Patent Publication Number: US-11638828-B2

Title: Demand driven capacitor charging for cardiac pacing

Description:
RELATED APPLICATION 
     This application is a Division of U.S. patent application Ser. No. 15/676,066, filed Aug. 14, 2017, the content of both of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to an implantable medical device (IMD) system and method that delivers cardiac pacing pulses and particularly to an IMD system and method for controlling the charging of capacitors used for generating and delivering cardiac pacing pulses based on pacing demand. 
     BACKGROUND 
     Medical devices, such as cardiac pacemakers and implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include cardiac pacing pulses or cardioversion/defibrillation (CV/DF) shocks. 
     The medical device may sense cardiac electrical events attendant to the intrinsic heart activity for detecting an abnormal intrinsic heart rhythm. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation therapy may be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver CV/DF shocks to the heart upon detecting tachycardia or fibrillation. 
     The ICD may sense the cardiac electrical signals from a heart chamber and deliver electrical stimulation therapies to the heart chamber using endocardial electrodes carried by transvenous medical electrical leads. In other cases, a non-transvenous lead may be coupled to the ICD, in which case the ICD may sense cardiac electrical signals and deliver electrical stimulation therapy to the heart using extra-cardiovascular electrodes. The energy of a therapeutic electrical stimulation pulse required to effectively stimulate the heart using the extra-cardiovascular electrodes is typically greater than the energy required to stimulate the heart using endocardial electrodes. A pacing circuit may include a holding capacitor that is charged to a pacing voltage amplitude for generating a pacing pulse according to the pacing pulse energy required to capture the pacing heart using the pacing electrode vector that is available. 
     SUMMARY 
     In general, the disclosure is directed to techniques for controlling charging of at least one holding capacitor that is used to deliver a cardiac electrical stimulation pulse by a therapy delivery circuit of an implantable medical device. An IMD operating according to these techniques may withhold capacitor charging when increased intrinsic heart rate criteria are satisfied. Charging of the capacitor may be withheld for at least a portion of a pacing interval, e.g., by charging after a delay interval has expired. The charging delay interval may be equal to, greater than or less than the pacing interval. In response to decreased heart rate criteria being satisfied, the IMD may switch back to charging the holding capacitor without delay, e.g., at the beginning or throughout a pacing interval as needed to maintain the holding capacitor charge at the pacing voltage amplitude in a ready state for delivering a pacing pulse. In some examples, the IMD may be configured to control when the function of switching between two different charging modes, e.g., a delayed capacitor charging mode and a capacitor charging without delay mode, is enabled (turned on) or disabled (turned off). When this charging mode switching function is disabled, the IMD may operate to charge the holding capacitor according to one, default charging mode. When the charging mode switching function is enabled, the IMD may operate to switch between two different charging modes based on intrinsic heart rate criteria and/or other pacing demand criteria. 
     In one example, the disclosure provides an IMD system including a therapy delivery circuit, a sensing circuit and a control circuit coupled to the therapy delivery circuit and the sensing circuit. The therapy delivery circuit has a holding capacitor and a charging circuit configured to charge the holding capacitor to a pacing voltage amplitude. The sensing circuit is configured to receive a cardiac electrical signal from a patient&#39;s heart. The control circuit is configured to control the therapy delivery circuit to deliver a pacing pulse, start a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse, control the therapy delivery circuit to charge the holding capacitor during the first pacing interval according to a first charging mode, detect an increased intrinsic heart rate from the cardiac electrical signal that is at least a threshold rate faster than the pacing rate, switch from the first charging mode to a second charging mode in response to detecting the increased intrinsic heart rate, start a second pacing interval in response to an intrinsic cardiac event sensed from the cardiac electrical signal, and control the therapy delivery circuit to withhold charging of the holding capacitor for at least a portion of the second pacing interval according to the second charging mode. 
     In another example, the disclosure provides a method including delivering a pacing pulse by a therapy delivery circuit having a holding capacitor and a charging circuit configured to the charge the holding capacitor to a pacing voltage amplitude and starting a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse. The method further includes charging the holding capacitor during the first pacing interval according to a first charging mode and detecting an increased intrinsic heart rate that is at least a threshold rate faster than the pacing rate from a cardiac electrical signal received by a sensing circuit, switching from the first charging mode to a second charging mode in response to detecting the increased intrinsic heart rate, starting a second pacing interval in response to a first intrinsic cardiac event sensed from the cardiac electrical signal; and withholding charging of the holding capacitor for at least a portion of the second pacing interval according to the second charging mode. 
     In another example, the disclosure provides a non-transitory, computer-readable storage medium comprising a set of instructions which, when executed by a control circuit of an implantable medical device, cause the device to deliver a pacing pulse by a therapy delivery circuit having a holding capacitor and a charging circuit configured to the charge the holding capacitor to a pacing voltage amplitude and start a first pacing interval corresponding to a pacing rate in response to the delivered pacing pulse. The instructions further cause the device to charge the holding capacitor during the first pacing interval according to a first charging mode, detect an increased intrinsic heart rate that is at least a threshold rate faster than the pacing rate from a cardiac electrical signal received by a sensing circuit, switch from the first charging mode to a second charging mode in response to detecting the increased intrinsic heart rate, start a second pacing interval in response to a first intrinsic cardiac event sensed from the cardiac electrical signal; and withhold charging of the holding capacitor for at least a portion of the second pacing interval according to the second charging mode. 
     This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the accompanying drawings and description below. Further details of one or more examples are set forth in the accompanying drawings and the description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are conceptual diagrams of an extra-cardiovascular ICD system according to one example. 
         FIGS.  2 A- 2 C  are conceptual diagrams of a patient implanted with the extra-cardiovascular ICD system of  FIG.  1 A  in a different implant configuration. 
         FIG.  3    is a schematic diagram of the ICD of  FIGS.  1 A- 2 C  according to one example. 
         FIG.  4    is diagram of a high voltage therapy circuit of the ICD of  FIG.  3    according to one example. 
         FIG.  5    is a diagram of a low voltage therapy circuit of the ICD of  FIG.  3    according to one example. 
         FIG.  6    is a flow chart of a method for controlling capacitor charging for pacing pulse delivery according to one example. 
         FIG.  7    is a flow chart of a method for controlling holding capacitor charging based on intrinsic heart rate criteria according to one example. 
         FIGS.  8 A through  8 D  are timing diagrams depicting operations performed by an ICD in controlling holding capacitor charging based on the timing of sensed intrinsic events. 
         FIGS.  9 A- 9 C  are timing diagrams depicting operations performed by an ICD or pacemaker for withholding capacitor charging according to a delayed capacitor charging mode and switching back to a charging without delay. 
         FIG.  10    is a flow chart of a method for controlling capacitor charging according to yet another example. 
         FIG.  11    is a flow chart of a method for controlling the capacitor charging mode based on the rate or slope of a change in heart rate. 
         FIG.  12    is flow chart of a method for controlling capacitor charging for cardiac pacing according to another example. 
         FIG.  13    is a flow chart of a method for controlling capacitor charging based on different pacing therapies according to one example. 
         FIG.  14    is a diagram of another IMD system that may be configured to control capacitor charging for pacing therapy delivery using the techniques disclosed herein. 
         FIG.  15    is a flow chart of a method for controlling holding capacitor charging according to yet another example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes techniques for controlling charging of a holding capacitor in a therapy delivery circuit of a cardiac medical device or system. A holding capacitor, or a combination of holding capacitors, is generally charged by a charging circuit to a pacing voltage amplitude for generating and delivering a cardiac pacing pulse. The holding capacitor may be charged as needed at the beginning or throughout a pacing interval that is started immediately after a pacing pulse or sensed intrinsic cardiac event, such as an R-wave or P-wave. In this way, the holding capacitor is maintained at the pacing voltage amplitude in a ready state until a pacing timing interval expires. Maintaining the charge of a holding capacitor or combination of capacitors at the pacing voltage amplitude, however, consumes energy supplied by the power source of the IMD. The techniques disclosed herein may be used to conserve energy normally required to charge and maintain a holding capacitor in a ready state for delivering cardiac pacing pulses by withholding capacitor charging during at least a portion or all of a pacing interval when increased heart rate criteria are met and/or low pacing demand criteria are met. 
     In some examples, the cardiac medical device system implementing the techniques disclosed herein may be an extra-cardiovascular ICD system. As used herein, the term “extra-cardiovascular” refers to a position outside the blood vessels and heart of a patient. Implantable electrodes carried by extra-cardiovascular leads may be positioned extra-thoracically (outside the ribcage and sternum, e.g., subcutaneously) or intra-thoracically (beneath the ribcage or sternum, e.g., substernally) but generally not in intimate contact with myocardial tissue. The techniques disclosed herein for controlling capacitor charging may be applied to a therapy delivery circuit that is coupled to extra-cardiovascular electrodes and likely to require relatively higher pacing voltage amplitude and/or longer pulse widths than a cardiac medical device coupled to endocardial or epicardial electrodes. 
     As such, techniques disclosed herein are described in conjunction with an ICD and implantable medical lead carrying extra-cardiovascular electrodes, but aspects disclosed herein may be utilized in conjunction with other cardiac medical devices or systems. For example, the techniques for controlling capacitor charging as described in conjunction with the accompanying drawings may be implemented in any implantable or external medical device enabled for delivering cardiac electrical stimulation pulses, including implantable pacemakers or ICDs coupled to transvenous, pericardial or epicardial leads carrying sensing and therapy delivery electrodes; intra-cardiac or leadless pacemakers or ICDs having housing-based electrodes; or external or wearable pacemakers or defibrillators coupled to external, surface or skin electrodes. 
       FIGS.  1 A and  1 B  are conceptual diagrams of an extra-cardiovascular ICD system  10  according to one example.  FIG.  1 A  is a front view of ICD system  10  implanted within patient  12 .  FIG.  1 B  is a side view of ICD system  10  implanted within patient  12 . ICD system  10  includes an ICD  14  connected to an extra-cardiovascular electrical stimulation and sensing lead  16 .  FIGS.  1 A and  1 B  are described in the context of an ICD system  10  capable of providing defibrillation and/or cardioversion shocks and pacing pulses. 
     ICD  14  includes a housing  15  that forms a hermetic seal that protects internal components of ICD  14 . The housing  15  of ICD  14  may be formed of a conductive material, such as titanium or titanium alloy. The housing  15  may function as an electrode (sometimes referred to as a “can” electrode). Housing  15  may be used as an active can electrode for use in delivering cardioversion/defibrillation (CV/DF) shocks or other electrical pulses including pacing pulses that may be delivered using a high voltage therapy circuit. In other examples, housing  15  may be available for use in delivering unipolar, cardiac pacing pulses from a low voltage therapy circuit and/or for sensing cardiac electrical signals in combination with electrodes carried by lead  16 . In other instances, the housing  15  of ICD  14  may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing  15  functioning as an electrode(s) may be coated with a material, such as titanium nitride, e.g., for reducing post-stimulation polarization artifact. 
     ICD  14  includes a connector assembly  17  (also referred to as a connector block or header) that includes electrical feedthroughs crossing housing  15  to provide electrical connections between conductors extending within the lead body  18  of lead  16  and electronic components included within the housing  15  of ICD  14 . As will be described in further detail herein, housing  15  may house one or more processors, memories, transceivers, electrical cardiac signal sensing circuitry, therapy delivery circuitry, power sources and other components for sensing cardiac electrical signals, detecting a heart rhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm. Elongated lead body  18  has a proximal end  27  that includes a lead connector (not shown) configured to be connected to ICD connector assembly  17  and a distal portion  25  that includes one or more electrodes. In the example illustrated in  FIGS.  1 A and  1 B , the distal portion  25  of lead body  18  includes defibrillation electrodes  24  and  26  and pace/sense electrodes  28  and  30 . In some cases, defibrillation electrodes  24  and  26  may together form a defibrillation electrode in that they may be configured to be activated concurrently. Alternatively, defibrillation electrodes  24  and  26  may form separate defibrillation electrodes in which case each of the electrodes  24  and  26  may be activated independently. 
     Electrodes  24  and  26  (and in some examples housing  15 ) are referred to herein as defibrillation electrodes because they are utilized, individually or collectively, for delivering high voltage stimulation therapy (e.g., cardioversion or defibrillation shocks). Electrodes  24  and  26  may be elongated coil electrodes and generally have a relatively high surface area for delivering high voltage electrical stimulation pulses compared to pacing and sensing electrodes  28  and  30 . However, electrodes  24  and  26  and housing  15  may also be utilized to provide pacing functionality, sensing functionality or both pacing and sensing functionality in addition to or instead of high voltage stimulation therapy. In this sense, the use of the term “defibrillation electrode” herein should not be considered as limiting the electrodes  24  and  26  for use in only high voltage cardioversion/defibrillation shock therapy applications. For example, electrodes  24  and  26  may be used in a sensing vector used to sense cardiac electrical signals and detect and discriminate abnormal rhythms such as asystole, bradycardia, non-sinus tachycardia or fibrillation and/or used in a pacing electrode vector for delivering cardiac pacing pulses to heart  8 . 
     Electrodes  28  and  30  are relatively smaller surface area electrodes which are available for use in sensing electrode vectors for sensing cardiac electrical signals and may be used for delivering pacing pulses in some configurations. Electrodes  28  and  30  are referred to as pace/sense electrodes because they are generally configured for use in low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses and/or sensing of cardiac electrical signals, as opposed to delivering high voltage cardioversion/defibrillation shocks. In some instances, electrodes  28  and  30  may provide only pacing functionality, only sensing functionality or both. ICD  14  may obtain cardiac electrical signals corresponding to electrical activity of heart  8  via one or more sensing vectors that include combinations of electrodes  24 ,  26 ,  28  and/or  30 . In some examples, housing  15  of ICD  14  is used in combination with one or more of electrodes  24 ,  26 ,  28  and/or  30  in a sensing electrode vector. 
     In the example illustrated in  FIGS.  1 A and  1 B , electrode  28  is located proximal to defibrillation electrode  24 , and electrode  30  is located between defibrillation electrodes  24  and  26 . Electrodes  28  and  30  may be positioned at other locations along lead body  18  and are not limited to the positions shown. Fewer or more pace/sense electrodes may be carried by lead  16 . For instance, a third pace/sense electrode may be located distal to defibrillation electrode  26  in some examples. Electrodes  28  and  30  are illustrated as ring electrodes; however, electrodes  28  and  30  may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, or the like. 
