

The timing of ATP pulses may be important in successfully terminating ventricular tachycardia (VT). In order to increase the likelihood of terminating VT, each ATP pulse is delivered during a time interval that the myocardium is likely to be in a non-refractory, excitable state between tachyarrhythmia depolarization wavefronts. This time interval is referred to as the "excitable gap. " During the excitable gap, an ATP pulse that captures the myocardial tissue evokes a myocardial depolarization that collides with a propagating tachycardia depolarization wavefront and blocks the re-entrant circuit of the tachycardia thereby resetting the re-entrant circuit or terminating the VT. The timing and duration of the excitable gap may change beat-by-beat due to TWA, which may potentially reduce the effectiveness of ATP pulses delivered at a fixed inter-pulse interval. Techniques for delivering ATP are presented herein for accounting for changes in the excitable gap timing and duration due to TWA to increase the likelihood of successfully terminating VT even in the presence of TWA.

<FIG> and <FIG> are conceptual diagrams of an extra-cardiovascular ICD system <NUM> according to one example. <FIG> is a front view of ICD system <NUM> implanted within patient <NUM>. <FIG> is a transverse sectional view of ICD system <NUM> implanted within patient <NUM>. ICD system <NUM> includes an ICD <NUM> connected to an extra-cardiovascular electrical stimulation and sensing lead <NUM>. An "extra-cardiovascular" lead as used referred to herein, refers to a lead that is implanted outside the heart and blood vessels of the patient's cardiovascular system. An extra-cardiovascular lead may extend subcutaneously, sub-muscularly or intra-thoracically, for example. <FIG> and <FIG> are described in the context of an ICD system <NUM> capable of providing defibrillation/cardioversion (CV/DF) shocks and pacing pulses, including ATP pulses.

ICD <NUM> includes a housing <NUM> that forms a hermetic seal that protects internal components of ICD <NUM>. The housing <NUM> of ICD <NUM> may be formed of a conductive material, such as titanium or titanium alloy. The housing <NUM> may function as a housing electrode (sometimes referred to as a can electrode). In examples described herein, housing <NUM> may be used as an active can electrode for use in delivering CV/DF shocks or other high voltage pulses delivered using a high voltage therapy circuit. In other examples, housing <NUM> may be available for use in delivering unipolar, low voltage cardiac pacing pulses in conjunction with lead-based cathode electrodes. In other instances, the housing <NUM> of ICD <NUM> may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing <NUM> functioning as an electrode(s) may be coated with a material, such as titanium nitride to reduce post-stimulation polarization artifact.

ICD <NUM> includes a connector assembly <NUM> (also referred to as a connector block or header) that includes electrical feedthroughs crossing housing <NUM> to provide electrical connections between conductors extending within the lead body <NUM> of lead <NUM> and electronic components included within the housing <NUM> of ICD <NUM>. As will be described in further detail herein, housing <NUM> may house one or more processors, memories, transceivers, sensors, electrical sensing circuitry, therapy delivery circuitry, power sources and other components for sensing cardiac electrical signals, detecting a heart rhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm.

Lead <NUM> includes an elongated lead body <NUM> having a proximal end <NUM> that includes a lead connector (not shown) configured to be connected to ICD connector assembly <NUM> and a distal portion <NUM> that includes one or more electrodes. In the example illustrated in <FIG> and <FIG>, the distal portion <NUM> of lead <NUM> includes defibrillation electrodes <NUM> and <NUM> and pace/sense electrodes <NUM> and <NUM>. In some cases, defibrillation electrodes <NUM> and <NUM> may be configured to be activated concurrently. Alternatively or additionally, each of the electrodes <NUM> and <NUM> may be activated independently.

Electrodes <NUM> and <NUM> (and in some examples housing <NUM>) are referred to herein as defibrillation electrodes because they are utilized, individually or collectively, for delivering high voltage stimulation therapy (e.g., cardioversion or defibrillation shocks). Electrodes <NUM> and <NUM> may be elongated coil electrodes and generally have a relatively high surface area for delivering high voltage electrical stimulation pulses compared to low voltage pacing and sensing electrodes <NUM> and <NUM>. However, electrodes <NUM> and <NUM> and housing <NUM> may also be utilized to provide pacing functionality, sensing functionality or both pacing and sensing functionality in addition to or instead of high voltage stimulation therapy. In this sense, the use of the term "defibrillation electrode" herein should not be considered as limiting the electrodes <NUM> and <NUM> for use in only high voltage CV/DF shock therapy applications. Electrodes <NUM> and <NUM> may be used in a pacing electrode vector for delivering extra-cardiovascular pacing pulses such as ATP pulses and/or in a sensing vector used to sense cardiac electrical signals and detect ventricular tachycardia (VT) and ventricular fibrillation (VF).

Electrodes <NUM> and <NUM> are relatively smaller surface area electrodes for delivering relatively lower voltage pacing pulses and for sensing cardiac electrical signals. Electrodes <NUM> and <NUM> are referred to herein as pace/sense electrodes because they are generally configured for use in low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses, which may include ATP pulses, and/or sensing of cardiac electrical signals. In some instances, electrodes <NUM> and <NUM> may provide only pacing functionality, only sensing functionality or both.

In the example illustrated in <FIG>, electrode <NUM> is located proximal to defibrillation electrode <NUM>, and electrode <NUM> is located between defibrillation electrodes <NUM> and <NUM>. In other examples, electrodes <NUM> and <NUM> may be positioned at other locations along lead <NUM>, which may include one or more pace/sense electrodes. Electrodes <NUM> and <NUM> are illustrated as ring electrodes, however electrodes <NUM> and <NUM> may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, or the like.

Lead <NUM> extends subcutaneously or submuscularly over the ribcage <NUM> medially from the connector assembly <NUM> of ICD <NUM> toward a center of the torso of patient <NUM>, e.g., toward xiphoid process <NUM> of patient <NUM>. At a location near xiphoid process <NUM>, lead <NUM> bends or turns and extends superiorly within anterior mediastinum <NUM> in a substernal position. Anterior mediastinum <NUM> (seen in <FIG>) may be viewed as being bounded laterally by pleurae <NUM>, posteriorly by pericardium <NUM>, and anteriorly by sternum <NUM>. In some instances, the anterior wall of anterior mediastinum <NUM> may also be formed by the transversus thoracis muscle and one or more costal cartilages. Anterior mediastinum <NUM> includes a quantity of loose connective tissue (such as areolar tissue), adipose tissue, some lymph vessels, lymph glands, substernal musculature, small side branches of the internal thoracic artery or vein, and the thymus gland. In one example, the distal portion <NUM> of lead <NUM> extends along the posterior side of sternum <NUM> substantially within the loose connective tissue and/or substernal musculature of anterior mediastinum <NUM>.

A lead implanted such that the distal portion <NUM> is substantially within anterior mediastinum <NUM> may be referred to as a "substernal lead. " In the example illustrated in <FIG> and <FIG>, lead <NUM> is located substantially centered under sternum <NUM>. In other instances, however, lead <NUM> may be implanted such that it is offset laterally from the center of sternum <NUM>. In some instances, lead <NUM> may extend laterally such that distal portion <NUM> of lead <NUM> is underneath/below the ribcage <NUM> in addition to or instead of sternum <NUM>. In other examples, the distal portion <NUM> of lead <NUM> may be implanted in other extra-cardiovascular, intra-thoracic locations, including the pleural cavity or around the perimeter of and adjacent to the pericardium <NUM> of heart <NUM>. Other implant locations and lead and electrode arrangements that may be used in conjunction with the ATP techniques described herein are generally disclosed in <CIT>) and <CIT>). For example, lead <NUM> may extend superiorly and subcutaneously or submuscularly over the ribcage and/or sternum <NUM>, rather than substernally. Alternatively, lead <NUM> may be placed along other subcutaneous or submuscular paths. The path of lead <NUM> may depend on the location of ICD <NUM>, the arrangement and position of electrodes carried by the lead distal portion <NUM>, and/or other factors.

