Patent ID: 12186571

DETAILED DESCRIPTION

In general, this disclosure describes techniques for sensing cardiac electrical signals by an implantable medical device (IMD) using a multi-level cardiac event sensing threshold. The multi-level cardiac event sensing threshold is set by a sensing circuit of the IMD under the control of a control circuit and is adjusted between threshold value levels at determined time intervals. When a cardiac electrical signal received by the sensing circuit crosses the cardiac event sensing threshold, a cardiac event is sensed. In some examples, the cardiac electrical signal is received by the IMD using implanted, extra-cardiovascular electrodes. As used herein, the term “extra-cardiovascular” refers to a position outside the blood vessels, heart, and pericardium surrounding the heart of a patient. Implantable electrodes carried by extra-cardiovascular leads may be positioned extra-thoracically (outside the ribcage and sternum) or intra-thoracically (beneath the ribcage or sternum) but generally not in intimate contact with myocardial tissue. The techniques disclosed herein provide a method for reliably sensing R-waves, attendant to ventricular depolarization, using extra-cardiovascular electrodes by applying multiple sensing thresholds to avoid oversensing of T-waves attendant to ventricular repolarization and P-waves attendant to atrial depolarization.

The techniques are described in conjunction with an implantable medical lead carrying extra-cardiovascular electrodes, but aspects disclosed herein may be utilized in conjunction with other cardiac electrical sensing lead and electrode systems. For example, the techniques for controlling a cardiac event sensing threshold as described in conjunction with the accompanying drawings may be implemented in any implantable or external medical device enabled for sensing cardiac electrical signals, including implantable pacemakers, ICDs or cardiac monitors coupled to transvenous or epicardial leads carrying sensing electrodes; leadless pacemakers, ICDs or cardiac monitors having housing-based sensing electrodes; and external or wearable pacemakers, defibrillators, or cardiac monitors coupled to external, surface or skin electrodes.

FIGS.1A and1Bare conceptual diagrams of one example of an extra-cardiovascular ICD system10in which the presently disclosed techniques may be implemented.FIG.1Ais a front view of ICD system10implanted within patient12.FIG.1Bis a side view of ICD system10implanted within patient12. ICD system10includes an ICD14connected to an extra-cardiovascular electrical stimulation and sensing lead16.FIGS.1A and1Bare described in the context of an ICD system10capable of providing defibrillation and/or cardioversion shocks and pacing pulses.

ICD14includes a housing15that forms a hermetic seal that protects internal components of ICD14. The housing15of ICD14may be formed of a conductive material, such as titanium or titanium alloy. The housing15may function as a housing electrode (sometimes referred to as a can electrode). In examples described herein, housing15may be used as an active can electrode for use in delivering cardioversion/defibrillation (CV/DF) shocks or other high voltage pulses delivered using a high voltage therapy circuit. In other examples, housing15may be available for use in delivering unipolar, low voltage cardiac pacing pulses in conjunction with lead-based cathode electrodes. In other instances, the housing15of ICD14may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing15functioning as an electrode(s) may be coated with a material, such as titanium nitride for reducing polarization artifact.

ICD14includes a connector assembly17(also referred to as a connector block or header) that includes electrical feedthroughs crossing housing15to provide electrical connections between conductors extending within the lead body18of lead16and electronic components included within the housing15of ICD14. As will be described in further detail herein, housing15may house one or more processors, memories, transceivers, sensors, electrical cardiac signal 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.

Lead16includes an elongated lead body18having a proximal end27that includes a lead connector (not shown) configured to be connected to ICD connector assembly17and a distal portion25that includes one or more electrodes. In the example illustrated inFIGS.1A and1B, the distal portion25of lead body18includes defibrillation electrodes24and26and pace/sense electrodes28,30and31. In some cases, defibrillation electrodes24and26may together form a defibrillation electrode in that they may be configured to be activated concurrently. Alternatively, defibrillation electrodes24and26may form separate defibrillation electrodes in which case each of the electrodes24and26may be activated independently. In some instances, defibrillation electrodes24and26are coupled to electrically isolated conductors, and ICD14may include switching mechanisms to allow electrodes24and26to be utilized as a single defibrillation electrode (e.g., activated concurrently to form a common cathode or anode) or as separate defibrillation electrodes, (e.g., activated individually, one as a cathode and one as an anode or activated one at a time, one as an anode or cathode and the other remaining inactive with housing15as an active electrode).

Electrodes24and26(and in some examples housing15) 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). Electrodes24and26may 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 electrodes28,30and31. However, electrodes24and26and housing15may 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 electrodes24and26for use in only high voltage CV/DF shock therapy applications. Electrodes24and26may be used in a pacing electrode vector for delivering extra-cardiovascular pacing pulses such as anti-tachycardia pacing (ATP) pulses or bradycardia pacing pulses and/or in a sensing vector used to sense cardiac electrical signals and detect ventricular tachycardia (VT) and ventricular fibrillation (VF).

Electrodes28,30and31are relatively smaller surface area electrodes for delivering low voltage pacing pulses and for sensing cardiac electrical signals. Electrodes28,30and31are referred to as pace/sense electrodes because they are generally configured for use in relatively lower voltage applications than defibrillation electrodes24and26, e.g., used as either a cathode or anode for delivery of pacing pulses and/or sensing of cardiac electrical signals. In some instances, electrodes28,30and31may provide only pacing functionality, only sensing functionality or both.

In the example illustrated inFIGS.1A and1B, electrode28is located proximal to defibrillation electrode24, and electrode30is located between defibrillation electrodes24and26. A third pace/sense electrode31may be located distal to defibrillation electrode26. Electrodes28and30are illustrated as ring electrodes, and electrode31is illustrated as a hemispherical tip electrode in the example ofFIGS.1A and1B. However, electrodes28,30and31may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, or the like, and may be positioned at any position along lead body18. Further, electrodes28,30and31may be of similar type, shape, size and material or may differ from each other.

Lead16extends subcutaneously or submuscularly over the ribcage32medially from the connector assembly27of ICD14toward a center of the torso of patient12, e.g., toward xiphoid process20of patient12. At a location near xiphoid process20, lead16bends or turns and extends superior subcutaneously or submuscularly over the ribcage and/or sternum, substantially parallel to sternum22. Although illustrated inFIGS.1A and1Bas being offset laterally from and extending substantially parallel to sternum22, lead16may be implanted at other locations, such as over sternum22, offset to the right or left of sternum22, angled laterally from sternum22toward the left or the right, or the like. Alternatively, lead16may be placed along other subcutaneous or submuscular paths. The path of lead16may depend on the location of ICD14, the arrangement and position of electrodes along lead body18, and/or other factors.

Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body18of lead16from the lead connector at the proximal lead end27to electrodes24,26,28,30and31located along the distal portion25of the lead body18. Lead body18may be tubular or cylindrical in shape. In other examples, the distal portion25(or all of) the elongated lead body18may have a flat, ribbon or paddle shape. The lead body18may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. However, the techniques disclosed herein are not limited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body18are each electrically coupled with respective defibrillation electrodes24and26and pace/sense electrodes28,30and31. Each of pacing and sensing electrodes28,30and31are coupled to respective electrical conductors, which may be separate respective conductors within the lead body. The respective conductors electrically couple the electrodes24,26,28,30and31to circuitry, such as a therapy circuit and/or a sensing circuit, of ICD14via connections in the connector assembly17, including associated electrical feedthroughs crossing housing15. The electrical conductors transmit therapy from a therapy circuit within ICD14to one or more of defibrillation electrodes24and26and/or pace/sense electrodes28,30and31and transmit cardiac electrical signals from one or more of defibrillation electrodes24and26and/or pace/sense electrodes28,30and31to the sensing circuit within ICD14.

ICD14may obtain cardiac electrical signals corresponding to electrical activity of heart8via a combination of sensing vectors that include combinations of electrodes28,30, and/or31. In some examples, housing15of ICD14is used in combination with one or more of electrodes28,30and/or31in a sensing electrode vector. ICD14may even obtain cardiac electrical signals using a sensing vector that includes one or both defibrillation electrodes24and/or26, e.g., between electrodes24and26or one of electrodes24or26in combination with one or more of electrodes28,30,31, and/or housing15.

ICD14analyzes the cardiac electrical signals received from one or more of the sensing vectors to monitor for abnormal rhythms, such as bradycardia, ventricular tachycardia (VT) or ventricular fibrillation (VF). ICD14may 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 U.S. Pat. No. 7,761,150 (Ghanem, et al.), incorporated by reference herein in its entirety.

ICD14generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia (e.g., VT or VF). ICD14may 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 electrodes24,26,28,30,31and/or housing15. The pacing electrode vector may be different than the sensing electrode vector. In one example, cardiac electrical signals are sensed between pace/sense electrodes28and30, and ATP pulses (or other cardiac pacing pulses) are delivered between pace/sense electrode30used as a cathode electrode and defibrillation electrode24used as a return anode electrode. In other examples, cardiac pacing pulses may be delivered between pace/sense electrode28and either (or both) defibrillation electrode24or26or between defibrillation electrode24and defibrillation electrode26. These examples are not intended to be limiting, and it is recognized that other sensing electrode vectors and cardiac pacing electrode vectors may be selected according to individual patient need.

If ATP does not successfully terminate VT or when VF is detected, ICD14may deliver one or more CV/DF shocks via one or both of defibrillation electrodes24and26and/or housing15. ICD14may 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 electrodes24,26,28,30and31and the housing15of ICD14. As disclosed herein, ICD14may detect a need for an electrical stimulation therapy based on at least a rate of cardiac events sensed from a cardiac electrical signal received via one or more sensing electrode vector selected from the available electrodes24,26,28,30and31and housing15. A fast rate of sensed cardiac events may lead to determining a need for ATP and/or a CV/DF shock. A slow rate of cardiac events may lead to cardiac pacing according to a programmed pacing protocol or mode, e.g., VVI pacing or post-shock pacing. For example, ICD14may detect a need for a pacing therapy based on a pacing interval expiring before a cardiac event is sensed, indicating a slow rate of cardiac events below a programmed lower pacing rate.

FIGS.1A and1Bare illustrative in nature and should not be considered limiting of the practice of the techniques disclosed herein. In other examples, lead16may include less than three pace/sense electrodes or more than three pace/sense electrodes and/or a single defibrillation electrode or more than two electrically isolated or electrically coupled defibrillation electrodes or electrode segments. The pace/sense electrodes28,30and/or31may be located elsewhere along the length of lead16. For example, lead16may include a single pace/sense electrode30between defibrillation electrodes24and26and no pace/sense electrode distal to defibrillation electrode26or proximal defibrillation electrode24. Various example configurations of extra-cardiovascular leads and electrodes and dimensions that may be implemented in conjunction with the extra-cardiovascular pacing techniques disclosed herein are described in commonly-assigned, pending U.S. Publication No. 2015/0306375 (Marshall, et al.) and U.S. Publication No. 2015/0306410 (Marshall, et al.), both of which are incorporated herein by reference in their entirety.

ICD14is shown implanted subcutaneously on the left side of patient12along the ribcage32. ICD14may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient12. ICD14may, however, be implanted at other subcutaneous or submuscular locations in patient12. For example, ICD14may be implanted in a subcutaneous pocket in the pectoral region. In this case, lead16may extend subcutaneously or submuscularly from ICD14toward the manubrium of sternum22and bend or turn and extend inferior from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD14may be placed abdominally. Lead16may be implanted in other extra-cardiovascular locations as well. For instance, as described with respect toFIGS.2A-2C, the distal portion25of lead16may be implanted underneath the sternum/ribcage in the substernal space.

An external device40is shown in telemetric communication with ICD14by a communication link42. External device40may include a processor52, memory53, display54, user interface56and telemetry unit58. Processor52controls external device operations and processes data and signals received from ICD14. Display54, which may include a graphical user interface, displays data and other information to a user for reviewing ICD operation and programmed parameters as well as cardiac electrical signals retrieved from ICD14. For example, as described in conjunction withFIGS.10and11, a clinician may view cardiac electrical signals received from ICD14during VF induction for testing programmed sensitivity settings and during normal sinus rhythm for reviewing and selecting programmable R-wave sensing threshold parameter settings.

User interface56may include a mouse, touch screen, key pad or the like to enable a user to interact with external device40to initiate a telemetry session with ICD14for retrieving data from and/or transmitting data to ICD14, including programmable parameters for controlling a cardiac event sensing threshold as described herein. Telemetry unit58includes a transceiver and antenna configured for bidirectional communication with a telemetry circuit included in ICD14and is configured to operate in conjunction with processor52for sending and receiving data relating to ICD functions via communication link42.