     In the example shown, lead  16  extends subcutaneously or submuscularly over the ribcage  32  medially from the connector assembly  27  of ICD  14  toward a center of the torso of patient  12 , e.g., toward xiphoid process  20  of patient  12 . At a location near xiphoid process  20 , lead  16  bends or turns and extends superior subcutaneously or submuscularly over the ribcage and/or sternum, substantially parallel to sternum  22 . Although illustrated in  FIG.  1 A  as being offset laterally from and extending substantially parallel to sternum  22 , the distal portion  25  of lead  16  may be implanted at other locations, such as over sternum  22 , offset to the right or left of sternum  22 , angled laterally from sternum  22  toward the left or the right, or the like. Alternatively, lead  16  may be placed along other subcutaneous or submuscular paths. The path of extra-cardiovascular lead  16  may depend on the location of ICD  14 , the arrangement and position of electrodes carried by the lead body  18 , and/or other factors. 
     Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body  18  of lead  16  from the lead connector at the proximal lead end  27  to electrodes  24 ,  26 ,  28 , and  30  located along the distal portion  25  of the lead body  18 . The elongated electrical conductors contained within the lead body  18  are each electrically coupled with respective defibrillation electrodes  24  and  26  and pace/sense electrodes  28  and  30 , which may be separate respective insulated conductors within the lead body  18 . The respective conductors electrically couple the electrodes  24 ,  26 ,  28 , and  30  to circuitry, such as a therapy delivery circuit and/or a sensing circuit, of ICD  14  via connections in the connector assembly  17 , including associated electrical feedthroughs crossing housing  15 . The electrical conductors transmit therapy from a therapy delivery circuit within ICD  14  to one or more of defibrillation electrodes  24  and  26  and/or pace/sense electrodes  28  and  30  and transmit sensed electrical signals from one or more of defibrillation electrodes  24  and  26  and/or pace/sense electrodes  28  and  30  to the sensing circuit within ICD  14 . 
     The lead body  18  of lead  16  may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. Lead body  18  may be tubular or cylindrical in shape. In other examples, the distal portion  25  (or all of) the elongated lead body  18  may have a flat, ribbon or paddle shape. Lead body  18  may be formed having a preformed distal portion  25  that is generally straight, curving, bending, serpentine, undulating or zig-zagging. 
     In the example shown, lead body  18  includes a pre-formed curving distal portion  25  having two “C” shaped curves, which together may resemble the Greek letter epsilon, “ϵ” Defibrillation electrodes  24  and  26  are each carried by one of the two respective C-shaped portions of the lead body distal portion  25 . The two C-shaped curves are seen to extend or curve in the same direction away from a central axis of lead body  18 , along which pace/sense electrodes  28  and  30  are positioned. Pace/sense electrodes  28  and  30  may, in some instances, be approximately aligned with the central axis of the straight, proximal portion of lead body  18  such that mid-points of defibrillation electrodes  24  and  26  are laterally offset from pace/sense electrodes  28  and  30 . 
     Other examples of extra-cardiovascular leads including one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by curving, serpentine, undulating or zig-zagging distal portion of the lead body  18  that may be implemented with the techniques described herein are generally disclosed in pending U.S. Pat. Publication No. 2016/0158567 (Marshall, et al.), incorporated herein by reference in its entirety. The techniques disclosed herein are not limited to any particular lead body design, however. In other examples, lead body  18  is a flexible elongated lead body without any pre-formed shape, bends or curves. Various example configurations of extra-cardiovascular leads and electrodes and dimensions that may be implemented in an IMD system employing the techniques disclosed herein are described in U.S. Publication No. 2015/0306375 (Marshall, et al.) and U.S. Publication No. 2015/0306410 (Marshall, et al.), both of which are incorporated herein by reference in their entirety. 
     ICD  14  analyzes the cardiac electrical signals received from one or more sensing electrode vectors to monitor for abnormal rhythms, such as asystole, bradycardia, or tachyarrhythmias. ICD  14  may be configured to set pacing intervals for timing the delivery of cardiac pacing pulses according to programmed pacing therapy control parameters. ICD  14  may be configured to operate according to multiple pacing modes, e.g., VVI(R), VDI(R), VVO(R), etc., and set the pacing timing intervals accordingly. ICD  14  delivers a cardiac pacing pulse in response to a pacing timing interval expiring. For example, when a ventricular pacing interval expires without sensing an intrinsic R-wave during the pacing interval, ICD  14  delivers a pacing pulse to maintain at least a programmed minimum heart rate or provide back-up pacing during asystole, e.g., following a CV/DF shock. Cardiac pacing pulses may be delivered using defibrillation electrodes  24  and  26  as an anode and cathode pair, using pacing and sensing electrodes  28  and  30  as an anode and cathode pair, or one of pace/sense electrodes  28  or  30  paired with one of defibrillation electrodes  24  or  26 , or any one of electrodes  24 ,  26 ,  28  or  30  paired with housing  15 . 
     ICD  14  may also be configured to deliver electrical stimulation therapy in response to detecting a tachyarrhythmia (e.g., VT or VF) using a therapy delivery electrode vector which may be selected from any of the available electrodes  24 ,  26 ,  28   30  and/or housing  15 . ICD  14  may analyze the heart rate and morphology of the cardiac electrical signals to monitor for tachyarrhythmia in accordance with any of a number of tachyarrhythmia detection techniques. One example technique for detecting tachyarrhythmia is described in U.S. Pat. No. 7,761,150 (Ghanem, et al.), incorporated herein by reference in its entirety. ICD  14  may deliver anti-tachycardia pacing (ATP) in response to VT detection and in some cases may deliver ATP prior to a CV/DF shock or during high voltage capacitor charging in an attempt to avert the need for delivering a CV/DF shock. If ATP 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 . 
       FIGS.  1 A and  1 B  are illustrative in nature and should not be considered limiting of the practice of the techniques disclosed herein. ICD  14  is shown implanted subcutaneously on the left side of patient  12  along the ribcage  32 . ICD  14  may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient  12 . ICD  14  may, however, be implanted at other subcutaneous or submuscular locations in patient  12 . For example, ICD  14  may be implanted in a subcutaneous pocket in the pectoral region. In this case, lead  16  may extend subcutaneously or submuscularly from ICD  14  toward the manubrium of sternum  22  and bend or turn and extend inferiorly from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD  14  may be placed abdominally. Lead  16  may be implanted in other extra-cardiovascular locations as well. For instance, as described with respect to  FIGS.  2 A- 2 C , the distal portion  25  of lead  16  may be implanted underneath the sternum/ribcage in the substernal space. 
     An external device  40  is shown in telemetric communication with ICD  14  by a communication link  42 . External device  40  may include a processor, display, user interface, telemetry unit and other components for communicating with ICD  14  for transmitting and receiving data via communication link  42 . Communication link  42  may be established between ICD  14  and external device  40  using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) or other RF or communication frequency bandwidth or protocol. 
     External device  40  may be embodied as a programmer used in a hospital, clinic or physician&#39;s office to retrieve data from ICD  14  and to program operating parameters and algorithms in ICD  14  for controlling ICD functions. External device  40  may be used to program cardiac event sensing parameters (e.g., R-wave sensing parameters), cardiac rhythm detection parameters (e.g., VT and VF detection parameters and SVT discrimination parameters) and therapy control parameters used by ICD  14 . Data stored or acquired by ICD  14 , including physiological signals or associated data derived therefrom, results of device diagnostics, and histories of detected rhythm episodes and delivered therapies, may be retrieved from ICD  14  by external device  40  following an interrogation command. External device  40  may alternatively be embodied as a home monitor or hand held device. 
       FIGS.  2 A- 2 C  are conceptual diagrams of patient  12  implanted with extra-cardiovascular ICD system  10  in a different implant configuration than the arrangement shown in  FIGS.  1 A- 1 B .  FIG.  2 A  is a front view of patient  12  implanted with ICD system  10 .  FIG.  2 B  is a side view of patient  12  implanted with ICD system  10 .  FIG.  2 C  is a transverse view of patient  12  implanted with ICD system  10 . In this arrangement, extra-cardiovascular lead  16  of system  10  is implanted at least partially underneath sternum  22  of patient  12 . Lead  16  extends subcutaneously or submuscularly from ICD  14  toward xiphoid process  20  and at a location near xiphoid process  20  bends or turns and extends superiorly within anterior mediastinum  36  in a substernal position. 
     Anterior mediastinum  36  may be viewed as being bounded laterally by pleurae  39 , posteriorly by pericardium  38 , and anteriorly by sternum  22  (see  FIG.  2 C ). The distal portion  25  of lead  16  may extend along the posterior side of sternum  22  substantially within the loose connective tissue and/or substernal musculature of anterior mediastinum  36 . A lead implanted such that the distal portion  25  is substantially within anterior mediastinum  36 , may be referred to as a “substernal lead.” 
     In the example illustrated in  FIGS.  2 A- 2 C , lead  16  is located substantially centered under sternum  22 . In other instances, however, lead  16  may be implanted such that it is offset laterally from the center of sternum  22 . In some instances, lead  16  may extend laterally such that distal portion  25  of lead  16  is underneath/below the ribcage  32  in addition to or instead of sternum  22 . In other examples, the distal portion  25  of lead  16  may be implanted in other extra-cardiovascular, intra-thoracic locations, including the pleural cavity or around the perimeter of or within the pericardium  38  of heart  8 . Other implant locations and lead and electrode arrangements that may be used in conjunction with the capacitor charging techniques described herein are generally disclosed in the above-incorporated references. 
       FIG.  3    is a schematic diagram of ICD  14  according to one example. The electronic circuitry enclosed within housing  15  (shown schematically as an electrode in  FIG.  3   ) includes software, firmware and hardware that cooperatively monitor cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver electrical stimulation therapies as needed according to programmed therapy delivery algorithms and control parameters. ICD  14  is coupled to an extra-cardiovascular lead, such as lead  16  carrying extra-cardiovascular electrodes  24 ,  26 ,  28 , and  30 , for delivering electrical stimulation pulses to the patient&#39;s heart and for sensing cardiac electrical signals. 
     ICD  14  includes a control circuit  80 , memory  82 , therapy delivery circuit  84 , sensing circuit  86 , and telemetry circuit  88 . In some examples, ICD  14  includes one or more sensors  90  for producing a signal that is correlated to a physiological function, state or condition of the patient. A power source  98  provides power to the circuitry of ICD  14 , including each of the components  80 ,  82 ,  84 ,  86 ,  88  and  90  as needed. Power source  98  may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source  98  and each of the other components  80 ,  82 ,  84 ,  86  and  88  are to be understood from the general block diagram of  FIG.  3   , but are not shown for the sake of clarity. For example, power source  98  is coupled to one or more charging circuits included in therapy delivery circuit  84  for providing the power needed to charge holding capacitors included in therapy delivery circuit  84  that are discharged at appropriate times under the control of control circuit  80  for producing electrical stimulation pulses according to a therapy protocol, such as for bradycardia pacing, post-shock pacing, ATP and CV/DF shock pulses. Power source  98  is also coupled to components of sensing circuit  86 , such as sense amplifiers, analog-to-digital converters, switching circuitry, etc., sensors  90 , telemetry circuit  88  and memory  82  to provide power to various circuits or components as needed. 
     The functional blocks shown in  FIG.  3    represent functionality included in ICD  14  and may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to ICD  14  herein. The various components may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine, or other suitable components or combinations of components that provide the described functionality. The particular form of software, hardware and/or firmware employed to implement the functionality disclosed herein will be determined primarily by the particular system architecture employed in the ICD and by the particular detection and therapy delivery methodologies employed by the ICD. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern cardiac medical device system, given the disclosure herein, is within the abilities of one of skill in the art. 
     Memory  82  may include any volatile, non-volatile, magnetic, or electrical non-transitory computer readable storage media, such as random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other memory device. Furthermore, memory  82  may include non-transitory computer readable media storing instructions that, when executed by one or more processing circuits, cause control circuit  80  and/or other ICD components to perform various functions attributed to ICD  14  or those ICD components. The non-transitory computer-readable media storing the instructions may include any of the media listed above. 
     The functions attributed to ICD  14  herein may be embodied as one or more integrated circuits. Depiction of different features as circuits is intended to highlight different functional aspects and does not necessarily imply that such circuits must be realized by separate hardware or software components. Rather, functionality associated with one or more circuits may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, therapy control operations for delivering electrical stimulation pulses may be performed cooperatively by therapy delivery circuit  84  and control circuit  80  and may include operations implemented in a processor or other signal processing circuitry included in control circuit  80  executing instructions stored in memory  82 . These therapy control operations may include controlling when holding capacitor charging to a pacing voltage amplitude is performed according to capacitor charging management techniques disclosed herein. 
     Control circuit  80  may include fixed function circuitry and/or programmable processing circuitry. Control circuit  80  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or analog logic circuitry. In some examples, control circuit  80  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to control circuit  80  herein may be embodied as software, firmware, hardware or any combination thereof. 
     Control circuit  80  communicates, e.g., via a data bus, with therapy delivery circuit  84  and sensing circuit  86  for sensing cardiac electrical activity, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals. Therapy delivery circuit  84  and sensing circuit  86  are electrically coupled to electrodes  24 ,  26 ,  28 ,  30  carried by lead  16  and the housing  15 , which may function as a common or ground electrode or as an active can electrode for delivering CV/DF shock pulses or cardiac pacing pulses. 
     Sensing circuit  86  may be selectively coupled to electrodes  28 ,  30  and/or housing  15  in order to monitor electrical activity of the patient&#39;s heart. Sensing circuit  86  may additionally be selectively coupled to defibrillation electrodes  24  and/or  26  for use in a sensing electrode vector together or in combination with one or more of electrodes  28 ,  30  and/or housing  15 . Sensing circuit  86  may be enabled to selectively receive cardiac electrical signals from one or more sensing electrode vectors from the available electrodes  24 ,  26 ,  28 ,  30 , and housing  15 . Sensing circuit  86  may monitor one or more cardiac electrical signals at a time for sensing cardiac electrical events, e.g., P-waves attendant to the depolarization of the atrial myocardium and/or R-waves attendant to the depolarization of the ventricular myocardium, and providing digitized cardiac signal waveforms for analysis by control circuit  80 . For example, sensing circuit  86  may include switching circuitry for selecting which of electrodes  24 ,  26 ,  28 ,  30 , and housing  15  are coupled to cardiac event detection circuitry. Switching circuitry may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple components of sensing circuit  86  to selected electrodes. 