In the example shown, lead body <NUM> includes a curving distal portion <NUM> having two "C" shaped curves, which together may resemble the Greek letter epsilon, "ε. " Defibrillation electrodes <NUM> and <NUM> are each carried by one of the two respective C-shaped portions of the lead body distal portion <NUM>. The two C-shaped curves are seen to extend or curve in the same direction away from a central axis of lead body <NUM>, along which pace/sense electrodes <NUM> and <NUM> are positioned. Pace/sense electrodes <NUM> and <NUM> may, in some instances, be approximately aligned with the central axis of the straight, proximal portion of lead body <NUM> such that mid-points of defibrillation electrodes <NUM> and <NUM> are laterally offset from pace/sense electrodes <NUM> and <NUM>.

Other examples of extra-cardiovascular leads including one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by curving, serpentine, undulating or zig-zagging distal portion of the lead body <NUM> that may be implemented with the techniques described herein are generally disclosed in <CIT>). The techniques disclosed herein are not limited to any particular lead body design, however. In other examples, lead body <NUM> is a flexible elongated lead body without any pre-formed shape, bends or curves.

Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body <NUM> of lead <NUM> from the lead proximal end <NUM> to electrodes <NUM>, <NUM>, <NUM>, and <NUM> located along the distal portion <NUM> of the lead body <NUM>. The conductors electrically couple respective ones of the electrodes <NUM>, <NUM>, <NUM>, and <NUM> to circuitry, such as a therapy delivery circuit and/or a sensing circuit, of ICD <NUM> via connections in the connector assembly <NUM>, including associated electrical feedthroughs crossing housing <NUM>. The electrical conductors transmit therapy from a therapy circuit within ICD <NUM> to one or more of defibrillation electrodes <NUM> and <NUM> and/or pace/sense electrodes <NUM> and <NUM> and transmit sensed electrical signals produced by the patient's heart <NUM> from one or more of defibrillation electrodes <NUM> and <NUM> and/or pace/sense electrodes <NUM> and <NUM> to the sensing circuit within ICD <NUM>.

ICD <NUM> may obtain electrical signals corresponding to electrical activity of heart <NUM> via a combination of sensing vectors that include combinations of electrodes <NUM> and <NUM>. In some examples, housing <NUM> of ICD <NUM> is used in combination with one or more of electrodes <NUM> and/or <NUM> in a sensing electrode vector. ICD <NUM> may even obtain cardiac electrical signals using a sensing vector that includes one or both defibrillation electrodes <NUM> and/or <NUM>, e.g., between electrodes <NUM> and <NUM> or between one of electrodes <NUM> or <NUM> in combination with one of electrodes <NUM>, <NUM> and/or housing <NUM>.

ICD <NUM> analyzes the cardiac electrical signals received from one or more of the sensing vectors to monitor for abnormal rhythms, such as bradycardia, VT and VF. ICD <NUM> may analyze the heart rate and/or morphology of the cardiac electrical signals to monitor for tachyarrhythmia in accordance with any of a number of tachyarrhythmia detection techniques. One example technique for detecting tachyarrhythmia is described in <CIT>).

ICD <NUM> generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia (e.g., VT or VF). ICD <NUM> may deliver ATP in response to VT detection, and in some cases may deliver ATP prior to a CV/DF shock or during high voltage capacitor charging in an attempt to avert the need for delivering a CV/DF shock. ATP may be delivered using an extra-cardiovascular pacing electrode vector selected from any of electrodes <NUM>, <NUM>, <NUM>, <NUM> and/or housing <NUM>. The pacing electrode vector may be different than the sensing electrode vector. In one example, cardiac electrical signals are sensed between pace/sense electrodes <NUM> and <NUM>, and ATP pulses are delivered between pace/sense electrode <NUM> used as a cathode electrode and defibrillation electrode <NUM> used as a return anode electrode. In other examples, ATP pulses may be delivered between pace/sense electrode <NUM> and either (or both) defibrillation electrode <NUM> or <NUM> or between defibrillation electrode <NUM> and defibrillation electrode <NUM>. These examples are not intended to be limiting, and it is recognized that other sensing electrode vectors and ATP electrode vectors may be selected according to individual patient need.

The myocardial site that is first captured by an ATP pulse delivered by the selected extra-cardiovascular pacing electrode vector is referred to herein as the "capture site" which is spaced apart from the pacing cathode electrode and the pacing anode electrode that are not in direct contact with the myocardium in an extra-cardiovascular ICD system, such as system <NUM>. In order to successfully terminate a detected VT, it is desirable that all ATP pulses capture the myocardium to overdrive pace the heart back into a normal sinus rhythm. In order to overdrive pace the heart, each pacing pulse of the ATP sequence should arrive at the capture site during the excitable gap and before the next expected intrinsic ventricular depolarization. During TWA, the onset and duration of the excitable gap may vary beat to beat. TWA may be present at the time of a VT detection and may persist during ATP delivery, with or without being altered. In other instances, TWA may not be present at the time of VT detection but may arise during ATP delivery. As described below, ICD <NUM> is configured to deliver ATP pulses at time intervals that take into account beat-to-beat variations in the timing of the excitable gap due to TWA.

If ATP does not successfully terminate VT or when VF is detected, ICD <NUM> may deliver one or more cardioversion or defibrillation (CV/DF) shocks via one or both of defibrillation electrodes <NUM> and <NUM> and/or housing <NUM>. ICD <NUM> may deliver the CV/DF shocks using electrodes <NUM> and <NUM> individually or together as a cathode (or anode) and with the housing <NUM> as an anode (or cathode). ICD <NUM> 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 the electrodes <NUM>, <NUM>, <NUM>, <NUM> and the housing <NUM> of ICD <NUM>.

<FIG> and <FIG> are illustrative in nature and should not be considered limiting of the practice of the techniques disclosed herein. For instance, ICD <NUM> is shown implanted subcutaneously on the left side of patient <NUM> along the ribcage <NUM>. ICD <NUM> may, in other instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient <NUM>. ICD <NUM> may be implanted at other subcutaneous or submuscular locations in patient <NUM> such as in a subcutaneous pocket in the pectoral region. In this case, lead <NUM> may extend subcutaneously or submuscularly from ICD <NUM> toward the manubrium of sternum <NUM> and bend or turn and extend inferior from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD <NUM> may be placed abdominally. Lead <NUM> may be implanted in other extra-cardiovascular locations as well and include other electrode and lead body configurations.

An external device <NUM> is shown in telemetric communication with ICD <NUM> by a communication link <NUM>. External device <NUM> may include a processor, display, user interface, telemetry unit and other components for communicating with ICD <NUM> for transmitting and receiving data via communication link <NUM>. Communication link <NUM> may be established between ICD <NUM> and external device <NUM> using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) or other RF or communication frequency bandwidth.

External device <NUM> may be embodied as a programmer used in a hospital, clinic or physician's office to retrieve data from ICD <NUM> and to program operating parameters and algorithms in ICD <NUM> for controlling ICD functions. External device <NUM> may be used to program cardiac rhythm detection parameters and therapy control parameters used by ICD <NUM>. Control parameters used to generate and deliver ATP according to techniques disclosed herein may be programmed into ICD <NUM> using external device <NUM>.

Data stored or acquired by ICD <NUM>, including physiological signals or associated data derived therefrom, results of device diagnostics, and histories of detected rhythm episodes and delivered therapies, may be retrieved from ICD <NUM> by external device <NUM> following an interrogation command. External device <NUM> may alternatively be embodied as a home monitor or hand held device.

In some examples, ICD <NUM> may be co-implanted with an intracardiac pacemaker <NUM>. For example, when lead <NUM> is implanted subcutaneously, pacing pulses of an ATP therapy may require amplitudes that are uncomfortable or painful for a patient. Intracardiac pacemaker <NUM> may be a leadless device implantable wholly within a heart chamber, e.g., within the right ventricle or left ventricle, for delivering electrical stimulation pulses, including ATP, via electrodes coupled to the housing of pacemaker <NUM>. Pacemaker <NUM> may be capable of delivering ATP therapy, either in response to detecting VT or VF or in response to receiving a communication signal from ICD <NUM>.

