Device for charge balancing during delivery of electrical stimulation

Abstract

The medical device is configured to deliver a series of electrical stimulation pulses including pulses of opposite polarity. The medical device delivers a charge balancing pulse by modifying every nth pulse in the electrical stimulation pulses to reduce a net charge delivered with the series of electrical stimulation pulses. In some examples, the medical device may be an implantable medical device coupled to a cardiovascular external lead for delivering cardiac pacing pulses.

Description

Technical Field

[0001] The present disclosure relates to a medical device and method for charge balancing electrical stimulation pulses, such as cardiac pacing pulses, delivered to a patient. Background Art

[0002] There are various medical devices for delivering electrical stimulation therapy, monitoring the physiological condition of a patient, or a combination thereof. Some of these medical devices include implantable medical devices (IMDs) that deliver electrical stimulation pulses using implanted electrodes. In some examples, the IMD may employ one or more elongated electrical leads carrying stimulation electrodes, sensing electrodes, and / or other sensors. The IMD may deliver therapy to various organs, nerves, muscles, or tissues (such as the heart, brain, stomach, spinal cord, pelvic floor, etc.) or monitor their conditions. Implantable medical electrical leads may be configured to position electrodes or other sensors at desired locations to deliver electrical stimulation or sense physiological conditions. For example, electrodes or sensors may be carried along the distal portion of a lead extending subcutaneously, under a muscle, in the chest, in the abdomen, in the cranium, or through a vein. The proximal portion of the lead may be coupled to the housing of the IMD, thereby providing an electrical connection to a circuit system (such as a signal generation circuit system and / or a sensing circuit system) contained within the housing.

[0003] Some IMDs, such as pacemakers or implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to the patient's heart via electrodes carried by one or more implantable leads and / or the housing of the pacemaker or ICD. The leads can be transvenous, for example, advanced into the heart through one or more veins to position the endocardial electrodes in close contact with the heart tissue. Other leads can be non-transvenous leads implanted outside the heart (e.g., in the epicardium, pericardium, or subcutaneously). The electrodes are used to deliver electrical pulses to the heart to address abnormal heart rhythms.

[0004] An IMD capable of delivering electrical pulses for treating abnormal heart rhythms typically senses a signal representing the intrinsic depolarization of the heart and analyzes the sensed signal to identify an abnormal rhythm. Upon detecting an abnormal rhythm, the device can deliver appropriate electrical stimulation therapy to restore a more normal rhythm. For example, a pacemaker or ICD can deliver pacing pulses to the heart upon detecting bradycardia or tachycardia. An ICD can deliver high-voltage cardioversion or defibrillation shocks to the heart upon detecting rapid ventricular tachycardia or fibrillation. Summary of the invention

[0005] The technology disclosed herein generally relates to a medical device and method for charge balancing electrical stimulation pulses (such as cardiac pacing pulses or neural stimulation pulses) delivered to a patient's body tissue via one or more electrodes. Electrode corrosion can occur when the electrical stimulation pulses delivered by the electrodes deliver a net charge that accumulates over time. This effect can be minimized by charge balancing the electrical stimulation pulses, thereby reducing or eliminating the net charge accumulated due to the delivered pulses. A medical device operating according to the technology disclosed herein controls the polarity, amplitude, and pulse width of each electrical stimulation pulse to provide charge balance within a series of electrical stimulation pulses.

[0006] In one example, the present disclosure provides a medical device having a therapy delivery circuit and a control circuit. The therapy delivery circuit is configured to generate an electrical stimulation pulse and deliver the electrical stimulation pulse via an electrode coupled to the therapy delivery circuit. The control circuit is coupled to the therapy delivery circuit and is configured to control the therapy delivery circuit to deliver an electrical stimulation pulse comprising electrical stimulation pulses of opposite polarity and to deliver a charge balancing pulse by modifying every nth pulse in the electrical stimulation pulse. The charge balancing pulse is delivered to reduce a net charge delivered with the electrical stimulation pulse.

[0007] In another example, the present disclosure provides a method performed by a medical device. The method includes delivering an electrical stimulation pulse including electrical stimulation pulses of opposite polarity, and delivering a charge balancing pulse by modifying every nth pulse in the electrical stimulation pulse. The charge balancing pulse is delivered to reduce a net charge delivered with the electrical stimulation pulse.

[0008] In yet another example, the present disclosure provides a non-transitory computer-readable medium storing a set of instructions that, when executed by control circuitry of a medical device, cause the medical device to: deliver an electrical stimulation pulse comprising electrical stimulation pulses of opposite polarity, and deliver a charge balancing pulse by modifying every nth pulse in the electrical stimulation pulse. The charge balancing pulse is delivered to reduce a net charge delivered with the electrical stimulation pulse.

[0009] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1A and Figure 1B is a conceptual diagram of an extracardiovascular ICD system according to an example.

[0011] Figure 2A and Figure 2B 2 are front and side views, respectively, of a patient implanted with an ICD system according to another example.

[0012] Figure 3 is a schematic diagram of an ICD according to an example.

[0013] Figure 4 is a schematic diagram of a high voltage therapy circuit coupled to a processor and an HV therapy control circuit.

[0014] Figure 5 is a conceptual diagram of a low voltage therapy circuit according to an example.

[0015] Figure 6 is a conceptual diagram of a series of alternating polarity pacing pulses including charge-balanced pulses.

[0016] Figure 7 is a diagram of a cardiac pacing pulse delivered according to another example technique for charge balancing.

[0017] Figure 8 is a diagram of another technique for delivering charge balanced cardiac pacing pulses.

[0018] Fig. 9 is a flow chart of a method for delivering charge balanced cardiac pacing pulses according to one example.

[0019] Fig.10 is a conceptual diagram of one example of a lookup table that may be stored in the memory of an ICD.

[0020] Fig.11 is a flow chart of a modified method for establishing a charge balanced pacing pulse by an ICD according to one example. DETAILED DESCRIPTION

[0021] In general, the present disclosure describes techniques for charge balancing electrical stimulation pulses delivered to a patient. For example, the technique may be particularly useful in the delivery of cardiac pacing pulses, but may be applied in the delivery of neural stimulation pulses or any other electrical stimulation pulses delivered to a patient's body tissue via medical electrodes. The delivered electrical stimulation pulses may be delivered for therapeutic purposes, such as cardiac pacing pulses, but in some examples may be delivered for diagnostic or testing purposes. The disclosed charge balancing techniques may be implemented in any medical device configured to deliver electrical stimulation pulses, particularly electrical stimulation pulses that are repeatedly delivered over time resulting in accumulated charge at the stimulation electrode(s). In some examples, the charge balancing techniques are used during the delivery of repetitive opposite polarity electrical stimulation pulses that have an inherent imbalance between the charge delivered by the positive polarity pulses and the charge delivered by the negative polarity pulses.

[0022] Charge-balanced electrical stimulation pulses are intended to deliver net zero or nearly net zero charge over time. Charge-unbalanced electrical stimulation pulses, i.e., electrical stimulation pulses in which non-net zero charge accumulates at the tissue-electrode interface over time, can cause electrode corrosion and lead to polarization artifacts that can interfere with sensing of cardiac electrical signals. Charge accumulation can occur when electrical stimulation pulses are unbalanced during the delivery of relatively high energy pulses (e.g., high current or voltage amplitude and / or long pulse duration) and relatively low energy pulses (e.g., low current or voltage amplitude and / or short pulse duration).

[0023] When the cardiac capture threshold is high, a relatively high pulse energy pulse is delivered for cardiac pacing. The capture threshold is the lowest pulse energy that captures the heart and causes an extrinsic depolarization or induced response. Factors that may contribute to a high cardiac capture threshold include pacing lead impedance and electrode position. For example, the cardiac capture threshold is significantly higher when the heart is paced using electrodes carried by non-transvenous cardiovascular external leads than when the heart is paced by endocardial electrodes carried by transvenous leads (which are in close contact with myocardial tissue). As used herein, the term "extra-cardiovascular" refers to a location outside the blood vessels, heart, and pericardium surrounding the patient's heart. Implantable electrodes carried by cardiovascular external leads may be positioned outside the chest (outside the thorax and sternum) or inside the chest (below the thorax or sternum), but may not be in close contact with myocardial tissue. Therefore, the charge balancing techniques disclosed herein may be used in some examples to deliver extra-cardiovascular pacing pulses or any pacing pulses delivered using electrodes that are not in direct contact with the heart.

[0024] Even relatively low energy electrical stimulation pulses (e.g., cardiac pacing pulses delivered using endocardial or epicardial electrodes in direct contact with the heart) can cause electrode erosion over time if the pulses are unbalanced. Charge balancing the pacing pulses can minimize these effects. In endocardial or epicardial pacing applications, the effects of unbalanced charge delivery can be mitigated by delivering a non-therapeutic recharge pulse of opposite polarity to the pacing pulse during the physiological refractory period of the myocardium following the pacing pulse. The recharge pulse of opposite polarity may not be exactly charge equivalent to the pacing pulse, but electrode erosion can be substantially avoided because in endocardial or epicardial pacing, the charge accumulation over time is small due to the relatively small charge delivered during each pulse. In an extracardiac cardiac pacing system, the pacing pulse energy delivered to capture the heart can be on the order of twice, or even 10 to 100 times, that during endocardial pacing. For example, endocardial pacing pulses may have amplitudes of up to 8 volts and pulse widths of up to 1.5 ms, and more typically have amplitudes of less than 5 volts (e.g., 1 V) and pulse widths of less than 0.5 ms. Extracardiovascular pacing pulses, on the other hand, may have amplitudes of 10 to 40 volts and pulse widths of up to 8 ms. Thus, in the absence of charge balancing, corrosive processes occurring on pacing electrodes during extracardiovascular pacing may be amplified 100 times or more compared to during endocardial pacing. Because the non-therapeutic recharge pulses that may be applied during endocardial pacing applications are typically not exactly charge equivalent to the therapeutic pacing pulses, techniques using recharge pulses may not provide adequate charge balancing in all applications, particularly during delivery of high pulse energy electrical stimulation pulses, due to the greater charge accumulation and corrosive effects that may occur over time.

[0025] One way to provide charge balance in high voltage extracardiovascular pacing applications is to deliver therapeutic pulses with alternating polarities so as to provide charge balance within each pair of pacing cycles. Examples of charge balanced pacing pulses delivered by high voltage pacing circuits are generally disclosed in U.S. Patent Application No. 15 / 425,169 (Grinberg et al.), filed on February 6, 2017. However, in some examples, there may be an inherent imbalance of pulses of opposite polarity, such that alternating therapeutic pulses may still result in a net charge accumulation over time and an associated risk of electrode corrosion, particularly in high voltage pacing applications such as extracardiovascular pacing. For these reasons, charge balancing for maintaining the integrity of the pacing system during relatively high voltage (or high current amplitude) pacing applications presents a greater challenge than during endocardial pacing applications using much lower pacing pulse voltage (or current) amplitudes. Disclosed herein are charge balancing techniques for providing cardiac pacing by an IMD while preventing or minimizing electrode corrosion over time. These techniques may include delivering a series of cardiac pacing pulses having opposite polarities, in some examples alternating positive and negative polarity pulses, with each nth pacing pulse in the series of cardiac pacing pulses being modified to balance the net charge delivered during the previous one or more pulses in the series of cardiac pacing pulses.

[0026] Figure 1A and Figure 1B is a conceptual diagram of an extracardiovascular ICD system 10 according to one example. Figure 1A is a front view of ICD system 10 implanted in patient 12 . Figure 1B 1 is a cross-sectional view of an ICD system 10 implanted in a patient 12. The ICD system 10 includes an ICD 14 connected to an extracardiac electrical stimulation and sensing lead 16. As used herein, an "external cardiovascular lead" refers to a lead that is implanted outside of the heart and blood vessels of the patient's cardiovascular system. For example, the extracardiac leads may extend subcutaneously, under the muscle, or within the chest. The ICD system 10 is described in the context of being able to provide cardioversion / defibrillation shocks and pacing pulses. Figure 1A and Figure 1B . However, it is contemplated that the techniques disclosed herein for providing charge balance of cardiac pacing pulses may be implemented in systems that are not configured for delivering electrical shock therapy. The disclosed charge balanced cardiac pacing techniques may be implemented in any medical device configured to deliver cardiac pacing pulses, and may also be beneficial in any medical device that delivers electrical stimulation pulses, particularly when alternating polarity pulses (or more generally, a series of pulses including pulses of opposite polarity) are delivered that have an inherent imbalance in the charge delivered by the positive and negative pulses.