     The cardiac event detection circuitry may be configured to amplify, filter and digitize the cardiac electrical signal received from selected electrodes to improve the signal quality for detecting cardiac electrical events, such as R-waves or performing other signal analysis. The cardiac event detection circuitry within sensing circuit  86  may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), timers or other analog or digital components. A cardiac event sensing threshold may be automatically adjusted by sensing circuit  86  under the control of control circuit  80 , based on timing intervals and sensing threshold values determined by control circuit  80 , stored in memory  82 , and/or controlled by hardware, firmware and/or software of control circuit  80  and/or sensing circuit  86 . 
     Upon detecting a cardiac electrical event based on a sensing threshold crossing, sensing circuit  86  may produce a sensed event signal, such as an R-wave sensed event signal, that is passed to control circuit  80 . The R-wave sensed event signal is used by control circuit  80  for restarting a pacing escape interval timer that controls the basic time intervals used for scheduling cardiac pacing pulses. For example, in a VVI pacing mode, a ventricular pacing interval (or VV interval) may be restarted in response to each R-wave sensed event signal that is received outside a blanking period to inhibit a scheduled pacing pulse. The ventricular pacing interval is started in response to each delivered pacing pulse to control the minimum heart rate and timing of pacing pulses delivered by therapy delivery circuit  84 . 
     Control circuit  80  may also use the R-wave sensed event signals corresponding to intrinsic (non-paced) heart depolarizations to determine RR intervals (RRIs) for detecting tachyarrhythmia and determining a need for therapy. An RRI is the time interval between two consecutively sensed intrinsic R-waves and may be determined between two consecutive R-wave sensed event signals received from sensing circuit  86 . Control circuit  80  may be configured to detect a tachyarrhythmia based on RRIs and/or the morphology of QRS waveforms received as multi-bit digitized signals from sensing circuit  86 . Therapy delivery circuit  84  is controlled to deliver ATP and/or CV/DF shock pulses according to programmed therapy protocols in response to detecting ventricular tachycardia or fibrillation. 
     In this example, therapy delivery circuit  84  includes a high voltage (HV) therapy circuit  83  and may include a low voltage (LV) therapy circuit  85 . Each therapy circuit  83  and  85  includes charging circuitry, one or more charge storage devices such as one or more high voltage holding capacitors or low voltage holding capacitors, respectively, and switching circuitry that controls when the capacitor(s) are charged and discharged across a selected CV/DF shock vector or pacing electrode vector. Charging of capacitors to a programmed pacing voltage amplitude and discharging of the capacitors for a programmed pacing pulse width may be performed by therapy delivery circuit  84  according to control signals received from control circuit  80 . For example, a pace timing circuit included in control circuit  80  may include programmable digital counters set by a microprocessor of the control circuit  80  for controlling the basic pacing time intervals associated with various pacing modes or ATP sequences delivered by ICD  14 . The microprocessor of control circuit  80  may also set the amplitude, pulse width, polarity or other characteristics of the cardiac pacing pulses, which may be based on programmed values stored in memory  82 . 
     Therapy delivery circuit  84  is controlled by control circuit  80  to charge one or more holding capacitors according to a capacitor charging mode. As described herein, the therapy delivery circuit  84  may be configured to charge one or more holding capacitors according to a delayed charging mode in which capacitor charging is withheld for at least a portion of or all of a pacing interval. The holding capacitor voltage may be allowed to fall below the pacing voltage amplitude during a pacing interval without being recharged. At other times, therapy delivery circuit  84  may be controlled to charge one or more holding capacitors according to a charging without delay mode, during which capacitor charging is not withheld and may be performed from the beginning or throughout a pacing interval as needed to maintain the holding capacitor charge at the pacing voltage amplitude. Control circuit  80  may be configured to control therapy delivery circuit to switch between the delayed capacitor charging mode and the capacitor charging without delay mode based on intrinsic heart rate criteria determined from the cardiac electrical signal(s) received by sensing circuit  86  and/or other signals received from sensor(s)  90 . 
     Components that may be included in HV therapy circuit  83  and LV therapy circuit  85  are described below in conjunction with  FIGS.  4  and  5   , respectively. It is recognized that the methods disclosed herein for controlling capacitor charging for pacing therapy may be implemented in an IMD system that includes only a HV therapy circuit  83  configured to deliver cardiac pacing pulses, which may be in addition to high voltage CV/DF shock delivery capabilities, or in an IMD system that includes only LV therapy circuit  85  without CV/DF shock therapy capabilities. In some systems, HV therapy circuit  83  delivers only high voltage CV/DF shock pulses, and LV therapy circuit  85  delivers relatively lower voltage pacing pulses. In other examples, control circuit  80  may selectively control which one of HV therapy circuit  83  or LV therapy circuit  85  is utilized for generating and delivering cardiac pacing pulses based on the type of pacing therapy, the pacing threshold voltage amplitude required to capture the heart, or other factors. 
     ICD  14  may include other sensors  90  for sensing signals from the patient for use in determining a need for and/or controlling electrical stimulation therapies delivered by therapy delivery circuit  84 . In some examples, a sensor indicative of a need for increased cardiac output may be included in ICD  14 , such as a tissue oxygen sensor, an impedance sensor, or a pressure sensor. A sensor indicative of a need for increased cardiac output may include a patient activity sensor, such as an accelerometer, or an impedance sensor for determining minute volume or other respiratory metrics. An increase in the metabolic demand of the patient due to increased activity may be determined by control circuit  80  from a sensor signal received from sensors  90  for use in determining a need for pacing or a need for an increased pacing rate. Likewise, a sensor signal may be used by control circuit  80  for determining when a need for pacing no longer exists or when the pacing rate may be decreased. 
     Control circuit  80  may be configured to use a sensor signal from sensors  90  to detect a need for pacing and/or an expected increased pacing burden. “Pacing burden” as used herein may be defined as the percentage of time the patient&#39;s heart rhythm is a paced rhythm (as opposed to an intrinsic rhythm) over a predetermined period of time. For example, the patient may be paced 10% of the time over a 24-hour period. In other examples, pacing burden may be determined as the proportion of paced events to sensed intrinsic events or the proportion of paced events to all paced and sensed intrinsic events combined over a predetermined time period or total number of cardiac events. Control circuit  80  may be configured to detect an expected increase in pacing burden based on a physiological condition of the patient, such as reduced cardiac output, increased patient activity, low tissue oxygenation, or other condition for which an increased pacing frequency and/or rate is expected to improve or alleviate. 
     The control circuit  80  may respond to detecting an expected change in pacing burden by enabling switching between different capacitor charging modes. For example, control circuit  80  may respond to an increase in expected pacing burden by enabling therapy delivery circuit  84  to switch between capacitor charging without delay and delayed capacitor charging based on whether increased intrinsic heart rate criteria and/or decreased intrinsic heart rate criteria are met. Enabling (turning on) the function of switching between capacitor charging modes does not necessarily require immediately making the switch from one charging mode to another. Rather, after criteria required to be satisfied for enabling the function of switching between capacitor charging modes, additional criteria relating to the intrinsic heart rate and/or sensor signals may be required to be met before actually performing the switch from one capacitor charging mode to another. 
     In other examples, control circuit  80  may respond to detecting a change in expected pacing burden based on a signal from sensors  90  by directly switching the capacitor charging mode. For example, when the pacing burden is expected to be decreased, e.g., due to a restored cardiac output, tissue oxygenation, blood pressure, or reduced patient activity, the control circuit  80  may control the therapy delivery circuit  84  to switch the capacitor charging pacing mode to delayed capacitor charging to conserve energy of power source  98 . Methods for enabling the function of automatic switching between capacitor charging modes and methods for controlling the timing of the switching between capacitor charging modes after the switching function is enabled are described in greater detail below in conjunction with the flow charts and timing diagrams presented herein. 
     Control parameters utilized by control circuit  80  for sensing cardiac events, detecting cardiac arrhythmias and controlling therapy delivery, including controlling capacitor charging techniques as disclosed herein, may be programmed into memory  82  via telemetry circuit  88 . Telemetry circuit  88  includes a transceiver and antenna for communicating with external device  40  (shown in  FIG.  1 A ) using RF communication or other communication protocols as described above. Under the control of control circuit  80 , telemetry circuit  88  may receive downlink telemetry from and send uplink telemetry to external device  40 . In some cases, telemetry circuit  88  may be used to transmit and receive communication signals to/from another medical device implanted in patient  12 . 
       FIG.  4    is schematic diagram  150  of HV therapy circuit  83  included in ICD  14  according to one example. HV therapy circuit  83  includes a HV charging circuit  154  and a HV charge storage and output circuit  160 . HV therapy circuit  83  is shown coupled to a processor and HV therapy control circuit  152  which may be included in control circuit  80  for controlling HV charging circuit  154  and HV charge storage and output circuit  160 . HV charge storage and output circuit  160  includes a HV holding capacitor  162  coupled to switching circuitry  166  via a pulse control switch  164  for coupling the HV holding capacitor  162  to electrodes  24 ,  26  and/or housing  15  to deliver a desired electrical stimulation pulse, which may be a pacing pulse or a CV/DF shock pulse, to the patient&#39;s heart  8 . In other examples, pacing and sensing electrodes  28  and  30  (not shown in  FIG.  4   ) may be selectively coupled to HV holding capacitor  162  via switching circuitry  166  for using one or both of electrodes  28  and  30  in a pacing electrode vector for delivering a pacing pulse from HV therapy module  83 . 
     HV holding capacitor  162  is shown schematically as a single capacitor, but it is recognized that a bank of two or more capacitors or other energy storage devices may be used to store energy for producing electrical signals delivered to heart  8 . In one example, HV capacitor  162  is a series of three capacitors having an effective capacitance of 148 microfarads. HV holding capacitor  162  is charged to a desired pacing pulse voltage amplitude (or shock voltage amplitude in the case of a CV/DF shock delivery) by HV charging circuit  154  under the control of processor and HV therapy control  152 . It is to be understood that charging to a pacing voltage amplitude may include charging to a specified tolerance within or greater than the pacing voltage amplitude. For example, charging to the pacing voltage amplitude may include charging to 113% (or another predetermined percentage) of the programmed pacing voltage amplitude. 
     HV charging circuit  154  receives a voltage regulated signal from power source  98  ( FIG.  3   ). HV charging circuit  154  includes a transformer  156  to step up the battery voltage of power source  98  in order to achieve charging of HV holding capacitor  162  to a voltage that is much greater than the battery voltage. Charging of capacitor  162  by HV charging circuit  154  is performed under the control of processor and HV therapy control  152 , which receives feedback signals from HV charge storage and output circuit  160  to determine when capacitor  162  is charged to a programmed voltage. A charge completion signal is passed to HV charging circuit  154  to terminate charging by processor and HV therapy control module  152 . One example of a high voltage charging circuit and its operation is generally disclosed in U.S. Pat. No. 8,195,291 (Norton, et al.), incorporated herein by reference in its entirety. 
     When the capacitor charging is being controlled in a capacitor charging without delay mode, charging of HV holding capacitor  162  may occur continuously or semi-continuously throughout a pacing interval started in response to a delivered pacing pulse or sensed intrinsic event. Continuous charging during a pacing interval may be achieved by comparing the feedback signal from HV charge storage and output circuit  160  to the targeted pacing voltage amplitude (plus or minus any tolerance) and performing top-off charging of capacitor  162  as needed to maintain the capacitor at a desired voltage. For example, the capacitor charge feedback signal may be compared to the targeted pacing voltage amplitude on every interrupt signal from control circuit  80  or other predetermined frequency during a pacing interval. Whenever the charge is below the pacing voltage amplitude, the capacitor  162  is charged as needed to maintain the charge at the programmed pacing voltage amplitude throughout the pacing interval. 
     In other examples of a charging without delay mode, the charge of capacitor  162  may be compared to the targeted pacing voltage amplitude at the start of a pacing interval and charged up to the pacing voltage amplitude (plus any specified tolerance) one time during the pacing interval, without monitoring/charging throughout the pacing interval. If a pacing pulse is delivered or leakage of the capacitor charge occurs during the pacing interval after charging, the capacitor charge is topped off at the beginning of the next pacing interval. 
     As described in conjunction with the timing diagrams and flow charts presented herein, the processor and HV therapy control  152  may be configured to withhold charging of HV holding capacitor  162  when increased heart rate criteria and/or decreased pacing burden criteria are satisfied. The capacitor charging may be delayed or withheld by processor and HV therapy control  152  until a pacing interval expires or until a capacitor charging delay interval expires. 
     When HV holding capacitor  162  is charged to a desired pacing voltage amplitude and a pacing interval expires, the HV holding capacitor  162  is coupled across the desired pacing electrode vector via pulse control switch  164  and switching circuitry  166  to deliver the pacing pulse. Switching circuitry  166  may be in the form of an H-bridge and may include switches  180   a - 180   c  and  182   a - 182   c  that are controlled by signals from processor and HV control circuit  152 . Switches  180   a - 180   c  and  182   a - 182   c  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. Switches  180   a - 180   c  and  182   a - 182   c  are controlled to be open or closed by processor and HV therapy control circuit  152  at the appropriate times for delivering a monophasic, biphasic or other desired pacing pulse by discharging capacitor  162  across the pacing load presented by heart  8  and a selected pacing electrode vector. The HV holding capacitor  162  is coupled across the selected pacing electrode vector for the programmed pacing pulse width via pulse control switch  164 . 
     For instance, the selected electrodes  24 ,  26  and/or housing  15  may be coupled to HV holding capacitor  162  by opening (i.e., turning off or disabling) and closing (i.e., turning on or enabling) the appropriate switches of switching circuitry  166  to pass a desired electrical signal to the therapy delivery electrode vector. The electrical signal may be a monophasic, biphasic or other shaped signal. The signal may be a monophasic or biphasic pacing pulse delivered in response to a pacing interval expiring such as a VVI pacing interval, a post-shock pacing interval or an ATP interval. At other times, the signal may be a CV/DF shock for terminating a ventricular tachyarrhythmia when VT or VF is detected. 