As shown in <FIG>, pacemaker <NUM> includes electrodes <NUM> and <NUM> spaced apart along its housing <NUM> for sensing cardiac electrical signals and delivering pacing pulses. Electrode <NUM> is shown as a tip electrode extending from a distal end <NUM> of pacemaker <NUM>, and electrode <NUM> is shown as a ring electrode along a mid-portion of housing <NUM>, for example adjacent proximal end <NUM>. Distal end <NUM> is referred to as "distal" in that it is expected to be the leading end as pacemaker <NUM> is advanced through a delivery tool, such as a catheter, and placed against a targeted pacing site.

Electrodes <NUM> and <NUM> form an anode and cathode pair for bipolar cardiac pacing and sensing. In alternative examples, pacemaker <NUM> may include two or more ring electrodes, two tip electrodes, and/or other types of electrodes exposed along pacemaker housing <NUM> for delivering electrical stimulation to heart <NUM> and sensing cardiac electrical signals. Electrodes <NUM> and <NUM> may be, without limitation, titanium, platinum, iridium or alloys thereof and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black among others. Electrodes <NUM> and <NUM> may be positioned at locations along pacemaker <NUM> other than the locations shown.

Housing <NUM> is formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housing <NUM> may include an insulating coating. Examples of insulating coatings include parylene, urethane, polyether ether ketone (PEEK), or polyimide among others. The entirety of the housing <NUM> may be insulated, but only electrodes <NUM> and <NUM> uninsulated. Electrode <NUM> may serve as a cathode electrode and be coupled to internal circuitry, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, enclosed by housing <NUM> via an electrical feedthrough crossing housing <NUM>. Electrode <NUM> may be formed as a conductive portion of housing <NUM> as a ring electrode. In other examples, the entire periphery of the housing <NUM> may function as an electrode that is electrically isolated from tip electrode <NUM>, instead of providing a localized ring electrode such as anode electrode <NUM>. Electrode <NUM> formed along an electrically conductive portion of housing <NUM> serves as a return anode during pacing and sensing.

The housing <NUM> includes a control electronics subassembly <NUM>, which houses the electronics for sensing cardiac signals, producing pacing pulses and controlling therapy delivery and other functions of pacemaker <NUM> as attributed to an IMD performing the ATP delivery techniques described herein. Housing <NUM> further includes a battery subassembly <NUM>, which provides power to the control electronics subassembly <NUM>.

Pacemaker <NUM> may include a set of fixation tines <NUM> to secure pacemaker <NUM> to patient tissue, e.g., by actively engaging with the ventricular endocardium and/or interacting with the ventricular trabeculae. Fixation tines <NUM> are configured to anchor pacemaker <NUM> to position electrode <NUM> in operative proximity to a targeted tissue for delivering therapeutic electrical stimulation pulses. Numerous types of active and/or passive fixation members may be employed for anchoring or stabilizing pacemaker <NUM> in an implant position.

Pacemaker <NUM> may optionally include a delivery tool interface <NUM>. Delivery tool interface <NUM> may be located at the proximal end <NUM> of pacemaker <NUM> and is configured to connect to a delivery device, such as a catheter, used to position pacemaker <NUM> at an implant location during an implantation procedure, for example within a heart chamber, such as the right or left ventricle.

In some examples, pacemaker <NUM> may be a triggered pacemaker that delivers a pacing pulse in response to a trigger signal transmitted by ICD <NUM>. Pacemaker <NUM> may receive a command from ICD <NUM> to initiate ATP therapy and deliver the ATP pulses according to timing interval data received from ICD <NUM>. In other examples, pacemaker <NUM> may analyze cardiac signals received via electrodes <NUM> and <NUM> for determining when a pacing pulse is needed, including detecting VT or VF and delivering an ATP therapy. As such, pacemaker <NUM> may be configured to detect TWA and adjust ATP pulse timing according to the techniques disclosed herein.

<FIG> is a conceptual diagram of patient <NUM> implanted with an ICD <NUM> according to another example. In this example, ICD <NUM> is coupled to one or more transvenous leads carrying electrodes for sensing cardiac electrical signals and for delivering electrical stimulation therapy, e.g., bradycardia pacing, ATP, cardiac resynchronization therapy (CRT) and/or CV/DF. ICD <NUM> is shown implanted in a right pectoral position in <FIG>; however it is recognized that ICD <NUM> may be implanted in a left pectoral position, particularly when ICD <NUM> includes sensing, pacing, cardioversion and/or defibrillation capabilities using housing <NUM> as an electrode.

ICD <NUM> is illustrated as a dual chamber device for sensing and therapy delivery in an atrial chamber <NUM> and a ventricular chamber <NUM> of heart <NUM>. As such, ICD <NUM> includes connector assembly <NUM> having two connector bores for receiving proximal connectors of a right atrial (RA) lead <NUM> and a right ventricular (RV) lead <NUM>. In other examples ICD <NUM> may be a single chamber device, e.g., connectable only to RV lead <NUM>, or a multichamber device including a third connector bore, e.g., for receiving a coronary sinus lead to enable ICD <NUM> to sense left ventricular signals and deliver electrical stimulation pulses to the LV.

RA lead <NUM> may carry a distal tip electrode <NUM> and ring electrode <NUM> spaced proximal from the tip electrode <NUM> for delivering pacing pulses to the right atrium <NUM> and obtaining atrial electrical signals for producing an atrial intra-cardiac electrogram (EGM) signal by ICD <NUM>. RV lead <NUM> may carry pacing and sensing electrodes <NUM> and <NUM> for delivering RV pacing pulses to the right ventricle <NUM> and obtaining ventricular electrical signals for producing an RV EGM signal by ICD <NUM>. RV lead <NUM> may also carry RV defibrillation electrode <NUM> and a superior vena cava (SVC) defibrillation electrode <NUM>. Defibrillation electrodes <NUM> and <NUM> are shown as coil electrodes spaced apart proximally from the distal pacing and sensing electrodes <NUM> and <NUM>.

ICD housing <NUM> encloses circuitry, as further described below, configured to detect arrhythmias and provide electrical stimulation therapy, such as bradycardia pacing, post-shock pacing, ATP, CRT and/or CV/DF shock therapy, using the electrodes <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of transvenous leads <NUM> and <NUM>. As described below, ICD <NUM> may adjust the time of ATP pulse delivery pulse-by-pulse based on detecting TWA and identifying the phase of the TWA.

<FIG> is a schematic diagram of ICD <NUM> according to one example. The electronic circuitry enclosed within housing <NUM> (shown schematically as an electrode in <FIG>) includes software, firmware and hardware that cooperatively monitor one or more cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver therapies as needed according to programmed therapy delivery algorithms and control parameters. The software, firmware and hardware are configured to detect VT and VF for determining when ATP or CV/DF shocks are required. ICD <NUM> shown schematically in <FIG> may generally correspond to ICD <NUM> shown in <FIG> and1B coupled to an extra-cardiovascular lead (such as lead <NUM> carrying extra-cardiovascular electrodes <NUM>, <NUM>, <NUM>, and <NUM>) or to ICD <NUM> shown in <FIG> coupled to at least one transvenous lead (e.g., lead <NUM> carrying defibrillation electrodes <NUM> and <NUM> and pace/sense electrodes <NUM> and <NUM>). Functions attributed to the circuitry described in conjunction with <FIG> may be adapted as needed for detecting VT and VF and delivering ATP via extra-cardiovascular electrodes or via endocardial electrodes. Furthermore, it is contemplated that the functionality for delivering ATP pulses adjusted to account for changes in the timing of the excitable gap due to TWA may be implemented in an intracardiac pacemaker, e.g., pacemaker <NUM> shown in <FIG>, which may include all or a portion of the example components illustrated in <FIG>.