[0027] The ICD 14 includes a housing 15 that forms an airtight seal that protects the internal components of the ICD. The internal components include a pulse generating circuit system that may include: a high voltage therapy circuit for generating high voltage CV / DF shocks and high voltage pacing pulses; and a low voltage therapy circuit for generating relatively low voltage pacing pulses delivered using an external cardiovascular electrode carried by a lead 16. The housing 15 of the ICD 14 may be formed of a conductive material such as titanium or a titanium alloy. The housing 15 may serve as a housing electrode (sometimes referred to as a can electrode). In the examples described herein, the housing 15 may be used as an active can electrode for use in delivering CV / DF shocks or other high voltage pulses (including cardiac pacing pulses) delivered using a high voltage therapy circuit. In other examples, the housing 15 may be used in unipolar, cardiac pacing pulses delivered using a low voltage therapy circuit in conjunction with a lead-based cathode electrode. In other cases, the housing 15 of the ICD 14 may include a plurality of electrodes on an external portion of the housing. The outer portion(s) of the housing 15 that serve as the electrode(s) may be coated with a material, such as titanium nitride, for reducing post-stimulation polarization artifacts.

[0028] ICD 14 includes a connector assembly 17 (also referred to as a connector block or header) that includes electrical feedthroughs through housing 15 to provide electrical connections between conductors extending within lead bodies 18 of leads 16 and electronic components included within housing 15 of ICD 14. As will be described in further detail herein, housing 15 may house one or more processors, memory, transceivers, sensors, electrical cardiac signal sensing circuitry, therapy delivery circuitry, power supplies, and other components for sensing cardiac electrical signals, detecting cardiac rhythms, and controlling and delivering electrical stimulation pulses to treat abnormal cardiac rhythms.

[0029] Lead 16 includes an elongated lead body 18 having a proximal end 27 including a lead connector (not shown) configured to connect to ICD connector assembly 17 and a distal portion 25 including one or more electrodes. Figure 1A and Figure 1B In the example shown, the distal portion 25 of the lead 16 includes defibrillation electrodes 24 and 26 and pacing / sensing electrodes 28 and 30. In some cases, the defibrillation electrodes 24 and 26 can form a defibrillation electrode together because they can be configured to be activated at the same time. Alternatively, the defibrillation electrodes 24 and 26 can form separate defibrillation electrodes, in which case each of the electrodes 24 and 26 can be activated independently.

[0030] Electrodes 24 and 26 (and in some examples, housing 15) are referred to herein as defibrillation electrodes because they are used, alone or together, to deliver high voltage stimulation therapy (e.g., cardioversion or defibrillation shock). Electrodes 24 and 26 may be elongated coil electrodes and generally have a relatively large surface area compared to the surface area of ​​pacing and sensing electrodes 28 and 30 (which may be used to deliver cardiac pacing pulses having a lower pulse energy than CV / DF shock pulses) for delivering high voltage shock therapy pulses. However, electrodes 24 and 26 and housing 15 may also be used to provide pacing functions, sensing functions, or both pacing and sensing functions in addition to or in lieu of high voltage CV / DF shock therapy. In this sense, the use of the term "defibrillation electrodes" herein should not be viewed as limiting electrodes 24 and 26 to use only for CV / DF shock therapy applications. Electrodes 24 and 26 may be used in a pacing electrode vector for delivering extracardiac pacing pulses, such as, but not limited to, bradycardia pacing pulses, anti-tachycardia pacing (ATP) pulses, entrainment pulses for tachyarrhythmia induction, cardiac resynchronization therapy (CRT) pulses, and / or post-shock pacing pulses. Electrodes 24 and 26 may also be used in a sensing vector for sensing cardiac electrical signals and detecting abnormal heart rhythms, such as ventricular tachycardia (VT) and ventricular fibrillation (VF).

[0031] Electrodes 28 and 30 are electrodes with relatively small surface areas for delivering relatively low voltage pacing pulses and for sensing cardiac electrical signals. Electrodes 28 and 30 are referred to herein as pacing / sensing electrodes because they are generally configured for relatively lower voltage applications than defibrillation electrodes 24 and 26. Electrodes 28 and 30 can be used as cathodes or anodes for delivering pacing pulses (including any of the examples given above), and / or sensing cardiac electrical signals. In some instances, electrodes 28 and 30 can provide only pacing functions, only sensing functions, or both pacing functions and sensing functions. Although electrodes 28 and 30 are generally referred to as being used to deliver relatively low voltage pacing pulses as compared to the high voltage CV / DF shock pulses delivered by defibrillation electrodes 24 and 26, it should be understood that the voltage amplitude and / or pulse width of cardiac pacing pulses delivered using any combination of extracardiovascular electrodes 24, 26, 28, 30 and housing 15 is generally higher than the voltage amplitude and / or pulse width of cardiac pacing pulses delivered using endocardial or epicardial electrodes. Thus, the extracardiovascular pacing pulses delivered by ICD system 10 are considered relatively high voltage pacing pulses (as compared to pacing pulses delivered by transvenous ICD systems) and can be controlled using the charge balancing techniques disclosed herein.

[0032] exist Figure 1AIn the example shown, electrode 28 is located proximal to defibrillation electrode 24, and electrode 30 is located between defibrillation electrodes 24 and 26. In other examples, electrodes 28 and 30 may be positioned at other locations along lead 16, and they may include one or more pacing / sensing electrodes. Electrodes 28 and 30 are shown as ring electrodes, however, electrodes 28 and 30 may include any of a variety of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, etc.

[0033] Lead 16 extends subcutaneously or submuscularly from connector assembly 17 of ICD 14 over thorax 32 centrally toward the center of the torso of patient 12 (e.g., toward xiphoid process 20 of patient 12). At a location near xiphoid process 20, lead 16 bends or turns in a substernal position within anterior mediastinum 36 and extends superiorly. Figure 1B The anterior mediastinum 36 (as seen in FIG. 1 ) can be considered to be bounded laterally by the pleura 39, from behind by the pericardium 38, and from the front by the sternum 22. In some instances, the anterior wall of the anterior mediastinum 36 can also be formed by the transverse thoracic muscle and one or more costal cartilages. The anterior mediastinum 36 includes a certain amount of loose connective tissue (such as, cellulite), adipose tissue, some lymphatic vessels, lymph glands, substernal musculature, small side branches of the internal thoracic artery or vein, and the thymus. In one example, the distal portion 25 of the lead 16 extends substantially along the posterior side of the sternum 22 within the loose connective tissue and / or substernal musculature of the anterior mediastinum 36. A lead that is implanted so that the distal portion 25 is substantially within the anterior mediastinum 36 can be referred to as a "substernal lead."

[0034] exist Figure 1A and Figure 1B In the example shown, lead 16 is substantially centrally located below sternum 22. However, in other examples, lead 16 may be implanted such that it is laterally offset from the center of sternum 22. In some examples, lead 16 may extend laterally such that a distal portion 25 of lead 16 is below / under thorax 32 in addition to or in lieu of sternum 22. In other examples, distal portion 25 of lead 16 may be implanted in other extra-cardiovascular, intra-thoracic locations, including the pleural cavity or the periphery of pericardium 38 surrounding heart 8 and adjacent to pericardium 38 of heart 8. Other implant locations and lead and electrode arrangements that may be used in conjunction with the cardiac pacing techniques described herein are generally disclosed in U.S. Patent Application Publication No. 2015 / 0306375 (Marshall et al.) and U.S. Patent No. 9,855,414 (Marshall et al.). For example, as Figure 2A and Figure 2BAs shown, lead 16 can extend upwardly and subcutaneously or submuscularly above the thorax and / or sternum 22 (rather than below the sternum). Alternatively, lead 16 can be placed along other subcutaneous or submuscular paths. The path of lead 16 can depend on the location of ICD 14, the arrangement and location of electrodes carried by lead distal portion 25, and / or other factors.

[0035] Lead body 18 can be a flexible lead body without preformed curves or bends. In other examples, a cardiovascular external lead including one or more defibrillation electrodes and / or one or more pacing and sensing electrodes can be carried by a lead body having one or more preformed curves or bends or a meandering, wavy, or zigzag distal portion of lead body 18. Examples of other cardiovascular external leads that can be implemented with the technology described herein are generally disclosed in U.S. Patent Application Publication No. 2016 / 0158567 (Marshall et al.). However, the technology disclosed herein is not limited to any particular electrode arrangement or lead body design.

[0036] Electrical conductors (not shown) extend from lead proximal end 27 through one or more lumens of elongated lead body 18 of lead 16 to electrodes 24, 26, 28, and 30 positioned along distal portion 25 of lead body 18. The conductors electrically couple electrodes 24, 26, 28, and 30 to circuitry (such as therapy delivery circuitry and / or sensing circuitry) of ICD 14 via connections in connector assembly 17, including associated electrical feedthroughs through housing 15. The electrical conductors transmit therapy from therapy delivery circuitry within ICD 14 to one or more of defibrillation electrodes 24 and 26 and / or pacing / sensing electrodes 28 and 30, and transmit cardiac electrical signals generated by the patient's heart 8 from one or more of defibrillation electrodes 24 and 26 and / or pacing / sensing electrodes 28 and 30 to sensing circuitry within ICD 14.

[0037] The ICD 14 may obtain cardiac electrical signals corresponding to the electrical activity of the heart 8 via a combination of sensing vectors including a combination of electrodes 28 and 30. In some examples, the housing 15 of the ICD 14 is used in combination with one or more of the electrodes 28 and / or 30 in the sensing electrode vector. The ICD 14 may even obtain cardiac electrical signals using a sensing vector including one or both of the defibrillation electrodes 24 and / or 26 (e.g., between electrodes 24 and 26, or between one of the electrodes 24 or 26 and one of the electrodes 28, 30 and / or the housing 15 in combination).

[0038] ICD 14 analyzes cardiac electrical signals received from one or more of the sensing vectors to monitor abnormal heart rhythms, such as bradycardia, VT, and VF. ICD 14 can analyze the heart rate and / or the morphology of the cardiac electrical signals to monitor tachyarrhythmias based on any of a variety of tachyarrhythmia detection techniques. ICD 14 generates and delivers electrical stimulation therapy in response to detection of a tachyarrhythmia (e.g., VT or VF). ICD 14 can deliver ATP in response to VT detection, and in some cases, can deliver ATP before CV / DF shock or during charging of a high voltage capacitor in an attempt to avoid the need to deliver CV / DF shock. ATP can be delivered using an extracardiac pacing electrode vector selected from electrodes 24, 26, 28, 30 and / or any of the electrodes in housing 15.

[0039] In the event that ATP is not delivered or does not successfully terminate VT or when VF is detected, ICD 14 may deliver one or more CV / DF shocks via one or both of defibrillation electrodes 24 and 26 and / or housing 15. ICD 14 may deliver CV / DF shocks using electrodes 24 and 26 alone, or using electrodes 24 and 26 together as the cathode (or anode) and housing 15 as the anode (or cathode).

[0040] Bradycardia pacing pulses may be delivered in response to a slow heart rate or asystole. The ICD 14 may generate and deliver cardiac pacing pulses, such as post-shock pacing pulses or bradycardia pacing pulses, using a pacing electrode vector including electrodes 24, 26, 28, 30 and one or more of the housing 15 of the ICD 14. The timing of the pacing pulses may be controlled by a controller included in the ICD 14 by setting a pacing escape interval timer or counter according to a lower pacing rate or a backup pacing interval. The charge balancing techniques disclosed herein may be used to control bradycardia pacing, post-shock pacing, CRT, atrial tracking ventricular pacing pulses, or any other type of pacing therapy delivered by the ICD 14.

[0041] External device 40 is shown in telemetric communication with ICD 14 via communication link 42. External device 40 may include a processor, a display, a user interface, a telemetry unit, and other components for communicating with ICD 14 for transmitting and receiving data via communication link 42. A radio frequency (RF) link such as Wi-Fi or Medical Implant Communications Service (MICS) or other RF or selected communications band) establishes a communications link 42 between the ICD 14 and the external device 40.

[0042] External device 40 may be implemented as a programmer used in a hospital, clinic, or physician's office to retrieve data from ICD 14 and program operating parameters and algorithms in ICD 14 for controlling ICD functions. External device 40 may be used to program cardiac rhythm detection parameters and therapy control parameters used by ICD 14. External device 40 may be used to program control parameters for generating and delivering cardiac pacing pulses according to the charge balancing techniques disclosed herein into ICD 14.

[0043] Data stored or acquired by ICD 14, including physiological signals or associated data derived therefrom, results of device diagnostics, and a history of detected rhythm episodes and delivered therapies, may be retrieved from ICD 14 by external device 40 following an interrogation command. External device 40 may alternatively be implemented as a home monitor or a handheld device.

[0044] Figure 2A and Figure 2B The front view and side view of a patient implanted with an ICD system 10 are shown in FIG. Figure 1A-1B In some examples, extracardiovascular electrical stimulation therapy can be delivered using electrodes that remain outside the chest. Figure 2A In the example of the embodiment, the distal portion 25 of the lead 16 extends over the thorax 32, laterally offset from the sternum 22 and substantially parallel to the sternum 22. In other examples, the lead 16 may extend over the sternum 22, or may extend diagonally over the sternum 22 and a portion of the thorax 32.