     To deliver a pacing pulse, for example, one of switches  180   a ,  180   b  or  180   c  may be closed simultaneously with one of switches  182   a ,  182   b , or  182   c  without closing both of the “a,” “b” or “c” switches across a given electrode  24 ,  26  or housing  15 , respectively, at the same time. To deliver a biphasic pulse using electrode  24  and housing  15 , for instance, switch  180   a  and  182   c  may be closed to deliver a first phase of the biphasic pulse. Switches  180   a  and  182   c  are opened after the first phase, and switches  180   c  and  182   a  are closed to deliver the second phase of the biphasic pulse. Switches  180   b  and  182   b  remain open or disabled in this example with electrode  26  not selected or used in the therapy delivery vector. In other examples, electrode  26  may be included instead of electrode  24  or simultaneously activated with electrode  24 . 
     After delivering of a pacing pulse, the HV holding capacitor  162  may be re-charged to the programmed pacing pulse amplitude if increased intrinsic heart rate detection criteria are not satisfied. Processor and HV therapy control  152 , however, may withhold charging of HV holding capacitor  162  to the pacing voltage amplitude in response to determining that increased intrinsic heart rate criteria are satisfied using the techniques disclosed herein. When the rate of intrinsic events sensed by sensing circuit  86  of ICD  14 , the slope of the rate of intrinsic events, and/or other criteria satisfy increased intrinsic heart rate detection criteria and/or decreased pacing burden criteria, charging of HV holding capacitor  162  may be withheld until a pacing interval expires or until a predetermined charging delay interval expires. Processor and HV therapy control circuit  152  may revert to charging HV holding capacitor  162  without delay, e.g., at the beginning of a pacing interval or throughout a pacing interval started in response to a pacing pulse or a sensed event signal as needed, in response to determining that decreased intrinsic heart rate criteria and/or decreased pacing burden criteria are met as described below. 
       FIG.  5    is a conceptual diagram of LV therapy circuit  85  according to one example. LV therapy circuit  85  may include a LV charging circuit  330 , a capacitor selection and control circuit  332 , and a capacitor array  350 . Capacitor array  350  may include multiple LV holding capacitors  352 ,  354 ,  356  and  358  that can each be charged by LV charging circuit  350  to a programmed pacing voltage amplitude. The LV holding capacitors  352 ,  354 ,  356  and  358  are coupled to a respective output capacitor  372   a - 372   d  (collectively  372 ),  376 , or  378  via respective switches  362 ,  364 ,  366 , and  368  to deliver pacing pulses. Each of LV holding capacitors  352 ,  354 ,  356  and  358  may have a capacitance that is less than the effective capacitance of HV holding capacitor  162  of HV therapy circuit  83 . For example, each of holding capacitors  352 ,  354 ,  356  and  358  may have a capacitance of up to 6 microfarads, up to 10 microfarads, up to 20 microfarads or other selected capacitance, but all may have a capacitance significantly less than the effective capacitance of HV holding capacitor  162  and have a lower voltage rating than HV holding capacitor  162 . 
     Power source  98  ( FIG.  3   ) may provide regulated power to LV charging circuit  330 . LV charging circuit  330  may be controlled by a state machine in capacitor selection and control circuit  332  to charge all or selected LV holding capacitors  352 ,  354 ,  356  and  358  using a multiple of the battery voltage of power source  98 , e.g., four times the battery voltage. LV charging circuit  330  charges one or more of capacitors  352 ,  354 ,  356  and  358  as needed for delivering a pacing pulse to the patient&#39;s heart via a selected pacing electrode vector. The pacing pulse may be a single pacing pulse delivered by discharging a single LV holding capacitor for a programmed pulse width. In other examples, two or more of LV holding capacitors  352 ,  354 ,  356  and  358  may be discharged sequentially to deliver two or more fused pulses within a pacing pulse width to deliver a composite pacing pulse. 
     In some examples, the LV therapy circuit  85  includes three pacing channels  342 ,  344  and  346 . Each pacing channel is capable of producing a single pacing pulse when a respective LV holding capacitor  352 ,  356  or  358  is discharged across an output capacitor  372 ,  376 , or  378 , respectively. Pacing channel  342  includes a back-up holding capacitor  354  that may be used for delivering back-up pacing pulses. Back-up holding capacitor  354  may be used to deliver an individual pulse of a composite pacing pulse. Depending on the number of extra-cardiovascular electrodes coupled to ICD  14 , one or more channels may include multiple selectable output signal lines. For example, channel  342  is shown in this example to include multiple selectable pacing output signal lines  382   a - 382   d  that may be selectively coupled to LV holding capacitor  352  and back-up holding capacitor  354  via closure of one or more of electrode selection switches  374   a - 374   d . For example, multiple electrodes carried by lead  16  may be coupled to pacing channel  342 , and a pacing electrode vector may be selected from the multiple electrodes by closing certain ones of switches  374   a - 374   d.    
     Pacing channels  344  and  346  are shown having single output signal lines  386  and  388  that are coupled to respective LV holding capacitors  366  and  368  via respective switches  366  and  368 . In other examples, all three pacing channels  342 ,  344  and  346  may be provided with a single output signal line or with multiple output signal lines to enable selection of a pacing electrode vector from among multiple extra-cardiovascular electrodes coupled to ICD  14 , e.g., any of electrodes  24 ,  26 ,  28 , or  30  of lead  16  shown in  FIG.  1 A . 
     When a pacing therapy is needed, control circuit  80  may control LV therapy circuit  85  to select any one or combination of the pacing channels  342 ,  344  and  346  to deliver a pacing pulse. The pacing pulse may be a single-pulse pacing pulse delivered by discharging one of the holding capacitors  352 ,  354 ,  356  or  358  across a selected pacing electrode vector via a respective output capacitor  374 ,  376  or  378  when a respective switch  362 ,  364 ,  366  or  368  is closed. The output line  382   a ,  382   b ,  382   c , or  382   d  used to deliver pacing current from pacing channel  342  may be selected via a respective electrode selection switch  372   a - 372   d . The switch  362 ,  364 ,  366  or  368  that enables discharge of a holding capacitor  352 ,  354 ,  356  or  358 , respectively, may be enabled by capacitor selection and control circuit  332  at the appropriate time when a pacing pulse is needed and maintained in an active, enabled (closed) state until the single-pulse pacing pulse width is expired. 
     In some patients, a single-pulse pacing pulse generated by LV therapy circuit  85  may not have the pulse energy required to capture the patient&#39;s heart. Control circuit  80  may control LV therapy circuit  85  to deliver fused pulses in a multi-pulse composite, pacing pulse. Two or all three pacing channels  342 ,  344  and  346  are tied together by switches  360   a - d  and  370  to enable individual pulses to be delivered across a selected pacing electrode vector from a single output signal line  344 . For example, control circuit  80  may control LV therapy circuit  85  to deliver a multi-pulse, composite pacing pulse by activating at least one of switches  360   a - d  and switch  370  to tie at least one of pacing output lines  382   a - d  and pacing output line  388  to pacing channel  344 . Control circuit  80  controls capacitor selection and control circuit  332  to enable pacing channel switches  362 ,  364 ,  366  and  368  (and at least one electrode selection switch  372   a - d  of pacing channel  342 ) in a sequential manner to sequentially couple two or more of the respective holding capacitors  352 ,  354 ,  356  or  358  to output signal line  386  to deliver a sequence of at least two fused, individual pulses to produce a composite pacing pulse. 
     In various examples, depending on the particular pacing channel and lead and electrode configuration used with ICD  14 , some electrode selection switches shown in  FIG.  5    may not be required. Furthermore, it is recognized that less than four holding capacitors or more than four holding capacitors may be included in a capacitor array  350  for use in delivering a sequence of fused pulses in a composite pacing pulse when the LV therapy circuit  85  is controlled to deliver a pacing pulse. 
     Capacitor selection and control circuit  332  selects which holding capacitors  352 ,  354 ,  356  or  358  are coupled to output line  386  and in what sequence by controlling respective switches  362 ,  364 ,  366  and  368 . A sequence of pulses may be delivered to produce a composite pacing pulse by sequentially discharging holding capacitors  352 ,  354 ,  356  or  358  one at a time (or one combination at a time) across a respective output capacitor  372 ,  376  or  378  by sequentially enabling or closing the respective switches  362 ,  364 ,  366  or  368 . For example, at least two of holding capacitors  352 ,  354 ,  356  or  358  are sequentially discharged to produce a composite pacing pulse produced by at least two fused individual pulses. Output line  386  may be electrically coupled to a pacing cathode electrode carried by lead  16  and a return anode electrode carried by lead  16  (or housing  15 ) may be coupled to ground. The pacing cathode electrode and return anode electrode may correspond to electrodes  28  and  24 , respectively, as shown in  FIG.  1 A  in one example, however any pacing electrode vector may be selected from electrodes  24 ,  26 ,  28 , and  30  and/or housing  15  shown in  FIG.  1 A . 
     In some examples, a low-voltage, fused pacing pulse is delivered by delivering an individual pulse from pacing channel  344  and  346  sequentially followed by a third, longer individual pulse delivered by pacing channel  342  by discharging both capacitors  352  and  354  simultaneously. The first two individual pulses may be 2.0 ms in pulse width and the third pulse may be 4.0 ms in pulse width for a composite pacing pulse width of 8 ms. The higher capacitance of the parallel capacitors  312  and  314  allows for the third individual pulse to be longer in pulse width while maintaining a pulse amplitude that successfully captures the heart. All three individual pulses are delivered via output line  386  by controlling output configuration switches  360  and  370  to couple the capacitors  352 ,  354  and  358  to output line  386 . Other examples of a LV therapy circuit and pacing pulse generation techniques that may be used in conjunction with the techniques disclosed herein are generally disclosed in U.S. patent application Ser. No. 62/262,412 and the corresponding U.S. patent application Ser. No. 15/368,197. 
     The LV holding capacitors  352 ,  354 ,  356  and/or  358  selected for use in generating a single-pulse pacing pulse or a multi-pulse, composite pacing pulse may be charged by LV charging circuit  330  according to the capacitor charging control techniques disclosed herein. When criteria for detecting an increased intrinsic heart rate are satisfied, charging of LV holding capacitors  352 ,  354 ,  356  and/or  358  used for generating pacing pulses may be withheld or delayed according to a delayed capacitor charging pacing mode. Charging is delayed until a pacing interval expires in some examples or until a capacitor charging delay interval expires in other examples. If criteria for detecting a decreased intrinsic heart rate are satisfied, charging of the LV holding capacitors  352 ,  354 ,  356  or  358  used for generating and delivering pacing pulses is performed without delay, e.g., at the beginning of a pacing interval or throughout the pacing interval as generally described above in conjunction with  FIG.  4   . 
     Control circuit  80  may enable the function of automatic switching between capacitor charging modes based on pacing burden criteria in some examples, as described in further detail herein. After automatic switching between charging modes is enabled based on a change in the actual or expected pacing burden, control circuit  80  may switch between controlling LV charging circuit  330  according to a delayed charging mode and a charging without delay mode. The switching between the charging modes may be controlled based on intrinsic heart rate criteria and/or pacing burden criteria. 
       FIG.  6    is a flow chart  100  of one method for controlling holding capacitor charging for pacing pulse delivery. At block  101 , control circuit  80  enables automatic switching between a first capacitor charging mode and a second capacitor charging mode. In some examples, charging mode switching is enabled in response to a user command received from external device  40  via telemetry circuit  88 . In other examples, control circuit  80  may automatically enable and/or disable charging mode switching based on determining an actual or expected change in pacing burden, e.g., as described in conjunction with  FIG.  12    below. The two charging modes may include a delayed charging mode and a charging without delay mode. 
     In the example shown in  FIG.  6   , ICD  14  may initially be operating in a capacitor charging without delay mode. In this charging mode, one or more holding capacitors are charged to the pacing voltage amplitude during each pacing interval according to a selected pacing output configuration, e.g., using either HV therapy circuit  83  or LV therapy circuit  85  as described above. The pacing interval is set by control circuit  80  in response to a delivered pacing pulse or sensed cardiac event, e.g., an intrinsic R-wave, to control the timing of pacing pulses, e.g., according to a VVI or other pacing mode. The holding capacitor charging is performed without delay such that charging may begin at the start of the pacing interval and/or occur at any time during the pacing interval in response to a comparison of the holding capacitor charge to the programmed pacing voltage. If the holding capacitor charge is less than the programmed pacing voltage amplitude (or less than a tolerance below the pacing voltage amplitude), control circuit  80  controls therapy delivery circuit  84  to charge the holding capacitor to the programmed pacing voltage amplitude. 
     While operating in the capacitor charging without delay mode, control circuit  80  monitors the cardiac electrical signal received by sensing circuit  86  for determining if increased intrinsic heart rate criteria are met at block  104 . Increased intrinsic heart rate criteria may require that the intrinsic heart rate be equal to or greater than a predetermined mode switching heart rate threshold. The mode switching heart rate threshold may be defined to be faster than the current pacing rate such that mode switching does not necessarily occur in response to one or more sensed events occurring at event intervals shorter than the pacing interval and resulting in one or more inhibited pacing pulses. For instance, charging mode switching may not occur if the heart rate is between the pacing rate and the mode switching heart rate threshold. Various techniques for determining if increased intrinsic heart rate criteria are met are described below, e.g., in conjunction with  FIGS.  7  and  12   . 
     If the increased intrinsic rate criteria are satisfied at block  104 , control circuit  80  switches to the second charging mode, the delayed capacitor charging mode in this example, at block  106 . In this mode, control circuit  80  may withhold comparisons between the holding capacitor charge and the programmed pacing voltage amplitude and/or withhold charging of the holding capacitor(s) even when the capacitor charge is less than the pacing voltage amplitude. Capacitor charging is withheld for at least a portion of the pacing interval started in response to a cardiac event, either a delivered pacing pulse or a sensed intrinsic event. Charging may be withheld for the entire pacing interval in some examples. In other examples, charging is withheld until expiration of a charging delay interval. If an intrinsic event is sensed prior to expiration of the charging delay interval, no charging occurs and the pacing interval is restarted. 
     During the delayed capacitor charging mode, control circuit  80  monitors the cardiac electrical signal for determining whether decreased intrinsic heart rate criteria are met at block  108 . The decreased intrinsic heart rate criteria may be satisfied in response to one or more pacing intervals expiring and/or one or more charging delay intervals expiring. As such, in some examples decreased intrinsic heart rate criteria may be satisfied in response to a decreasing intrinsic heart rate before the intrinsic heart rate falls below the pacing rate, before a pacing interval expires. In response to the decreased intrinsic heart rate criteria being met, control circuit  80  switches back to the capacitor charging without delay mode at block  102 . Methods for determining if decreased intrinsic heart rate criteria are described in greater detail below, e.g., in conjunction with  FIGS.  7 ,  8 A -D, and  12 . 