ICD <NUM> includes a control circuit <NUM>, memory <NUM>, therapy delivery circuit <NUM>, sensing circuit <NUM>, and telemetry circuit <NUM>. A power source <NUM> provides power to the circuitry of ICD <NUM>, including each of the components <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> as needed. Power source <NUM> may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source <NUM> and each of the other components <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are to be understood from the general block diagram of <FIG>, but are not shown for the sake of clarity. For example, power source <NUM> may be coupled to one or more charging circuits included in therapy delivery circuit <NUM> for charging holding capacitors included in therapy delivery circuit <NUM> that are discharged at appropriate times under the control of control circuit <NUM> for producing electrical pulses according to a therapy protocol, such as for bradycardia pacing, CRT, post-shock pacing, ATP and/or CV/DF shock pulses. Power source <NUM> may also be coupled to components of sensing circuit <NUM>, such as sense amplifiers, analog-to-digital converters, switching circuitry, etc., telemetry circuit <NUM>, and memory <NUM> to provide power as needed.

The functional blocks shown in <FIG> represent functionality included in an ICD configured to sense cardiac electrical signals and deliver cardiac electrical stimulation therapy and may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to an ICD (or pacemaker) herein. The various components may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine, 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 by the particular detection and therapy delivery methodologies employed by the ICD. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modem IMD system, given the disclosure herein, is within the abilities of one of skill in the art.

Control circuit <NUM> communicates, e.g., via a data bus, with therapy delivery circuit <NUM> and sensing circuit <NUM>. Therapy delivery circuit <NUM> and sensing circuit <NUM> are electrically coupled to electrodes <NUM>, <NUM>, <NUM>, <NUM> and the housing <NUM>, which may function as a common or ground electrode or as an active can electrode for delivering CV/DF shock pulses or cardiac pacing pulses. Sensing circuit <NUM> may be selectively coupled to electrodes <NUM>, <NUM> and/or housing <NUM> in order to monitor electrical activity of the patient's heart. Sensing circuit <NUM> may additionally be selectively coupled to defibrillation electrodes <NUM> and/or <NUM> for use in a sensing electrode vector together or in combination with one or more of electrodes <NUM>, <NUM> and/or housing <NUM>. Sensing circuit <NUM> may be enabled to selectively receive cardiac electrical signals from at least two sensing electrode vectors from the available electrodes <NUM>, <NUM>, <NUM>, <NUM>, and housing <NUM>. For example, sensing circuit <NUM> may include switching circuitry (not shown) for selecting which of electrodes <NUM>, <NUM>, <NUM>, <NUM>, and housing <NUM> are coupled to a first sensing channel <NUM> and which are coupled to a second sensing channel <NUM> of sensing circuit <NUM>. Switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple components of sensing circuit <NUM> to selected electrodes. In other instances, ICD <NUM> may include only a single sensing channel.

In the case of two sensing channels, two cardiac electrical signals from two different sensing electrode vectors may be received simultaneously by sensing circuit <NUM>. The two sensing electrode vectors may include two different ventricular sensing electrode vectors each coupled to a respective sensing channel <NUM> and <NUM>. In other examples, when an atrial sensing electrode vector is available, e.g., when RA lead <NUM> is present carrying atrial pacing and sensing electrodes <NUM> and <NUM> (as shown in <FIG>), one sensing channel <NUM> may be an atrial sensing channel and one sensing channel <NUM> may be a ventricular sensing channel.

Sensing circuit <NUM> may monitor one or both or the cardiac electrical signals for sensing cardiac electrical events, e.g., P-waves attendant to the depolarization of the atrial myocardium, R-waves attendant to the depolarization of the ventricular myocardium, and T-waves attendant to myocardial repolarization in the ventricles. Sensing circuit <NUM> may produce digitized cardiac signal waveforms for analysis by control circuit <NUM> for detecting a cardiac rhythm, including detection of TWA.

In some examples, one sensing channel, e.g., channel <NUM>, may be configured to sense R-waves from a cardiac electrical signal obtained using a first sensing electrode vector selected from the available electrodes <NUM>, <NUM>, <NUM>, <NUM> and housing <NUM>. The second sensing channel <NUM> may be configured to sense T-waves from the same cardiac electrical signal or a different cardiac electrical signal obtained using a second sensing electrode vector different than the first vector. In some examples, T-wave sensing includes rejecting R-waves based on the timing of R-waves sensed by the first sensing channel.

Each sensing channel <NUM> and <NUM> may be configured to amplify, filter and digitize the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for detecting cardiac electrical events, such as R-waves and T-waves or performing other signal analysis. The cardiac event detection circuitry within sensing circuit <NUM> may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), timers or other analog or digital components. A cardiac event sensing threshold may be automatically adjusted by sensing circuit <NUM> under the control of control circuit <NUM>, based on timing intervals and sensing threshold values determined by control circuit <NUM>, stored in memory <NUM>, and/or controlled by hardware, firmware and/or software of control circuit <NUM> and/or sensing circuit <NUM>. For instance, an R-wave sensing threshold and a T-wave sensing threshold may each be stored in memory <NUM> and applied by sensing circuit <NUM> as auto-adjusting thresholds that include one or more decay rates and/or step adjustments. When the cardiac electrical signal crosses the respective sensing threshold, a cardiac event is sensed. For example, when the cardiac electrical signal crosses a R-wave sensing threshold an R-wave is sensed. As another example, when the cardiac electrical signal crosses a T-wave sensing threshold and is is not concurrent with a sensed R-wave, the sensing circuit may sense a T-wave. These T-waves may be used in detecting TWA by control circuit <NUM> as described further herein.

Upon sensing a cardiac electrical signal (e.g., an R-wave, T-wave or P-wave) based on a sensing threshold crossing, sensing circuit <NUM> may produce a sensed event signal, such as an R-wave sensed event signal, T-wave sensed event signal or P-wave sensed event signal, which is passed to control circuit <NUM>. The R-wave sensed event signals may be used by control circuit <NUM> for determining ventricular event intervals, referred to herein as "RR intervals" or "RRIs" for detecting tachyarrhythmia and determining a need for therapy. A ventricular event interval or RRI is the time interval between two consecutively sensed R-waves and may be determined between two consecutive R-wave sensed event signals received from sensing circuit <NUM>. For example, control circuit <NUM> may include a timing circuit <NUM> for determining RRIs between consecutive R-wave sensed event signals received from sensing circuit <NUM> and for controlling various timers and/or counters used to control the timing of therapy delivery by therapy delivery circuit <NUM>, including the timing of ATP pulses as described herein. Timing circuit <NUM> may additionally set time windows such as morphology template windows, morphology analysis windows or perform other timing related functions of ICD <NUM> including synchronizing CV/DF shocks or other therapies delivered by therapy delivery circuit <NUM> with sensed cardiac events.

T-wave sensed event signals may be passed from sensing circuit <NUM> to control circuit <NUM> for use in detecting and characterizing TWA. In some examples, timing circuit <NUM> may receive R-wave sensed event signals and T-wave sensed event signals from sensing circuit <NUM> for determining R-T intervals. TWA may be detected by control circuit <NUM> in response to beat-to-beat variation in R-T intervals. An R-T interval is the time interval between a sensed R-wave and a subsequently sensed T-wave of the cardiac cycle determined using sensed event signals received from sensing circuit <NUM>. The phase of the TWA may be determined based on the R-T intervals. For example, the R-T intervals may alternate between a first phase, which may be a relatively longer R-T interval, and a second phase, which may be a relatively shorter R-T interval. Detection of TWA and identification of the phase, e.g., short or long phase based on R-T interval, may be used by control circuit <NUM> in controlling the timing of ATP pulses delivered by therapy delivery circuit <NUM>.