[0045] Figures 1A-2B The embodiments of the present invention are exemplary in nature and should not be considered to limit the practice of the technology disclosed herein. For example, ICD 14 is shown as being implanted subcutaneously on the left side of patient 12 along thorax 32. In other examples, ICD 14 may be implanted between the left posterior axillary line and the left anterior axillary line of patient 12. ICD 14 may be implanted at other subcutaneous or submuscular locations in the body of patient 12, such as in a subcutaneous pocket in the chest area. In this case, lead 16 may extend from ICD 14 toward the manubrium of sternum 22 subcutaneously or submuscularly, and bend or turn from the manubrium subcutaneously or submuscularly and extend downward to a desired position. In yet another example, ICD 14 may be placed in the abdomen. Lead 16 may also be implanted in other extracardiac locations and include other electrode and lead body structures.

[0046] Figure 3 is a schematic diagram of an ICD 14 according to an example. Figure 3The electronic circuit system within the ICD 14 (schematically shown as can electrodes in FIG. 1 ) includes software, firmware, and hardware that cooperatively monitor one or more cardiac electrical signals, determine when pacing therapy is needed, and provide prescribed pacing therapy as needed. The software, firmware, and hardware are also configured to determine when CV / DF shock is needed and deliver the prescribed CV / DF shock therapy. The ICD 14 is coupled to cardiovascular external leads such as lead 16 carrying external cardiovascular electrodes 24, 26, 28, and 30 for delivering pacing therapy, CV / DF shock therapy, and sensing cardiac electrical signals.

[0047] ICD 14 includes control circuitry 80, memory 82, therapy delivery circuitry 84, cardiac electrical signal sensing circuitry 86, and telemetry circuitry 88. ICD 14 may include impedance measurement circuitry 90 for delivering drive signals across the pacing electrode vectors and measuring the resulting voltage to determine the electrical impedance of the pacing electrode vectors.

[0048] Power supply 98 provides power as needed to the circuitry of ICD 14 (including each of circuits 80, 82, 84, 86, 88, and 90). Power supply 98 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. Connections between power supply 98 and each of the other circuits 80, 82, 84, 86, and 88 will be based on the Figure 3 For example, the power supply 98 is coupled to a low voltage (LV) charging circuit and a high voltage (HV) charging circuit included in the therapy delivery circuit 84 for charging the LV capacitor and the HV capacitor, respectively, or is coupled to other energy storage devices included in the therapy delivery circuit 84 for generating electrical stimulation pulses.

[0049] Figure 3 The functional blocks shown represent example functions that may be included in ICD 14. ICD 14 may include Figure 3More or fewer components as shown. The ICD 14 may include any discrete and / or integrated electronic circuit components that implement analog and / or digital circuits capable of producing the functions attributed to the ICD 14 herein, which may include one or more of the following: an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, a state machine, or other suitable components that provide the described functions. The specific form of software, hardware, and / or firmware used to implement the functions disclosed herein will be determined primarily by the specific system architecture employed in the device and the specific detection and therapy delivery methods employed by the ICD 14. In view of the disclosure herein, it is within the capabilities of those skilled in the art to provide software, hardware, and / or firmware for implementing the described functions in the context of any modern IMD system.

[0050] Memory 82 may include any volatile, nonvolatile, magnetic, or electrically non-transitory computer-readable storage medium, such as random access memory (RAM), read-only memory (ROM), nonvolatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, or any other memory device. In addition, memory 82 may include a non-transitory computer-readable medium that stores instructions that, when executed by one or more processing circuits, cause control circuit 80 or other ICD circuits to perform various functions attributed to ICD 14 or those ICD circuits. The non-transitory computer-readable medium that stores instructions may include any of the media listed above.

[0051] The functions attributed to ICD 14 may be performed by separate hardware, firmware, or software components, or integrated within common hardware, firmware, or software components. For example, cardiac pacing operations may be performed by therapy delivery circuitry 84 under the control of control circuitry 80, and may include operations implemented in a processor executing instructions stored in memory 82.

[0052] The control circuit 80 communicates with the therapy delivery circuit 84 and the sensing circuit 86 for sensing cardiac electrical activity, detecting cardiac rhythm, and controlling the delivery of cardiac electrical stimulation therapy in response to sensed cardiac signals. The therapy delivery circuit 84 and the electrical sensing circuit 86 are each electrically coupled to Figures 1A-2B Any or all of the electrodes 24, 26, 28 and 30 shown carried by lead 16 and housing 15, housing 15 may be used as a common or ground electrode or as an active can electrode for delivering CV / DF shock pulses.

[0053] The cardiac electrical signal sensing circuit 86 may be selectively coupled to the electrodes 24, 26, 28, and 30 and the housing 15 to monitor the electrical activity of the patient's heart. The sensing circuit 86 may include a switching circuit system for selecting which of the electrodes 24, 26, 28, and 30 and the housing 15 are coupled to a sensing amplifier or other cardiac event detection circuit system included in the sensing circuit 86. The switching circuit system may include a switch array, a switch matrix, a multiplexer, or any other type of switching device suitable for selectively coupling a sensing amplifier to a selected electrode. The cardiac event detection circuit system within the electrical sensing circuit 86 may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), or other analog or digital components.

[0054] In some examples, the electrical sensing circuit 86 includes a plurality of sensing channels for acquiring cardiac electrical signals from a plurality of sensing vectors selected from electrodes 24, 26, 28, and 30 and housing 15. Each sensing channel can be configured to amplify, filter, and rectify cardiac electrical signals received from selected electrodes coupled to the corresponding sensing channel to improve the signal quality for sensing cardiac events (e.g., P waves and / or R waves). Each sensing channel includes a cardiac event detection circuit system for sensing cardiac events from received cardiac electrical signals generated across the selected sensing electrode vector(s). For example, each sensing channel in the sensing circuit 86 can include: an input or pre-filter and amplifier for receiving cardiac electrical signals from the corresponding sensing vector, an analog-to-digital converter, a post-amplifier and filter, and a rectifier for generating a digitized, rectified, and amplified cardiac electrical signal, which is transmitted to a cardiac event detector included in the sensing circuit 86 and / or is transmitted to the control circuit 80 for performing signal analysis.

[0055] The cardiac event detector may include a sense amplifier, a comparator, or other circuitry for comparing the rectified cardiac electrical signal to a cardiac event sensing threshold amplitude (such as an R-wave sensing threshold amplitude) which may be an automatically adjusted threshold. The sensing circuit 86 may generate a sensed cardiac event signal in response to a sensing threshold crossing. Sensed cardiac events (or the lack of sensed cardiac events) such as R-waves are used to detect cardiac rhythm and determine the need for treatment by the control circuit 80. In some examples, the cardiac electrical signal (such as a sensed R-wave) is used to detect capture of a pacing pulse delivered by the ICD 14.

[0056] The therapy delivery circuit 84 may include a low voltage (LV) therapy circuit 85 for delivering low voltage pacing pulses using an extracardiac pacing electrode vector selected from electrodes 24, 26, 28 and 30 and housing 15. The LV therapy circuit 85 may be configured to deliver low voltage pacing pulses, for example, a pulse amplitude of 8V or less, or a pulse amplitude of 10V or less. One or more capacitors included in the LV therapy circuit 85 are charged to a voltage according to a programmed pacing pulse amplitude by an LV charging circuit, which may include a state machine. The LV charging circuit may charge the capacitor to a multiple of the voltage of a battery included in the power supply 98 without the need for a transformer. At the appropriate time, the LV therapy circuit 85 couples the holding capacitor(s) to the pacing electrode vector, for example via a tip or output capacitor, to deliver pacing pulses to the heart 8. The following is in conjunction with Figure 5 An example of a LV therapy circuit is described.

[0057] The high voltage (HV) therapy circuit 83 includes one or more high voltage capacitors. When a shockable rhythm is detected, the HV capacitor (s) are charged to a shock voltage amplitude according to the programmed shock energy by the HV charging circuit. The HV charging circuit 83 may include a transformer and may be a processor-controlled charging circuit controlled by the control circuit 80. When a feedback signal from the therapy delivery circuit 84 is detected, the control circuit 80 applies a signal to trigger the discharge of the HV capacitor (s), and the feedback signal indicates that the HV capacitor has reached the shock voltage amplitude required to deliver the programmed shock energy. In this way, the control circuit 80 controls the operation of the HV therapy circuit 83 to deliver CV / DF shocks using the defibrillation electrodes 24, 26 and / or the housing 15.

[0058] In some applications, the HV therapy circuit 83 can be used to deliver cardiac pacing pulses. In this case, the HV capacitor (s) are charged to a voltage that is much lower than the voltage used to deliver electric shock therapy, but can be higher than the maximum available pulse voltage amplitude produced by the LV therapy circuit 85. For example, the HV capacitors can be charged to 40V or less, 30V or less, or 20V or less for generating extracardiovascular pacing pulses. In some examples, the HV therapy circuit 83 is enabled to deliver cardiac pacing pulses by applying at least the minimum current required to maintain a switch included in the HV therapy circuit 83 in a closed state as needed to couple the HV capacitor (s) to the pacing electrode vector. The following is combined with Figure 4 The circuitry included in the HV therapy circuit 83 is described.

[0059] The pulses delivered by the HV therapy circuit 83 may have a higher voltage amplitude and / or a relatively longer pulse width than the pacing pulses delivered by the LV therapy circuit 85 to deliver higher energy pacing pulses to capture the heart. More current may be delivered using a low impedance pacing electrode vector (e.g., between electrodes 24 and 26). Because the HV capacitor(s) have a higher capacitance (and therefore a higher RC time constant), a longer pulse width may be obtained. The LV therapy circuit 85 may be capable of producing a maximum pulse voltage amplitude of up to and including 10V. The maximum single pulse pacing pulse width produced by the LV therapy circuit 85 may be 2ms. In some examples, the LV therapy circuit 85 may be configured to produce a composite pacing pulse comprising two or more individual pulses fused in time to deliver a cumulative composite pacing pulse energy to capture the heart. Techniques for delivering compound pacing pulses are generally disclosed in U.S. Patent Application Publication No. 2017 / 0157413 (Anderson et al.) and U.S. Patent Application Publication No. 2017 / 0157399 (Anderson et al.). The compound pacing pulse width can be, for example, 4 ms or higher.

[0060] Due to the higher capacitance of the high voltage capacitor included in the HV therapy circuit 83, the HV therapy circuit 83 can be capable of generating pulse voltage amplitudes of 10V or higher, and can generate monophasic pulses or multiphasic pulses with relatively long pacing pulse widths (e.g., 10ms or longer). A typical HV pacing pulse width can be 10ms; however, an example range of available pulse widths can be 2ms to 20ms. An example of a maximum voltage amplitude that can be used to deliver a high voltage pacing pulse can be 40V. When a patient can tolerate a relatively high pacing pulse voltage amplitude (e.g., greater than 10V), a relatively short pacing pulse width, e.g., 2 to 5ms, can be used during an extracardiac pacing output configuration. However, longer pacing pulse widths, such as 10V, 20ms pacing pulses, can be used as needed to capture the patient's heart without causing pain or discomfort.

[0061] For comparison, the HV capacitor(s) of the HV therapy circuit 83 may be charged to an effective voltage greater than 100 V for delivering CD / DF shocks. For example, two or three HV capacitors having an effective capacitance of 148 to 155 microfarads may be provided in series in the HV therapy circuit 83. These series capacitors may be charged to produce 100 to 800 V for series combination to deliver a shock having a pulse energy of 5 joules or greater, and more typically 20 joules or greater.

[0062] In contrast, depending on the pacing electrode impedance, the pacing pulses delivered by the HV therapy circuit 83 may have pulse energies less than 1 joule, and even pulse energies in the millijoule range or one-tenth of a millijoule range. For example, a pacing pulse generated by the HV therapy circuit 83, having a 10V amplitude and a 20ms pulse width, delivered using a pacing electrode vector with an impedance in the range of 20 to 200 ohms between the defibrillation electrodes 24 and 26, may have a delivered energy of 5 to 7 millijoules. When a relatively short pulse width (e.g., as low as 2ms) is used, the pacing pulses delivered by the HV therapy circuit 83 using the defibrillation electrodes 24 and 26 may be as low as 1 millijoule. The pacing pulses delivered by the HV therapy circuit 83 are expected to have a pacing voltage amplitude of less than 100V and typically no more than 40V, and to deliver at least 1 millijoule but less than 1 joule of energy. The delivered energy for a given pacing voltage amplitude will vary depending on the pulse width and the pacing electrode vector impedance.

[0063] If pace / sense electrode 28 or 30 is included in the pacing electrode vector, resulting in a relatively high impedance (e.g., in the 400 to 1000 ohm range), the delivered pacing pulse energy may be in the 2 to 5 mJ range. HV therapy circuit 83 may deliver more current via a lower impedance pacing electrode vector (e.g., between defibrillation electrodes 24 and 26) than the current delivered by LV therapy circuit 85 via a pacing electrode vector including pace / sense electrode 28 or 30 (relatively high impedance), even if the pacing voltage amplitude is the same.