Communication link42may be established between ICD14and external device40using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) or other RF or communication frequency bandwidth or communication protocols. Data stored or acquired by ICD14, 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 ICD14by external device40following an interrogation command.

External device40may be embodied as a programmer used in a hospital, clinic or physician's office to retrieve data from ICD14and to program operating parameters and algorithms in ICD14for controlling ICD functions. External device40may alternatively be embodied as a home monitor or hand held device. External device40may be used to program cardiac signal sensing parameters, cardiac rhythm detection parameters and therapy control parameters used by ICD14. At least some control parameters used to control a cardiac event sensing threshold, e.g., the R-wave sensing threshold, according to techniques disclosed herein may be programmed into ICD14using external device40.

FIGS.2A-2Care conceptual diagrams of patient12implanted with extra-cardiovascular ICD system10in a different implant configuration than the arrangement shown inFIGS.1A-1B.FIG.2Ais a front view of patient12implanted with ICD system10.FIG.2Bis a side view of patient12implanted with ICD system10.FIG.2Cis a transverse view of patient12implanted with ICD system10. In this arrangement, lead16of system10is implanted at least partially underneath sternum22of patient12. Lead16extends subcutaneously or submuscularly from ICD14toward xiphoid process20and at a location near xiphoid process20bends or turns and extends superiorly within anterior mediastinum36in a substernal position.

Anterior mediastinum36may be viewed as being bounded laterally by pleurae39, posteriorly by pericardium38, and anteriorly by sternum22. In some instances, the anterior wall of anterior mediastinum36may also be formed by the transversus thoracis muscle and one or more costal cartilages. Anterior mediastinum36includes 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 portion25of lead16extends along the posterior side of sternum22substantially within the loose connective tissue and/or substernal musculature of anterior mediastinum36.

A lead implanted such that the distal portion25is substantially within anterior mediastinum36may be referred to as a “substernal lead.” In the example illustrated inFIGS.2A-2C, the distal portion25of lead body18is located substantially centered under sternum22. In other instances, however, the distal portion25may be implanted such that it is offset laterally from the center of sternum22. In some instances, lead16may extend laterally such that distal portion25is underneath/below the ribcage32in addition to or instead of sternum22. In other examples, the distal portion25of lead16may be implanted in other extra-cardiovascular, intra-thoracic locations, including the pleural cavity or around the perimeter of or adjacent to the pericardium38of heart8. Other implant locations and lead and electrode arrangements that may be used in conjunction with the techniques described herein are generally disclosed in the incorporated patent references.

FIG.3is a conceptual diagram illustrating a distal portion25′ of another example of extra-cardiovascular lead16ofFIGS.1A-2Chaving a curving distal portion25′ of lead body18′. Lead body18′ may be formed having an undulating, curving, bending, serpentine, or zig-zagging shape along distal portion25′. In the example shown, defibrillation electrodes24′ and26′ are carried along pre-formed curving portions of the lead body18′. Pace/sense electrode30′ is carried in between defibrillation electrodes24′ and26′. Pace/sense electrode28′ is carried proximal to the proximal defibrillation electrode24′. No electrode is provided distal to defibrillation electrode26′ in this example.

As shown inFIG.3, lead body18′ may be formed having a curving distal portion25′ that includes two “C” shaped curves, which together may resemble the Greek letter epsilon, “ε.” Defibrillation electrodes24′ and26′ are each carried by one of the two respective C-shaped portions of the lead body distal portion25′, which extend or curve in the same direction away from a central axis33of lead body18′. In the example shown, pace/sense electrode28′ is proximal to the C-shaped portion carrying electrode24′, and pace/sense electrode30′ is proximal to the C-shaped portion carrying electrode26′. Pace/sense electrodes28′ and30′ may, in some instances, be approximately aligned with the central axis33of the straight, proximal portion of lead body18′ such that mid-points of defibrillation electrodes24′ and26′ are laterally offset from electrodes28′ and30′. 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 that may be implemented with the pacing techniques described herein are generally disclosed in pending U.S. Pat. Publication No. 2016/0158567 (Marshall et al.), incorporated herein by reference in its entirety.

FIG.4is a schematic diagram of ICD14according to one example. The electronic circuitry enclosed within housing15(shown schematically as an electrode inFIG.4) includes software, firmware and hardware that cooperatively monitor one or more cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver therapy as needed according to programmed therapy delivery algorithms and control parameters. The software, firmware and hardware are configured to detect and discriminate VT and VF for determining when ATP or CV/DF shocks are required and may determine when bradycardia pacing, post-shock pacing, rate-responsive pacing or other types of electrical stimulation is needed. ICD14is coupled to an extra-cardiovascular lead, such as lead16carrying extra-cardiovascular electrodes24,26,28,30, and31, as shown inFIG.1A, for delivering electrical stimulation pulses to the patient's heart and for sensing cardiac electrical signals.

ICD14includes a control circuit80, memory82, therapy delivery circuit84, sensing circuit86, and telemetry circuit88. A power source98provides power to the circuitry of ICD14, including each of the circuits80,82,84,86, and88as needed. Power source98may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source98and each of the other circuits80,82,84,86and88are to be understood from the general block diagram ofFIG.4, but are not shown for the sake of clarity. For example, power source98may be coupled to a low voltage (LV) charging circuit and to a high voltage (HV) charging circuit included in therapy delivery circuit84for charging low voltage and high voltage capacitors, respectively, included in therapy delivery circuit84for producing respective low voltage pacing pulses, such as bradycardia pacing, post-shock pacing or ATP pulses, or for producing relatively higher voltage pulses, such as CV/DF shock pulses or higher voltage pacing pulses. In some examples, high voltage capacitors are charged and utilized for delivering pacing pulses instead of low voltage capacitors.

The circuits shown inFIG.4represent functionality included in ICD14and may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to ICD14herein. The various circuits may include one or more of 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 combination 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 modern IMD system, given the disclosure herein, is within the abilities of one of skill in the art.

Memory82may include any volatile, non-volatile, magnetic, or electrical non-transitory computer readable storage media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other memory device. Furthermore, memory82may include non-transitory computer readable media storing instructions that, when executed by one or more processing circuits, cause control circuit80and/or other ICD circuits to perform various functions attributed to ICD14or those ICD circuits. The non-transitory computer-readable media storing the instructions may include any of the media listed above.

The functions attributed to ICD14herein may be embodied as one or more integrated circuits. Depiction of different circuits is intended to highlight different functional aspects and does not necessarily imply that such circuits must be realized by separate hardware or software components. Rather, functionality associated with one or more circuits may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, sensing operations may be performed by sensing circuit86under the control of control circuit80and may include operations implemented in a processor executing instructions stored in memory82and control signals such as timing and sensing threshold amplitude signals sent from control circuit80to sensing circuit86.

Control circuit80communicates, e.g., via a data bus, with therapy delivery circuit84and sensing circuit86for sensing cardiac electrical activity, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals. Therapy delivery circuit84and sensing circuit86are electrically coupled to electrodes24,26,28,30and31(if present) carried by lead16and the housing15, 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 circuit86may be selectively coupled to electrodes28,30and31and/or housing15in order to monitor electrical activity of the patient's heart. Sensing circuit86may additionally be selectively coupled to defibrillation electrodes24and/or26for use in a sensing electrode vector. Sensing circuit86is enabled to selectively monitor one or more sensing vectors at a time selected from the available electrodes24,26,28,30,31and housing15. For example, sensing circuit86may include switching circuitry for selecting which of electrodes24,26,28,30,31and housing15are coupled to sense amplifiers or other cardiac event detection circuitry included in sensing circuit86. Switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple components of sensing circuit86to selected electrodes. In some instances, control circuit80may control the switching circuitry to selectively couple sensing circuit86to one or more sense electrode vectors. The cardiac event detection circuitry within sensing circuit86may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), or other analog or digital components.

In some examples, sensing circuit86includes multiple sensing channels for acquiring cardiac electrical signals from multiple sensing vectors selected from electrodes24,26,28,30,31and housing15. Each sensing channel may be configured to amplify, filter and rectify the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for sensing cardiac events, such as R-waves. For example, each sensing channel may include a pre-filter and amplifier for filtering and amplifying a signal received from a selected pair of electrodes. The resulting raw cardiac electrical signal may be passed from the pre-filter and amplifier to a post-filter and amplifier, analog-to-digital converter, rectifier, and cardiac event detector that compares the digitized, filtered and rectified cardiac electrical signal to a cardiac event sensing threshold for sensing cardiac events from the received cardiac electrical signal. The cardiac event detector may include a sense amplifier, comparator or other detection circuitry that senses a cardiac event when the cardiac electrical signal crosses the cardiac event sensing threshold. The cardiac event sensing threshold is automatically adjusted by sensing circuit86under the control of control circuit80, based on timing intervals and sensing threshold values determined by control circuit80, stored in memory82, and/or controlled by hardware of control circuit80and/or sensing circuit86. Some sensing threshold control parameters may be programmed by a user and passed from control circuit80to sensing circuit86via a data bus.

As described herein, sensing circuit86may sense R-waves according to a sensing threshold that is automatically adjusted to multiple threshold levels at specified times after a sensing threshold crossing or after an electrical stimulation pulse delivered by therapy delivery circuit84. Multiple threshold levels and the time intervals over which each threshold level or value is applied may be used to provide accurate R-wave sensing while minimizing T-wave oversensing and P-wave oversensing. If T-waves and/or P-waves are falsely sensed as R-waves, due to a cardiac electrical signal crossing the R-wave sensing threshold, a tachyarrhythmia may be falsely detected potentially leading to an unnecessary cardiac electrical stimulation therapy, such as ATP or shock delivery. This situation is avoided using the multi-level sensing threshold techniques disclosed herein while still providing VT and VF detection with a high sensitivity. Oversensing may also cause ICD14to inhibit bradycardia pacing pulses when pacing is actually needed. By avoiding oversensing using the multi-level sensing threshold, inhibiting of bradycardia pacing pulses when pacing is actually needed is avoided.

Upon sensing a cardiac event based on a sensing threshold crossing, sensing circuit86may produce a sensed event signal, such as an R-wave sensed event signal, that is passed to control circuit80. The sensed event signals are used by control circuit80for detecting cardiac rhythms and determining a need for therapy. For example, time intervals between consecutive sensed event signals may be determined and compared to tachyarrhythmia detection intervals for detecting a tachyarrhythmia and thereby determine a need for an electrical stimulation therapy to treat the detected tachyarrhythmia. Sensing circuit86may also pass a digitized electrocardiogram (ECG) signal to control circuit80for morphology analysis performed for detecting and discriminating heart rhythms. In some examples, analysis of the digitized cardiac electrical signal is performed for determining R-wave sensing threshold control parameters as described in conjunction withFIG.11.

Signals from the selected sensing vector may be passed through a bandpass 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 circuit86, for storage in random access memory included in memory82under control of a direct memory access circuit via a data/address bus. Control circuit80may be a microprocessor based controller that employs digital signal analysis techniques to characterize the digitized signals stored in random access memory of memory82to recognize and classify the patient's heart rhythm employing any of numerous signal processing methodologies for analyzing cardiac signals and cardiac event waveforms, e.g., R-waves. Examples of algorithms that may be performed by ICD14for detecting, discriminating and treating tachyarrhythmia which may be adapted to utilize techniques disclosed herein for controlling a multi-level cardiac event sensing threshold for sensing cardiac electrical signals are generally disclosed in U.S. Pat. No. 5,354,316 (Keimel); U.S. Pat. No. 5,545,186 (Olson, et al.); U.S. Pat. No. 6,393,316 (Gillberg et al.); U.S. Pat. No. 7,031,771 (Brown, et al.); U.S. Pat. No. 8,160,684 (Ghanem, et al.), and U.S. Pat. No. 8,437,842 (Zhang, et al.), all of which patents are incorporated herein by reference in their entirety.

Therapy delivery circuit84includes charging circuitry, one or more charge storage devices, such as one or more high voltage capacitors and in some examples one or more 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 circuit84according to control signals received from control circuit80. Control circuit80may include various timers or counters that control when ATP or other cardiac pacing pulses are delivered.