     In this way, capacitor charging is controlled according to a delayed capacitor charging mode after increased intrinsic heart rate criteria are met and the potential need for a pacing pulse is expected to be low. Capacitor charging is controlled according a charging without delay mode after decreased intrinsic heart rate criteria are met and the potential need for a pacing pulse is relatively higher. In some examples, control circuit  80  may monitor one or more signals from sensors  90  in addition to or alternatively to the intrinsic cardiac electrical events sensed by sensing circuit  86  for determining if increased intrinsic heart rate criteria and/or decreased intrinsic heart rate criteria are met at blocks  104  and  108 , respectively. 
       FIG.  7    is a flow chart  110  of a method for controlling holding capacitor charging based on intrinsic heart rate criteria according to one example. The method of  FIG.  7    may be implemented for delivering a pacing therapy by either HV therapy circuit  83  or LV therapy circuit  85 . The method of flow chart  110  controls the timing of holding capacitor charging, which may be HV holding capacitor  162  of HV therapy circuit  83  or one or more of the LV holding capacitors  352 ,  354 ,  356 , or  358  of LV therapy circuit  85  depending on which of HV therapy circuit  83  or LV therapy circuit  85  is selected for delivering the pacing therapy. 
     At block  112 , a pacing pulse is delivered by the selected HV therapy circuit  83  or LV therapy circuit  85 . The pacing pulse is delivered upon expiration of a pacing interval, which may be a ventricular pacing interval during VVI pacing or another pacing interval according to another pacing therapy or pacing mode. Control circuit  80  restarts the pacing interval at block  114  in response to delivery of the pacing pulse. Control circuit  80  controls the charging circuitry of the selected HV therapy circuit  83  or LV therapy circuit  85  to charge the holding capacitor(s) back up to the pacing voltage amplitude at block  115  during the pacing interval without delay. In this way, if the pacing interval expires without a sensed event, the therapy circuit  83  or  85  is prepared to deliver the next pacing pulse upon pacing interval expiration. 
     At block  116 , the control circuit  80  waits for the pacing interval to expire. If the pacing interval expires without sensing circuit  86  sensing an intrinsic cardiac event, e.g., an R-wave, at block  116 , the scheduled pacing pulse is delivered in response to the expired pacing interval at block  112 . Control circuit  80  restarts the pacing interval at block  114 . 
     If sensing circuit  86  does sense an intrinsic cardiac event during the pacing interval at block  118 , a sensed event signal may be passed from sensing circuit  86  to control circuit  80 . In response to receiving the sensed event signal, e.g., an R-wave sensed event signal, from sensing circuit  86  before the pacing interval expires (“yes” branch of block  116 ), control circuit  80  inhibits the scheduled pacing pulse by re-starting the pacing interval at block  118 . 
     Control circuit  80  may be configured to monitor for an increased intrinsic heart rate that is a predetermined rate greater than the programmed pacing rate for controlling capacitor charging mode switching. In one example, control circuit  80  may detect an increase in the intrinsic heart rate by starting a hysteresis interval at block  118  at the same time that the pacing interval is started. The hysteresis interval may be set equal to or shorter than the pacing interval. For instance, the hysteresis interval may be at least 10 to 30 ms shorter than the pacing interval. In this way, detection of an increased intrinsic heart rate for switching to a delayed capacitor charging mode may require a higher intrinsic heart rate than the intrinsic heart rate required to withhold pacing. 
     In one example the hysteresis interval is 15 ms shorter than the pacing interval when the pacing interval is 1 second, corresponding to a pacing rate of 60 pulses per minute. The hysteresis interval of approximately 0.985 ms corresponds to a heart rate of 65 beats per minute, about 5 pulses per minute faster than the pacing rate. The hysteresis interval may be determined by control circuit  80  as a fixed interval less than the pacing interval currently in effect. In other examples, control circuit  80  may determine the hysteresis interval by determining a sensed event interval corresponding to an intrinsic heart rate that is a fixed rate less than the pacing rate currently in effect, e.g., 5 to 15 beats less than the current pacing rate. In other examples, the hysteresis interval may be the same as the pacing interval. 
     Control circuit  80  may control the therapy delivery circuit  84  to maintain the charge of the holding capacitor at the programmed pacing voltage amplitude at block  112  after inhibiting the scheduled pacing pulse. The capacitor charge may be monitored during the pacing interval set in response to a sensed event, and if the charge drops more than a voltage tolerance below the pacing voltage amplitude, the charge of the holding capacitor may be topped off back to the pacing voltage amplitude. 
     Different protocols or techniques may be used to control topping off or maintaining the capacitor charge during pacing intervals. In one example, when LV therapy circuit  85  is used to deliver pacing pulses, one or more LV holding capacitors may be charged to the pacing voltage amplitude at the start of each pacing interval. In some cases, charging to the pacing voltage amplitude is controlled by charging to the pacing voltage amplitude plus a tolerance, e.g., to 110% to 115% of the programmed pacing voltage amplitude. In one example, the LV holding capacitor(s) used for delivering a pacing pulse are charged to 113% of the programmed pacing voltage amplitude at the start of each pacing interval that is reset in response to a delivered pacing pulse or sensed intrinsic event. 
     In other examples, for instance if the HV therapy circuit  83  is controlled to deliver pacing pulses, the HV holding capacitor  162  is charged to the pacing voltage amplitude (or the pacing voltage amplitude plus a tolerance), and continuous top-off charging is performed at block  120  until a pacing interval expires and a pacing pulse is delivered. In this example, processor and HV therapy control  152  may receive a capacitor charge signal from HV therapy circuit  83  indicating the voltage across HV holding capacitor  162 , e.g., on each interrupt clock signal. Processor and HV therapy control  152  may compare the capacitor charge signal to a capacitor charge threshold and control HV charging circuit  154  to perform top-off charging following any interrupt clock signal during an unexpired pacing interval as needed to maintain the HV holding capacitor charge at the pacing voltage amplitude (plus or minus a specified tolerance). The specific protocol used to maintain a holding capacitor at the pacing voltage amplitude in a ready state for pacing pulse delivery may vary between devices but generally includes top-off charging to the pacing voltage amplitude (or the pacing voltage amplitude plus a tolerance) during each pacing interval as needed to maintain the holding capacitor charge at the pacing voltage amplitude. 
     If sensing circuit  86  does not sense an intrinsic event during the pacing interval started at block  118 , “no” branch of block  122 , control circuit  80  controls the therapy delivery circuit  84  to deliver the scheduled pacing pulse at block  112  in response to the expiration of the pacing interval. Control circuit  80  continues to charge the holding capacitor(s) according to the charging without delay mode during each pacing interval set in response to each delivered pacing pulse and sensed cardiac event. 
     If an intrinsic event is sensed by the sensing circuit  86  during the pacing interval started at block  118  (“yes” branch of block  122 ), control circuit  80  determines at block  124  if the sensed event occurred before the hysteresis interval expired. If the sensed event occurred after the hysteresis interval expired but before the pacing interval expired (“no” branch of block  124 ), the control circuit  80  inhibits the scheduled pacing pulse by restarting the pacing and hysteresis intervals at block  118 . The holding capacitor charge is maintained at the pacing voltage amplitude at block  120  according the charging without delay mode. 
     If an intrinsic event is sensed by the sensing circuit  86  before the hysteresis interval expires (“yes” branch of block  124 ), control circuit  80  may increase the value of a counter at block  125 . The counter may be previously initialized to zero and is used to count the number of cardiac cycles during which an intrinsic event is sensed prior to the expiration of the hysteresis interval. Control circuit  80  may be configured to detect an increased intrinsic heart rate based on a threshold number of cardiac cycles having a sensed event occurring during the hysteresis interval. If the counter has not reached the threshold number of cardiac cycles for detecting an increased intrinsic heart rate, “no” branch of block  126 , control circuit  80  restarts the pacing and hysteresis intervals at block  118  and continues to maintain the charge of the holding capacitor at the pacing voltage amplitude in a ready state for pacing pulse delivery, according to the charging without delay mode. 
     If the counter reaches the threshold number of cardiac cycles having a sensed event during the hysteresis interval, as determined at block  126 , control circuit  80  detects an increased intrinsic heart rate that is equal to or greater than the rate corresponding to the hysteresis interval. The threshold number of cardiac cycles having an intrinsic event sensed within the hysteresis interval required to detect an increased intrinsic heart rate may be one or more. In some examples, an intrinsic event may be required to be sensed within the hysteresis interval for at least five cardiac cycles in order to detect an increased intrinsic heart rate. The threshold number of cardiac cycles may be required to be consecutive in some examples, e.g., at least three consecutively sensed intrinsic events at or above the hysteresis rate corresponding to the hysteresis interval. 
     In other examples, the control circuit  80  may include an X of Y counter such that the cardiac cycles having intrinsic events sensed within respective hysteresis intervals may not be required to be consecutive, e.g., three out of five cardiac cycles, four out of six cardiac cycles, eight out of ten cardiac cycles or other ratio or percentage. In some examples, all Y cardiac cycles may be required to be sensed cardiac cycles with no paced cardiac cycles. For example, if four out of six cardiac cycles are required to include intrinsic events sensed within the hysteresis interval, the other two cardiac cycles may be required to include sensed intrinsic events within the pacing interval. None of the six cardiac cycles are paced cardiac cycles. In other examples, the Y cardiac cycles may include both paced and sensed cardiac cycles but at least X cardiac cycles are required to include a sensed intrinsic event during the hysteresis interval in order for an increased intrinsic heart rate to be detected. 
     At block  127 , control circuit  80  determines that increased intrinsic heart rate criteria are met in response to the counter reaching the threshold number of cardiac cycles that each include an intrinsic sensed event during the respective hysteresis interval. In response to sensing the latest intrinsic event within the hysteresis interval that causes the increased intrinsic heart rate to be detected, the next scheduled pacing pulse is inhibited by restarting the pacing interval at block  128  without delivering a pacing pulse. Control circuit  80  switches to a delayed capacitor charging mode by not starting the hysteresis interval at block  128  and withholding capacitor charging at block  129 . The holding capacitor charge is not maintained at the pacing voltage amplitude during the pacing interval after the heart rate has reached or exceeded the rate corresponding to the hysteresis interval for the threshold number of cardiac cycles. 
     If an intrinsic event is sensed by the sensing circuit  86  during the pacing interval started at block  128 , as determined at block  130 , control circuit  80  inhibits the scheduled pacing pulse by restarting the pacing interval at block  128  and continues to withhold capacitor charging at block  129 . If the pacing interval expires at block  130  without an intrinsic event being sensed by the sensing circuit  86 , the counter used to count the number of events within the hysteresis interval may be reset to zero at block  132 . In response to the pacing interval expiring at block  130  without a sensed intrinsic event, control circuit  80  controls therapy delivery circuit  84  to charge the holding capacitor at block  130  and deliver the scheduled pacing pulse at block  112  as soon as the selected holding capacitor reaches the programmed pacing voltage amplitude. The time required to charge the holding capacitor(s) at block  136 , after the pacing interval has expired, may delay the delivery of the pacing pulse at block  112 . After the pacing pulse is delivered, the pacing interval is restarted at block  114 , and the control circuit  80  controls the therapy delivery circuit  84  to charge the holding capacitor(s) during the pacing interval at block  115  according to the charging without delay mode. 
     In the example shown, expiration of the pacing interval a single time at block  130  during delayed capacitor charging mode may cause the control circuit  80  to return to the charging without delay mode for controlling charging of the holding capacitor(s) beginning at the start of the pacing interval after each pacing pulse or sensed event and maintaining the holding capacitor charge at the pacing voltage amplitude during the pacing interval as needed. In other examples, more than one pacing pulse may be required to be delivered due to an expired pacing interval during the delayed charging mode before reverting back to charging the holding capacitor without delay after each delivered pacing pulse. As a result, more than one pacing pulse may be delivered at a delayed time interval after expiration of the pacing interval due to the time required for charging the holding capacitor to the pacing voltage amplitude after expiration of the pacing interval. 
     After returning to block  112 , maintaining the holding capacitor in a “ready” state by charging to the pacing voltage amplitude and topping off the charge as needed until a pacing interval expires may continue until an increased intrinsic heart rate is detected again. The increased intrinsic heart rate is detected according to predetermined criteria which may include a hysteresis interval and a required number of cardiac cycles having an intrinsic event sensed within the hysteresis interval, where the hysteresis interval may be shorter than the pacing interval. 
       FIGS.  8 A through  9 C  are timing diagrams depicting operations performed by ICD  14  in controlling holding capacitor charging based on the timing of sensed intrinsic events.  FIGS.  8 A and  8 B  are timing diagrams depicting operations performed by ICD  14  during the capacitor charging without delay mode. In  FIG.  8 A , timing diagram  200  shows two pacing pulses  201  and  203  delivered by ICD  14  separated in time by a pacing interval  205 . During pacing, control circuit  80  may control therapy delivery circuit  84  to charge a holding capacitor during each pacing interval, until increased intrinsic heart rate criteria are met, which may be based on a hysteresis interval  206  shorter than the pacing interval  205 . 
     Upon delivering pacing pulse  201 , therapy delivery circuit  84  may be controlled to charge the holding capacitor(s) used for delivering pacing pulse  201  during a capacitor charging time  204 , according to the charging without delay mode. As described above, the capacitor charged during charging time  204  may be the HV holding capacitor  162  ( FIG.  4   ) or any combination of LV holding capacitors  352 ,  354 ,  356 , and/or  358  ( FIG.  5   ). The charging time  204  is not necessarily a fixed time interval. Rather the charging time  204  is the time required to recharge the holding capacitor(s) back to the pacing voltage amplitude after pacing pulse  201  is delivered and will depend on the pacing voltage amplitude, the residual charge left on the holding capacitor after pacing pulse delivery, the capacitance of the holding capacitor, and other factors. While charging time  204  is shown as a single discrete time interval at the beginning of pacing interval  206 , it is recognized that the capacitor charge may be monitored throughout pacing interval  205  and topped off as needed if the capacitor charge decreases below the pacing voltage amplitude due to leakage current in the ICD circuitry. The charging time  204  represented as a block of time is intended to represent the capacitor charging without delay mode, which may include charging at the beginning and/or throughout the pacing interval as needed to recharge the holding capacitor to the pacing voltage amplitude during the pacing interval. 