[0064] For the pacing load ranges given in the previous examples, a composite pacing pulse having an 8V amplitude and an 8ms pulse width delivered by the LV therapy circuit 85 can be in the range of 0.5-1.3 millijoules. For a pacing load of 400 to 1000 ohms, an extracardiac single pulse pacing pulse having an amplitude of 8V and a pulse width of 2ms delivered by the LV therapy circuit 83 can be in the range of 0.2 to 0.3 millijoules. In contrast, for a typical intracardiac pacing pulse having an amplitude of 2V, a pulse width of 0.5ms, and applied across a pacing electrode vector impedance of 400 to 1000 ohms, the pacing pulse delivered using the endocardial electrodes or epicardial electrodes can be on the order of microjoules, e.g., 2 microjoules to 5 microjoules. Thus, even though LV therapy circuit 83 is referred to herein as a "low voltage" therapy circuit, cardiac pacing delivered by LV therapy circuit 83 may still be considered a high voltage pacing application relative to very low voltage pacing applications utilizing endocardial or epicardial electrodes because LV therapy circuit 85 is configured to deliver pacing pulses via extracardiac electrodes. As will be described below, control circuit 80 may control therapy circuit 84 to deliver cardiac pacing pulses using the charge balancing techniques disclosed herein. Cardiac pacing pulses may be generated by either HV therapy circuit 83 or LV therapy circuit 85.

[0065] In some instances, the control circuit 80 may control the impedance measurement circuit 90 to determine the impedance of the pacing electrode vector. The impedance measurement circuit 90 may be electrically coupled to the available electrodes 24, 26, 28, 30 and the housing 15 to perform impedance measurements on one or more candidate pacing electrode vectors. The control circuit 80 may control the impedance measurement circuit 90 to perform impedance measurements by transmitting a signal to the impedance measurement circuit 90 for initiating impedance measurements on the pacing electrode vector. The impedance measurement circuit 90 is configured to apply a drive current or an excitation current across the pacing electrode vector and determine the resulting voltage. The voltage signal may be used directly as an impedance measurement, or the impedance may be determined based on the applied current and the measured voltage. The impedance measurement may be transmitted to the control circuit 80.

[0066] The control circuit 80 can use the impedance measurement from the impedance measurement circuit 90 to control the delivery of pacing pulses using the charge balancing techniques disclosed herein. For example, the pacing vector impedance can be used to select a frequency at which to inject a modified pacing pulse in a series of alternating polarity pacing pulses to provide charge balance. As described below, the pacing electrode vector impedance can be used as an input to an algorithm for selecting to modify the nth pacing pulse in a series of alternating polarity pacing pulses, or for determining to modify the nth pacing pulse based on a lookup table stored in the memory 82 based on a plurality of impedance ranges.

[0067] The control parameters used by the control circuit 80 for detecting cardiac rhythm and delivering electrical stimulation therapy and tachyarrhythmia inducing pulses, including charge balancing pacing pulse modification, can be programmed into the memory 82 via the telemetry circuit 88. The telemetry circuit 88 includes a transceiver and an antenna for communicating with the external device 40 ( Figure 1A Under the control of control circuit 80, telemetry circuit 88 may receive downlink telemetry from external device 40 and send uplink telemetry to external device 40. In some cases, telemetry circuit 88 may be used to transmit communication signals to and receive communication signals from another medical device implanted in patient 12.

[0068] Figure 4 2 is a schematic diagram 200 of an HV therapy circuit 83 coupled to a processor and an HV therapy control circuit 230. The HV therapy circuit 83 includes an HV charging circuit 240 and an HV charge storage and output circuit 202. The processor and the HV therapy control circuit 230 may be included in the control circuit 80 for controlling the HV charging circuit 240 and the HV charge storage and output circuit 202.

[0069] The HV charge storage and output circuit 202 includes an HV capacitor 210 coupled to the switch circuit system 204 via a pulse control switch 206, which is used to electrically couple the HV capacitor 210 to the electrodes 24 and 26 and / or the housing 15 to deliver the desired HV electrical stimulation pulses to the patient's heart 8. Although the HV capacitor 210 is shown as a single capacitor, it is recognized that a group of two or more capacitors or other energy storage devices can be used to store energy for generating electrical signals for delivery to the heart 8. In one example, the HV capacitor 210 is a series of three capacitors having an effective capacitance of 148 microfarads, 155 microfarads, or other selected capacitances. In contrast, the holding capacitors included in the LV therapy circuit 85 that are charged to a multiple of the battery voltage by the state machine may have a capacitance of up to 6 microfarads, up to 10 microfarads, up to 20 microfarads, or other selected capacitances, but all of the holding capacitors have a capacitance significantly less than the effective capacitance of the HV capacitor 210. LV therapy circuit 85 has a lower breakdown voltage than HV therapy circuit 83, thereby allowing HV capacitor 210 to be charged to the shock voltage amplitude required to deliver a CV / DF shock, for example, 100 V or more.

[0070] The switching circuit system 204 may take the form of an H-bridge including switches 212a-212c and 214a-214c controlled by signals from the processor and the HV control circuit 230. The switches 212a-212c and 214a-214c may be implemented as silicon controlled rectifiers (SCRs), insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), and / or other switching circuit components or combinations thereof.

[0071] When the control circuit 80 determines that an electrical stimulation pulse needs to be delivered from the HV therapy circuit 83, the switch circuit system 204 is controlled by a signal from the processor and the HV therapy control circuit 230 to electrically couple the HV capacitor 210 to the therapy delivery vector so that the capacitor 210 is discharged across a vector selected from the electrodes 24, 26 and / or the housing 15. The selected electrodes 24, 26 and / or the housing 15 are coupled to the HV capacitor 210 by opening (i.e., disconnecting or disabling) and closing (i.e., connecting or enabling) the appropriate switches of the switch circuit system 204 to pass the desired electrical signal to the therapy delivery electrode vector. Although only the electrodes 24, 26 and the housing 15 are indicated as being coupled to the switch circuit system 204, it should be understood that the pacing / sensing electrodes 28 and 30 can be coupled to the switch circuit system 204 and can be used in the pacing electrode vector.

[0072] When the control circuit 80 determines that electric shock therapy is required based on the detected heart rhythm (e.g., VT or VF), the electrical signal delivered by the HV therapy circuit 83 can be a monophasic, biphasic, or other shaped CV / DF electric shock pulse for terminating ventricular tachyarrhythmias. When the control circuit 80 determines that pacing therapy is required based on the detected heart rhythm or the expiration of the pacing escape interval, the electrical signal delivered by the HV therapy circuit 83 includes alternating polarity pulses, wherein every nth pulse is modified to provide charge balance. The modification of every nth pulse may include modification of polarity, pulse amplitude, and / or pulse width compared to the expected, unmodified pattern of alternating polarity pulses.

[0073] The pacing pulse may be terminated when the pulse voltage amplitude decays according to a programmed "tilt." The tilt is the percentage of the leading edge voltage amplitude to which the pulse decays. For example, when the pulse amplitude has decayed to 50% of the leading edge voltage amplitude, the pacing pulse may be terminated with a 50% tilt. If the programmed tilt is 20%, the pacing pulse may be terminated when the pulse amplitude has decayed by 20% (i.e., down to 80% of the leading edge voltage amplitude). In other examples, the pacing pulses delivered by the HV pacing circuit 83 are terminated according to a predetermined pulse width.

[0074] The processor and HV therapy control circuit 230 control the capacitor 210 to charge to a programmed pacing pulse voltage amplitude or a modified charge-balanced pacing pulse voltage amplitude. The switches 212a-212c and 214a-214c are controlled to be open or closed by the processor and HV therapy control circuit 230 at appropriate times for delivering monophasic, biphasic, or other desired pacing pulses by discharging the capacitor 210 across the pacing load presented by the heart 8 and the selected pacing electrode vector (e.g., electrodes 24 and 26). The capacitor 210 is coupled across the selected pacing electrode vector for a sustained programmed or modified pacing pulse width or according to a programmed tilt.

[0075] The processor and HV therapy control 230 can control the HV therapy circuit 83 to deliver cardiac pacing pulses by controlling the HV therapy circuit 83 to deliver alternating polarity pacing pulses. Inherent limitations of the therapy delivery circuit 84 can result in charge-unbalanced alternating polarity pacing pulses. For example, there may be a net 10% charge difference between a positive-going pulse and a negative-going pulse controlled by the therapy delivery circuit 84 according to the same programmed pulse amplitude (although opposite polarity) and the same programmed pulse width (or tilt). Over time, this small charge difference accumulates and can cause electrode corrosion. In this example, assuming that each positive polarity pulse delivers approximately 10% more charge than each negative polarity pulse, the control circuit 80 can deliver ten alternating polarity pulses (+-+-+-…) and modify the polarity of every eleventh pulse to a negative polarity pulse (rather than the expected positive polarity) so that two consecutive negative polarity pulses are delivered to balance the net positive charge accumulated with the previous ten alternating polarity pulses. In other examples, the processor and HV therapy control 230 can control each nth pacing pulse delivered by the HV therapy circuit 83 to have a modified pulse amplitude and / or a modified pulse width (compared to the programmed pacing pulse amplitude and pulse width of the previous alternating polarity pulse) to balance the charge accumulated in one or more previous pacing pulses. The examples presented herein generally refer to the nth pulse in a series of n pulses as a modified charge balancing pulse. However, it is contemplated that the modified charge balancing pulse is not necessarily the last pulse in a series of n pulses. Any pulse in a series of n pulses can be a modified charge balancing pulse that reduces or eliminates the net charge accumulated with the other n-1 pulses in the series of n pulses.

[0076] Prior to the first pulse, the HV capacitor 210 may be charged to the programmed pacing pulse voltage amplitude. The HV capacitor 210 is recharged to the programmed pacing pulse voltage amplitude between successive pulses. The HV charging circuit 240 is powered by a power supply 98 ( Figure 3) is powered. The HV charging circuit 240 includes a transformer 242 for stepping up the battery voltage of the power source 98 so as to enable charging of the capacitor 210 to a voltage much greater than the battery voltage. The charging of the capacitor 210 by the HV charging circuit 240 is performed under the control of the processor and the HV therapy control 230, which receives a feedback signal from the HV charge storage and output circuit 202 to determine when the capacitor 210 is charged to the programmed voltage. A charge completion signal is transmitted to the HV charging circuit 240 so that the charging is terminated by the processor and the HV therapy control circuit 230. An example of a high voltage charging circuit and its operation is generally disclosed in U.S. Patent No. 8,195,291 (Norton et al.).

[0077] The switches 212a-212c and switches 214a-214c may require a minimum current flow to keep them closed (i.e., turned on or enabled) in order to pass current when the HV capacitor 210 is discharged. Depending on the pacing load impedance and other conditions, the minimum current to enable (close) the switches 214a-214c may be approximately 10 mA or less. When the capacitor 210 is discharged across the selected pacing electrode vector, the current flowing through the enabled switches 212a-212c and 214a-214c may drop below the minimum current required to keep the enabled switches closed. If the current flowing through the corresponding switch drops below the minimum current required to keep the switch closed, the switch may open (or become disabled), resulting in premature truncation of the pacing pulse, which may result in loss of capture and / or inadequate charge balance. Thus, a minimum pacing pulse voltage amplitude can be set for delivering pulses from the HV therapy circuit 83 so as to reduce the likelihood of the current falling below the minimum current required to maintain a stable state of enabled switches of the switching circuit system 204 during the programmed pacing pulse width (or before the programmed tilt is reached).

[0078] In other examples, the HV charge storage and output circuit 202 may include optional shunt resistors 250, 252 in parallel with the pacing load to maintain a minimum current through the switching circuit system 204. When electrodes 24 and 26 are selected as the anode and cathode (or cathode and anode, respectively) of the pacing electrode vector, the shunt resistor 250 is shown in parallel with the pacing load, which is schematically shown as the heart 8. It should be recognized that for any selected pacing electrode vector, the shunt resistor can be set in parallel with the pacing load, for example, the shunt resistor 252 is schematically shown where the pacing electrode vector includes electrode 26 and housing 15. Similarly, when the pacing electrode vector includes electrode 24 and housing 15, the shunt resistor can be set in parallel with the pacing load. The shunt resistor 250 or 252 can be a variable resistor that is set to match the pacing electrode vector impedance so that the load across the heart 8 using the selected pacing electrode vector is matched to the shunt resistor. In this manner, the current through the switch circuitry 204 may be maintained equal to or greater than the minimum current required to maintain a stable state of the enabled switches in the switch circuitry 204 during the pacing pulse. Other examples of using shunt resistors in the switch circuitry 204 are generally disclosed in the above-referenced U.S. Patent Application No. 15 / 425,169.