For example, control circuit80may include pacer timing and control circuitry having programmable digital counters set by the microprocessor of the control circuit80for controlling the basic time intervals associated with various pacing modes or anti-tachycardia pacing sequences delivered by ICD14. The microprocessor of control circuit80may also set the amplitude, pulse width, polarity or other characteristics of the cardiac pacing pulses, which may be based on programmed values stored in memory82.

During pacing, escape interval counters within the pacer timing and control circuitry are reset upon sensing of an intrinsic R-wave as indicated by an R-wave sensed event signal from sensing circuit86and upon generation of a pacing pulse. In accordance with the selected mode of pacing, pacing pulses are generated by a pulse output circuit of therapy delivery circuit84if an escape interval expires without being reset due to an R-wave sensed event signal. The pace output circuit is coupled to the desired electrodes via switch matrix for discharging one or more capacitors across the pacing load. The durations of the escape intervals are determined by control circuit80via a data/address bus. The value of the count present in the escape interval counters when reset by sensed R-waves can be used to measure R-R intervals for detecting the occurrence of a variety of arrhythmias. As described in conjunction with the diagrams and flow charts that follow, sensing circuit86may be configured to control the R-wave sensing threshold used for sensing R-waves from a received cardiac electrical signal according to post-sense control parameters and according to post-pulse control parameters. One or more time intervals and/or sensing threshold values used to control the R-wave sensing threshold following an electrical stimulation pulse, e.g., a pacing pulse or CV/DF shock pulse, may be different than the time intervals and/or sensing threshold values used to control the R-wave sensing threshold following a sensed intrinsic R-wave.

Memory82includes read-only memory (ROM) in which stored programs controlling the operation of the control circuit80reside. Memory82may further include random access memory (RAM) configured as a number of recirculating buffers capable of holding a series of measured intervals, counts or other data for analysis by the control circuit80for predicting or diagnosing an arrhythmia.

In response to the detection of VT, ATP therapy can be delivered by loading a regimen from the microprocessor included in control circuit80into the pacer timing and control circuit according to the type and rate of tachycardia detected. In the event that higher voltage cardioversion or defibrillation pulses are required, the control circuit microprocessor activates cardioversion and defibrillation control circuitry included in control circuit80to initiate charging of the high voltage capacitors of via a charging circuit, both included in therapy delivery circuit84, under the control of a high voltage charging control line. The voltage on the high voltage capacitors is monitored via a voltage capacitor line, which is passed to control circuit80. When the voltage reaches a predetermined value set by the microprocessor of control circuit80, a logic signal is generated on a capacitor full line passed to therapy delivery circuit84, terminating charging. The CV/DF pulse is delivered to the heart by an output circuit of therapy delivery circuit84under the control of the pacer timing and control circuitry via a control bus. The output circuit determines the electrodes used for delivering the CV/DF pulse and the pulse wave shape. Therapy delivery and control circuitry generally disclosed in any of the above-incorporated patents may be implemented in ICD14.

Control parameters utilized by control circuit80for detecting cardiac rhythms and controlling therapy delivery may be programmed into memory82via telemetry circuit88. Telemetry circuit88includes a transceiver and antenna for communicating with external device40(shown inFIG.1A), e.g., using RF communication as described above. Under the control of control circuit80, telemetry circuit88may receive downlink telemetry from and send uplink telemetry to external device40. In some cases, telemetry circuit88may be used to transmit and receive communication signals to/from another medical device implanted in patient12.

FIG.5is a timing diagram100showing a band-pass filtered and rectified cardiac electrical signal102and R-wave sensed event signals103produced by sensing circuit86. The cardiac electrical signal102includes R-waves104,104′, T-wave106, and P-wave108. As used herein, “R-wave sensing” generally refers to sensing the intrinsic QRS complex of a cardiac electrical signal for the purposes of detecting and discriminating intrinsic ventricular rhythms, e.g., for detecting and discriminating ventricular fibrillation, ventricular tachycardia, supraventricular tachycardia, bradycardia, asystole or other types of intrinsic heart rhythms. Sensing circuit86automatically adjusts an R-wave sensing threshold110to multiple threshold values116,118, and120. The multiple threshold values116,118, and120may be determined by control circuit80based on the maximum peak amplitude112of a sensed R-wave104and passed to sensing circuit86along with multiple timing intervals130,132,134and136for controlling the R-wave sensing threshold110for detecting of the next R-wave104′. Threshold values116,118and120of R-wave sensing threshold110may also be referred to as threshold “levels” or “settings” or merely as “thresholds” but all refer to different voltage amplitudes which, when crossed by a positive-going, rectified band-pass filtered cardiac electrical signal, result in an R-wave sensed event signal being produced by sensing circuit86.

In the example shown, R-wave104is sensed by sensing circuit86when the cardiac electrical signal102crosses R-wave sensing threshold110, which is set to threshold value120at the time of the threshold crossing. An R-wave sensed event signal150is generated. Upon sensing the R-wave104, a post-sense blanking interval130may be started. The post-sense blanking interval may be a fixed time interval controlled by hardware that prevents the R-wave104from being sensed twice. The post-sense blanking interval130is 120 ms in one example, and may be 120 ms to 160 ms in other examples. During the post-sense blanking interval130, a peak detector circuit included in sensing circuit86or control circuit80determines the maximum peak amplitude112of R-wave104.

At the expiration of the post-sense blanking interval130, the maximum peak amplitude112is used to determine the sensing threshold values116and118. In one example, a second blanking interval132is started upon sensing R-wave104and is slightly longer than the first blanking interval130. Upon expiration of the first blanking interval130, a microprocessor within control circuit80may fetch the R-wave peak amplitude and determine the first, starting threshold value116prior to expiration of the second blanking interval132. The R-wave sensing threshold110may be set to the starting threshold value116upon expiration of the second blanking interval132.

The second blanking interval132may be at least 20 ms longer than the first blanking interval130. For example, second blanking interval132may be 140 ms to 180 ms long in some examples. In some implementations, the first blanking interval130is a hardware controlled blanking interval, and the second blanking interval132is a digital blanking interval controlled by firmware or software stored in memory82. The second blanking interval132may be a user-programmable value so that it may be tailored to the patient, e.g., based on the width of R-wave104.

The control circuit80may determine the first, starting threshold value116as a percentage of the R-wave peak amplitude112during the interval between the expiration of the first blanking interval130and the expiration of the second blanking interval132. This interval difference between the first and second blanking intervals130and132may be minimized in some examples to enable firmware processing time to determine the first, starting threshold value116but enable R-wave sensing as early as possible after expiration of the first blanking interval130.

By implementing the second blanking interval132and computation of the starting threshold value116in firmware stored in memory82and executed by a microprocessor of control circuit80, the multi-level R-wave sensing threshold110disclosed herein may be implemented in many existing IMD systems that already include a hardware-implemented blanking interval without requiring hardware modifications. The longer second blanking interval132may be fixed or programmable to account for wider R-waves that typically appear in a cardiac signal obtained using extra-cardiovascular electrodes compared to the R-wave width in intracardiac electrogram signals. Furthermore, implementation of the second blanking interval132as a digital blanking interval allows R-wave sensing threshold control techniques disclosed herein to operate in conjunction with other algorithms or methods being executed by hardware or firmware of ICD14for heart rhythm detection without modification. For example, ICD14may be configured to execute T-wave oversensing rejection algorithms implemented in hardware or firmware configured to determine a differential filtered cardiac electrical signal as generally disclosed in U.S. Pat. No. 7,831,304 (Cao, et al.), incorporated herein by reference in its entirety. By setting the second blanking interval132as a digital blanking interval controlled by firmware, the R-wave sensing threshold110may be controlled without altering operations performed by a T-wave oversensing rejection algorithm operating concurrently, which may be implemented in hardware.

In other examples, the first blanking interval130, peak detector for determining R-wave peak amplitude112, and the second blanking interval132may all be implemented in hardware, all be implemented in firmware or a combination of both. In some examples, the second blanking interval132may be programmable such that the time of the onset of R-wave sensing at the expiration of the second blanking interval132, after the expiration of the first blanking interval130, may be selected by a user according to patient need. When peak detection and determination of the starting threshold value116are implemented in hardware, a single hardware implemented blanking interval130may be used without requiring a second blanking interval132for providing firmware processing time during which the starting threshold value116is determined.

The first sensing threshold value116may set to a percentage of the R-wave peak amplitude112. For example, the first sensing threshold value116may be 50% of peak amplitude112, and may be from 40% to 60% in other examples. The percentage of R-wave peak amplitude112used to determine the starting threshold value116is selected to promote a high likelihood that the threshold value116is greater than the maximum amplitude of T-wave106. The percentage of peak amplitude112used to determine starting threshold value116may be selected based on a previous baseline T-wave amplitude measurement or T/R amplitude ratio. The starting threshold value116is held constant over a sense delay interval134in the example shown to maintain the R-wave sensing threshold110above a maximum T-wave amplitude until a time point near the end or after the T-wave106. In other examples, the R-wave sensing threshold110may have a starting threshold value116that slowly decays over the sense delay interval134. The decay rate, however, is selected to be relatively slow so that the ending threshold value at the expiration of the sense delay interval134is still greater than an expected T-wave amplitude.

Sense delay interval134may be started upon sensing R-wave104, as shown inFIG.5. Alternatively, sense delay interval134may be started upon expiration of the second blanking interval132. Sense delay interval134may be a user-programmable interval which may be tailored to patient need to encompass the T-wave106, or at least the peak of the T-wave or a majority of the T-wave106, to avoid T-wave oversensing. Sense delay interval134is 360 ms in one example and may be, with no limitation intended, 300 ms to 400 ms in other examples. By allowing a user to program the sense delay interval134, the user has the ability to make adjustments to how early after a sensed R-wave the sensing threshold110is adjusted to a lower value, e.g., threshold value118. In this way, if T-wave oversensing is being detected or reported by the ICD14or is being observed in cardiac electrical signal episodes that are stored by ICD14and transmitted to external device40, the clinician has the ability to increase the sense delay interval134to avoid future T-wave oversensing without compromising detection of ventricular fibrillation or ventricular tachycardia.

Control circuit80may be configured to detect T-wave oversensing when it occurs and reject RR-intervals or other evidence of VT or VF when T-wave oversensing is detected. Examples of T-wave oversensing rejection algorithms that may be included in ICD14are generally disclosed in the above-incorporated '304 patent (Cao, et al.) and in U.S. Pat. No. 8,886,296 (Patel, et al.) and U.S. Pat. No. 8,914,106 (Charlton, et al.), also incorporated herein by reference in their entirety. In some examples, control circuit80may automatically increase the sense delay interval134and/or increase the starting value116of R-wave sensing threshold110in response to T-wave oversensing detection. Sense delay interval134may be increased up to a predefined maximum limit, e.g., 440 ms. If there is no TWOS detected for a predetermined time interval, for example one minute, one hour or one day, or if a tachyarrhythmia episode is being detected (e.g., three or more VT or VF intervals detected), sense delay interval134may be automatically reduced to a shorter interval or to a previous setting by control circuit80.

In some examples, sense delay interval134is set equal to the tachycardia detection interval (TDI) used by control circuit80for detecting ventricular tachycardia (VT). Alternatively, sense delay interval134may be set slightly longer than the TDI, e.g., 10 to 20 ms longer than the TDI. Intervals between consecutively sensed R-waves, for example RR interval140between two consecutive R-wave sensed event signals150and152shown inFIG.5, are compared to the TDI and to a fibrillation detection interval (FDI) by a cardiac rhythm analyzer included in control circuit80. If an RR interval is less than the TDI, the cardiac rhythm analyzer may increase a VT interval counter. If the RR interval is less than the FDI, the cardiac rhythm analyzer may increase a VF interval counter. If the VT counter reaches a number of intervals to detect (NID) VT, VT is detected. If the VF counter reaches an NID to detect VF, VF is detected. By setting sense delay interval134equal to a programmed TDI, the R-wave sensing threshold110is kept high, at the starting threshold value116, throughout the FDI and the TDI (which is longer than the FDI) such that the likelihood of a falsely sensed R-wave due to T-wave oversensing during the TDI is minimized, minimizing the likelihood of an oversensed T-wave contributing to a VT or VF detection. If the T-wave106exceeds a lower value of R-wave sensing threshold110at an interval after the R-wave104that is longer than the TDI, the T-wave oversensed event will not contribute to VT detection. Accordingly, the sense delay interval134may be set to match the TDI programmed for VT detection and may be automatically adjusted to track the TDI if the TDI is reprogrammed to a different value.