Tachyarrhythmia detector <NUM> is configured to analyze signals received from sensing circuit <NUM> for detecting tachyarrhythmia episodes. Tachyarrhythmia detector <NUM> may be implemented in control circuit <NUM> as software, hardware and/or firmware that processes and analyzes signals received from sensing circuit <NUM> for detecting VT and/or VF. In some examples, tachyarrhythmia detector <NUM> may include comparators and counters for counting RRIs determined by timing circuit <NUM> that fall into various rate detection zones for determining a ventricular rate or performing other rate- or interval-based assessments for detecting and discriminating VT and VF. For example, tachyarrhythmia detector <NUM> may compare the RRIs determined by timing circuit <NUM> to one or more tachyarrhythmia detection interval zones, such as a tachycardia detection interval zone and a fibrillation detection interval zone. RRIs falling into a detection interval zone are counted by a respective VT interval counter or VF interval counter and in some cases in a combined VT/VF interval counter included in tachyarrhythmia detector <NUM>.

When a VT or VF interval counter reaches a threshold count value, referred to as "number of intervals to detect" or "NID," a ventricular tachyarrhythmia may be detected by control circuit <NUM>. Tachyarrhythmia detector <NUM> may be configured to perform other signal analysis for determining if other detection criteria are satisfied before detecting VT or VF when an NID is reached. For example, cardiac signal analysis may be performed to determine if R-wave morphology criteria, onset criteria, and noise and oversensing rejection criteria are satisfied in order to determine if the VT/VF detection should be made or withheld.

To support additional cardiac signal analyses performed by tachyarrhythmia detector <NUM>, sensing circuit <NUM> may pass a digitized cardiac electrical signal to control circuit <NUM>. A cardiac electrical signal from the selected sensing channel, e.g., from first sensing channel <NUM> and/or the second sensing channel <NUM>, may be passed through a filter and amplifier, provided to a multiplexer and thereafter converted to multi-bit digital signals by an analog-to-digital converter, all included in sensing circuit <NUM>, for storage in memory <NUM>. Additional signal analyses may include morphological analysis of pre-determined time segments of the cardiac electrical signals or QRS waveforms. In some examples, additional analysis may be performed to detect TWA based on changes in the T-wave of a ventricular electrical signal, such as changes in T-wave amplitude, polarity, Q-T interval, R-T interval or the like. Detection of TWA performed by control circuit <NUM> may occur before and/or during ATP delivery in some examples.

Therapy delivery circuit <NUM> 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) are discharged across a selected pacing electrode vector or CV/DF shock vector. Charging of capacitors to a programmed pulse amplitude and discharging of the capacitors for a programmed pulse width may be performed by therapy delivery circuit <NUM> according to control signals received from control circuit <NUM>. Timing circuit <NUM> of control circuit <NUM> includes various timers or counters that control when ATP or other cardiac pacing pulses are delivered. For example, timing circuit <NUM> may include programmable digital counters set by a microprocessor of the control circuit <NUM> for controlling the basic pacing time intervals associated with various pacing modes or ATP sequences delivered by ICD <NUM>. The microprocessor of control circuit <NUM> may also set the amplitude, pulse width, polarity or other characteristics of the cardiac pacing pulses, which may be based on programmed values stored in memory <NUM>.

In response to detecting VT or VF, control circuit <NUM> may control therapy delivery circuit <NUM> to deliver therapies such as ATP and/or CV/DF therapy. Therapy can be delivered by initiating charging of high voltage capacitors via a charging circuit, both included in therapy delivery circuit <NUM>. Charging is controlled by control circuit <NUM>, which monitors the voltage on the high voltage capacitors passed to control circuit <NUM> via a charging control line. When the voltage reaches a predetermined value set by control circuit <NUM>, a logic signal is generated on a capacitor full line and passed to therapy delivery circuit <NUM>, terminating charging. A CV/DF pulse is delivered to the heart under the control of the timing circuit <NUM> by an output circuit of therapy delivery circuit <NUM> via a control bus. The output circuit may include an output capacitor through which the charged high voltage capacitor is discharged via switching circuitry, e.g., an H-bridge, which determines the electrodes used for delivering the cardioversion or defibrillation pulse and the pulse wave shape. In some examples, the high voltage therapy circuit configured to deliver CV/DF shock pulses can be controlled by control circuit <NUM> to deliver pacing pulses, e.g., for delivering ATP or post shock pacing pulses. In other examples, therapy delivery circuit <NUM> may include a low voltage therapy circuit for generating and delivering relatively lower voltage pacing pulses for a variety of pacing needs, including ATP.

Control parameters utilized by control circuit <NUM> for detecting cardiac arrhythmias and controlling therapy delivery may be programmed into memory <NUM> via telemetry circuit <NUM>. Telemetry circuit <NUM> may include a transceiver and antenna for communicating with external device <NUM> (shown in <FIG>) using RF communication as described above. Under the control of control circuit <NUM>, telemetry circuit <NUM> may receive downlink telemetry from and send uplink telemetry to external device <NUM>. Telemetry circuit <NUM> may be used to transmit and receive communication signals to/from another medical device implanted in patient <NUM>, such as pacemaker <NUM>.

<FIG> is a flow chart <NUM> of a method for delivering ATP according to one example. <FIG> and other flow charts (e.g., <FIG>) presented herein are described in conjunction with the circuitry of ICD <NUM> of <FIG>. As indicated above, ICD <NUM> may correspond to ICD <NUM> of <FIG> and <FIG> coupled to an extra-cardiovascular lead or to ICD <NUM> of <FIG>, coupled to one or more transvenous leads. It is to be understood, however, that the ATP delivery techniques described in conjunction with the flow charts presented herein are not limited for use by an ICD. A pacemaker, such as intracardiac pacemaker <NUM> of <FIG>, may be capable of delivering ATP therapy and may be configured to control the timing of ATP pulses in the presence of TWA using the techniques disclosed herein. In other examples, any device capable delivering ATP may be adapted to perform the techniques disclosed herein, including external pacemakers and defibrillators.

At block <NUM>, control circuit <NUM> detects a ventricular tachyarrhythmia for which ICD <NUM> is programmed to deliver ATP. ATP may be programmed as a therapy delivered in response to detecting VT but may also be programmed as a therapy delivered in response to detecting VF since ATP is sometimes delivered during high voltage capacitor charging in an attempt to avert the need for CV/DF shock delivery. The ventricular tachyarrhythmia may be detected according to a detection protocol implemented in ICD <NUM>. Practice of the techniques for controlling ATP delivery as presented herein is not limited to use with a particular tachyarrhythmia detection protocol.

At block <NUM>, control circuit <NUM> determines the timing of T-waves sensed by sensing circuit <NUM>. The timing of T-waves sensed by the sensing circuit <NUM> is used by control circuit <NUM> for controlling the therapy delivery circuit <NUM> to deliver ATP at block <NUM>. Therapy delivery circuit <NUM> is configured to deliver the ATP pulses at alternating ATP time intervals that are controlled based on the T-wave timing determined at block <NUM>. As will be described in greater detail below, the T-wave timing may be determined at block <NUM> prior to ATP delivery. The T-wave timing determined prior to ATP delivery may be used to detect TWA and used to establish at least two alternating ATP intervals that separate consecutive ATP pulses. The alternating ATP intervals provide for delivery of each ATP pulse early during the excitable gap following each T-wave when TWA persists during ATP delivery.

In other examples, the T-wave timing may be determined at block <NUM> contemporaneously with ATP delivery. A T-wave may be sensed by sensing circuit <NUM> after each of at least two consecutive ATP pulses. Two different ATP time intervals may be established based on two different times determined from each delivered pulse to a subsequent T-wave. The two ATP time intervals may be used for delivering the ATP pulses at alternating ATP time intervals corresponding to the alternating timing of T-waves during TWA persisting or arising during ATP delivery. In still other examples, a combination of determining T-wave timing preceding ATP delivery and determining T-wave timing during ATP delivery may be performed at block <NUM> in order to control the therapy delivery circuit <NUM> to deliver ATP pulses at alternating ATP time intervals at block <NUM>, based on T-wave timing in the presence of TWA.