[0079] In some examples, the pacing electrode vector coupled to the HV capacitor 210 via the switch circuit system 204 may include electrodes 24, 26, 28, and / or 30 carried by the lead 16. The housing 15 may be used for cardiac pacing pulse delivery by keeping switches 212c and 214c open without using the housing 15. Depending on the implantation location of the ICD 14 and lead 16 and the resulting electrical stimulation delivery vector between the housing 15 and the electrodes 24, 26, 28, and / or 30, more skeletal muscle recruitment may occur when the housing 15 is included in the pacing electrode vector. A larger volume of skeletal muscle tissue may be placed along the vector extending between the distal portion 25 of the lead 16 and the housing 15 than along the vector extending between the two electrodes carried by the distal portion 25 of the lead. However, in other electrode configurations and implant locations, electrodes (which may include the ICD housing 15) for delivering extracardiovascular pacing pulses via the HV therapy circuit 83 may be selected to provide a pacing electrode vector that minimizes the volume of skeletal muscle included in the pacing electrode vector while directing sufficient energy to the heart 8 to capture and pace the heart.

[0080] Figure 56 is a conceptual diagram of an LV therapy circuit 85 according to an example. The LV therapy circuit 85 may include a capacitor selection and control circuit 504, an LV charging circuit 514, and a capacitor array 610. The capacitor array 610 may include a plurality of holding capacitors 612, 614, 616, and 618, each of which may be charged to a programmed pacing pulse amplitude by the LV charging circuit 514. The holding capacitors 612, 614, 616, and 618 are coupled to respective output capacitors 632a-632d (collectively 632), 636, or 638 via respective switches 622, 624, 626, and 628 to deliver pacing pulses. Each of the holding capacitors 612, 614, 616, and 618 has a capacitance that is less than the effective capacitance of the high voltage capacitor 210 of the HV therapy circuit 83. For example, each of holding capacitors 612 , 614 , 616 , and 618 may have a capacitance of up to 6 microfarads, up to 10 microfarads, up to 20 microfarads, or other selected capacitance, but all of the holding capacitors have a capacitance that is significantly less than the effective capacitance of high voltage capacitor 210 .

[0081] Power supply 98( Figure 3 ) can provide regulated power to the LV charging circuit 514. The LV charging circuit 514 can be controlled by a state machine in the capacitor selection and control circuit 504 to charge all or selected capacitors of the holding capacitors 612, 614, 616, and 618 using a multiple of the battery voltage of the power supply 98 (e.g., four times the battery voltage). The LV charging circuit 514 charges the capacitors 612, 614, 616, and / or 618 as needed for delivering a pacing pulse, which can be a single pacing pulse (e.g., monophasic or biphasic) or can be a composite pacing pulse during fusion pacing. A composite pacing pulse includes two or more individual pulses delivered in succession so that the pulse energies of the consecutive individual pulses are effectively "fused" in time to deliver a cumulative pacing pulse energy greater than a cardiac capture threshold. Fusion pacing using composite pacing pulses delivered by a LV pacing circuit for extracardiovascular pacing is generally disclosed in U.S. Patent Application Publication No. 2017 / 0157399 (Anderson et al.).

[0082] In some examples, LV therapy circuit 85 includes three pacing channels 602, 604, and 606. Each pacing channel is capable of generating a single pacing pulse when a corresponding holding capacitor 612, 616, or 618 is discharged across an output capacitor 632, 636, or 638, respectively. Pacing channel 602 includes a backup holding capacitor 614 that can be used to deliver a backup pacing pulse, for example, when a single pulse pacing output is selected. When fusion pacing is delivered, backup holding capacitor 614 can be used to deliver individual pulses in a composite pacing pulse.

[0083] One or more channels may include multiple selectable output signal lines depending on the number of extracardiovascular electrodes coupled to ICD 14. For example, channel 602 is shown in this example as including multiple selectable pacing output signal lines 642a-642d that may be selectively coupled to holding capacitor 612 and backup holding capacitor 614 via closing one or more of electrode selection switches 634a-634d. For example, multiple electrodes carried by lead 16 may be coupled to pacing channel 602, and a pacing electrode vector may be selected from the multiple electrodes by closing certain of switches 634a-634d.

[0084] Pacing channels 604 and 606 are shown as having single output signal lines 646 and 648 coupled to respective holding capacitors 616 and 618 via respective switches 626 and 628. In other examples, all three pacing channels 602, 604, and 606 may be provided with a single output signal line or with multiple output signal lines to enable selection of a pacing electrode vector from a plurality of extracardiovascular electrodes coupled to ICD 14 (e.g., any of electrodes 24, 26, 28, or 30 of lead 16).

[0085] The single channel pacing output configuration may be enabled by the control circuit 80 using any one of the pacing channels 602, 604, and 606 to deliver a single pulse pacing pulse. When the corresponding switch 622, 624, 626, or 628 is closed, single pulse pacing may be delivered by discharging one of the holding capacitors 612, 614, 616, or 618 across the selected pacing electrode vector via the corresponding output capacitor 632, 636, or 638. The output line 642a, 642b, 642c, or 642d for delivering the pacing current from the pacing channel 602 may be selected via the corresponding electrode selection switch 634a-634d. The switch 622, 624, 626, or 628 that enables the discharge of the holding capacitor 612, 614, 616, or 618, respectively, may be enabled by the capacitor selection and control circuit 504 at the appropriate time when a pacing pulse is required and maintained in an active, enabled state until the single pacing pulse width expires.

[0086] When the control circuit 80 is configured to deliver fusion pacing using the LV therapy circuit 85, the pacing channels 602, 604, and 606 are tied together by switches 620a-620d and 630 to enable individual pulses to be delivered across selected pacing electrode vectors from a single output signal line (e.g., line 646). For example, the control circuit 80 can enable fusion pacing pulse output by activating switches 620a-620b and 630 to tie pacing output lines 642a-642d and pacing output line 648 to the pacing channel 604. The control circuit 80 controls the capacitor selection and control circuit 504 to enable the pacing channel switches 622, 624, 626, and 628 (and at least one electrode selection switch 634a-634d of the pacing channel 602) in a sequential manner to couple the corresponding holding capacitor 612, 614, 616, or 618 to the output signal line 646 to deliver a sequence of fused individual pulses to produce a composite pacing pulse.

[0087] In various examples, depending on the specific pacing channel and lead and electrode configuration used with the ICD 14, it may not be necessary Figure 5 Certain electrode selection switches are shown. In addition, it is recognized that fewer than four holding capacitors or more than four holding capacitors may be included in capacitor array 610 for delivering a sequence of fusion pacing pulses.

[0088] Capacitor selection and control circuit 504 selects which holding capacitors 612, 614, 616, and 618 are coupled to output line 646 and in what order by controlling corresponding switches 622, 624, 626, and 628. Composite pacing pulses can be generated by delivering a sequence of pulses in the following manner: sequentially discharging holding capacitors 612, 614, 616, and 618 one at a time (or one combination at a time) across corresponding output capacitors 632, 636, and 638 by sequentially enabling or closing corresponding switches 622, 624, 626, and 628. For example, at least two of holding capacitors 612, 614, 616, and 618 are sequentially discharged to produce a composite pacing pulse generated by at least two fused individual pulses. Output line 646 can be electrically coupled to a pacing cathode electrode carried by lead 16, and a return anode electrode carried by lead 16 (or housing 15) can be coupled to ground. The pacing cathode electrode and the return anode electrode may correspond to electrodes 24 and 28 or may be selected from Figures 1A-2B Any pacing electrode vectors of electrodes 24, 26, 28 and 30 and / or housing 15 are shown.

[0089] In some examples, a fusion pacing pulse is delivered by delivering two consecutive individual pulses from pacing channels 604 and 606 one at a time, followed by delivering a third, longer individual pulse from pacing channel 602 by discharging both capacitors 612 and 614 simultaneously. For a composite pacing pulse width of 8 ms, the pulse widths of the first two individual pulses may be 2.0 ms, while the pulse width of the third pulse may be 4.0 ms. The higher capacitance of parallel capacitors 612 and 614 allows the pulse width of the third individual pulse to be longer while maintaining the pulse amplitude to successfully capture the heart for the duration of the pulse width. Because output configuration switches 620 and 630 are enabled for fusion pacing output configuration, all three individual pulses are delivered via output line 646. In other examples, selected ones of holding capacitors 612, 614, 616, and 618 are discharged sequentially, individually, or in combination to deliver a series of pulses that are close together in time to form a composite pacing pulse.

[0090] Each nth pulse in a series of alternating polarity pacing pulses delivered by the LV therapy circuit 85 may be controlled by the control circuit 80 to have a modified polarity, a modified pulse amplitude, and / or a modified pulse width to balance the unbalanced charge delivered with the previous n-1 pacing pulses. The n-1 unmodified alternating polarity pacing pulses may be delivered as alternating polarity single pulse pacing pulses, or as alternating polarity composite pacing pulses including two or more fused pulses. The modification of the nth pulse may include modifying the intended polarity of the nth pulse in the pattern of alternating polarity pulses by delivering two consecutive positive pulses or two consecutive negative pulses (the n-1th pulse and the nth pulse) having the same polarity. Modification of the nth pulse may include modifying the desired pulse amplitude and / or pulse width by increasing or decreasing the pulse amplitude and / or pulse width from a programmed pacing pulse amplitude and / or a programmed pacing pulse width, respectively, the programmed pacing pulse amplitude and / or the programmed pacing pulse width being used to control the previous n-1 pulses.

[0091] In the examples presented herein, the electrical stimulation pulses comprising opposite polarity pulses are voltage-controlled pulses such that the nth charge-balanced pulse in a series of pulses may have a modified voltage amplitude. However, it is recognized that, depending on the medical device, the electrical stimulation pulses may be current-controlled pulses and, therefore, the modification to the nth charge-balanced pulse may be a modified current amplitude.

[0092] Figure 6300 is a conceptual diagram of a series of alternating polarity pacing pulses 302 including charge balanced pulses 322. The series of n pulses 302 includes positive polarity pulses 304 and negative polarity pulses 306 delivered alternately. Although the pulses are shown as monophasic pulses, in other examples the pulses may be biphasic or other shaped pulses. The series of n pulses 302 including opposite polarity pulses and the pulses 304 and 306 are delivered alternately. Figure 7 and Figure 8 The series of pulses 350 and 380 shown may represent various electrical stimulation pulses delivered to the patient's body tissue by the medical device. In the illustrative examples described herein, the pulses may be cardiac pacing pulses that include pacing pulses of opposite polarity and nth charge-balanced pacing pulses. In one example, the control circuit 80 may control the HV therapy circuit 83 or the LV therapy circuit 85 to deliver alternating polarity pacing pulses 304 and 306 in the series of n pulses 302.

[0093] The alternating polarity pulses 304 and 306 can be separated in time by a pacing pulse interval 320. The pacing pulse interval 320 can be a lower frequency interval used to pace the heart when the natural frequency is lower than the programmed lower frequency. In some examples, the lower frequency interval can be adaptively controlled based on patient activity indicated by the sensor to provide frequency-responsive pacing. The pacing pulse interval 320 can alternatively be an ATP interval, a post-shock pacing interval, or other pacing interval set by the control circuit 80 based on a programmed cardiac pacing therapy protocol. Depending on the pacing protocol, the pacing pulse interval 320 can remain constant during the series of n alternating pacing pulses 302, or can vary. In some examples, the pacing pulse interval 320 can be controlled by the expiration of an escape interval counter included in the control circuit 80.

[0094] In other examples, the pacing pulse 302 may represent a ventricular pacing pulse delivered in an atrial tracking pacing mode or delivered during CRT such that each pulse 304 and 306 is delivered upon expiration of an AV pacing interval (not shown), in which case the pulses 304 and 306 may be separated by a variable interval that depends on the atrial rate being tracked. In this case, the interval 320 between the pulses does not necessarily represent a pacing escape interval. In addition, it should be understood that in some cases, consecutive alternating polarity pacing pulses may occur for an extended period of time (e.g., one or more unexpired pacing escape intervals) during which an intervening intrinsic cardiac event occurs. Therefore, it is not necessary for a positive pacing pulse to appear consecutively with a negative pacing pulse in a pair of alternating polarity pulses on consecutive cardiac cycles. No intrinsic cardiac cycle, one or more intrinsic cardiac cycles may occur between the positive and negative polarity pacing pulses in a pair of charge-unbalanced alternating polarity cardiac pacing pulses.

[0095] Both the positive polarity pulse 304 and the negative polarity pulse 306 may be delivered according to the same programmed pulse amplitude 310 and pulse width 312 (or tilt), which may be selected based on a cardiac capture threshold test. However, inherent system limitations may result in a difference in the net charge delivered during a pair of pacing pulses having one positive pacing pulse 304 and one negative pacing pulse 306. Depending on the pacing frequency, even a small charge imbalance of each pair of alternating polarity pulses may result in an accumulation of unbalanced charge over time. Each nth pacing pulse 322 of the series 302 is modified by reversing the polarity of the nth pulse in the pacing pulse alternation pattern. A modified nth pacing pulse 322 having a polarity opposite to that of the charge imbalance of the pair of pulses having one positive pulse and one negative pulse is delivered to balance or cancel the accumulated charge. The charge delivered during each positive pacing pulse 304 may be a known or approximate percentage greater than the charge delivered during each negative polarity pulse 306, such that the pair of pulses has a net positive charge delivered or an unbalanced positive charge. In other cases, the negative polarity pulses 306 may deliver a greater charge than the positive pulses 304, resulting in a net negative polarity charge accumulation that is balanced by the positive polarity charge balancing pulses.