Upon expiration of the sense delay interval134, the sensing circuit86adjusts R-wave sensing threshold110to a second threshold value118, lower than the starting value116. The second threshold value118may be determined as a percentage of the R-wave peak amplitude112. In one example, threshold value118is set to approximately 28% of the R-wave peak amplitude112. Threshold value118may be set to 20% to 30% of the R-wave peak amplitude112in other examples. The second threshold value118is set to a value that is expected to be greater than the peak amplitude of the P-wave108. P-waves are generally much lower in amplitude than R-waves, however, depending on the alignment of the sensing electrode vector relative to the cardiac axis and other factors, P-wave oversensing can occur in some patients, particularly when the lead16is positioned substernally as shown inFIG.2A.

R-wave sensing threshold110is held at the second threshold value118until the expiration of drop time interval136. Drop time interval136may be started at the time R-wave104is sensed, as shown inFIG.5, or upon expiration of blanking interval130, blanking interval132, or sense delay interval134. When drop time interval136is started upon sensing R-wave104, it may be set to 1.5 seconds or other relatively long interval to promote a high likelihood of maintaining the R-wave sensing threshold at the second value118until after P-wave108, or at least until after the peak amplitude or majority of P-wave108. Drop time interval136may be a fixed interval or may be programmable by the user. The drop time interval136may range from 0.8 to 2.0 seconds in other examples. In some examples, the drop time interval136may be adjusted with changes in heart rate. For example, as heart rate increases based on measurements of RR intervals such as RR interval140, the drop time interval136may be shortened. As heart rate decreases, the drop time interval136may be increased.

The second threshold value118is shown to be a constant value from the expiration of sense delay interval134until the expiration of drop time interval136. In other examples, the second threshold value188may slowly decay until the expiration of drop time interval136. The decay rate would be selected to be slow, however, so that the ending R-wave sensing threshold at the expiration of the drop time interval136is still expected to be greater than the P-wave amplitude to avoid P-wave oversensing. An example decay rate might be 10% of the maximum peak amplitude112per second.

If the cardiac electrical signal has not crossed R-wave sensing threshold110prior to expiration of the drop time interval136, the R-wave sensing threshold110is adjusted from the second sensing threshold value118to a minimum sensing threshold value120, which may be referred to as the “sensing floor.” The R-wave sensing threshold110remains at the minimum sensing threshold120until the cardiac electrical signal102crosses the threshold120. In the example shown, R-wave104′ is sensed when the minimum sensing threshold120is crossed, causing sensing circuit86to generate R-wave sensed event signal152.

In some examples, the minimum sensing threshold value120is set equal to the programmed sensitivity setting122which may be, for example, 0.07 millivolts (mV), 0.15 mV, 0.3 mV, 0.6 mV or higher. The programmed sensitivity setting122may establish the minimum possible sensing threshold value in some examples, in which case the R-wave sensing threshold110is never set below the programmed sensitivity setting122. The sensitivity setting122may be programmable between 0.075 and 1.2 millivolts (mV) in one example and may be selected by a user as the minimum voltage threshold required to sense a cardiac event from cardiac signal102. As the value of the sensitivity setting122decreases, sensitivity of the sensing circuit for sensing low amplitude signals increases. As such, a low sensitivity setting122corresponds to high sensitivity for sensing R-waves. The lowest setting, e.g., 0.07 mV, corresponds to the highest sensitivity, and the highest setting, e.g., 1.2 mV, corresponds to the lowest sensitivity of sensing circuit86for sensing R-waves.

Pulses of the cardiac electrical signal102that have a maximum peak voltage below the programmed sensitivity setting122are considered noise or events that are not intended to be sensed, which may include T-waves and P-waves. When T-wave or P-wave sensing is detected or observed, the user may reprogram the sensitivity setting122to a higher setting (lower sensitivity). However, by providing the multi-threshold R-wave sensing threshold110, controlled using a programmable sense delay time interval134and drop time interval136, the programmed sensitivity setting122may be kept at a low value to provide high sensitivity for sensing R-waves and low amplitude fibrillation waves while still minimizing the likelihood of T-wave and P-wave oversensing.

In addition to determining the starting threshold value116and the second threshold value118, control circuit80may establish a maximum R-wave sensing threshold limit114that limits the maximum starting value of R-wave sensing threshold110. If the starting value116of the R-wave sensing threshold110determined based on peak amplitude112of R-wave104is greater than the maximum threshold limit114, the starting value of R-wave sensing threshold110may be set to the maximum threshold limit114. In some cases, a maximum R-wave sensing threshold limit114is set as a fixed multiple or fixed gain of the programmed sensitivity setting122, for example a gain of eight to ten times the sensitivity setting122. In other examples, the gain applied to the programmed sensitivity setting122for establishing a maximum R-wave sensing threshold limit114is a variable gain. The variable gain may be defined to be dependent on the programmed sensitivity setting122as described below.

FIG.6is a diagram of a filtered and rectified cardiac electrical signal200including R-wave202and T-wave204. Two examples of maximum R-wave sensing threshold limits216and218are each set as a fixed multiple of a respective programmed sensitivity setting220or222. As can be seen in this example, in some cases, when large amplitude R-waves and T-waves occur, the maximum R-wave sensing threshold limit218set as a fixed multiple of the lower programmed sensitivity setting222may result in T-wave oversensing because T-wave204crosses the maximum R-wave sensing threshold limit218. In an illustrative example, the maximum peak amplitude212of R-wave202is 10 mV, and the sensitivity setting222is programmed to 0.3 mV. The maximum R-wave sensing threshold limit218is set to 3 mV, when a fixed gain of 10 times the programmed sensitivity setting is used to set the maximum threshold limit218. In this situation of a very large R-wave202, the first sensing threshold value214determined as a percentage (50% in the example shown) of the maximum peak amplitude212of the R-wave202is greater than the maximum sensing threshold limit218. As such, the R-wave sensing threshold is set to the maximum sensing threshold limit218at the expiration of the second blanking interval132until the expiration of sense delay interval134. The R-wave sensing threshold set to the maximum threshold limit218would result in T-wave oversensing in this example since the maximum limit218is less than the amplitude of T-wave204.

In order to prevent T-wave oversensing, a higher sensitivity setting220could be programmed, for example 0.6 mV. The maximum sensing threshold limit216is 6 mV in the example of the programmed sensitivity setting220being 0.6 mV and a fixed gain of 10 being used to determine the maximum limit216. This maximum threshold limit216is greater than the starting sensing threshold value214determined as a percentage of R-wave amplitude peak212, which is 50% of 10 mV or 5 mV in this example. This starting threshold value214is applied as the R-wave sensing threshold upon expiration of the second blanking interval132since it is less than the maximum threshold limit216. The starting threshold value214does not result in T-wave oversensing because the amplitude of T-wave204is less than the starting sensing threshold value214.

As can be seen by the illustrative example ofFIG.6, in the presence of large amplitude R-waves and T-waves, T-wave oversensing can occur when a maximum sensing threshold limit is determined as a fixed gain of the sensitivity setting and the sensitivity setting is low. In order to avoid T-wave oversensing, the sensitivity setting can be increased to lower the sensitivity, e.g., to 0.6 mV from 0.3 mV as represented by sensitivity setting220and sensitivity setting222, respectively, inFIG.6. The higher sensitivity setting, however, makes sensing circuit86less sensitive to low amplitude R-waves that may occur during VT or VF, potentially resulting in under-detection of ventricular tachyarrhythmia episodes.

FIG.7is a diagram of the cardiac electrical signal200shown inFIG.6shown with two different examples of maximum sensing threshold limits256and258determined using a variable, sensitivity-dependent gain applied to the programmed sensitivity. The gain or multiple of the programmed sensitivity setting used by control circuit80to determine the maximum sensing threshold limit following a sensed event is a function of the programmed sensitivity setting in some examples. The maximum sensing threshold limit may be inversely related to the programmed sensitivity setting such that a higher gain is applied to a lower programmed sensitivity setting for obtaining the maximum threshold limit.

For instance, control circuit80may compute a variable gain (G) for determining a maximum sensing threshold limit by determining an inverse proportion of the sensitivity setting and adding a constant using the equation G=A+B/S where A and B are constants and S is the programmed sensitivity (B/S being an inverse proportion of the programmed sensitivity setting). In some examples, the gain determined for each available programmable sensitivity setting is stored in a look-up table in memory80and is retrieved by control circuit80each time a new sensitivity setting is programmed.

In one example, A is at least 5 and B is at least 1.5. For instance, A may be equal to 6 and B may be equal to 2.5 in the equation given for the gain G above. The minimum possible value of the maximum sensing threshold limit will approach 2.5 since the maximum sensing threshold limit is the product of the gain and the programmed sensitivity setting, or 6S+2.5 where S equals the programmed sensitivity setting. For a programmable range of sensitivity settings from 0.075 mV to 1.2 mV, the sensitivity-dependent gain ranges from approximately 39.3 for the lowest sensitivity setting of 0.075 mV (corresponding to highest sensitivity) to approximately 8.1 for the highest sensitivity setting of 1.2 mV (corresponding to the lowest sensitivity). The higher the sensitivity, i.e., the lower the sensitivity setting, the higher the sensitivity-dependent gain is.

For a programmed sensitivity of 0.3 mV, the sensitivity-dependent gain is given by G=6+2.5/0.3 or approximately 14.3 using the constants given in the foregoing example. The maximum sensing threshold limit258determined when the sensitivity setting222is programmed to 0.3 mV is the sensitivity-dependent gain, 14.3, multiplied by the sensitivity setting, 0.3 mV, or approximately 4.3 mV. This maximum sensing threshold limit258is less than the first sensing threshold value254determined as a percentage (50% in this example) of R-wave peak amplitude212. As a result, the R-wave sensing threshold210will be set to the maximum sensing threshold limit258, but in this case the sensing threshold limit258set using the variable gain is greater than the amplitude of T-wave204, thereby avoiding T-wave oversensing while still allowing a high sensitivity (low sensitivity setting) to be used for sensing low amplitude waveforms during VT or VF (especially spontaneous fine VF) after the drop time interval134expires.

Continuing with the illustrative example given above, the maximum sensing threshold limit256determined for a programmed sensitivity setting of 0.6 mV220is determined using a sensitivity-dependent gain of approximately 10.2 (G=6+2.5/0.6). The maximum sensing threshold limit256is 6.1 mV in this case (0.6 mV multiplied by the gain of 10.2), which is greater than the starting sensing threshold value254determined as a percentage (e.g., 50%) of the R-wave peak amplitude212. In this case, the R-wave sensing threshold210is set to the starting sensing threshold value254at the expiration of the second blanking interval132(shown inFIG.5).

In both cases of 0.6 mV sensitivity setting220and 0.3 mV sensitivity setting222, the R-wave sensing threshold value during the sense delay interval134avoids T-wave oversensing in the presence of large amplitude R-waves and T-waves. Even when a low sensitivity setting is used, e.g., 0.3 mV or less, so that sensing circuit86remains highly sensitive to small R-waves that may occur during a ventricular tachyarrhythmia, T-wave oversensing is avoided by using a sensitivity-dependent variable gain for determining the maximum R-wave sensing threshold limit.

At the expiration of the sense delay interval134, the R-wave sensing threshold is adjusted from the starting threshold254or258, to the second threshold260which is determined as 25% of the R-wave peak amplitude212in this example. The second threshold260remains in effect until the drop time interval136expires (described inFIG.5) after which the R-wave sensing threshold210drops to the programmed sensitivity setting, either the 0.6 mV sensitivity setting220or the 0.3 mV sensitivity setting222in the example shown inFIG.7.

FIG.8Ais a diagram of a filtered and rectified cardiac electrical signal300including an R-wave302, a T-wave304, and a P-wave306and an automatically adjusted R-wave sensing threshold310having multiple sensing threshold values316,317,318and320. In the examples ofFIGS.5,6and7, the R-wave sensing threshold310is set to the first, starting threshold value and second threshold value before dropping to the programmed sensitivity setting. In other examples, the R-wave sensing threshold310may be adjusted to three or more threshold values before dropping to the programmed sensitivity setting.

As shown inFIG.8A, the starting threshold value316may be determined as a first percentage of the peak R-wave amplitude312that is detected during first blanking interval330, e.g., 62.5% or between 55% and 70% of the peak R-wave amplitude312. The starting threshold value316may be maintained from the expiration of the second blanking interval332until the expiration of a first sense delay interval333. The first sense delay interval333may be approximately 180 ms, for example 30 to 60 ms longer than the second blanking interval332. The higher starting threshold value316applied for a short interval may reduce the likelihood of double sensing the R-wave302, particularly in patients exhibiting a wide QRS complex.