     Pacing interval  205 , which may be a VV interval during VVI pacing for example, is started upon delivery of the first pacing pulse  201 . If no intrinsic events are sensed during the pacing interval  205 , pacing pulse  203  is delivered by therapy delivery circuit  84  in response to the expiration of pacing interval  205 . The holding capacitor is recharged during charging time  208  following pacing pulse  203  according to the charging without delay mode. 
       FIG.  8 B  is a timing diagram  210  showing inhibition of a pacing pulse in response to a sensed intrinsic event during the pacing interval  205 . Pacing pulse  211  is delivered, and the holding capacitor is recharged to the pacing voltage amplitude during pacing interval  205  without delay, as indicated capacitor charging time  214 . If an intrinsic event is sensed during the pacing interval  205 , the scheduled pacing pulse  213  is withheld (as indicated by dashed line). Sensing circuit  86  may be configured to produce a sensed event signal  216 , e.g., an R-wave sensed event signal, that is passed to control circuit  80 . In response to the sensed event signal  216 , the pacing interval  205  is restarted as new pacing interval  215 , inhibiting the scheduled pacing pulse  213 . 
     The control circuit  80  may monitor the charge of the holding capacitor and top off the capacitor charge during recharging time  218  if needed to maintain the holding capacitor voltage at the pacing voltage amplitude (or within a specified tolerance voltage of the pacing voltage amplitude) during the pacing interval  215 . The sensed event signal  216  that is within the pacing interval  205  but not within the hysteresis interval  206  does not alter the control of capacitor charging in this example. The holding capacitor continues to be recharged during each pacing interval  205  (or  215 ) as needed according to the charging without delay mode in order to prepare and maintain the holding capacitor at the pacing voltage amplitude. The holding capacitor may be charged during pacing interval  205  following a pacing pulse  211  as well as during a pacing interval  215  following a sensed event signal  216  if charging is needed to top off the capacitor charge to the pacing voltage amplitude. Even though a pacing pulse has not been delivered, the holding capacitor charge may fall below the pacing voltage amplitude due to inherent leakage current in the ICD circuitry. Top off capacitor charging may not be required immediately following every sensed event. Control circuit  80  may be configured to monitor the holding capacitor voltage at the start of each pacing interval and/or throughout each pacing interval, start charging if the voltage is less than a tolerance below the pacing voltage amplitude, and terminate charging when the charge is back up to the pacing voltage amplitude. 
     If the intrinsic heart rate is less than a hysteresis rate corresponding to hysteresis interval  206  but greater than the pacing rate corresponding to pacing interval  205 , the holding capacitor charge continues to be monitored and maintained at the pacing voltage amplitude according to the charging without delay mode. As such, if the sensed event signal  216  occurs after expiration of the hysteresis interval  206 , capacitor charging is performed as needed without delay. 
       FIGS.  8 C and  8 D  are timing diagrams depicting operations of ICD  14  performed for switching from the charging without delay mode to the delayed charging mode in response to increased intrinsic heart rate criteria being met.  FIG.  8 C  is a timing diagram showing pacing pulse  221  followed by capacitor charge time  214  according to the charging without delay mode. The pacing interval  205  and the hysteresis interval  206  are started simultaneously in response to the delivered pacing pulse  221 . The next scheduled pacing pulses  223  is inhibited due to a sensed intrinsic event signal  226  that is received from sensing circuit  86  by control circuit  80  during pacing interval  205 . The pacing interval  205  is restarted as new pacing interval  225  in response to sensed event signal  226 . 
     During the charging without delay mode, control circuit  80  may monitor for an increase in heart rate, faster than the pacing rate corresponding to pacing interval  205 , by counting the number of cardiac cycles having an intrinsic event sensed during the hysteresis interval  206 . In the example shown, control circuit  80  detects an increased intrinsic heart rate in response to the sensed event signal  226  occurring within hysteresis interval  206 . Control circuit  80  switches from the charging without delay mode to the delayed charging mode in response to detecting the increased intrinsic heart rate by withholding capacitor charging during the next pacing interval  225 . 
     This withholding of capacitor charging is illustrated schematically by dashed capacitor charging time block  224 , during which no capacitor charging actually occurs. If the sensed event signal  226  had occurred after hysteresis interval  206  as described in conjunction with  FIG.  8 B , capacitor charging time  224  would occur during pacing interval  225  as needed to maintain the holding capacitor charge at the pacing voltage amplitude. Capacitor charging may be withheld during pacing interval  225  by withholding monitoring of the capacitor charge during the pacing interval  225  or by withholding charging even when the capacitor charge falls below the pacing voltage amplitude. In the example of  FIG.  8 C , a sensed event signal  226  during a single hysteresis interval  206  is shown to cause control circuit  80  to switch from charging without delay to the delayed capacitor charging mode and withhold capacitor charging. In other examples, a sensed event signal during each of more than one respective hysteresis interval as described above in conjunction with  FIG.  7    may be required for increased intrinsic heart rate criteria to be met to cause switching to the delayed charging mode. For instance, sensed event signal  226  may be the Xth event sensed within a respective number of X hysteresis intervals out of Y cardiac cycles, causing the increased intrinsic heart rate criteria to be met. 
       FIG.  8 D  is a timing diagram  230  depicting operations performed by ICD  14  in controlling holding capacitor charging in response to detecting an increased heart rate based on hysteresis interval  206  according to another example. In the example of  FIG.  8 D , three consecutive sensed event signals  226 ,  227  and  228  are required to each be sensed within a respective hysteresis interval  206 ,  207  and  209  in order to detect an increased intrinsic heart rate and switch the capacitor charging mode to the delayed charging mode. Each hysteresis interval  206 ,  207  and  209  is started in response to the preceding cardiac event, pacing pulse  221 , sensed event signal  226  and sensed event signal  227 , respectively. In this example, criteria for detecting an increased intrinsic heart rate is not met until the three consecutively sensed events  226 ,  227  and  228  are sensed within the hysteresis interval  206 ,  207  and  208  set for the respective cardiac cycles. As such, control circuit  80  does not withhold or delay capacitor charging until the increased heart rate detection criteria are satisfied. Control circuit  80  controls the charging circuitry of therapy delivery circuit  84  to charge the holding capacitor(s) used for generating pacing pulses to the pacing voltage amplitude as needed during each of pacing intervals  225  and  229  started in response to sensed event signals  226  and  227  even though the sensed event signals  226  and  227  each occurred above the hysteresis rate, within respective hysteresis intervals  206  and  207 . 
     Recharging of the holding capacitor(s) may occur during charging times  232  and  234  at the beginning of the respective pacing intervals  225  and  229  or throughout the pacing intervals  225  and  229  as needed to top-off the holding capacitor charge within a tolerance of the pacing voltage amplitude. It is recognized, that depending on the duration of the pacing interval, the inherent leakage current and other factors, recharging of the holding capacitor(s) may not be required during every pacing interval in order to maintain the capacitor charge within a specified tolerance of the pacing voltage amplitude. However, control circuit  80  may monitor or check the charge of the holding capacitor during each pacing interval  225  and  229  that is started in response to the sensed event signals  226  and  227 , respectively, until criteria for detecting an increase in heart rate are satisfied. 
     Upon detecting the third sensed event signal  228  within hysteresis interval  209 , an increased intrinsic heart rate that is faster than or equal to the hysteresis rate is detected in this example. In response, control circuit  80  switches to the delayed charging mode by withholding capacitor charging during the next pacing interval  231 . The HV or LV charging circuit of therapy delivery circuit  84  being used for pacing pulse generation may be turned “off” by withholding power supplied to the charging circuit for charging the holding capacitor. Withholding of capacitor charging is illustrated by the dashed box  236 . No charging occurs following sensed event signal  228  during the first pacing interval  231  after increased intrinsic heart rate criteria are met. Monitoring of the holding capacitor voltage, e.g, by a comparator comparing the capacitor voltage to the programmed pacing voltage amplitude, that may normally be performed to control top-off charging of the holding capacitor during a pacing interval may be disabled in response to detecting the increased heart rate since no charging of the holding capacitor is performed again until at least one pacing interval expires or until other decreased intrinsic heart rate criteria are subsequently satisfied. 
       FIGS.  9 A- 9 C  are timing diagrams depicting operations performed by ICD  14  for withholding capacitor charging according to a delayed capacitor charging mode and switching back to the charging without delay mode in response to decreased intrinsic heart rate criteria being met. Holding capacitor charging is withheld after increased intrinsic heart rate criteria are met as described above in conjunction with  FIGS.  8 C and  8 D . As shown in  FIG.  9 A , the pacing interval  245  is started in response to a sensed event signal  242 . No capacitor charging is performed during pacing interval  245  because capacitor charging is being withheld according to the delayed charging mode due to increased intrinsic heart rate criteria being previously met. Pacing interval  245  expires without a sensed intrinsic event. Control circuit  80  controls therapy delivery circuit  84  to charge the holding capacitor(s) in response to the pacing interval  245  expiring as indicated by charging time  244 . Once the holding capacitor voltage has reached the pacing voltage amplitude, pacing pulse  243  is delivered. Pacing pulse  243  may be delivered at a delay after the expiration of pacing interval  245  that is equal to the charging time  244  required to charge the holding capacitor up to the pacing voltage amplitude. 
     Control circuit  80  may be configured to detect a decreased intrinsic heart rate in response to expiration of a single pacing interval  245  without as sensed intrinsic event. In response to detecting the decreased heart rate that is slower than the pacing rate based on no intrinsic event being sensed during the pacing interval  245 , the control circuit  80  switches to charging without delay by charging the holding capacitor during each pacing interval as needed to maintain the holding capacitor(s) in a ready state for pacing pulse delivery. Pacing interval  246  is started in response to delivering pacing pulse  243 . Capacitor charging may be initiated at the beginning or onset of pacing interval  246  after detecting a decreased intrinsic heart rate based on at least one expired pacing interval  245 . Control circuit  80  may control the therapy delivery circuit  84  to recharge the holding capacitor to the pacing voltage during charging time  247  following pacing pulse  243 . 
     In addition to starting pacing interval  246 , control circuit  80  may start hysteresis interval  206  to begin monitoring for an increased intrinsic heart rate as described above in conjunction with  FIGS.  8 C- 8 D . Control circuit  80  continues to charge the holding capacitor(s) during each pacing interval as needed according to the charging without delay mode in order to maintain the capacitor charge within a specified tolerance of the pacing voltage amplitude during each pacing interval until an increased intrinsic heart rate is detected. Control circuit  80  may enable power source  98  to provide power to the HV therapy circuit  83  or LV therapy circuit  85  for charging selected holding capacitor(s) until an increased intrinsic heart rate is detected. Control circuit  80  may compare a capacitor charge signal from the selected HV therapy circuit  83  or LV therapy circuit  85  at the start of pacing interval  246  and each pacing interval thereafter until an increased intrinsic heart rate is detected again. If the capacitor charge signal indicates that the holding capacitor charge is less than a tolerance below the pacing voltage amplitude, control circuit  80  enables charging of the selected holding capacitor(s) as needed during pacing interval  246 . 
     In  FIG.  9 A , the decreased intrinsic heart rate is detected by control circuit  80  in response to a single expired pacing interval for the purposes of switching back to charging without delay. In other examples, more than one expired pacing interval may be required for detecting a decreased intrinsic heart rate to cause control circuit  80  to change the timing of capacitor charging. For example, capacitor charging may occur upon expiration of the pacing interval for two or more pacing cycles resulting in pacing pulses being delivered at the pacing interval plus a delay interval equal to the required charging time for the two or more pacing cycles. A decreased intrinsic heart rate may be detected in response to a predetermined number of consecutive or non-consecutive (X out of Y) expired pacing intervals. 
     To illustrate, if a decreased intrinsic heart rate is detected in response to three consecutively expired pacing intervals, up to three pacing pulses may be delivered at a rate less than the pacing rate corresponding to pacing interval  245 . The actual pacing rate for the first three pacing pulses may be the rate corresponding to the pacing interval  245  plus the charging time  244  required to charge the holding capacitor to the pacing voltage amplitude. For instance, the pacing interval  245  may be set to 1.5 seconds for a lower pacing rate of 40 pulses per minute. The charge time  244  may average 0.5 seconds, resulting in an actual pacing rate of 30 pulses per minute for the three pacing cycles leading up to satisfaction of the decreased heart rate detection criteria. After the third pacing pulse, the control circuit  80  may detect a decreased intrinsic heart rate based on decreased heart rate criteria and re-enable the therapy delivery circuit  84  to charge the holding capacitor during each pacing interval without delay so that subsequent pacing pulses are each delivered at the expiration of the programmed pacing interval without delay 
     In some instances, a sensed event signal could be received by control circuit  80  during the charging time  244 . In this case, the pacing pulse  243  would be inhibited and capacitor charging could be terminated or allowed to continue. If a required number of expired pacing intervals has been reached for decreased intrinsic heart rate criteria to be met, control circuit  80  may switch back to the charging without delay mode even if a sensed event signal was received during the charging time, after the pacing interval expired, causing the pacing pulse to be inhibited. As such, a required number of pacing intervals for detecting a decreased intrinsic heart rate may be reached without requiring the same number of delivered pacing pulses. The number of delivered pacing pulses may be less than the number of expired pacing intervals due to sensed event signals being received during the capacitor charging. 
       FIG.  9 B  is a timing diagram of a method for controlling holding capacitor charging during a decreasing intrinsic heart rate according to another example. In some examples, capacitor charging is withheld for the entire pacing interval according to the delayed capacitor charging mode, as shown in  FIG.  9 A . In other examples, capacitor charging is withheld for a portion of the pacing interval but may be started during the pacing interval after a charging delay interval. In the example of  FIG.  9 B , a capacitor charging delay interval  256  is set in response to sensed event signal  251 , along with starting pacing interval  255 . In response to detecting an increased heart rate based on the hysteresis interval as described above in conjunction with  FIG.  8 C or  8 D , control circuit  80  may withhold capacitor charging during the pacing interval  255  until after the capacitor charging delay interval  256  expires, according to the delayed charging mode. If a sensed event  252  occurs during the capacitor delay interval  256 , holding capacitor charging is withheld and the capacitor charging delay interval is restarted as interval  258 . The pacing interval is restarted as interval  257  in response to the sensed event signal  252 . 