[0096] The value of n used to control the frequency of delivering modified charge-balanced pacing pulses can be based on the net charge difference between the positive polarity pulses 304 and the negative polarity pulses 306. For example, n can be determined as two times the inverse of the charge imbalance plus 1 or n=2*(1 / c)+1, where c is the charge imbalance percentage (in decimal form) for a pair of alternating cardiac pacing pulses having one positive polarity cardiac pacing pulse and one negative polarity cardiac pacing pulse. Every nth pulse 322 can be modified by delivering an nth pulse of opposite polarity having an unbalanced charge. To illustrate, if the positive polarity pulses 304 deliver approximately 10% more charge than the negative polarity pulses 306, a net positive charge equal to approximately 100% of the charge of one positive pulse will accumulate after a series of 20 alternating pulses (10 positive-going pulses and 10 negative-going pulses). To balance the 10% accumulated positive charge for each alternating pacing pulse pair, the control circuit 80 can determine n=21 (21=2(1 / 0.1)+1). Thus, every 21st pulse in the alternating polarity pulses is modified by delivering every 21st pulse with negative polarity to balance the unbalanced positive charge accumulated in the previous 20 pulses. In this way, any net charge accumulated with the previous n-1 alternating pulses is substantially balanced by the nth pulse.

[0097] The examples presented herein relate to a series of pulses having the following pulses: n-1 alternating polarity pulses, followed by the nth pulse in the series that is modified to provide charge balance. However, it is contemplated that the n-1 pulses may include any series of electrical stimulation pulses that result in a net charge accumulation that is offset by the nth pulse in the series. For example, the n-1 pulses may include charge-unbalanced, opposite polarity pulses, but not necessarily in an alternating pattern where every other pulse is positive and every other pulse is negative. A series of pulses may include two, three or more consecutive positive polarity pulses, followed by the same number of negative polarity pulses, and the pattern may be repeated until the nth modified charge-balanced pulse. For illustration, a series of pulses including opposite polarity pulses may include two positive pulses, followed by two negative pulses (+, +, -, -, +, +, -, -, +, + ...), and the series may terminate with a modified charge-balanced pulse.

[0098] Despite Figure 6 Continuing cardiac pacing pulses following series 302 are not shown in FIG. 3 , but it should be understood that series 302 may be n pacing pulses in a continuous series of cardiac pacing pulses delivered according to a programmed pacing therapy or pacing mode. For example, ICD 314 may be programmed to deliver cardiac pacing in a VVI, VDI, or VDD pacing mode (with or without a frequency response) such that pacing may occur over an extended period of time, and series 302 represents n pacing pulses delivered during the continuous pacing. During the continuous pacing, every nth pulse may be modified by delivering every nth pulse with the same polarity as a pulse having an inherently less net charge. As a result, during the continuous pacing, every n-1 pulse and the nth pulse may have the same polarity to balance the net charge accumulated with the n-1 pulses. The polarity alternation may restart with the next n+1 pulse being of the opposite polarity to the nth pulse and being the first pulse in the next series of n pulses ending with the modified nth pulse. For example, if every 5th pulse is modified to balance the net charge delivered during the previous four alternating polarity pulses, the pacing pulse polarity may be represented as: +-+--+--+--+--+-, etc. In other examples, pacing pulse series 302 may represent all pulses delivered during an ATP or other pacing therapy, where the last pacing pulse may be nth pulse 322, which is modified to balance the charge delivered with the previous n-1 pulses in the pacing sequence.

[0099] Figure 7350 is a diagram of cardiac pacing pulses delivered according to another example technique for charge balancing. In this example, each nth pulse 372 in each series of n pulses 352 is modified by adjusting the pacing pulse amplitude 374 of the nth pulse 352. Each unmodified positive polarity pulse 354 and each unmodified negative going pulse 356 are delivered by therapy circuit 84 according to a programmed pacing pulse amplitude 360 ​​and pulse width 362 (or ramp). Although delivered according to the programmed pacing pulse amplitude 360 ​​and pulse width 362, the alternating positive pulses 354 and negative pulses 356 may each deliver inherently different charges, resulting in unbalanced charge delivery after each pair of pulses having one positive pulse 354 and one negative pulse 356. In the example shown, the nth pulse 372 in each series of n pulses 352 is modified by increasing the pulse amplitude 374 by a predetermined amount to balance the net positive charge accumulated during the previous n-1 pulses. In this example, the increased pulse amplitude 374 is based on the inherently higher charge delivered with each positive polarity pulse 354 .

[0100] For example, if each positive pulse 354 delivers 10% more charge than each negative pulse 356, the modified negative polarity pulse 372 can have a modified pulse amplitude 374 that is adjusted to be greater than the unmodified pulse amplitude 360 ​​to balance the 20% net positive charge imbalance of the previous two pulse pairs having one positive pulse and one negative pulse. The value of the increased amplitude 374 can be determined based on a bench test or modeling of the charge imbalance between the positive pulses 354 and the negative pulses 356. The amplitude adjustment made to the modified pulse amplitude can depend on the lead and electrode vector impedance and other electrical stimulation pulse output factors. In the example shown, n=4, so that every fourth pulse is delivered with an increased pulse amplitude. In other examples, every other pulse (e.g., every positive pulse or every negative pulse) can be delivered with a modified pulse amplitude so that the series of pacing pulses 352 includes alternating polarity and alternating pulse amplitude pulses. In this case, each pair of pulses having one positive polarity pulse 354 and one negative polarity pulse 356 is delivered as a series of n=2 pulses and in a manner that minimizes any inherent charge imbalance.

[0101] In some examples, the control circuit 80 can be configured to determine the frequency of delivering the modified pulses and the amount to adjust the pulse amplitude to balance the previous n-1 pulses based on the inherent charge difference between the positive polarity pulse 354 and the negative polarity pulse 356 and the maximum acceptable pulse amplitude of the modified charge balanced pulse. If the charge imbalance of a pair of alternating polarity pulses is relatively high, a relatively low value of n can be selected so that the modified nth pulse occurs relatively frequently with a reasonable and acceptably increased pulse amplitude. If the charge imbalance of a pair of alternating polarity pulses is relatively low, a larger value of n can be selected so that the frequency of the modified charge balanced pulses can be relatively low while still having an acceptably high pulse amplitude. Unacceptably high pulse amplitudes can result in unintended capture of skeletal muscle or nerves near the pacing electrode and cause patient discomfort.

[0102] Although the modified pulse 372 Figure 7 372 as having an increased pulse amplitude compared to the unmodified alternating polarity pulses 354 and 352 to offset the charge imbalance of the opposite polarity of the modified pulse 372, but it is contemplated that the modified pulses in the series of pulses having n pulses may have a reduced pulse amplitude rather than an increased pulse amplitude. The reduction in amplitude may offset the inherently higher charge of the previous pulse having the same polarity as the modified charge-balanced pulse. However, the reduced pulse amplitude may result in cardiac loss of capture for the nth pulse, which may or may not be acceptable, depending on the particular pacing application.

[0103] Figure 8 is a diagram 380 of another technique for delivering charge balanced cardiac pacing pulses. Figure 8, the nth pulse 392 in each series 382 of alternating cardiac pacing pulses 304 and 306 is modified by adjusting the pulse width 384 from the unmodified pulse width 312. The pulse width 384 may be increased or decreased to balance the accumulated net charge delivered with the previous n-1 pulses. It should be appreciated that a reduced pulse width may result in pacing loss of capture, which may or may not be acceptable. In the illustrated example, the nth pulse 392 is modified by increasing the pulse width 384 from the pulse width 312 of the unmodified pulses 304 and 306 to offset or balance the net positive charge accumulated during the delivery of the previous n-1 pulses. The modified pacing pulse width may be determined based on the known inherent charge imbalance of each pair of unmodified alternating polarity pacing pulses and a selected value of n. The selected value of n may have an upper limit that is based on the maximum pacing pulse width achievable based on the RC time constant of the pacing circuit and is acceptable in terms of the overall pacing pulse energy delivered to the patient's heart. Thus, if a relatively high charge imbalance exists, a lower value of n may be selected so that every nth pacing pulse is delivered with an increased pulse width 384 of an acceptable and achievable modified charge balanced pulse 392 .

[0104] In the above examples, positive polarity pulses are described as inherently delivering a greater charge than negative polarity pulses, such that after a pair of pacing pulses having one positive pacing pulse and one negative pacing pulse, a net positive charge is delivered. In these examples, the modified nth pulse is described as a negative polarity pulse. However, it should be appreciated that the modified nth pulse can be a positive polarity pulse that is adjusted to balance the charge imbalance. The charge imbalance can be attributed to a net positive polarity charge, or to a net negative polarity charge, because in some cases, a negative polarity pulse can inherently deliver a greater charge than a positive polarity pulse.

[0105] Figure 6-8 Each of the pulses shown is intended to be a therapeutic cardiac pacing pulse, and is therefore delivered with a pulse amplitude and width that is expected to cause cardiac capture and depolarization of myocardial tissue. Therefore, each nth charge-balanced pacing pulse is also a therapeutic cardiac pacing pulse. However, in other examples, a modified nth pulse may be delivered during the physiological refractory period of the myocardium after n-1 pulses, so that the nth pulse is a non-therapeutic pulse that is delivered as a charge-balanced pulse without the intention of causing cardiac capture. In other examples, the modification of the nth pulse may change the delivered pacing pulse energy to a level that is less than the pacing capture threshold. In this case, loss of capture may occur, but in some applications, one loss of capture in every n pacing pulses may be acceptable.

[0106] Fig. 9400 is a flow chart of a method for delivering charge balanced cardiac pacing pulses according to an example. At box 402, the control circuit 80 establishes one or more charge balanced pacing pulse modifications to be performed during cardiac pacing. The control circuit 80 can control the HV therapy circuit 83 or the LV therapy circuit 85 to deliver extracardiac pacing pulses. The extracardiac pacing pulses generated by the HV therapy circuit 83 or the LV therapy circuit 85 are relatively higher energy pacing pulses than pulses delivered by endocardial or epicardial electrodes. Therefore, the control circuit 80 can establish charge balanced pulse modifications to be applied when the HV therapy circuit 83 is used to generate and deliver cardiac pacing pulses, and the same or different modifications to be used when the LV therapy circuit 85 is selected for generating and delivering cardiac pacing pulses. The charge balanced pacing pulse modifications when using the LV therapy circuit 85 to generate and deliver cardiac pacing pulses may be different from the charge balanced pacing pulse modifications when using the HV therapy circuit 83 because there may be different inherent charge imbalances between the positive polarity pulses and the negative polarity pulses generated by the two different circuits.

[0107] In some examples, the charge balance pacing pulse modification can be stored in a lookup table (LUT) for one or more pacing output conditions in the memory 82. The known intrinsic charge difference between the positive and negative polarity pulses generated by the therapy circuit 84 can be determined when the ICD 14 is manufactured, such as through experience or benchmark testing. The intrinsic charge difference can be determined for one or more pacing output conditions. For example, there may be a different charge imbalance between the positive and negative polarity pulses generated by the HV therapy circuit 83 than between the charge imbalance between the positive and negative polarity pulses generated by the LV therapy circuit 85. Other pacing output conditions that can change the intrinsic charge difference between the positive and negative polarity pulses can include, for example, but not limited to: the pacing electrode vector used to deliver the pulse, the pacing electrode vector impedance, the pacing pulse amplitude, the pacing pulse width, and the pacing frequency. For each of the multiple pacing output conditions, the intrinsic charge difference and / or the associated pacing pulse modification can be stored in the memory 82. The intrinsic charge difference (multiple) and / or the corresponding pacing pulse modification (multiple) can be stored in one or more LUTs.

[0108] Thus, the control circuit 80 may establish charge balance modifications at box 402 by retrieving modifications from a LUT stored in the memory 82 for a given pacing output condition or combination of conditions. For example, one LUT may be stored in the memory 82 for use when the HV therapy circuit 83 is used to deliver cardiac pacing pulses. The LUT may include multiple pacing electrode vector impedance ranges paired with multiple pacing pulse amplitude ranges. Each cell in the LUT may store modifications to be applied for a given combination of impedance and pacing pulse amplitude when the HV therapy circuit 83 is selected to generate and deliver pacing pulses. Another LUT may be stored in the memory 82 for use when the LV therapy circuit 85 is selected to deliver cardiac pacing pulses. The LV therapy circuit LUT may include the same or different pacing output condition combinations as the HV therapy circuit LUT, because different conditions may have a greater or lesser effect on the inherent charge imbalance generated by the two different therapy circuits 83 and 85. For example, the LV therapy circuit LUT may include multiple pacing electrode vector impedance ranges paired with multiple pacing pulse width ranges.