Upon expiration of the first sense delay interval333, the R-wave sensing threshold310is adjusted to a lower, second sensing threshold value317, which may be between 30% and 60% of the R-wave peak amplitude312, such as 50% of the R-wave peak amplitude312. The second sensing threshold value317is maintained until expiration of the second sense delay interval334, which may be between 300 and 360 ms, and may be set equal to a programmed TDI as described previously in conjunction withFIG.5.

Upon expiration of the second sense delay interval334, the third sensing threshold value318is applied until a drop time interval336expires, and the R-wave sensing threshold310falls to a minimum sensing threshold value320, which may be equal to the programmed sensitivity setting. The third sensing threshold value318may be approximately 28% of the R-wave peak amplitude312, or between 20% and 30% in other examples, and extend for a drop time interval336of one to two seconds, e.g., 1.5 seconds.

FIG.8Bis a diagram of a non-monotonic, multi-level R-wave sensing threshold350according to another example. In the examples ofFIGS.5and8A, R-wave sensing threshold110and R-wave sensing threshold310, respectively, are monotonically decreasing sensing thresholds. In other examples, the multi-level R-wave sensing threshold controlled by control circuit80is non-monotonic, including one or more step increases in the value of R-wave sensing threshold in addition to the decreasing step changes in the R-wave sensing threshold value.

The filtered and rectified cardiac electrical signal300, including R-wave302, T-wave304, and P-wave306, and an automatically adjusted R-wave sensing threshold350are shown inFIG.8B. R-wave sensing threshold350may include a starting sensing threshold value316beginning upon expiration of second blanking interval332and a second sensing threshold value318beginning after expiration of the sense delay interval334. R-wave sensing threshold350drops to the programmed sensitivity setting320upon expiration of drop time interval336.

If cardiac signal300does not cross the R-wave sensing threshold350before a maximum sensitivity interval338expires, the R-wave sensing threshold350is increased to a third sensing threshold value322. The third sensing threshold value may be equal to the second sensing threshold value318or set as a percentage of a previously determined baseline P-wave maximum peak amplitude, e.g., 1.5 times a previously determined P-wave maximum peak amplitude. The maximum sensitivity interval338controls how long the R-wave sensitivity threshold350is held at the maximum sensitivity, e.g., the programmed sensitivity setting320, before being increased to the third sensing threshold value322. In some examples, the maximum sensitivity interval338is approximately 200 ms longer than the drop time interval336so that the R-wave sensing threshold350is set to the programmed sensitivity setting320for up to 200 ms if an R-wave sensing threshold crossing does not occur.

Upon expiration of the maximum sensitivity interval338, R-wave sensing threshold350is increased to the third sensing threshold value322to minimize the likelihood of oversensing the P-wave306during very slow heart rates and when the P-wave306has an amplitude greater than the programmed sensitivity setting320. By allowing the R-wave sensing threshold350to drop to the programmed sensitivity setting320, to provide high sensitivity for up to a predefined time interval as controlled by interval338, undersensing of low amplitude, fine VF waveforms is avoided. Sensing circuit86may sense low amplitude ventricular tachyarrhythmia waveforms after expiration of drop time interval336and before expiration of maximum sensitivity interval338. If the heart rate is very slow, however, such that the P-wave306arrives relatively late after R-wave302and after expiration of drop-time interval336, P-wave oversensing may be avoided by increasing the R-wave sensing threshold350to the third threshold value322while still providing an interval of high sensitivity to low amplitude tachyarrhythmia waveforms. The third sensing threshold value322may be maintained until a sensing threshold crossing occurs. In other examples, as shown inFIG.8B, the third sensing threshold value322is held until a second drop time interval340expires, at which time the R-wave sensing threshold350is adjusted back to the programmed sensitivity setting320.

FIG.9is a flow chart400of a method for controlling the R-wave sensing threshold according to one example. At block402, the control circuit80establishes the maximum threshold limit. The maximum threshold limit may be set based on a sensitivity-dependent gain as described in conjunction withFIG.7. Control circuit80determines the sensitivity-dependent gain then computes the maximum threshold limit as the product of the gain and the programmed sensitivity setting. Alternatively, the maximum threshold limit may be set as a fixed multiple of the programmed sensitivity setting, using a gain that is independent of the programmed sensitivity, as described in conjunction withFIG.6. The maximum threshold limit is re-established at block402each time the sensitivity is reprogrammed to a different sensitivity setting.

At block404, an R-wave is sensed in response to the cardiac electrical signal crossing the R-wave sensing threshold, which may initially be set to the maximum sensing threshold, a nominal sensing threshold, the programmed sensitivity setting or other starting value. In response to sensing an R-wave, sensing circuit86produces an R-wave sensed event signal at block408, and control circuit80sets various timers or counters as described in conjunction withFIGS.5and8A and8Bfor controlling the multi-threshold R-wave sensing threshold. For example, a first blanking interval, which may be a hardware controlled blanking interval, a second blanking interval, which may be a digital blanking interval controlled by firmware stored in memory82, a sense delay interval, a drop time interval, and a maximum sensitivity interval may be started upon sensing an R-wave due to a positive-going R-wave sensing threshold crossing of the cardiac electrical signal received by sensing circuit86.

At block408, the maximum peak amplitude of the sensed R-wave is determined during the first post-sense blanking interval. The R-wave peak amplitude may be determined by a peak track and hold circuit or other hardware or firmware. The R-wave peak amplitude is fetched by control circuit80at the expiration of the first post-sense blanking interval. Control circuit80determines the starting and second threshold values at block410as two different percentages of the R-wave peak amplitude. For example, the starting threshold value may be determined as 40 to 60% of the R-wave peak amplitude, and the second threshold value may be determined as 20 to 30% of the R-wave peak amplitude. Control circuit80may execute firmware after expiration of the first blanking interval for determining the starting, first threshold value and the second threshold value before expiration of the second blanking interval. The starting and second threshold values may be passed to sensing circuit86as control values used by circuitry of sensing circuit86for controlling the R-wave sensing threshold. In other examples, three or more threshold values are determined as described in conjunction withFIGS.8A and8B.

Upon expiration of the second blanking interval, as determined at block412, the sensing circuit86sets the starting R-wave sensing threshold at block414to the starting threshold value determined as a percentage of the R-wave peak amplitude or to the maximum threshold limit, whichever is less, under the control of control circuit80. If the cardiac electrical signal crosses the starting R-wave sensing threshold, as determined at block416, the process returns to block406where sensing circuit86produces another R-wave sensed event signal and restarts the various control time intervals as described above, e.g., first post-sense blanking interval130, second post-sense blanking interval132, sense delay interval134and drop time interval136shown inFIG.5.

If the sense delay interval expires at block418before the cardiac electrical signal crosses the R-wave sensing threshold, sensing circuit86adjusts the R-wave sensing threshold at block420to the second threshold value received from control circuit80. If the cardiac electrical signal crosses the R-wave sensing threshold adjusted to the second threshold value, as determined at block422, the process returns to block406to generate an R-wave sensed event signal and reset the control time intervals as described above. If the drop time interval expires at block424without the cardiac electrical signal crossing the R-wave sensing threshold (block422), the sensing circuit86adjusts the R-wave sensing threshold to the minimum threshold value or sensing floor, which may be the programmed sensitivity setting, at block426. In other examples, more than two drop steps in the sensing threshold value may be implemented, as described in conjunction with FIG.8A, and/or a step increase in the sensing threshold value may be included as described in conjunction withFIG.8B.

Sensing circuit86waits for the cardiac electrical signal to cross the sensing floor at block404and the process repeats by advancing to block406if the cardiac electrical signal crosses the sensing floor. If a cardiac electrical stimulation therapy is enabled, however, for example a cardiac pacing therapy, the control circuit80may start a pacing interval at block406when various timers are set in response to the R-wave sensed event signal. If the pacing interval expires at block430before the cardiac electrical signal crosses the R-wave sensing threshold, control circuit80may advance to block756ofFIG.15, described below, to deliver a cardiac pacing pulse and control the R-wave sensing threshold according to post-pace R-wave sensing control parameters.

While not shown inFIG.9, it is recognized that a pacing interval may expire during the sense delay interval (e.g., an ATP pulse) or the drop time interval (e.g., a rate responsive pacing pulse) or any time prior to an R-wave sensing threshold crossing causing a pacing pulse to be delivered. Control circuit80may operate according to the methods described in conjunction withFIG.15for controlling the R-wave sensing threshold post-pulse. The various timing intervals and threshold values shown in any ofFIGS.5-8Bmay be determined and applied for controlling the R-wave sensing threshold following a delivered electrical stimulation pulse and are not limited to being used following only intrinsic R-wave sensed events. However, control parameters used to control the R-wave sensing threshold following an electrical stimulation pulse may be different than the post-sense R-wave sensing threshold control parameters described in conjunction withFIGS.5-9. Examples of post-pulse R-wave sensing threshold control parameters are described in conjunction withFIGS.14-16below.

Furthermore, while the techniques have been described for controlling an R-wave sensing threshold for sensing R-waves attendant to ventricular depolarization, it is to be understood that aspects of the disclosed techniques may be used for controlling a cardiac event sensing threshold for sensing other cardiac event signals, such as P-waves attendant to atrial depolarization or T-waves attendant to ventricular repolarization. For example, a maximum P-wave sensing threshold limit may be set based on a sensitivity-dependent gain and a programmed sensitivity; a maximum T-wave sensing threshold limit may be set based on a sensitivity-dependent gain and programmed sensitivity. P-wave and/or T-wave sensing thresholds may be controlled using multiple threshold levels and multiple time intervals.

FIG.10is a flow chart of a method for selecting a sensitivity setting in ICD14. The process shown inFIG.10may be performed at the time of ICD14implant or during a lead replacement procedure to determine a reliable sensitivity setting for detecting low amplitude fibrillation waves during VF. The process shown by flow chart500may be a semi-automated process executed by ICD14in response to programming commands received from external device40.

At block502, a test sensitivity setting is selected. The test sensitivity setting may be programmed by a user using external device40or may be a value two times a nominal sensitivity setting or a preferred sensitivity setting, which may be based on a measured R-wave amplitude. For example, if the R-wave amplitude observed on an electrocardiogram signal produced by ICD14and transmitted to external device40is at least 3 mV, a test sensitivity setting of 0.6 mV may be set at block502for a preferred sensitivity setting of 0.3 mV, half of the test setting.

At block504, a user transmits a VF induction command to ICD14using external device40. ICD14may induce VF using any implemented induction method, such as a T-shock, which is a large energy electrical pulse delivered during the vulnerable period associated with the T-wave. If VF is detected at the programmed sensitivity setting, “Yes” branch of block506, a defibrillation shock is delivered according to programmed shock therapy control parameters to terminate the VF. If VF is not detected at the programmed sensitivity within a predetermined time limit, a shock is delivered to terminate the induced VF, and the sensitivity setting may be decreased, to increase the sensitivity to VF waveforms, at block508. VF may be induced again at block506to test the new sensitivity setting. This process may be repeated one or more times as needed to determine the highest sensitivity setting that allows successful detection of VF. Alternatively, if VF is not detected at block506using the first sensitivity setting, a recording of the cardiac electrical signal during the induced VF may be used to determine the voltage amplitude of the VF waveforms, and the sensitivity setting may be programmed lower than the VF waveform amplitude at block512. In still other examples, if sensing circuit86includes two sensing channels, the cardiac electrical signal may be sensed using two different test sensitivity settings simultaneously to determine if one or both result in VF detection.

When VF is detected at the current test sensitivity setting, the sensitivity setting is programmed to one-half to one-third the test sensitivity setting. For example, if the sensitivity setting tested is 0.6 mV, the sensitivity setting is programmed to 0.3 mV at block512. By using a sensitivity-dependent gain for setting the maximum R-wave sensing threshold limit, a lower sensitivity setting may be used with confidence in avoiding T-wave and P-wave oversensing while still providing high sensitivity for detecting VF, both acutely and chronically after implantation of the ICD system10.