Based on the T-wave timing, control circuit <NUM> controls therapy delivery circuit <NUM> to deliver a series of ATP pulses at alternating time intervals. The alternating time intervals may include at least a first ATP time interval separating an earliest occurring pair of the ATP pulses and a second ATP time interval different than the first ATP time interval and consecutively following the first ATP time interval. The second ATP time interval separates a second pair of the ATP pulses consecutively following the earliest occurring pair. For example, a first ATP time interval may separate the first and second ATP pulses of a series of ATP pulses, and a second ATP time interval different than the first may separate the second and third ATP pulses of the series of ATP pulses. The first and second ATP intervals may continue to alternate between consecutively delivered ATP pulses of the series until the programmed number of ATP pulses in the series has been delivered. In another example, the alternating ATP time intervals may be ceased upon no longer detecting TWA.

Depending on the phase of the TWA during the cardiac cycle in which ATP is initiated, the first ATP time interval may be longer or shorter than the second ATP time interval. In some examples, as described below, TWA may be detected prior to delivering ATP. The R-T interval of each phase of the TWA may be determined. In this case, the phase of the TWA (e.g., a short R-T interval phase or a long R-T interval phase) on the cycle that the first ATP pulse is being delivered may be determined by control circuit <NUM>. Additionally or alternatively, the phase of the TWA may be determined on a cycle preceding the cycle during which the first ATP pulse is to be delivered so that the TWA phase of the cardiac cycle during which the first ATP pulse is delivered can be predicted with relatively high confidence. The first ATP interval following the first, leading ATP pulse may be set based on the expected phase and R-T interval of the detected TWA. In other examples, previous determination of the phase and R-T intervals of TWA is not required. TWA may not be present or may not be detected prior to ATP delivery. In these cases, sensing of T-waves during ATP delivery may be used by control circuit <NUM> to control the timing of ATP pulses at alternating ATP time intervals corresponding to alternating phases of TWA that are present during ATP delivery.

The alternating ATP time intervals may be short-long-short-long, etc., until the series of ATP pulses is complete. In other instances, the alternating ATP time intervals may be long-short-long-short, etc., until the series of ATP pulses is complete, depending on the TWA phase of the first cycle following the first ATP pulse. Furthermore, it is to be understood that the alternating ATP time intervals may generally follow a short-long-short-long or long-short-long-short alternating pattern that does not necessarily require each short ATP time interval to be equal to each other short ATP time interval or each long ATP time interval be equal to each other long ATP time interval. Some variation in the T-wave timing during each short and long phase of the TWA may occur. T-wave sensing during ATP delivery may enable control circuit <NUM> to make adjustments to the timing of an ATP pulse cycle-by-cycle based on T-wave sensing during ATP delivery. As long as TWA is present, however, these cycle-by-cycle adjustments may still produce alternating cycles of relatively shorter and relatively longer ATP intervals.

After delivering ATP, control circuit <NUM> may return to block <NUM> to re-detect the tachyarrhythmia, if not successfully terminated by the ATP, or detect the next tachyarrhythmia episode. If the tachyarrhythmia is not successfully terminated, another ATP attempt may be performed according to a programmed menu of therapies. One or more additional attempts at terminating the tachyarrhythmia using ATP, which may include alternating ATP time intervals, may be made after the first ATP therapy. Adjustments to the subsequent ATP therapy attempts may be made. In some cases, if a maximum number of attempts of ATP therapies fail to terminate the tachyarrhythmia, a cardioversion/defibrillation shock is delivered.

<FIG> is a flow chart <NUM> of a method for controlling ATP pulses by a medical device according to another example. At block <NUM>, control circuit <NUM> of ICD <NUM> detects a ventricular tachyarrhythmia. At block <NUM>, control circuit <NUM> may set a coupling interval by determining a cycle length of the detected tachyarrhythmia (e.g., a median, minimum or most recent RR interval of the detected tachyarrhythmia). The coupling interval may be a time interval set to a percentage or pre-determined interval less than the tachyarrhythmia cycle length. The first, leading ATP pulse of a series of ATP pulses may be delivered at block <NUM> at the coupling interval following an R-wave sensed by sensing circuit <NUM>.

In another example, control circuit <NUM> may set the coupling interval at block <NUM> based on the timing of a T-wave sensed prior to the first ATP pulse. Sensing circuit <NUM> may pass an R-wave sensed event signal to control circuit <NUM> followed by an immediately consecutive T-wave sensed event signal passed to control circuit <NUM>. Control circuit <NUM> may deliver the first, leading ATP pulse at block <NUM> consecutive with the sensed T-wave, at a coupling interval set based on the timing of the immediately preceding T-wave. As such, the coupling interval may be set as a fixed time interval following the sensed T-wave.

After delivering the first, leading ATP pulse, sensing circuit <NUM> is configured to sense a T-wave following the first ATP pulse at block <NUM>. At block <NUM>, control circuit <NUM> delivers the next ATP pulse after the sensed T-wave such that the second ATP pulse is delivered following the first ATP pulse at a first ATP interval based on the timing of the T-wave sensed after the first ATP pulse. If all ATP pulses of the programmed ATP therapy have not been delivered, as determined at block <NUM>, sensing circuit <NUM> senses the next T-wave, consecutively following the second ATP pulse, at block <NUM> and controls the therapy delivery circuit <NUM> to deliver the third ATP pulse at block <NUM> at a second ATP interval, consecutively following the first ATP interval, and separating the second and third ATP pulses. The second ATP interval will either be longer or shorter than the first ATP interval depending on the phase of TWA that has either arisen or persisted during the delivery of the ATP pulses.

This process of sensing a T-wave consecutively following each ATP pulse and delivering an ATP pulse at a fixed interval after each sensed T-wave continues until all ATP pulses of the series of ATP pulses have been delivered. In the presence of TWA, the ATP intervals separating consecutively delivered ATP pulses alternate between relatively longer and relatively shorter ATP intervals based on the timing of the sensed T-waves. Detection of TWA prior to ATP delivery is not required to establish the alternating ATP intervals. The ATP pulses may be delivered at a fixed interval after sensing a T-wave. Since the T-waves occur at alternating R-T intervals in the presence of TWA, the resulting ATP time intervals between consecutive ATP pulses will alternate between relatively longer and shorter ATP time intervals.

<FIG> is a flow chart <NUM> of a method for controlling ATP delivery after detecting TWA according to another example. At block <NUM>, control circuit <NUM> detects a ventricular tachyarrhythmia episode for which ATP therapy is programmed to be delivered. At block <NUM>, control circuit <NUM> may set a base ATP interval by determining the cycle length of the detected VT or VF. The base ATP interval may be a fixed interval or percentage less than the determined cycle length. The base ATP interval may be the ATP interval used for delivering ATP pulses in the absence of TWA.

At block <NUM>, control circuit <NUM> may analyze the cardiac electrical signal(s) received from sensing circuit <NUM> for detecting TWA. Control circuit <NUM> may determine R-T intervals between consecutive R-wave sensed event signals and T-wave sensed event signals received from sensing circuit <NUM>. If the differences between consecutive R-T intervals are greater than a threshold difference and represent alternating R-T intervals, TWA may be detected. Various TWA detection criteria may be applied to the cardiac electrical signal(s) received from sensing circuit <NUM>. The techniques used for detecting the presence of TWA prior to delivering ATP are not limited to any particular TWA detection technique. The techniques for detecting TWA, however, generally include detection of beat-to-beat variation of the R-T or Q-T time interval, or more generally the relative timing of the T-wave in the cardiac cycle, indicating that the onset of the excitable gap and its duration may be varying from beat-to-beat.

If TWA is not present, ATP may be delivered at block <NUM> using the base ATP interval determined at block <NUM>. After a leading ATP pulse is delivered at a coupling interval, which may or may not be equal to the base ATP interval, each ATP pulse may be delivered at the base ATP interval following a preceding ATP pulse, at progressively shorter (decrementing) intervals starting from the base ATP interval (e.g., a ramp ATP therapy) or other ATP protocol based on the base ATP interval that does not include alternating ATP time intervals to account for TWA.