[0109] Each cell in the charge balance modification LUT may define at least a value of n, where n is the number of a modified pulse in a series of consecutive alternating polarity cardiac pacing pulses. Each cell in the LUT may also include one or more pacing pulse modifications to be applied to the nth pacing pulse. Figure 6-8 The modification may include polarity inversion, pulse amplitude adjustment, pulse width adjustment, or any combination thereof.

[0110] In other examples, the LUT stored in the memory 82 may store the inherent charge imbalance of a pair of alternating polarity pacing pulses for a given pacing output condition or combination of pacing output conditions. The control circuit 80 may be configured to determine the charge balance modification at box 402 based on the inherent charge imbalance stored in the memory 82. For example, one or more LUTs may be stored in the memory 82, which list the inherent charge imbalance for the HV therapy circuit 83 for one or more output conditions (such as pacing electrode vector, pacing impedance, pacing frequency, pacing pulse amplitude and / or pacing pulse width). Similarly, one or more LUTs may store the inherent charge imbalance for the LV therapy circuit 83 for one or more output conditions. The control circuit 80 may retrieve the charge imbalance for a given pacing output configuration or condition and determine the pacing modification to be applied to every nth cardiac pacing pulse. In some cases, the control circuit 80 only determines the value of n to control the frequency of delivering charge-balanced pacing pulses with predetermined modifications. The predetermined modification may be a reversal of polarity, a fixed pulse amplitude change, and / or a fixed pulse width change.

[0111] When the charge balancing pulse is modified by switching its polarity, such as in combination with Figure 6 As described above, establishing the frequency at which the modified pulse polarity is applied (e.g., every n pulses) may be the only parameter that the control circuit 80 needs to determine based on the stored inherent charge imbalance at block 402 to establish the charge balance modification. For example, the control circuit may determine based on the stored inherent charge imbalance as described above. Figure 6 The value of n is determined by the unbalanced charge of each pair of pulses described, having one positive polarity pulse and one negative polarity pulse.

[0112] In examples that include adjusting the pulse amplitude and / or pulse width of the modified pulse, establishing the charge balance modification at block 402 may additionally or alternatively include: determining a pulse amplitude and / or pulse width adjustment to apply to every nth pulse based on a stored inherent charge imbalance. In some examples, n may be fixed such that: using a known charge imbalance between positive polarity pulses and negative polarity pulses, the control circuitry may calculate an appropriate polarity, amplitude, and / or pulse width adjustment that should be applied to the nth pulse to balance the charge accumulated during the previous n-1 pulses. Fig.11 Other examples of methods for establishing charge balance modifications are described.

[0113] The control circuit 80 can control the timing of pacing pulses delivered by the therapy delivery circuit 84 by setting a pacing escape interval timer or counter according to the pacing mode or therapy. The pacing escape interval can be a programmed value stored in the memory 82 (e.g., a fixed lower frequency interval or an atrioventricular pacing interval), or an automatically adjusted pacing interval (e.g., a temporary lower frequency interval of the frequency response). When the pacing interval expires at box 404, the control circuit 80 determines at box 406 whether the scheduled pacing pulse is the nth pulse in a series of consecutive alternating pacing pulses. For example, the control circuit 80 may include a counter that counts each delivered pacing pulse until n-1 delivered pacing pulses (or counts down from n-1) so that the nth pulse is delivered as a modified charge balancing pulse at box 408. If the scheduled pacing pulse is not the nth pulse as determined at block 406 , the control circuit 80 controls the therapy delivery circuitry to deliver unmodified cardiac pacing pulses according to the programmed pacing pulse amplitude, width, and alternating polarity at block 410 .

[0114] In some examples, once the charge balance modification is established at box 402, no adjustment to the modification may be required. In other examples, the control circuit 80 may determine at box 412 that a change in the pacing output has occurred. One example of a change in the pacing output is a change in the impedance of the pacing electrode vector. The control circuit 80 may control the impedance measurement circuit 90 to monitor the pacing electrode vector impedance periodically (and / or in response to detecting loss of capture or selecting a new pacing electrode vector). If a change in the impedance of the pacing electrode vector is detected (which may be due to a change in the selected pacing electrode vector), the charge imbalance between the positive polarity pulse and the negative polarity pulse may change. Other changes in the pacing output detected at box 412 may include: a change in selecting the HV therapy circuit 83 or the LV therapy circuit 85 for generating pacing pulses, a different pacing electrode vector selected for delivering pacing pulses, an adjustment to the pacing pulse amplitude, an adjustment to the pacing pulse width, and / or an adjustment to the pacing mode (e.g., resulting in an increased or decreased pacing frequency or rate).

[0115] The control circuit 80 may be configured to detect or identify a change in a pacing output condition associated with a change in charge imbalance between positive polarity pulses and negative polarity pulses, thereby warranting a change in the charge balance modification established at block 402. In response to detecting a change in a pacing output condition associated with a change in charge imbalance, the control circuit 80 may return to block 402 to select or determine a different charge balance modification to account for the change in charge imbalance for each pair of pacing pulses having one positive polarity pacing pulse and one negative polarity pacing pulse. As described above, the control circuit 80 may reestablish the charge balance modification by retrieving data from the LUT stored in the memory 82. If a change in the pacing output is not detected at block 412, the control circuit 80 continues to count pacing pulses of alternating polarity (blocks 404 and 406) and controls the therapy delivery circuit 84 to deliver each nth pulse according to the established charge balance modification at block 408.

[0116] Fig.10 4 is a conceptual diagram of one example of a LUT 450 that may be stored in memory 82 for control circuit 80 to retrieve charge balance pacing pulse modifications from. In this example, pacing output conditions are shown as pacing electrode vector impedance Z in first column 452. Three impedance ranges are shown in first column 452, but any desired number of pacing electrode vector impedances may be included in LUT 450. Other examples of pacing output conditions that may be listed in the rows of first column 452 may include, but are not limited to, a range of unmodified pacing pulse amplitudes, a range of unmodified pacing pulse widths, or a range of pacing frequencies.

[0117] The first row across LUT 450 shows different pacing conditions. In this example, an HV therapy circuit output configuration 454 and an LV therapy circuit output configuration 456 may be included in LUT 450. In other examples, a separate LUT may be provided for each of the HV therapy circuit and the LV therapy circuit, and the separate LUT may be a one-dimensional or multi-dimensional table that stores parameters used by control circuit 80 to establish charge balance modifications for a given set of pacing output conditions. In the example shown, control circuit 80 may be configured to: determine the pacing electrode vector impedance (using impedance measurement circuit 90) and the selected HV or LV therapy circuit for delivering the pacing pulse, and retrieve the number n of the charge balance modified pacing pulse from the corresponding cell 458. The nth pulse may be modified by reversing the polarity of the nth pulse in a pattern of alternating polarity pulses.

[0118] In other examples, each cell 458 of the LUT 450 can indicate a pulse amplitude modification and / or a pulse width modification to be applied to every nth pacing pulse, where n is a fixed predetermined value. In still other examples, each cell 458 can include a value of n, a pulse amplitude adjustment, a pulse width adjustment, and an indication of whether the polarity should be reversed from a normal alternating order, or any combination thereof. The range of stored pulse amplitude adjustments can be from 0 to a maximum amplitude adjustment, e.g., up to 1 to 5V increase from an unmodified pacing pulse amplitude. The range of stored pulse width adjustments can be from 0 to a maximum pulse width adjustment, e.g., up to 5 to 10ms increase from an unmodified pacing pulse width. One or more LUTs can be stored in the memory 82 to enable the control circuit 80 to retrieve a charge balancing pacing pulse modification to be applied to a combination of determined pacing output conditions.

[0119] In other examples, LUT 450 may store in each cell 458 a net charge imbalance for a pair of pacing pulses having one positive polarity pacing pulse and one negative polarity pacing pulse for a given impedance and selected therapy circuit combination (HV therapy circuit 83 or LV therapy circuit 85). Control circuit 80 may perform a calculation or algorithm for determining a charge balance modification based on the retrieved net charge imbalance. For example, control circuit 80 may store ... Figure 6 The value of n is determined based on the net charge imbalance as described. Other mathematical functions that model the relationship between pacing output conditions may be implemented in the processor of control circuit 80 for determining the charge balance modification required to balance the detected net charge imbalance within predefined limits of pacing pulse amplitude and pacing pulse width.

[0120] Fig.115 is a flowchart 500 of a modified method for establishing a charge balancing pacing pulse by the control circuit 80 of the ICD 314 according to one example. The process of the flowchart 500 may be performed at Fig. 9 402, or may be performed to generate a LUT, such as Fig.10 The control circuit 80 may be configured to execute an algorithm for determining the value of n (where n is the number of the modified pacing pulse in a series of consecutive pacing pulses), the polarity, pulse amplitude, and / or pulse width of each modified nth pacing pulse. To determine the modification(s) of the charge balancing pacing pulses, the control circuit 80 may determine one or more pacing output conditions that may alter the inherent charge imbalance between a pair of pacing pulses having one positive polarity pacing pulse and one negative polarity pacing pulse.

[0121] The control circuit 80 may select a pacing output configuration at block 501. The pacing output configuration may be selected by selecting a therapy delivery output circuit and / or a pacing electrode vector. For example, the control circuit 80 may select the HV therapy circuit 83 or the LV therapy circuit 85 for delivering pacing pulses via the selected pacing electrode vector. The LV therapy circuit 85 may be selected to deliver a single pulse pacing pulse or a composite pacing pulse comprising multiple fused pulses delivered rapidly together.

[0122] The control circuit 80 may determine the impedance of the selected pacing electrode vector at box 502 by controlling the impedance measurement circuit 90. The impedance may be determined by delivering a drive current signal across the selected pacing electrode vector and determining the resulting voltage generated across the electrode vector. At box 505, the control circuit 80 may determine the pacing pulse amplitude and / or pacing pulse width of the cardiac pacing pulse. This determination may be made by performing a cardiac pacing threshold test to establish a pacing pulse amplitude (or width) at a safety margin above a threshold pulse amplitude (or width). In other examples, the pacing pulse amplitude and width may be retrieved from the memory 82 as stored pacing control parameters. Although in Fig.11 The determination of the pacing output configuration, electrode vector impedance, and pulse amplitude and pulse width are shown as separate sequential steps, but it should be understood that since the pacing output configuration selected at box 501 can depend on the determined pacing electrode vector impedance, and the pacing pulse amplitude and pacing pulse width will depend on the capture threshold determined for a given pacing electrode vector, these determinations can be performed in a semi-simultaneous or integrated manner.

[0123] At block 506, the control circuit 80 may determine an expected or historical pacing burden or frequency. The expected pacing frequency may be based on the programmed lower frequency and / or pacing mode. For example, the expected average pacing frequency during a frequency-responsive VVIR pacing mode may be higher than the expected average pacing frequency during a non-frequency-responsive VVI pacing mode having a low programmed lower frequency (such as, 40 pulses per minute) or during VDD pacing with minimized ventricular pacing.

[0124] At block 510, the control circuit 80 may determine the expected accumulated charge imbalance after each pair of pacing pulses having one positive polarity pacing pulse and one negative polarity pacing pulse based on the impedance, pacing pulse amplitude and width, the desired pacing frequency, and the output configuration. In some examples, the control circuit 80 may use a pacing electrode vector having an impedance Z when the charge of the delivered pacing pulse has a pulse amplitude V (in volts) and a pulse width PW in seconds to utilize the relationship Q=V*PW / Z for determining Q. In another example, the control circuit 80 may measure the current I (in amperes) delivered during a pacing pulse having a pulse width PW (in seconds) and utilize the relationship Q=I*PW to determine the charge Q delivered during the pacing pulse. The delivered charge for a set of pacing output conditions may be determined for positive polarity pulses and for negative polarity pulses to determine the net charge imbalance of the pair of pulses having one positive polarity pulse and one negative polarity pulse.

[0125] One or more pacing condition inputs may be used to model or estimate the charge imbalance obtained during empirical testing. For example, a mathematical model of net charge imbalance may be generated for one or more independent pacing output conditions during a baseline test of the ICD 314. The control circuit 80 may provide the input pacing conditions to the model (stored in the memory 82) to determine the predicted charge imbalance for each pair of alternating polarity pulses. After inputting the conditions determined at blocks 501 to 506, a predicted net charge imbalance for a pair of opposite polarity pulses may be output from the mathematical model. Based on the predicted net charge imbalance for each pair of opposite polarity pacing pulses, the control circuit 80 may determine a charge balance modification for application on each nth pacing pulse, which charge balance modification may include reversing polarity, adjusting pulse amplitude, and / or adjusting pulse width. Additionally or alternatively, the control circuit 80 may be configured to determine n (the number of pacing pulses delivered in a series of alternating polarity pacing pulses ending with a modified charge balance pulse), and a default modification (such as polarity reversal) may be applied to the nth pacing pulse. For example, if the net charge imbalance is determined to be 5% based on the pacing output configuration and the conditions determined at blocks 502 to 508, the control circuit 80 may determine n to be 41. The 41st pulse may be delivered with the opposite polarity of the charge imbalance of a pair of alternating polarity pulses to balance the charge accumulated during the previous 40 pacing pulses. Other examples of pacing pulse modifications that the control circuit 80 may determine at block 510 have been described above.