The R-wave amplitude of the cardiac electrical signal received by the extra-cardiovascular electrodes is similar during the acute phase (days or weeks) after implantation and after chronic implantation (months or years). Accordingly, the recommended sensitivity setting determined at block512need not change based on time since implant, unlike transvenous ICD systems which may have larger R-wave amplitude acutely and smaller R-wave amplitude chronically. A two-fold or three fold sensitivity safety margin (in other words using one-half to one-third of a tested sensitivity setting) may be used in the extra-cardiovascular ICD system10rather than higher safety margins which have been practiced in the past for transvenous ICD systems, such as a four-fold safety margin. Control of the R-wave sensing threshold as disclosed herein using a two- to three-fold sensitivity safety margin minimizes the risk of undersensing spontaneous fine VF (usually with small waveform amplitudes) while avoiding T-wave and P-wave oversensing.

FIG.11is a flow chart600of a method for selecting R-wave sensing threshold control parameters according to one example. As described above in conjunction withFIG.10, the sensitivity setting, which may define the minimum R-wave sensing threshold value, may be based at least in part on VF detection testing. The starting value of the R-wave sensing threshold may be set on a beat-by-beat basis based on the peak R-wave amplitude (as shown inFIG.5) or based on the programmed sensitivity following an electrical stimulation pulse as described below in conjunction withFIG.14. The gain applied to the sensitivity for setting the maximum R-wave sensing threshold value may be a variable gain that is dependent on the programmed sensitivity setting as described in conjunction withFIG.7. In addition to these R-wave sensing threshold control parameters, other R-wave sensing threshold control parameters may be variable and/or programmable based on cardiac signal features determined for an individual patient to tailor optimal R-wave sensing threshold control for that patient and/or based on empirical data from a population of patients.

For example, the percentage of the R-wave peak amplitude used to set the post-sense starting threshold value116, the second blanking interval132, the sense delay interval134, the post-sense second, lower threshold value118, and the drop time interval136(all shown inFIG.5) may all be programmable or variable values that may be tailored to an individual patient and/or based on sensitivity performance data obtained from a population of patients. Other control parameters such as a second sensing delay interval334as shown inFIG.8Aor a maximum sensitivity interval338as shown inFIG.8Bmay also be programmable and tailored individually to a patient.

At block602, population-based VT/VF detection sensitivity for one or more individual R-wave sensing threshold control parameter settings and/or combinations of R-wave sensing threshold control parameter settings may be stored in ICD memory82and/or in memory53of external device40. For example, a VT/VF detection sensitivity curve as a function of the programmed sensitivity setting122(FIG.5), second blanking interval132, drop time interval136, or other R-wave sensing control parameters described above may be determined from empirical data gathered from a population of ICD patients.

FIG.12is a plot650of an illustrative VT/VF detection sensitivity curve656. VT/VF detection sensitivity, expressed as the percentage of all VT/VF episodes actually detected, is plotted along the y-axis652as a function of an R-wave sensing threshold control parameter setting plotted along the x-axis654. The sensing control parameter is indicated generically inFIG.12but may be the second blanking interval132, the percentage used to determine the starting value116of the R-wave sensing threshold, the sense delay interval134, the percentage used to determine the second value118of the R-wave sensing threshold, the drop time interval136, the sensitivity setting122(all shown inFIG.5) or any of the other R-wave sensing threshold control parameters described herein.

An alert threshold658may be set, below which the VT/VF detection sensitivity falls below VT/VF detection performance expectations, e.g., 95%. When the control parameter setting has a programmed value greater than “X”, the VT/VF detection sensitivity falls below the alert threshold658. As described below, stored VT/VF detection sensitivity data may be used by control circuit80(or external device processor52) to look up an expected VT/VF detection sensitivity for a programmed R-wave sensing threshold control parameter individually or in combination with other parameter values in a multi-parameter n-dimensional model of detection sensitivity. If the detection sensitivity falls below an alert threshold658for ICD performance expectations, a clinician alert may be generated as described below.

FIG.13is a plot680of an example VT/VF detection sensitivity curve686. VT/VF detection sensitivity is plotted along y-axis682as a function of the programmed sensitivity setting plotted along x-axis684. When the programmed sensitivity setting is less than approximately 135 microvolts, the VT/VF detection sensitivity is greater than the alert threshold688, shown as 95% in this example though other alert threshold levels may be selected. In this example, when the programmed sensitivity setting is 140 microvolts or higher, the VT/VF detection sensitivity686falls below the alert threshold688, and the ICD system10may generate an alert displayed on external device display54as described below in response to a sensitivity setting greater than 140 microvolts being selected for programming.

FIGS.12and13represent VT/VF detection sensitivity as a function of a single sensing threshold control parameter setting. It is recognized that instead of a single parameter function as shown inFIG.12, VT/VF sensitivity may be modeled in a multi-parameter, n-dimensional model taking into account a combination of sensing threshold parameters. Furthermore, it is contemplated that instead of sensitivity or in addition to sensitivity, VT/VF detection specificity may be modeled for one or more sensing threshold parameters, individually or in a multi-parameter, n-dimensional model. Sensing threshold control parameters may be determined based on baseline cardiac electrical signal features and selected in order to achieve a targeted specificity and/or targeted sensitivity.

Returning toFIG.11, at block604, a processor included in control circuit80may determine baseline cardiac signal features. During a confirmed normal sinus rhythm, for example, one or more of the R-wave amplitude, R-wave width, T-wave amplitude, P-wave amplitude, R-T time interval, R-P time interval, T-P time interval, and/or baseline noise may be determined. The R-T, R-P and T-P time intervals may be determined as time intervals between the absolute maximum peak amplitude of the respective R-, T- and P-waves or between other predefined fiducial points of these waves. The normal sinus rhythm may be confirmed manually or based on R-R intervals being greater than a tachyarrhythmia detection interval, no cardiac pacing being delivered, and/or an R-wave morphology match score greater than a predetermined threshold. The cardiac signal features may be determined by control circuit80from a digitized, filtered and rectified cardiac signal received from sensing circuit86.

Alternatively, cardiac signal features may be determined manually from a cardiac electrical signal transmitted to and displayed by external device40or determined automatically by external device processor52from the transmitted cardiac electrical signal. The cardiac signal feature values may then be used by external device processor52for determining recommended R-wave sensing threshold control parameters, or the cardiac signal feature values may be programmed into ICD14, stored in ICD memory82, and retrieved by a processor included in control circuit80for use in determining R-wave sensing threshold control parameters.

With continued reference to the post-sense R-wave sensing threshold control parameters illustrated inFIG.5, at block606ofFIG.11, a processor of control circuit80may determine control parameters for setting the starting value116of the R-wave sensing threshold. As described above in conjunction withFIG.7, a variable gain applied to the sensitivity setting for determining a maximum R-wave sensing threshold limit may be determined based on the programmed sensitivity setting. The maximum R-wave sensing threshold limit is one control parameter used to determine the starting value116. Another control parameter is the percentage of the maximum peak amplitude112of the currently sensed R-wave104that is used to determine the starting value116. This percentage may be based on a T/R ratio of the peak T-wave voltage amplitude to the peak R-wave voltage amplitude112determined from the filtered, rectified cardiac electrical signal at block604. For example, if the T/R ratio is 0.5, the starting R-wave sensing threshold may be determined as at least 0.6 or 0.7 of the maximum peak R-wave amplitude112or a percentage of at least 60% or 70%. If the T/R ratio is 0.3, the percentage may be set to 50% or some other percentage greater than the T/R ratio.

In other examples, a minimum limit of starting value116may be set based on the T-wave amplitude determined at block604, e.g., a minimum limit of the starting value may be determined as a percentage greater than the maximum peak T-wave voltage amplitude, e.g., 125% of the peak T-wave voltage amplitude or 150% of the peak T-wave voltage amplitude, or a fixed interval greater than the peak T-wave voltage amplitude, e.g., 0.25 mV or 0.5 mV greater than the peak T-wave voltage amplitude.

At block608, control circuit80may determine one or more control parameters for use in setting the second threshold value118(shown inFIG.5). The second threshold value118may be set as a second percentage of the peak R-wave voltage amplitude112that is less than the percentage used to determine the starting value116. This second percentage may be based on the P/R ratio of the maximum peak P-wave voltage amplitude to the maximum peak R-wave voltage amplitude112determined from the filtered, rectified cardiac electrical signal at block604. For example, if the P/R ratio is 0.2, the second threshold value118may be determined as 0.4, or 40%, of the maximum peak R-wave amplitude. If the P/R ratio is 0.3, the percentage may be set to 50% or some other percentage greater than the P/R ratio.

In other examples, a minimum limit of the second threshold value118may be set based on the P-wave amplitude determined at block604. For example, a minimum limit of the second threshold value118may be determined as a percentage of the P-wave amplitude, e.g., 125% of the P-wave amplitude or 150% of the P-wave amplitude, or a fixed amount greater than the maximum peak P-wave voltage amplitude, e.g., 0.2 mV or 0.3 mV, or other fixed amount than the peak P-wave amplitude.

At block610, the control circuit80may set the second blanking interval132based on an R-wave width determined at block604. As described previously, the first blanking interval130may be a hardware implemented blanking interval that is absolute and may define a minimum possible value of the second blanking interval132. An R-wave width measurement may be determined at block604from a bandpass filtered cardiac electrical signal as the time interval from a fiducial point on the positive-going portion of the R-wave to a fiducial point on the negative-going portion of the R-wave, e.g., from the first positive crossing of a predetermined voltage to the last negative-going crossing of the predetermined voltage or from a maximum +dV/dt to a maximum −dV/dt. The second blanking interval132may be set to be at least equal to the determined R-wave width, a pre-determined portion of the R-wave width, or a fixed interval greater than or less than the R-wave width. The manner in which the second blanking interval132is determined based on an R-wave width may depend on how the R-wave width is determined. Example methods for determining an R-wave width are generally disclosed in U.S. Pat. No. 8,983,586 (Zhang) and U.S. Pat. No. 5,312,441 (Mader, et al.), both patents incorporated herein by reference in their entirety.

At block612, control circuit80may determine the sense delay interval134based on the R-T interval determined at block604. For example, sense delay interval134may be a fixed interval longer than the R-T interval, e.g., 20 ms longer than the measured R-T interval, or a predetermined percentage of the R-T interval, e.g., 120% of the R-T interval.

The drop time interval136may be determined by control circuit80at block614based on the R-P interval determined at block604, e.g., as a fixed interval or percentage greater than the R-P interval. Since the R-T and R-P intervals may change with heart rate, the control circuit80may adjust the sense delay interval134and the drop time interval136based on heart rate (e.g., based on a most recent RR interval or a running average of a predetermined number of recent RR intervals) in addition to or alternatively to basing the values on the measured R-T and R-P intervals.

The programmed sensitivity setting may be determined and set at block615based on the P-wave amplitude and/or baseline noise determined at block604. A baseline noise amplitude may be determined by measuring the peak cardiac signal amplitude during a baseline window set between cardiac events, e.g., after the T-wave and before an R-wave. The sensitivity setting may be determined as the lowest setting that is greater than the determined baseline noise amplitude or a fixed interval or percentage greater than the determined baseline noise amplitude.

At block616, control circuit80may be configured to compare the R-wave sensing threshold control parameters to the population-based VT/VF detection sensitivity data stored at block602. The R-wave sensing threshold control parameters may include a combination of automatically determined control parameters set by control circuit80as described above and/or user-programmed control parameters. Individual control parameter settings or combinations of control parameter settings may be compared to VT/VF detection sensitivity data to predict the expected sensitivity for detecting VT and VF when the currently selected R-wave sensing threshold control parameters are utilized.

If any of the control parameters, individually or in combination, result in an expected VT/VF detection sensitivity that is less than the alert threshold (e.g., threshold658inFIG.12), an alert condition is detected at block618. In response to detecting an alert condition, control circuit80may generate an alert notification at block620that is transmitted to external device40and displayed on user display54. A user may then review the programmed settings and make any adjustments needed to improve the expected VT/VF detection sensitivity or accept the programmed settings without adjustments. The programmed settings with any adjustments may be transmitted back to ICD14and stored in memory82at block622for use in controlling the R-wave sensing threshold.

In some examples, detection sensitivity data are stored in memory82of ICD14and the process of flow chart600is performed automatically by control circuit80for setting the sensing threshold control parameters. A targeted VT/VF detection sensitivity value may be programmed into ICD14by a user and ICD14may determine the sensing threshold control parameters based on the targeted sensitivity and the baseline cardiac signal features. This process may be repeated periodically for updating the sensing threshold control parameters.