     Charging delay interval  258  expires before the next sensed event signal  253 . Capacitor charging is initiated as indicated by charging time  254  in response to charging delay interval  258  expiring. In response to receiving a sensed event signal  253 , capacitor charging  254  may be terminated since the scheduled pacing pulse is inhibited and the pacing interval is restarted as pacing interval  259 . In other examples, charging to the pacing voltage amplitude may be completed during charging time  254  (which may extend past sensed event signal  253  and into the next pacing interval  259 ). The next charging delay interval  260  is started along with the pacing interval  259  in response to the sensed event signal  253 . 
     If charging delay interval  260  expires, control circuit  80  is configured to control the therapy delivery circuit  80  to initiate capacitor charging  254 . If the pacing interval  259  expires, pacing pulse  264  is delivered. A new pacing interval  261  is started and a hysteresis interval  270  may be set in response to delivered pacing pulse  264  for use in detecting an increased intrinsic heart rate again as described above, e.g., in conjunction with  FIG.  7 C or  7 D . In some instances, pacing pulse  264  may be delivered upon pacing interval expiration without delay if the holding capacitor is fully charged to the pacing voltage amplitude by the time the pacing interval  259  expires. In other instances, capacitor charging may be incomplete upon expiration of pacing interval  259 , and pacing pulse  264  may be delivered at a short delay after expiration of the pacing interval  259  as required to complete capacitor charging. 
     In some examples, control circuit  80  may be configured to determine an estimated capacitor charging time based on the pacing voltage amplitude and capacitance of the holding capacitor. In other examples, control circuit  80  may be configured to determine the capacitor charging time based on charging history. For instance, the time interval from the start of capacitor charging following a delivered pacing pulse until a charge completion signal is received from the therapy delivery circuit  84  may be determined by control circuit  80 . This time interval, e.g., charging interval  247  of  FIG.  9 A , or an average of multiple charge completion time intervals obtained in this manner during multiple capacitor charging occurrences, may be determined as the capacitor charging time. 
     Control circuit  80  may be configured to set the capacitor charging delay interval  260  based on the pacing interval  259  and the calculated or measured capacitor charging time. The capacitor charging delay interval  260  may be determined as the difference of the pacing interval  259  and the determined capacitor charging time. In some examples, the capacitor charging delay interval  260  may be the difference between pacing interval  259  and the determined capacitor charging time less a pacing safety interval (which may be set to zero) to promote charge completion prior to expiration of pacing interval  259  to avoid any delay of pacing pulse  264 . 
     In the example shown in  FIG.  9 B , control circuit  80  controls the therapy delivery circuit  84  to recharge the holding capacitor during charging time  266  after delivery of pacing pulse  264 . Expiration of a single pacing interval  259  may cause control circuit  80  to switch to the charging without delay mode to restore the holding capacitor charge to the pacing voltage during each pacing interval without waiting for a capacitor charging delay interval to expire. In other examples, a predetermined number of pacing intervals may be required to expire before reverting back to charging without delay during each pacing interval. For instance, charging may be performed after the capacitor charging delay interval after two or more consecutive or non-consecutive pacing pulses, e.g., after 3 consecutive pacing pulses or after 3 pacing pulses out of 5 consecutive cardiac cycles, before switching to charging without delay, without setting the capacitor charging delay interval. 
       FIG.  9 C  is a timing diagram of another example for controlling holding capacitor charging during a decreasing intrinsic heart rate. In the example of  FIG.  9 B , control circuit  80  detects a decreased intrinsic heart rate based on a predetermined number of expired pacing intervals. In other examples, control circuit  80  may be configured to detect a decreased intrinsic heart rate in response to a predetermined number of expired capacitor charging delay intervals. The decreased intrinsic heart rate detection criteria may be determined to be satisfied based on expired capacitor charging delay intervals, even if no pacing intervals have expired. 
     As shown in  FIG.  9 C  and described above in conjunction with  FIG.  9 B , a sensed event  252  during a capacitor charging delay interval  256  causes capacitor charging to be withheld. Expiration of a capacitor charging intervals  258  and  260  without sensed intrinsic events results in capacitor charging  254  to top off the holding capacitor charge at the pacing voltage amplitude. In the example of  FIG.  9 B , capacitor charging  254  was terminated in response to a sensed event signal  253  during the capacitor charging. In the example of  FIG.  9 C , capacitor charging  254  continues after sensed event signal  253  to complete charging to the pacing voltage amplitude. 
     Sensed events  253  and  282  during the capacitor charging  254  and prior to expiration of respective pacing intervals  257  and  288  cause scheduled pacing pulses to be inhibited. In response to the expiration of a predetermined number of charging delay intervals, two in this example ( 258  and  260 ), control circuit  80  detects a decreased intrinsic heart rate and switches the capacitor charging mode to charging without delay after sensed event signal  282 . A capacitor charging delay interval is not started concomitantly with pacing interval  290 . Capacitor charging may be completed as needed to top-off the capacitor charge following sensed event  282  and during the subsequent pacing interval  290 . The next event is a sensed event  284  during pacing interval  290 . Capacitor charging occurs without delay during pacing interval  292  as indicated by charge time  286 . A hysteresis interval  270  may be started following the sensed event  284  concomitantly with pacing interval  292  to facilitate detection of increased intrinsic heart rate criteria being satisfied. 
     In this example, if a sensed intrinsic event occurs during the capacitor charging delay interval, charging is withheld because the intrinsic heart rate is faster than the rate corresponding to the capacitor charging delay interval. If a predetermined number of capacitor charging delay intervals expire, however, the intrinsic heart rate may be decreasing toward a potential need for pacing. The control circuit  80  may detect a decreased intrinsic heart rate in response to a predetermined number of expired capacitor charging delay intervals and switch to charging without delay in response to detecting the decreased intrinsic heart rate. As such, switching to capacitor charging without delay may occur before the intrinsic heart rate falls below the programmed pacing rate and before any pacing pulses are delivered. 
       FIG.  10    is a flow chart  300  of a method for controlling holding capacitor charging according to another example. At block  302 , control circuit  80  controls the therapy delivery circuit  84  to charge the selected holding capacitor(s) for delivering pacing pulses after a capacitor charging delay interval, which may be equal to or less than the pacing interval currently being used for scheduling a pacing pulse. Operations performed at block  302  may include determination of a calculated or measured capacitor charge completion time interval used for setting the capacitor charging delay interval. Control circuit  80  withholds capacitor charging in response to receiving a sensed event signal during the capacitor charging delay interval, and the scheduled pacing pulse is inhibited, e.g., as shown in  FIGS.  9 B and  9 C . The capacitor charging delay interval and the pacing interval are restarted. In other examples, the pacing interval is restarted after each pacing pulse and sensed event signal and capacitor charging is delayed until a pacing interval expires without setting a separate capacitor charging delay interval, e.g., as in  FIG.  9 A . 
     Control circuit  80  may detect a decreased intrinsic heart rate at block  304  based on a threshold number of expired pacing intervals, e.g., as in  FIGS.  9 A and  9 B . The number of expired pacing intervals may be greater than the number of delivered pacing pulses since a sensed event after the pacing interval expiration may occur during delayed charging, causing the pacing pulse to be inhibited. In other examples, control circuit  80  may detect a decreased intrinsic heart rate in response to a predetermined number of expired charging delay intervals, e.g., as shown in  FIG.  9 C . In response to detecting the decreased intrinsic heart rate, control circuit  80  switches to controlling the therapy delivery circuit  84  to charge the holding capacitor used for pacing without delay at block  306 , e.g., without setting the capacitor charging delay interval. Re-charging or top-off charging to maintain the holding capacitor charge at the pacing voltage amplitude during the pacing interval may occur at the beginning of a pacing interval or throughout the pacing interval set after a delivered pacing pulse or sensed event. 
     At block  308 , control circuit  80  may determine the rate or slope of the decrease in the intrinsic heart rate. For example, the control circuit  80  may determine the rate of decrease over a predetermined number of heart beats or a predetermined time interval leading up to the time at which the decreased heart rate criteria were satisfied. If the heart rate decrease occurs abruptly, the decreased intrinsic heart rate detection criteria may be adjusted at block  310  to enable detection of the decreased heart rate in fewer cardiac cycles, e.g., as few as one cardiac cycle. If the rate of decrease is relatively slow, the decreased intrinsic heart rate detection criteria may be adjusted at block  310  by increasing the number of expired capacitor charging delay intervals and/or expired pacing intervals required to detect the decreased intrinsic heart rate before switching from delayed capacitor charging to charging the holding capacitor(s) without delay. 
     Control circuit  80  continues to control the therapy delivery circuit  84  to charge the holding capacitor without delay during each pacing interval to maintain the holding capacitor in a ready state until an increased intrinsic heart rate is detected at block  312 . As described above, criteria for detecting an increased intrinsic heart rate may require one or more cardiac cycles having a sensed event signal occurring within the hysteresis interval. 
     When increased intrinsic heart rate criteria are met at block  312 , control circuit  80  may be configured to determine the rate or slope of the increase in the intrinsic heart rate at block  314 . The rate of increase may be determined over a pre-determined time interval or number of cardiac cycles. Based on the rate of increase, control circuit  80  may adjust the criteria for detecting the increased intrinsic heart rate at block  316 . If the rate of increase occurs rapidly, the control circuit may set a relatively low number of sensed events occurring at or above the hysteresis interval rate as a requirement for detecting the increased intrinsic heart rate. Alternatively or additionally, the hysteresis interval may be set to a relatively longer interval, up to the pacing interval, to promote earlier detection of an increasing intrinsic heart rate. By adjusting the increased intrinsic rate criteria in a patient whose heart rate recovers rapidly, battery charge is conserved by switching back to delayed capacitor charging earlier. 
     If the rate of increase occurs gradually, e.g., with intermittent pacing and sensing or sensing near the pacing rate for a sustained time interval, the hysteresis interval may be adjusted to a relatively shorter interval and/or the required number of sensed hysteresis interval events may be reduced. By adjusting the increased intrinsic rate detection criteria, switching back and forth between delayed capacitor charging and charging without delay may be avoided and any pacing delays due to delayed capacitor charging may be avoided while the heart rate is gradually increasing. 
     After adjusting the increased rate detection criteria based on the rate of increase, the control circuit  80  operates to control the therapy delivery circuit  84  to delay charging of the holding capacitor during pacing intervals following sensed events and pacing pulses. Capacitor charging is delayed by withholding capacitor charging until a pacing interval expires, e.g., as shown in  FIG.  9 A , or until a capacitor charging delay interval expires, e.g., as shown in  FIG.  9 B or  9 C . Capacitor charging is delayed until a decreased intrinsic heart rate is detected according to adjusted decreased rate detection criteria. It is to be understood that while adjustment of both decreased rate detection criteria and increased rate detection criteria are indicated in  FIG.  10    based on determining the respective rate of decrease and rate of increase of intrinsic heart rate changes, the control circuit  80  may be configured to automatically adjust only the decreased intrinsic heart rate detection criteria, adjust only the increased intrinsic heart rate detection criteria, both or neither. 
       FIG.  11    is a flow chart  320  of a method for controlling the capacitor charging mode based on the rate or slope of a change in heart rate. At block  321 , ICD  14  may be delivering pacing pulses during a sustained run of pacing due to expired pacing intervals without sensed intrinsic events. The control circuit  80  is controlling the therapy delivery circuit  84  to charge the holding capacitor(s) without delay. 
     At block  322 , an intrinsic event is sensed, causing a scheduled pacing pulse to be inhibited. The sensed event interval is determined at block  323 . At block  324 , the slope of the heart rate over a predetermined time interval or a predetermined number of cardiac cycles is determined. For example, five to ten of the most recent sensed event intervals may be used to determine the slope of the heart rate change at block  324 . If the intrinsic heart rate is increasing rapidly, e.g., due to increased patient activity, capacitor charging may be withheld since the likelihood of a pacing interval expiring is now greatly reduced. Control circuit  80  compares the slope of the intrinsic heart rate change over the predetermined time interval or number of sensed intrinsic events to a threshold slope at block  325 . This threshold slope is a positive slope corresponding to a relatively rapid rise in intrinsic heart rate. 
     If the required time interval or number of sensed event intervals for determining the slope at block  324  has not been reached, the slope of the intrinsic heart rate change will be less than the slope threshold at block  325 . In that case, capacitor charging without delay continues at block  326 . The process returns to block  322  to wait for the next sensed event. 
     If the required time interval or number of sensed event intervals has been reached to enable slope determination at block  324 , but the slope is less than the slope threshold at block  325 , the control circuit  80  continues to control capacitor charging without delay at block  326 . The intrinsic heart rate may be increasing but may not be increasing rapidly enough or may not be increasing monotonically to justify switching from the charging without delay mode to the delayed charging mode. The probability of a pacing interval expiring is still high enough to warrant maintaining the holding capacitor in a ready state for pacing. 
     If the slope of the intrinsic heart rate change determined at block  324  is equal to or greater than the slope threshold at block  325 , control circuit  80  switches the capacitor charging mode to delayed capacitor charging at block  327 , e.g., by setting a capacitor charging delay interval as described previously or withholding capacitor charging during all or a portion of the next pacing interval. The slope threshold applied at block  325  may require that the intrinsic heart rate increase from the current pacing rate to 20 beats per minute faster than the pacing rate within one minute, for example, though other slope thresholds may be defined according to patient need. 
     After switching to delayed capacitor charging, the control circuit  80  may continue to monitor the slope of the intrinsic heart rate change at block  328 . The slope may be compared to a rate drop threshold at block  329 . The rate drop threshold may be applied to the slope of the intrinsic heart rate over a predetermined time interval or predetermine number (Y) of sensed event intervals to determine if the intrinsic heart rate is rapidly decreasing, which increases the likelihood of a pacing interval expiring. For example, the rate drop threshold may require that the slope of the intrinsic heart rate falls 30 beats per minute within one minute. This rate drop threshold is a negative slope threshold corresponding to a relatively rapid decrease in heart rate. The drop threshold and required time interval or number of sensed event intervals for determining the slope compared to the drop threshold may be defined differently than the respective slope and threshold determined and used at blocks  324  and  325  for detecting a rapid rise in heart rate. 
     If the slope of the intrinsic heart rate change is greater than (e.g., less negative than) the drop threshold, delayed charging of the holding capacitor(s) continues at block  327 . The heart rate may be increasing (a positive slope), stable (zero slope), or slowly decreasing (smaller negative slope than the drop threshold), such that there is a relatively low probability that a pacing interval will expire. If the slope is less than (more negative than) the drop threshold at block  329 , indicating a rapid decrease in heart rate and an increased likelihood of a pacing interval expiring, control circuit  80  may switch to charging the holding capacitor(s) without delay at block  328 . 