[0126] Although the above examples include modifying every nth pulse using the same modification(s) of polarity reversal, pulse amplitude, and / or pulse width, it is further contemplated that the charge balancing cardiac pacing techniques disclosed herein may include modifying more than one pulse in a sequence of pulses, e.g., modifying every nth pulse using a first modification, and modifying every mth pulse using a second modification that is different from the first modification. For example, every nth pulse may be delivered with a modified pulse amplitude, and every mth pulse may be delivered with a polarity opposite to the intended alternating polarity pattern. In one illustrative example, every 21st pulse is modified according to the first modification, and every 201st pulse is modified according to the second modification. Thus, in some examples, the control circuit 80 may establish more than one charge balancing modification at block 510. A first modification for every nth pulse and a second modification for every mth pulse may be determined.

[0127] The modification of every mth pulse can minimize any residual charge imbalance left after the previous number of pulses in a series of n pulses are delivered. For example, if the positive pulse delivers 10% more charge than the negative pulse, the modified nth pacing pulse can be the 21st pacing pulse delivered with a negative polarity. However, this nth pulse may not balance 100% of the positive charge accumulated with the previous 20 pulses. For example, this one negative pulse can balance 90% of the positive charge accumulated with the previous 20 pulses because it is 10% less than the charge delivered by the positive pulse. Therefore, every 201st pulse with a negative polarity can be delivered to minimize the residual charge after every 20th modified nth pulse.

[0128] In such an example, m can have a higher value than n. In other examples, m and n can be equal but staggered so that in a series of pacing pulses, the mth and nth pulses are spaced apart and can have different charge balance modifications. For example, if both m and n are equal to 5, then in a series of 5 pulses, the first two pulses can be unmodified (u), the third pulse can be modified according to a first modification (n), the fourth pulse can be unmodified, and the fifth pulse can be modified according to a second modification (m), and this pattern of five pulses can be repeated (uunumuunum...). The second modification of every mth pulse can further reduce any charge imbalance that is not eliminated by the first modification of every nth pulse.

[0129] It should be understood that, depending on the example, certain actions or events in any of the methods described herein may be performed in a different order, may be added, merged, or omitted entirely (e.g., not all described actions or events are necessary to practice the method). In addition, in some examples, actions or events may be performed simultaneously rather than sequentially, for example, through multithreading, interrupt processing, or multiple processors. In addition, although for clarity, certain aspects of the present disclosure are described as being performed by a single device, circuit, or unit, it should be understood that the techniques of the present disclosure may be performed by a combination of units or circuits associated with, for example, one or more medical devices.

[0130] In one or more examples, the described functions can be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, these functions can be stored as one or more instructions or codes on a computer-readable medium and executed by a hardware-based processing unit. A computer-readable medium may include a computer-readable storage medium, which corresponds to a tangible medium, such as a data storage medium (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and can be accessed by a computer).

[0131] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general-purpose microprocessors, application-specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuit systems. Accordingly, the term "processor" as used herein may refer to any of the above structures or any other structure suitable for implementing the techniques described herein. In addition, these techniques may be fully implemented in one or more circuits or logic elements.

[0132] Therefore, the medical device system has been presented in the foregoing description with reference to specific examples. It will be understood that the various aspects disclosed herein may be combined in combinations different from the specific combinations presented in the drawings. It will be understood that various modifications may be made to the referenced examples without departing from the scope of the present disclosure and the appended claims and examples.

[0133] Example 1 A method includes delivering a plurality of electrical stimulation pulses including electrical stimulation pulses of opposite polarity; and delivering a charge balancing pulse by modifying every nth pulse of the plurality of electrical stimulation pulses to reduce a net charge delivered with the plurality of electrical stimulation pulses.

[0134] Example 2 The method of Example 1, wherein delivering a plurality of electrical stimulation pulses comprises delivering the electrical stimulation pulses as alternating polarity pulses.

[0135] Example 3 The method of any of Examples 1 or 2, wherein delivering multiple electrical stimulation pulses includes: delivering at least one pair of opposite polarity pulses, wherein the opposite polarity pulses include a positive polarity pacing pulse and a negative polarity pacing pulse having unbalanced charge; and modifying the nth pulse to reduce the accumulation of unbalanced charge after the at least one pair of opposite polarity pulses.

[0136] Example 4 The method of Example 3 further includes determining a value of n based on a charge imbalance of each pair of pulses having one positive polarity pulse and one negative polarity pulse.

[0137] Example 5 The method of any of Examples 1-4, wherein modifying every nth pulse of the plurality of electrical stimulation pulses comprises reversing a polarity of the nth pulse.

[0138] Example 6 The method of any of Examples 1-5, wherein modifying every nth pulse of the plurality of electrical stimulation pulses comprises adjusting at least one of a pulse amplitude or a pulse width.

[0139] Example 7 The method of any of Examples 1-6, further comprising: storing a lookup table of pulse modifications in a memory; establishing a modification to every nth pulse by retrieving the modification from the lookup table; and modifying every nth pulse according to the established modification.

[0140] Example 8 The method of any of Examples 1-7, further comprising: selecting an electrical stimulation pulse output configuration, and establishing the modification of every nth pulse based on the selected electrical stimulation pulse output configuration.

[0141] Example 9 The method of Example 8 further includes: selecting an electrical stimulation pulse output configuration by selecting one of a first output circuit and a second output circuit in a therapy delivery circuit of a medical device for delivering multiple electrical stimulation pulses; and establishing a modification of each nth pulse by establishing at least one of the following: the value of n, the polarity of the nth pulse, the amplitude of the nth pulse, or the width of the nth pulse.

[0142] Example 10 The method of any of Examples 1-9, further comprising: detecting a change in an electrical stimulation pulse output condition, and changing the charge balancing pulse in response to detecting the change in the electrical stimulation pulse output condition.

[0143] Example 11 The method of Example 10 further includes detecting changes in electrical stimulation pulse output conditions by detecting at least one of the following: changes in cardiac pacing mode, changes in electrode vector impedance, changes in pulse amplitude of multiple electrical stimulation pulses, changes in pulse width of multiple electrical stimulation pulses, or changes in frequency of electrical stimulation pulses.

[0144] Example 12 The method of any of Examples 10-11, further comprising changing the charge balance pulse by adjusting at least one of: a value of n, a polarity of the nth pulse, a pulse amplitude of the nth pulse, or a pulse width of the nth pulse.

[0145] Example 13 The method of any of Examples 1-12, further comprising modifying every nth pulse using the first modification and modifying every mth pulse of the plurality of electrical stimulation pulses using the second modification for reducing a charge imbalance remaining after the nth pulse.

[0146] Example 14 The method of any of Examples 1-13, further comprising delivering a plurality of electrical stimulation pulses via at least one electrode carried by a cardiovascular external lead.

[0147] Example 15 The method of any of Examples 1-14, wherein delivering the electrical stimulation pulse comprises delivering a cardiac pacing pulse.

[0148] Example 16 A non-transitory computer-readable medium storing a set of instructions that, when executed by control circuitry of a medical device, cause the medical device to: deliver a plurality of electrical stimulation pulses comprising electrical stimulation pulses of opposite polarity, and deliver a charge balancing pulse by modifying every nth pulse in the plurality of electrical stimulation pulses to reduce a net charge delivered with the plurality of electrical stimulation pulses.

Claims

1. A medical device, comprising: a therapy delivery circuit comprising a pulse generator configured to generate electrical stimulation pulses and deliver the electrical stimulation pulses via electrodes coupleable to the therapy delivery circuit; as well as a control circuit coupled to the therapy delivery circuit and configured to control the therapy delivery circuit to: delivering a plurality of electrical stimulation pulses comprising electrical stimulation pulses of opposite polarity; and delivering a charge balancing pulse by modifying every nth pulse in the plurality of electrical stimulation pulses to reduce a net charge delivered with the plurality of electrical stimulation pulses, Wherein, each charge balanced pulse is preceded by a series of n-1 unmodified electrical stimulation pulses, the n-1 unmodified electrical stimulation pulses including at least one first pulse having a first polarity, and at least one second pulse having a second polarity opposite to the first polarity, and the at least one first pulse and the at least one second pulse are charge unbalanced pulses that deliver the net charge.

2. The device according to claim 1, characterized in that The control circuit is configured to control the therapy delivery circuit to deliver the plurality of electrical stimulation pulses as alternating polarity pulses.

3. A medical device, comprising: a therapy delivery circuit comprising a pulse generator configured to generate electrical stimulation pulses and deliver the electrical stimulation pulses via electrodes coupleable to the therapy delivery circuit; as well as a control circuit coupled to the therapy delivery circuit and configured to control the therapy delivery circuit to: delivering a plurality of electrical stimulation pulses comprising electrical stimulation pulses of opposite polarity; and delivering a charge balancing pulse by modifying every nth pulse in the plurality of electrical stimulation pulses to reduce a net charge delivered with the plurality of electrical stimulation pulses, Wherein, the control circuit is configured to: controlling the therapy delivery circuit to deliver the plurality of electrical stimulation pulses comprising at least one pair of opposite polarity pulses comprising a positive polarity pulse and a negative polarity pulse having unbalanced charges; and The therapy delivery circuit is controlled to modify the nth pulse to reduce accumulation of the unbalanced charge after the at least one pair of opposite polarity pulses.

4. The device according to claim 3, characterized in that The control circuit is further configured to: The value of n is determined based on the unbalanced charge of each pair of pulses having one positive polarity pulse and one negative polarity pulse.

5. The device according to claim 4, characterized in that The control circuit is configured to control the therapy delivery circuit to modify each nth pulse of the plurality of electrical stimulation pulses by reversing the polarity of the nth pulse.

6. The device according to claim 3, characterized in that The control circuit is configured to control the therapy delivery circuit to modify every nth pulse of the plurality of electrical stimulation pulses by adjusting at least one of a pulse amplitude or a pulse width.

7. The device according to claim 3, characterized in that The medical device further comprises a memory comprising a lookup table of pulse modifications, Wherein, the control circuit is configured to: establishing said modification for every nth pulse by retrieving a modification from said look-up table; and The therapy delivery circuit is controlled to modify every nth pulse according to the established modification.

8. The device according to claim 3, characterized in that The control circuit is configured to: Selecting an electrical stimulation pulse output configuration; and Based on the selected electrical stimulation output configuration, a modification of every nth pulse is established.

9. The device according to claim 8, characterized in that: The therapy delivery circuit includes a first output circuit for generating and delivering electrical stimulation pulses and a second output circuit for generating and delivering electrical stimulation pulses; and The control circuit is configured to: selecting the electrical stimulation pulse output configuration by selecting one of the first output circuit and the second output circuit for delivering the plurality of electrical stimulation pulses, and The modification of every nth pulse is established by establishing at least one of: a value of n, a polarity of the nth pulse, an amplitude of the nth pulse, or a width of the nth pulse.

10. The device according to claim 3, characterized in that The control circuit is further configured to: detecting a change in an electrical stimulation pulse output condition; and The charge balancing pulse is altered in response to detecting a change in the electrical stimulation pulse output condition.

11. The device according to claim 10, characterized in that The control circuit is configured to detect a change in the electrical stimulation output condition by detecting at least one of: a change in cardiac pacing mode, a change in electrode vector impedance, a change in pulse amplitude of the multiple electrical stimulation pulses, a change in pulse width of the multiple electrical stimulation pulses, or a change in frequency of the electrical stimulation pulses.

12. The device according to any one of claims 10 to 11, characterized in that The control circuit is configured to change the charge balancing pulse by adjusting at least one of: a value of n, a polarity of the nth pulse, a pulse amplitude of the nth pulse, or a pulse width of the nth pulse.

13. The device according to claim 3, characterized in that The control circuit is further configured to: Every nth pulse is modified using the first modification, and every mth pulse of the plurality of electrical stimulation pulses is modified using the second modification for reducing a charge imbalance remaining after the nth pulse.

14. The device according to claim 13, characterized in that Further including: A housing encloses the therapy delivery circuitry and the control circuitry and is coupleable to an extracardiovascular cardiac pacing lead carrying at least one of the electrodes.

15. The device according to claim 14, characterized in that The therapy delivery circuit is configured to generate and deliver the electrical stimulation pulses as cardiac pacing pulses.

16. The device according to claim 3, characterized in that The control circuit is configured to deliver the plurality of electrical stimulation pulses as alternating polarity pulses.

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