In other examples, the VT/VF detection sensitivity data stored at block602is stored in memory53of external device40. The operations of blocks604through618may be performed by a processor included in control circuit80, by external device processor52after receiving a cardiac electrical signal episode from ICD14via ICD telemetry circuit88and external device telemetry unit58, or cooperatively by a processor of control circuit80and external device processor52with some steps or operations performed by control circuit80and some performed by processor52. Processor52may perform the comparison at block616and generate the display of the alert notification at block620on display54in response to detecting an alert condition at block618. Upon user acceptance of the programmed settings of the R-wave sensing threshold control parameters, after any adjustments made based on an alert if generated, external device40transmits the programmable control parameter settings to ICD14.

FIG.14is a timing diagram700showing ventricular pacing pulses701and703and a band-passed filtered and rectified cardiac electrical signal702. The cardiac electrical signal702includes a pacing-evoked R-wave704followed by T-wave706and P-wave708. During atrio-ventricular block, P-wave708may not be conducted to the ventricles such that a pacing interval705expires. Pacing pulse703is delivered at the pacing interval705following the preceding pulse701if pacing interval expires without cardiac electrical signal702crossing the R-wave sensing threshold710. The pacing interval705may be a lower rate interval for bradycardia pacing, a back-up pacing interval to prevent asystole during post-shock pacing, an anti-tachycardia pacing pulse, or other pacing interval set according to a programmed pacing therapy protocol.

The post-pulse R-wave sensing threshold710is controlled by control circuit80and sensing circuit86. Upon delivery of ventricular pacing pulse701, a post-pulse blanking interval730is started. The post-pulse blanking interval may be a fixed time interval controlled by hardware that prevents sensing the pacing-evoked R-wave704and any post-pace polarization artifact. The post-pulse blanking interval730may be 250 ms in one example, and may be 200 ms to 500 ms in other examples. The post-pulse blanking interval730may be longer than the post-sense blanking intervals130and132(FIG.5). The example of post-pulse R-wave sensing threshold710shown inFIG.14generally relates to a pacing pulse, however the post-pulse control parameters used for controlling the post-pulse R-wave sensing threshold710may be used or adjusted for use following a CV/DF shock or other electrical stimulation pulses, such as an electrical stimulation pulse delivered for VT or VF induction as described in conjunction withFIG.10. For instance, an even longer post-pulse blanking interval, e.g., up to 2 seconds, may be used following a CV/DF shock pulse to allow longer post-stimulation polarization recovery.

At the expiration of the post-pulse blanking interval730, the R-wave sensing threshold710is set to a post-pulse starting threshold value716, which may be based directly on the programmed sensitivity setting or on a maximum threshold limit714. The maximum threshold limit714may be established by using a sensitivity-dependent gain as described above. For instance, the gain applied to a programmed sensitivity may be computed as 8+1.8/S where S is the programmed sensitivity. To obtain the maximum threshold limit714, this gain is multiplied by the sensitivity S resulting in the maximum threshold limit714being set to eight times the programmed sensitivity setting plus 1.8. This maximum threshold limit714may be the same as the maximum threshold limit114(FIG.5) established for use in controlling the starting R-wave sensing threshold value following a sensed intrinsic R-wave. During pacing, the maximum peak amplitude of an evoked R-wave704may not be determined for setting the starting value716of R-wave sensing threshold710. The pacing-evoked R-wave signal amplitude may not be predictive of the amplitude of intrinsic R-waves. The maximum threshold limit714may therefore be used for determining the post-pulse starting and second threshold values716and718of the R-wave sensing threshold710following pacing pulse701without determining a peak amplitude of pacing-evoked R-wave704.

The starting threshold value716may be a first percentage of the maximum threshold limit714, and the second threshold value718may be a second percentage of the maximum threshold limit714. In an illustrative example, the starting threshold value716may be approximately 31% of the maximum threshold limit714, and the second threshold value718may be approximately 15% of the maximum threshold value714. The percentages used to set the starting and second threshold values716and718may be programmable and may range from 10 to 50% or higher (with the second percentage being less than the first percentage).

The starting threshold value716is held during post-pulse delay interval732, from the expiration of the post-pace blanking interval730until expiration of the delay interval732. Delay interval732may be a fixed, programmable time interval, e.g., 500 ms to encompass the T-wave706, or at least the peak of the T-wave or a majority of the T-wave706, to avoid T-wave oversensing. The post-pulse delay interval732may be optimized to minimize the likelihood of T-wave oversensing as generally described previously in conjunction withFIG.5with regard to sense delay interval134. Delay interval732may be dependent on the post-pulse blanking period730, e.g., 250 ms longer than the post-pulse blanking period but not greater than a drop time interval734as described below.

Upon expiration of the delay interval732, the sensing circuit86adjusts R-wave sensing threshold710from the starting value716to the second threshold value718, lower than the starting value716. R-wave sensing threshold710is held at the intermediate threshold value718until the expiration of drop time interval734. Drop time interval734may be started upon delivery of pacing pulse701. Drop time interval734may be determined by control circuit80based on a paced heart rate, e.g., based on the pacing interval705. In one example, drop time interval734is set to 50% of the pacing interval705but may be set to other percentages, greater or less than 50%, of the pacing interval705.

In some examples, the drop time interval734may be set within a limited range, e.g., up to a maximum upper limit and/or down to a minimum lower limit. For example, the maximum drop time interval may be 600 to 750 ms following an electrical stimulation pulse. The drop time interval734may have a minimum lower limit so that the drop time interval734does not expire earlier than the delay interval732.

The drop time interval734determined as a selected percentage of the pacing interval705may be compared to a minimum drop time interval, which may be equal to delay interval732. If the drop time interval734determined as a percentage of the pacing interval705is equal to or less than the minimum drop time interval, sensing circuit86may set the drop time interval equal to the minimum drop time interval. When the minimum drop time interval equals the delay interval732, the R-wave sensing threshold710may be held at the starting threshold value716for the portion736of the delay interval732extending from the expiration of blanking period730to the expiration of delay interval732. Upon expiration of delay interval732, the R-wave sensing threshold710is adjusted to the minimum sensing threshold720.

In response to the drop time interval determined as a percentage of the pacing interval705being equal to or less than the delay interval732, the drop time interval may be set equal to the delay interval732such that the cardiac event sensing threshold is held at a first threshold value equal to the starting threshold value716from the expiration of the post-pulse blanking period730until expiration of the drop time interval. The intermediate sensing threshold718determined as a second percentage of the maximum threshold limit714may be skipped. The drop time interval734and the delay interval732may expire simultaneously such that the starting sensing threshold value716is adjusted to the minimum sensing threshold720without R-wave sensing threshold710being set to the intermediate sensing threshold value718before dropping to the minimum sensing threshold value720.

In another example, if the drop time interval734determined as a percentage of the pacing interval705is shorter than the sense delay interval732, the sense delay interval732may be truncated to be equal to the drop time interval734. Upon expiration of the post-pulse blanking interval, the R-wave sensing threshold710is set equal to the starting value716and held at that value until the simultaneous expiration of the drop time interval734and the sense delay interval732. The R-wave sensing threshold710may be adjusted from the starting value716to the minimum sensing threshold value720upon expiration of the drop time interval734and truncated sense delay interval732. The intermediate threshold value718may be skipped.

The pacing interval705may be set to various time intervals depending on the electrical stimulation therapy that is being delivered. As such, the drop time interval734may be determined as a percentage of the pacing interval705associated with the electrical stimulation therapy that is being delivered in accordance with a determined need for the therapy. Different minimum drop time intervals may be set for different electrical stimulation therapies. For example, the delay interval732, which may be set as a fixed interval longer than the post-pulse blanking interval730, may be longer following a CV/DF shock or post-shock pacing pulse than the delay interval following a bradycardia pacing pulse. If the minimum drop time interval is set to be equal to the delay interval732, the minimum drop time interval that may be set for controlling the R-wave sensing threshold following a CV/DF shock and/or a post-shock pacing pulse may be longer than the minimum drop time interval that may be set for controlling the R-wave sensing threshold following a bradycardia pacing pulse. For example, the minimum drop time interval post-shock and/or during post-shock pacing may be 800 ms, and the minimum drop time interval following a bradycardia pacing pulse, e.g., during VVI pacing, may be 500 ms. These minimum drop time intervals may equal corresponding delay intervals used during the respective electrical stimulation therapy. A maximum drop time interval may be set as a fixed value, e.g., up to 800 ms or more, and in some examples different fixed maximum drop time intervals may be stored in memory82to be used following different types of electrical stimulation pulses (e.g., following a shock pulse, a post-shock pacing pulse, a VVI pacing pulse, etc.)

When the drop time interval734determined as a percentage of the pacing interval705exceeds the delay interval732, the R-wave sensing threshold710is adjusted from the starting sensing threshold value716to the intermediate sensing threshold value718. R-wave sensing threshold710may be held at the second sensing threshold value718for the portion738of drop time interval734after expiration of sense delay interval732. Upon expiration of the drop time interval734, the R-wave sensing threshold710is adjusted to the minimum sensing threshold value720, or the “sensing floor,” if the cardiac electrical signal702has not yet crossed the R-wave sensing threshold710. The minimum sensing threshold value720may be set equal to the programmed sensitivity setting as described above, which may be, for example, 0.075 mV, 0.15 mV, 0.3 mV, 0.6 mV or higher. The R-wave sensing threshold710remains at the minimum sensing threshold720until the cardiac electrical signal702crosses the threshold710or pacing interval705expires, whichever occurs first. In the example shown, pacing interval705expires, and pacing pulse703is delivered.

FIG.15is a flow chart750of a method for controlling the R-wave sensing threshold following an electrical stimulation pulse. The methods described in conjunction with flow chart750generally relate to controlling the R-wave sensing threshold following a pacing pulse, however, the R-wave sensing threshold may be controlled in the same or similar manner following other electrical stimulation pulses, such as a CV/DF shock pulse or a T-wave shock for inducing ventricular tachyarrhythmia.

At block752, control circuit80establishes a maximum threshold limit. As described above, the maximum threshold limit may be set by determining a multiple of the programed sensitivity setting and/or adding a predetermined constant. In one example, the maximum threshold limit is set to eight times the programmed sensitivity setting plus 1.8. The maximum threshold limit determined at block752may be the same maximum threshold limit determined at block402ofFIG.9for establishing a maximum R-wave sensing threshold value that may be set following a sensed intrinsic R-wave. In some cases, the maximum threshold limit may be determined using a sensitivity-dependent gain. In other examples, the maximum threshold limit determined at block752may be determined differently than the maximum threshold limit determined at block402ofFIG.9.

The maximum R-wave sensing threshold limit determined at block752is used at block754to determine the starting and second, intermediate sensing threshold values based on the programmed sensitivity setting. The maximum R-wave sensing threshold determined at block402ofFIG.9sets a maximum limit of the starting R-wave sensing threshold value determined based on the peak amplitude determined during a post-sense blanking interval as described in conjunction withFIGS.5and9above.

Since the starting and second, intermediate threshold values used after an electrical stimulation pulse are not based on determined a peak amplitude of an R-wave, these values may be pre-determined, prior to a delivered pacing pulse. The starting and second threshold values used following a pacing pulse, therefore, do not need to be determined on a beat-by-beat basis during a blanking interval. The starting and second threshold values used following an intrinsic, sensed R-wave may be determined beat-by-beat since they are based on the peak R-wave amplitude determined during the post-sense blanking interval. The post-pulse starting and second threshold values used following an electrical stimulation pulse may be determined a single time for a given programmed sensitivity setting and may be re-determined only in response to a change in the programmed sensitivity setting.

The starting threshold value determined at block754may be a first percentage of the maximum threshold limit, and the second, intermediate threshold value may be a second percentage of the maximum threshold limit. As described above, the starting threshold value may be 31% of the maximum threshold limit and the intermediate threshold value may be 15% of the maximum threshold limit though other percentages or relative ratios of the maximum threshold limit (or programmed sensitivity setting) may be used.

A pacing pulse may be delivered at block756according to a programmed pacing protocol. The pacing pulse is delivered at a pacing interval following a preceding pacing pulse or sensed intrinsic R-wave. The pacing interval may be a lower rate interval set to control bradycardia pacing (e.g., VVI pacing) a post-shock back-up pacing rate interval, an ATP interval or other time interval used to control the delivery of an electrical stimulation pulse by therapy delivery circuit84.