In response to detecting TWA at block <NUM>, control circuit <NUM> may establish adjusted ATP time intervals at block <NUM> for delivering ATP in the presence of TWA. The R-T time interval for each short and long phase of the TWA may be determined at block <NUM>. The R-T time interval may be determined for each phase by averaging or determining a median, mode, minimum, maximum range or other characterization of multiple, alternating short R-T intervals and of multiple alternating long R-T intervals. In some examples, TWA detection may occur simultaneously with tachyarrhythmia detection such that setting a base ATP interval at block <NUM> is not required. Instead, two different ATP time intervals may be established at block <NUM> in response to detecting TWA simultaneously with detecting the tachyarrhythmia.

At block <NUM>, control circuit <NUM> receives an R-wave sensed event signal followed by a T-wave sensed event signal from sensing circuit <NUM>. Control circuit <NUM> determines if the current cardiac cycle including the sensed R-wave and T-wave (block <NUM>) is the short or long phase of the TWA at block <NUM>. Based on this determination of the TWA phase, the leading pulse of the ATP therapy may be delivered at block <NUM> at one of the two established ATP time intervals following the next R-wave sensed event signal. For instance, if the current R-T time interval is the short phase of the TWA, the next cardiac cycle is expected to have a relatively longer R-T time interval. The leading ATP pulse may be synchronized to the next sensed R-wave (the "synchronizing" R-wave) by delivering the leading ATP pulse after the next sensed R-wave following the longer one of the two ATP intervals established at block <NUM>. If the R-T interval determined at block <NUM> is the long phase of the TWA, the leading pulse of the ATP sequence may be scheduled after the shorter ATP time interval following the next sensed, synchronizing R-wave.

Subsequent ATP pulses are delivered at block <NUM> according to the established alternating ATP time intervals to provide correspondence with the alternating short and long phases of the TWA detected prior to the onset of ATP. The two different ATP time intervals are established to enable delivery of the ATP pulses as early as possible during the excitable gap without being delivered into the T-wave during either phase. After ATP delivery, control circuit <NUM> may return to block <NUM> to continue monitoring the heart rhythm and delivering additional therapies as needed and in accordance with a programmed sequence of tachyarrhythmia therapies.

In a variation of the method shown in <FIG>, the leading ATP pulse may be delivered at block <NUM> at a coupling interval determined by control circuit <NUM> based on the cycle length of the detected ventricular tachyarrhythmia. The second ATP pulse may be delivered following the leading ATP pulse at an ATP time interval set to one of the two established ATP intervals, either short or long, based on the expected phase of the TWA for the cycle beginning with the second ATP pulse. The expected phase of the TWA is known from determining the TWA phase of a cardiac cycle preceding the leading ATP pulse.

To illustrate, the leading ATP pulse may be delivered following a synchronizing R-wave at a coupling interval that is set based on the tachyarrhythmia cycle length. The coupling interval may be different than both of the ATP time intervals established at block <NUM> based on the alternating phases of the detected TWA. An R-T interval immediately preceding the leading ATP pulse may be determined to correspond to the short phase of the TWA. In this example, the leading ATP pulse starts an expected long phase of the TWA since it was immediately preceded by the short phase. As such, the second ATP pulse may be delivered at the long ATP interval following the leading ATP pulse to correspond to the expected long phase of the TWA. The second ATP pulse marks the start of an expected short phase of the TWA. The third ATP pulse is delivered at the short ATP interval established at block <NUM> to correspond to the short phase of the TWA. The third ATP pulse is separated from the second ATP pulse by the shorter one of the established ATP time intervals to coincide with the predicted short phase of the TWA that was detected prior to the start of ATP delivery. Subsequent ATP pulses are delivered at alternating short and long ATP time intervals to correspond to the phase of the TWA as predicted based on the TWA phase determined immediately prior to the leading ATP pulse.

<FIG> is a flow chart <NUM> of a method for controlling ATP pulses in the presence of TWA according to another example. Blocks <NUM> through <NUM> correspond to identically numbered blocks described above in conjunction with <FIG>. However instead of only applying ATP intervals established based on the R-T intervals and TWA phase detected before initiating ATP, ICD <NUM> may continue monitoring for TWA during ATP delivery to adjust ATP intervals on a cycle by cycle basis as needed. If TWA is detected prior to starting ATP ("yes" branch of block <NUM>), the ATP intervals for each phase of the TWA may be established at block <NUM> by control circuit <NUM>. The TWA phase of a cycle prior to the first, leading ATP pulse may be determined at block <NUM> based on determining an R-T interval. At block <NUM>, control circuit <NUM> delivers the leading ATP pulse synchronized to a sensed R-wave at a selected coupling interval, which may be the ATP interval established for the phase of the cardiac cycle beginning with the synchronizing R-wave. In some cases, the leading ATP pulse may be delivered at the shorter ATP interval established at block <NUM> to increase TWA instability, which may help to terminate the tachyarrhythmia. In other cases, the leading ATP pulse may be delivered at the longer ATP interval established at block <NUM> to promote cardiac capture by the leading ATP pulse. In other examples, a coupling interval different than one of the two established ATP intervals may be used to synchronize the leading ATP pulse to the sensed R-wave at block <NUM>. The coupling interval may be set based on the tachyarrhythmia cycle length to promote cardiac capture by the leading ATP pulse.

If TWA is not detected prior to initiating ATP ("no" branch of block <NUM>), the leading ATP pulse may be synchronized to the sensed R-wave at block <NUM> at the ATP interval set at block <NUM> based on the tachyarrhythmia cycle length. The leading pulse may be delivered at the base ATP interval or a coupling interval, which may be shorter than the base ATP interval, following a sensed R-wave.

After the leading ATP pulse delivered at either block <NUM> or at block <NUM>, sensing circuit <NUM> senses the T-wave following the leading ATP pulse at block <NUM> and determines the time interval between the leading ATP pulse and the sensed T-wave. At block <NUM>, control circuit <NUM> controls therapy delivery circuit <NUM> to deliver the next ATP pulse at a selected ATP interval following the leading ATP pulse. If TWA was detected prior to the leading ATP pulse, the selected ATP interval may be one of the adjusted ATP intervals established at block <NUM> and selected according to the expected TWA phase for the current cycle. If TWA was not detected prior to the leading ATP pulse, the selected ATP interval for controlling the second ATP pulse delivery is the base ATP interval established at block <NUM>.

After delivering the second ATP pulse, the subsequent T-wave is sensed by sensing circuit <NUM>. Control circuit <NUM> determines the pulse to T-wave time interval from the second ATP pulse to the sensed T-wave. At block <NUM>, control circuit <NUM> determines if TWA is present (or changed) based on the two T-wave time intervals determined at block <NUM> and <NUM>. If TWA is not detected based on the two T-wave time intervals determined after the onset of ATP, e.g., following the leading ATP pulse and second ATP pulse, the next, third ATP pulse may be delivered at block <NUM> at the base ATP interval established at block <NUM> according to the tachyarrhythmia cycle length. In some instances, even if TWA was present prior to the leading ATP pulse, the TWA may not be present after ATP is initiated. In this case, the base ATP interval set according to the tachyarrhythmia cycle length may be used for completing the ATP therapy.

If TWA is detected at block <NUM> during ATP delivery but was not detected prior to the leading ATP pulse, the base ATP interval determined at block <NUM> may be adjusted at block <NUM> according to the T-wave time intervals determined at blocks <NUM> and <NUM> and the expected phase of the TWA. In some examples, the next ATP pulse may be delivered at an ATP interval that is adjusted from the previously established base ATP interval. In other examples, the next ATP pulse may already be scheduled at the base ATP interval such that the adjustment to the ATP time interval based on TWA detected at block <NUM>, after ATP is started, is delayed for one ATP pulse interval. The second ATP pulse after detecting TWA may be delivered according to the expected TWA phase and the ATP interval adjustment made at block <NUM>.