Control circuit80determines the drop time interval at block758based on the pacing interval. The drop time interval may be set to a percentage of the pacing interval, e.g., one-half of the pacing interval. As described above, the drop time interval may be set to a predetermined percentage of the pacing interval but not less than a predetermined minimum interval. In some examples, the drop time interval may not be set less than the delay interval732shown inFIG.14. In other examples, if the drop time interval determined at block758is shorter than the sense delay interval, the sense delay interval is truncated to be equal to the drop time interval.

The drop time interval may be set to the predetermined percentage of the pacing interval but not more than a maximum drop time, which may be a fixed time interval, e.g., 600 to 750 ms. While the drop time is shown to be determined at block758after pacing pulse delivery, it is recognized that the drop time interval may be determined prior to pacing pulse delivery, while the pacing interval is running, in anticipation of a delivered pacing pulse and post-pulse R-wave sensing.

At block760, control circuit80sets timers or counters for controlling the multi-level R-wave sensing threshold following the pacing pulse. For example, the post-pulse blanking interval, which may be a digital blanking interval controlled by firmware stored in memory82, the sense delay interval, the determined drop time interval and a pacing interval may each be started upon delivery of the pacing pulse at block756. The post-pulse blanking interval may be longer than the blanking interval used after a sensed R-wave. The post-pulse blanking interval may be a digital blanking interval that is 250 ms long following a pacing pulse. The post-pulse blanking interval may be longer, e.g., 800 ms or longer, when the delivered electrical stimulation pulse is a CV/DF shock pulse. The post-pulse blanking interval may be programmable and set between 250 ms following a pacing pulse and up to 2 seconds following a shock pulse.

Upon expiration of the post-pulse blanking interval, as determined at block762, the sensing circuit86sets the starting R-wave sensing threshold at block764to the starting threshold value determined at block754. If the cardiac electrical signal crosses the starting R-wave sensing threshold, as determined at block766, the process returns to block406ofFIG.9where sensing circuit86produces an R-wave sensed event signal and restarts the various control time intervals for controlling the R-wave sensing threshold following a sensed R-wave as described above in conjunction withFIG.9.

If the sense delay interval expires at block768before the cardiac electrical signal crosses the R-wave sensing threshold, sensing circuit86adjusts the R-wave sensing threshold at block770to the second threshold value determined at block754. If the cardiac electrical signal received by sensing circuit86crosses the R-wave sensing threshold adjusted to the second threshold value, as determined at block772, the process advances to block406ofFIG.9to generate an R-wave sensed event signal and reset the post-sense R-wave sensing threshold control time intervals as described above. In the case of the drop time interval being set equal to the delay interval, such that both expire at block768, adjustment from the starting threshold to the second, intermediate threshold value at block770may be skipped and the process advances to block774.

If the drop time interval expires at block774without the cardiac electrical signal crossing the R-wave sensing threshold (block772), the sensing circuit86adjusts the R-wave sensing threshold to the minimum threshold value or sensing floor, which may be the programmed sensitivity setting, at block776. In other examples, more than two drop steps in the sensing threshold value may be implemented and adjusted at the expiration of respective drop time intervals, e.g., each set to different percentages of the pacing interval.

Sensing circuit86waits for the cardiac electrical signal to cross the sensing floor at block778and for the pacing interval to expire at block780, whichever occurs first. If the cardiac electrical signal crosses the minimum sensing threshold at block778, the process advances to block406ofFIG.9to generate an R-wave sensed event signal and control the R-wave sensing threshold using the post-sense R-wave sensing threshold control parameters. If the pacing interval expires without an R-wave sensing threshold crossing as determined at block780, a pacing pulse is delivered at block756. The process of flow chart750continues controlling the R-wave sensing threshold using the post-pace R-wave sensing threshold control parameters following the pacing pulse. The drop time may be re-determined at block758if the pacing interval has changed so that the drop time interval is adjusted dynamically with changes in pacing rate. The pacing interval may change as a result of user programming, rate-responsive pacing based on patient activity, an ATP protocol that uses different inter-pulse intervals, or a change in the electrical stimulation therapy being delivered.

FIG.16is a flow chart800of a method for controlling the R-wave sensing threshold by an implantable medical device according to another example. At block802, sensing circuit86of ICD14may sense cardiac events in a cardiac electrical signal based on cardiac event sensing threshold crossings by the received cardiac signal. Prior to detecting a need for an electrical stimulation therapy, control circuit80may control the cardiac event sensing threshold used by sensing circuit86at block802according the techniques for controlling the post-sense R-wave sensing threshold as described in conjunction with any ofFIGS.5-9.

ICD14may be configured to determine a need for a first electrical stimulation therapy at block804based on a rate of the sensed cardiac events, which may be a predetermined number of most recent sensed cardiac events. For example, ICD14may detect a tachyarrhythmia based on a required number of cardiac event intervals determined between consecutively sensed cardiac events, e.g., R-waves, falling into a tachyarrhythmia detection interval zone. The rate of the cardiac events may therefore lead to a tachyarrhythmia detection resulting in control circuit80determining a need for ATP and/or a CV/DF shock. In another example, the rate of sensed cardiac events may lead to control circuit80determining a need for a CV/DF shock and post-shock pacing in the case of a sensed cardiac event not occurring during a post-shock pacing interval following the CV/DF shock.

If a need for a first electrical stimulation therapy is determined, control circuit80determines the drop time interval at block806for use following an electrical stimulation pulse delivered according to the first stimulation therapy. The drop time interval may be determined at block806as a percentage of a pacing interval of the first stimulation therapy, e.g., as a percentage of a post-shock pacing interval when a need for a CV/DF shock is determined or as a percentage of an ATP interval when a need for ATP is determined.

The determined drop time interval may be compared to a first minimum drop time interval corresponding to the first stimulation therapy. The percentage of the pacing interval used to determine the drop time interval at block806may be a programmable value. The sense delay interval set in response to a delivered electrical stimulation pulse during the first stimulation therapy may be a fixed or programmable value. In some instances the drop time interval determined at block806as a percentage of a pacing interval of the first stimulation therapy may be less than the sense delay interval. The first minimum drop time interval may be equal to the sense delay interval used to control the cardiac event sensing threshold during the first stimulation therapy in some examples. In other examples, the first minimum drop time interval compared to the determined drop time interval at block808may be greater or less than the sense delay interval associated with the first stimulation therapy.

At block810, control circuit80sets the drop time interval to the greatest one of the first minimum drop time interval and the determined drop time interval. The process returns to block802to sense cardiac events according to the post-pulse cardiac event sensing threshold control parameters during the first stimulation therapy, until an intrinsic cardiac event is sensed. If the minimum drop time interval is set equal to or less than the sense delay interval at block810, the control circuit80may control the sensing circuit86to hold the cardiac event sensing threshold at a first threshold value set equal to the starting threshold value determined based on the maximum threshold limit for the portion of the drop time interval extending from the expiration of the post-pulse blanking period until the expiration of the drop time interval. If the minimum drop time interval is greater than the sense delay interval, the control circuit80may control sensing circuit86to hold the cardiac event sensing threshold at a starting threshold value set to a first percentage of the maximum threshold limit until the expiration of the sense delay interval and hold the sensing threshold at a next, lower threshold value set to a second percentage of the maximum threshold limit from the expiration of the delay interval until the expiration of the drop time interval.

If the determined drop time interval is greater than the first minimum drop time interval associated with the first stimulation therapy, “no” branch of block808, control circuit80sets the drop time interval to the determined drop time interval at block812and returns to block802to control the cardiac event sensing threshold according to the post-pulse cardiac event sensing threshold control parameters corresponding to the first stimulation therapy. The sensing circuit86is configured to hold the cardiac event sensing threshold at a predetermined percentage of the maximum threshold limit during the drop time interval. The cardiac event sensing threshold is held at the predetermined percentage of the maximum threshold for the portion of the drop time interval extending from the expiration of the sense delay interval until expiration of the drop time interval in some examples. The cardiac event sensing threshold may be set to a starting threshold value set as a first percentage of the maximum threshold limit during a delay interval and be held at the second threshold set to a second, lower percentage of the maximum threshold limit during the drop time interval. In this way, the cardiac event sensing threshold is equal to or greater than second threshold value from the expiration of the post-pulse blanking period until expiration of the drop time interval.

In some examples, control circuit80is configured to determine a need for a second electrical stimulation therapy. If a need for the first stimulation therapy is not detected, “no” branch of block84, control circuit80may determine a need for the second electrical stimulation therapy at block814. The need for the second electrical stimulation therapy may also be based on a rate of sensed cardiac events. For example, if a cardiac event is not sensed during a pacing interval such that the rate of sensed cardiac events is less than a programmed pacing rate, control circuit80determines a need for the second electrical stimulation therapy, e.g., a ventricular pacing pulse.

Determining a need for an electrical stimulation therapy at block804or814may be performed by control circuit80according to any implemented arrhythmia detection algorithm or as generally disclosed in any of the incorporated references. If control circuit80does not determine a need for an electrical stimulation therapy at either of blocks804or814, sensing circuit86continues to sense cardiac events at block802based on an R-wave sensing threshold controlled according to the post-sense control parameters as described herein, e.g., in conjunction with any ofFIGS.5-9.

Control circuit80is configured to respond to determining the need for the second electrical stimulation therapy by determining a drop time interval at block816based on a pacing interval of the second electrical stimulation therapy. The drop time interval determined as a percentage of the pacing interval of the second electrical stimulation therapy is compared to a second minimum drop time interval at block818. The second minimum drop time interval may be different than the first minimum drop time interval. For example, the sense delay interval used following an electrical stimulation pulse may be set to a post-pulse blanking interval plus a predetermined time interval, e.g., 250 ms. The post-pulse blanking interval may be longer during the first stimulation therapy, e.g., up to 800 ms, up to 1,500 ms or even up to two seconds, compared to the post-pulse blanking interval used during the second stimulation therapy, e.g., up to 250 ms. The post-pulse sense delay interval during the first stimulation therapy may be 1000 ms or more, for example and the post-pulse sense interval during the second stimulation therapy may be 500 ms, for example. The first minimum drop time interval may be set to the first sense delay interval, and the second minimum drop time interval may be set to the second sense delay interval, which may be shorter than the first delay interval.

The control circuit80is configured to set the drop time interval to the greatest one of the second minimum drop time interval (at block820) and the drop time interval determined as a percentage of a pacing interval of the second stimulation therapy (at block822). The process returns to block802to control the cardiac event sensing threshold according to the set drop time interval and other post-pulse cardiac event sensing threshold control parameters associated with the second stimulation therapy.

For example, control circuit80controls sensing circuit86to hold the R-wave sensing threshold at a threshold value determined as a percentage of the maximum threshold limit during the set drop time interval. As described above, the sensing circuit86may set the R-wave sensing threshold to a starting threshold value set as a first percentage of the maximum threshold limit after expiration of a post-pulse blanking interval, adjust the R-wave sensing threshold to a second threshold value determined as a second percentage of the maximum threshold limit upon expiration of the sense delay interval, hold the R-wave sensing threshold at the second threshold value until the expiration of the set drop time interval, and adjust the R-wave sensing threshold to a minimum sensing threshold value upon expiration of the set drop time interval, as long as the cardiac electrical signal does not cross the R-wave sensing threshold. If an intrinsic cardiac event is sensed at block802, control circuit80and sensing circuit86return to controlling the cardiac event sensing threshold according to post-sense control parameters.

In this way, control of the cardiac event sensing threshold may alternate between post-sense control parameters and post-pulse control parameters corresponding to one or more electrical stimulation therapies. In each of these situations, the drop time interval used to control adjustment from a threshold value to the minimum sensing threshold may be dynamically determined based on a heart rate, which may be a sensed intrinsic rate or a paced rate. When the drop time interval is based on a paced rate after determining a need for an electrical stimulation therapy, the drop time interval may be determined based on a pacing interval of an electrical stimulation therapy pulse that is scheduled or has been delivered. In other instances, the control of the cardiac event sensing threshold may alternate between post-sense control parameters which have fixed drop times and post-pulse control parameters corresponding to one or more electrical stimulation therapies that utilize dynamic drop times based on pacing interval or other heart rate intervals.

Thus, a method and apparatus for controlling a cardiac event sensing threshold by an IMD system have been presented in the foregoing description with reference to specific embodiments. In other examples, various methods described herein may include steps performed in a different order or combination than the illustrative examples shown and described herein. It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the disclosure and the following claims.