In other instances, TWA detected prior to the leading ATP pulse may still be present and relatively unchanged during ATP delivery. In this case, no additional adjustment to the next ATP interval is required at block <NUM>. The adjusted ATP intervals established at block <NUM> prior to ATP onset, and the predicted TWA phase, may continue to be used to complete the series of ATP pulses at alternating ATP time intervals. A comparison between the T-wave time intervals following ATP pulses determined at blocks <NUM> and <NUM> and the R-T time intervals determined prior to ATP for determining if the TWA has changed may account for an expected difference between the intrinsic R-T time interval and the pacing pulse-to-T-wave time interval.

If the TWA detected prior to the leading ATP pulse is still present but is altered by the initiation of ATP, the next ATP interval may be adjusted at block <NUM> based on the T-wave intervals determined after ATP pulses. The process may return to block <NUM> to deliver the next ATP pulse at the adjusted ATP interval. The process of sensing the T-wave following a delivered ATP pulse and determining if TWA is still present or altered at block <NUM> may be repeated pulse by pulse during ATP delivery until all pulses of the ATP series are delivered. In other examples, determining if TWA is still present or altered after the onset of ATP therapy may be performed by sensing only the first one, two, three, four or other predetermined number of T-waves during ATP delivery. In this way, monitoring of T-wave time intervals during ATP delivery may account for TWA that is started, terminated or altered by the delivery of ATP. If all ATP pulses of the scheduled series of pulses have been delivered, as determined at block <NUM>, the control circuit <NUM> may return to block <NUM> to await the next tachyarrhythmia detection.

<FIG> is a timing diagram <NUM> of ATP therapy delivered by a medical device according to one example of the techniques disclosed herein. A conceptual diagram of a filtered and rectified cardiac electrical signal <NUM>, produced by sensing circuit <NUM> from a cardiac electrical signal received from the patient's heart, includes intrinsic R-waves <NUM> and T-waves <NUM>. Sensing circuit <NUM> may apply an R-wave sensing threshold <NUM> for sensing R-waves <NUM> and a T-wave sensing threshold <NUM> for sensing T-waves <NUM>. Various methods may be used for sensing R-waves <NUM> and T-waves <NUM> using one or more sensing channels as described above. Such methods may include comparing one or more features of the cardiac signal <NUM> (such as amplitude, width, area, timing relative to a sensed R-wave, etc.) to T-wave sensing criteria and/or performing cardiac signal waveform morphology analysis for detecting a waveform morphology that matches an expected T-wave morphology. Examples of methods that may be used for sensing T-waves and detecting TWA are generally disclosed in <CIT>). Sensing circuit <NUM> may produce an R-wave sensed event signal <NUM> in response to sensing an R-wave <NUM> and a T-wave sensed event signal <NUM> in response to sensing a T-wave <NUM>.

Control circuit <NUM> receives the R-wave sensed event signals <NUM> and the T-wave sensed event signals <NUM>. R-wave sensed event signals <NUM> are used for determining RR intervals and detecting VT or VF. In the example shown, a VT detection <NUM> is made by control circuit <NUM>. Prior to and/or after VT detection <NUM>, control circuit <NUM> may determine R-T intervals <NUM> and <NUM> between a pair of consecutively received R-wave sensed event and T-wave sensed event signals <NUM> and <NUM>. Control circuit <NUM> may be configured to detect TWA based on alternating R-T intervals <NUM> and <NUM>, where every other R-T interval <NUM> is shorter (or longer) than the intervening R-T interval <NUM>. Based on the alternating R-T intervals <NUM> and <NUM>, control circuit <NUM> may establish two different ATP intervals <NUM> and <NUM> that are both shorter than the VT cycle length <NUM> but different from each other.

After the VT detection <NUM>, control circuit <NUM> may determine a subsequent R-T interval <NUM> or <NUM> to predict the phase of the TWA in the next cardiac cycle during which a leading ATP pulse <NUM> is delivered. In some instances, if the current R-T interval <NUM> immediately following VT detection <NUM> is the short phase, the next R-T interval <NUM> is predicted to be the long phase, which may be associated with a relatively late, short excitable gap <NUM>. Control circuit <NUM> may control therapy delivery circuit <NUM> to withhold ATP for one cardiac cycle corresponding to the long phase of the TWA (long R-T interval <NUM>) and deliver the leading ATP pulse <NUM> at a coupling interval <NUM> during a cardiac cycle corresponding to the short phase of the TWA (short R-T interval <NUM>), when the excitable gap <NUM> is expected to start earlier and be relatively longer. In other examples, the leading ATP pulse may be synchronized to the next sensed R-wave following VT detection <NUM>, independent of TWA phase.

The coupling interval <NUM> may be set based on the VT cycle length <NUM>, and may be the same as one of the established ATP intervals <NUM> and <NUM> or different than both ATP intervals <NUM> and <NUM>. In some examples, the coupling interval <NUM> is set to the ATP interval <NUM> or <NUM> corresponding to the expected phase of the TWA of the cardiac cycle during which the leading pulse <NUM> is being delivered. In the example shown, control circuit <NUM> detects a short RT interval <NUM> immediately following VT detection <NUM>, waits one cardiac cycle corresponding to the long RT interval <NUM> (and shortened excitable gap <NUM>) to deliver the leading ATP pulse <NUM> at a coupling interval <NUM>, which may be set to the short ATP interval <NUM> corresponding to the short TWA phase (short RT interval <NUM>) expected after the long RT interval <NUM> and corresponding to a relatively earlier and longer excitable gap <NUM>. Each ATP pulse <NUM> following the leading pulse <NUM> is delivered at an alternating ATP time interval <NUM> or <NUM> following the immediately preceding ATP pulse. Each pair of consecutive ATP pulses is separated by one of the alternating ATP time intervals <NUM> or <NUM>.

In some examples, the alternating ATP time intervals <NUM> and <NUM> are pre-determined, based on TWA detection prior to the leading ATP pulse <NUM>. In other examples, e.g., using the methods of <FIG> or <FIG>, the alternating ATP time intervals <NUM> and <NUM> between pairs of consecutive ATP pulses may occur by sensing T-waves <NUM> during ATP delivery and delivering ATP pulses <NUM> based on T-wave timing (e.g., a fixed interval after a T-wave sensed event signal) or based on T-wave time intervals (e.g., a fixed percentage or interval greater than the interval from an ATP pulse to a T-wave sensed event signal) determined during ATP delivery. In this way, each ATP pulse <NUM> has a high likelihood of being delivered relatively early during the excitable gap <NUM> or <NUM> even in the presence of TWA, increasing the likelihood of terminating the detected ventricular tachyarrhythmia.

It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). In addition, while certain aspects of this disclosure are described as being performed by a single device, circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, one or more medical devices.

If implemented in software, the functions may be stored as one or more instructions or code on a non-transitory computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include non-transitory computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

Claim 1:
A medical device comprising:
a sensing circuit (<NUM>) configured to receive a cardiac electrical signal from a patient's heart and sense R-waves and T-waves from the cardiac electrical signal;
a therapy delivery circuit (<NUM>) configured to generate and deliver anti-tachycardia pacing, ATP, pulses to the patient's heart via electrodes coupled to the therapy delivery circuit; and
a control circuit (<NUM>) coupled to the sensing circuit and to the therapy delivery circuit and configured to:
detect a ventricular tachyarrhythmia from the cardiac electrical signal received by the sensing circuit;
responsive to the detected ventricular tachyarrhythmia, control the therapy delivery circuit to:
deliver a plurality of ATP pulses at alternating ATP time intervals, the alternating ATP time intervals comprising at least a first ATP time interval separating a first pair of the plurality of the ATP pulses and a second ATP time interval separating a second pair of the plurality of ATP pulses, the second ATP time interval being different than the first ATP time interval and consecutively following the first ATP time interval,
wherein:
the first pair comprises a first ATP pulse and a second ATP pulse consecutively following the first ATP pulse at the first ATP time interval, the second pair comprises the second ATP pulse and a third ATP pulse consecutively following the second ATP pulse at the second ATP time interval, and
the control circuit is configured to control the therapy delivery circuit to deliver a fourth ATP pulse consecutively following the third ATP pulse at the first ATP time interval.