     It is to be understood that all or a part of the process of flow chart  320  may be combined with the techniques described above in conjunction with  FIGS.  6  through  9 C  such that control circuit  80  may switch the control of capacitor charging in response to the slope of a heart rate change crossing a slope threshold, e.g., being greater than the positive, increasing slope threshold at block  325  and/or less than the negative, drop slope threshold at block  329  before other increased intrinsic heart rate detection criteria or decreased intrinsic heart rate detection criteria are met. When combined with the techniques for switching capacitor charging mode based on other increased intrinsic heart rate detection criteria and decreased intrinsic heart rate detection criteria, control circuit  80  may respond appropriately to both rapid and relatively slower changes in intrinsic heart rate by switching the capacitor charging mode in a manner that conserves battery energy of power source  98  when expiration of a pacing interval is less likely and maintains the holding capacitor charge in a ready state when a pacing interval expiration is relatively more likely. 
       FIG.  12    is flow chart  400  of a method for enabling and disabling the capacitor charging mode switching function based on detecting changes in pacing burden according to one example. ICD  14  may operate according to the techniques described above in conjunction with any of  FIGS.  6  through  11    on a continuous basis. In other words, control circuit  80  may be enabled to automatically switch between a charging without delay mode and a delayed charging mode based on intrinsic heart rate criteria at any time of day or independent of other physiological conditions. In other examples, control circuit  80  may operate only in a default capacitor charging mode, e.g., charging without delay, with the function of switching between charging without delay and delayed charging modes disabled (turned off). The function of charging mode switching may be enabled (turned on) and disabled (turned off) by control circuit  80  in response to an actual or predicted change in pacing burden. 
     In  FIG.  12   , ICD  14  initially operates in a default capacitor charging mode at block  402 . In the example shown, the default mode is charging without delay during each pacing interval to maintain the capacitor charge in a ready state for delivering a pacing pulse the next time a pacing interval expires. The functions of detecting a change in intrinsic heart rate based on predefined criteria and switching between capacitor charging modes may be disabled or turned off during this default charging mode. 
     At block  404 , control circuit  80  may monitor one or more parameters for determining if decreased pacing burden criteria are satisfied. A decreased pacing burden may be determined or predicted based on pacing history, time of day, and/or other physiological signals received from sensors  90 . In one example, decreased pacing burden criteria are met when the time of day is determined to be nighttime or a programmable time of day that the patient is normally resting, goes to bed or is asleep. In response to the time of day reaching the programmed “night” time, control circuit  80  may enable or turn on the function of switching capacitor charging mode at block  406 . Enabling the function of automatic switching between charging modes does not necessarily mean that switching from one charging mode to the other occurs right away. Rather, after turning on the switching mode function, monitoring for detecting changes in intrinsic heart rate may be performed to control switching between the two charging modes. In order to actually switch from the charging without delay mode that is currently active to the delayed charging mode, increased intrinsic heart rate criteria need to be met, e.g., based on a sensed intrinsic event during at least one hysteresis interval as described above in conjunction with  FIGS.  8 C and  8 D . 
     After enabling the function of charging mode switching at block  406 , control circuit  80  may monitor one or more parameters for determining if increased pacing burden criteria are met at block  408 . The switching function may be turned off or disabled again at block  410  if increased pacing burden criteria are met. In the illustrative example of enabling charging mode switching at block  406  in response to determining that the time of day is “night” at block  404 , corresponding to an expected decrease in pacing demand, control circuit  80  may determine that increased pacing burden criteria are met at block  408  in response to the time of day reaching morning or a programmed time of day that the patient is expected to wake up or become active. In a given patient, the pacing burden may be expected to be higher during active daytime hours than during night time hours. The increased pacing burden criteria are met at block  408  based on the time of day and an expected higher pacing frequency during daytime hours. It is recognized that the time of day that is detected as meeting decreased pacing burden criteria and the time of day that is detected as meeting increased pacing burden criteria may be tailored according to a patient&#39;s individual habits and daily routine. 
     In other examples, the increased pacing burden criteria may be determined to be satisfied at block  408  based on an increase in the actual pacing burden determined as the number or percentage of delivered pacing pulses during a predetermined time period, e.g., over at least one hour or more. In response to the increased pacing burden criteria being met at block  408 , control circuit  80  disables the function of charging mode switching at block  410 . Control circuit  80  operates to control the therapy delivery circuit  84  to charge the holding capacitor(s) according to the default pacing mode, which may be the charging without delay pacing mode as indicated at block  402 , without monitoring for intrinsic heart rate changes and without switching between charging modes. 
     Other criteria for detecting a decreased pacing burden at block  404  may include an actual pacing burden falling below a predetermined threshold. For example, the pacing burden may be determined as the number of pacing pulses, the percentage of pacing pulses delivered out all cardiac events, or the ratio of paced events to intrinsic sensed events during a predetermined time interval, e.g., one hour, two hours, four hours, eight hours, twelve hours, twenty-four hours, one week or other time interval. If the pacing burden falls below a pacing burden threshold, the decreased pacing burden criteria are met at block  404 . To illustrate, if fewer than 10% of all events, sensed and paced, are delivered pacing pulses over the past twenty-four hours, control circuit  80  may enable charging mode switching at block  406  such that delayed capacitor charging may be performed when increased intrinsic heart rate criteria are met. 
     Other examples of criteria for determining that the decreased pacing burden criteria are met at block  404  may be based on a sensor signal received from sensors  90 . For instance a decrease in patient activity or a decreased sensor indicated pacing rate as determined from an activity sensor signal or other indicator of decreased metabolic demand, such as decreased respiratory minute volume, may be an indicator of decreased pacing burden. Similarly, criteria for detecting an increased pacing burden may be satisfied at block  408  in response to detecting an increase in patient activity or sensor indicated rate determined from a patient activity sensor, such as an accelerometer or an impedance sensor used to track respiratory minute volume as an indication of increased metabolic demand. 
     Other physiological sensor signals correlated to the patient&#39;s hemodynamic function may be used to determine that decreased pacing burden criteria are met at block  404  and/or increased pacing burden criteria are met at block  408 . For example a signal or metric derived from a pressure sensor, oxygen saturation sensor, impedance sensor, or other physiological sensor may be determined and compared to a threshold for determining a need for increased cardiac output. The increased pacing burden criteria may be satisfied based on a need to increase the cardiac output. For instance, blood pressure, tissue or blood oxygen saturation, or other parameter that is correlated to cardiac output may be determined by control circuit  80  from a sensor signal and compared to a threshold at block  408 . If the parameter indicates that cardiac output is low, e.g., below a predetermined threshold, such that an increase in cardiac output is needed, increased pacing burden criteria may be determined to be satisfied at block  408 , and charging mode switching may be disabled at block  410 . 
     The decreased pacing burden criteria and the increased pacing burden criteria may be defined differently such that different criteria, which may include different parameters and/or different thresholds applied to respective parameters, are used to determine when to enable and disable the function of switching charging mode. For example, the time of day may be used to detect satisfaction of decreased pacing burden criteria for enabling capacitor charging mode switching while a sensor signal parameter, e.g., indicative of an increased metabolic demand or a need for increased cardiac output, may be used to detect that increased pacing burden criteria are met at block  408  for disabling charging mode switching at block  410 . 
     After charging mode switching is enabled at block  406 , control circuit  80  may operate according to any of the techniques described above for detecting an increased intrinsic heart rate and switching to delayed capacitor charging. After switching to delayed capacitor charging, control circuit may monitor for a decreased intrinsic heart rate and switch back to capacitor charging without delay as long as the switching function has not been disabled due to increased pacing burden criteria being met. 
     It is further contemplated that the default charging mode is the delayed charging mode rather than the charging without delay mode as shown in  FIG.  12   . The capacitor charging mode switching function may be enabled and/or disabled based on an actual or predicted pacing burden change crossing a pacing burden threshold. A patient receiving ICD  14  may be expected to seldom require pacing. In that case, control circuit  80  may control therapy delivery circuit  84  to delay capacitor charging until a charging delay interval or pacing interval expires without a sensed intrinsic event. A slow intrinsic heart rate that requires pacing may occur without switching to the charging without pacing mode as long as the charging mode switching function remains disabled. Control circuit  80  may monitor actual pacing burden based on the frequency of delivered pacing pulses, the time of day, and/or one or more physiological signals from sensors  90  for determining if increased pacing burden criteria are met, e.g. as described above in conjunction with block  408 . In response to the increased pacing burden criteria being met, control circuit  80  may enable switching between capacitor charging modes. After capacitor charging mode switching is enabled, control circuit  80  monitors for intrinsic heart rate changes and may switch from delayed capacitor charging to charging without delay in response to decreased intrinsic heart rate criteria being met. 
     In some examples, after enabling the switching between charging modes, the switching function may never be disabled again. For example, upon initial implant, ICD  14  may operate according to a default capacitor charging mode, either charging with delay or delayed charging, with switching between the two modes disabled based on the anticipated pacing needs of the patient. Upon detecting that a change in pacing burden has occurred or is expected to occur, using any one or combination of the criteria described above, capacitor charging mode switching is enabled. After being enabled, control circuit  80  switches between the two charging modes based on detecting increased and decreased intrinsic heart rates according to predetermined criteria using any of the examples given herein. Capacitor charging mode switching may remain enabled without ever being automatically disabled by control circuit  80 , e.g., for the remaining life of the ICD  14  or until manually reprogrammed by a user. 
       FIG.  13    is a flow chart  450  of a method for controlling capacitor charging by ICD  14  based on different pacing therapies according to one example. Control circuit  80  may start a pacing interval at block  452  in response to a sensed event signal or a delivered electrical stimulation pulse or other determination of a need for a pacing therapy. In some cases, the delivered stimulation pulse may be a CV/DF shock pulse in which case the pacing interval may be post-shock pacing interval for preventing post-shock asystole. If the pacing interval is a post-shock pacing interval, as determined at block  454 , the control circuit  80  may be configured to disable delayed capacitor charging at block  460  in anticipation of a potential critical need for pacing without delay. Control circuit  80  may be configured to control the therapy delivery circuit  84  to deliver post-shock pacing by charging the selected LV or HV holding capacitor(s) to the pacing voltage amplitude during each post-shock pacing interval, e.g., starting at the beginning of each pacing interval and in some instances throughout the pacing interval as needed to maintain the holding capacitor charge at the programmed pacing voltage amplitude. 
     In other cases, the pacing interval started at block  452  may be started in response to a sensed event signal at the time of a tachyarrhythmia detection. In this case, the pacing interval may be an ATP interval set to control the delivery of a series of ATP pulses. If the pacing interval is an ATP pacing interval, control circuit  80  may be configured to disable delayed charging of the holding capacitor at block  460 . Capacitor charging is performed during each ATP pacing interval without delay to promote accurate timing of ATP pacing pulses and successful tachyarrhythmia termination. 
     At other times, the pacing interval started at block  452  may be a bradycardia pacing interval, e.g., a VVI pacing interval, started in response to a delivered bradycardia pacing pulse or R-wave sensed event signal. If the pacing interval was started as a bradycardia pacing interval (“no” branch of block  454 ) and the precipitating event is a sensed event signal that is detected as a premature ventricular contraction (PVC) by the control circuit  80 , the control circuit  80  may disable delayed capacitor charging at block  460  for one cycle. Capacitor charging may be performed during the pacing interval started in response to a sensed event identified as a PVC without delay as needed to top-off the capacitor charge to the pacing voltage amplitude. In this way, the therapy delivery circuit  84  is ready to deliver a pacing pulse in anticipation of a long, compensatory pause following the PVC. Control circuit  80  may be configured to detect a sensed R-wave as a PVC based on the sensed event interval (RR interval) since a most recent preceding ventricular event (paced or sensed) and/or whether or not an atrial P-wave was sensed prior to the sensed R-wave during the RR interval ending on the sensed R-wave. For example, an R-wave sensed event signal that is received at an RRI that is less than a PVC detection threshold interval may be detected as a PVC at block  406 . 
     In other examples, PVCs may be ignored for the purposes of controlling changes in the capacitor charging state. For example, a short sensed event interval determined to end on a sensed event signal identified as a PVC may be ignored in detecting an increased intrinsic heart rate. A long sensed event interval (the compensatory pause) following a sensed event signal identified as a PVC may be ignored in detecting a decreased intrinsic heart rate. In this way, a PVC or a run of PVCs will not alter the capacitor charging state by causing capacitor charging mode switching. If the control circuit  80  is presently operating to charge the holding capacitor(s) used for generating pacing pulses without delay, sensed event intervals immediately preceding and immediately following a PVC are ignored for the purposes of detecting a change in intrinsic heart rate. No change to the capacitor charging without delay is made. Likewise, if the control circuit  80  is presently operating in the delayed charging mode, the sensed event interval immediately preceding a sensed event identified as a PVC and the sensed event interval immediately following the PVC are ignored for the purposes of detecting a change in the intrinsic heart rate. 
     If the pacing interval started at block  452  is not an ATP pacing interval or a post-shock pacing interval and is not started in response to a sensed event signal that is detected as a PVC (“no” branch of block  456 ), the control circuit  80  may operate according to the current capacitor charging control mode at block  458 . If the control circuit  80  is operating to delay capacitor charging until the expiration of a capacitor charging delay interval or until expiration of a pacing interval, the capacitor charging is withheld during the pacing interval that was started at block  452  according to the delayed charging mode. Capacitor charging is delayed until the expiration of the pacing interval or a capacitor charging delay interval as described in conjunction with  FIGS.  9   a - 9   c   . If the control circuit  80  has recently detected a decreased intrinsic heart rate and is operating to charge without delay, capacitor charging may be performed without delay, e.g., at the beginning and/or throughout, the pacing interval started at block  452  as needed to maintain the holding capacitor in a ready state for pacing pulse delivery. 
     Control circuit  80  may be configured to enable delayed capacitor charging only during selected pacing therapies, e.g., during VVI pacing, so that pacing pulse delivery is not delayed during other pacing therapies such as post-shock pacing and ATP when pacing pulse timing may be critical. Depending on the types of electrical stimulation therapies that the ICD  14  or other IMD implementing the techniques disclosed herein is capable of, the control circuit  80  may be configured to disable delayed capacitor charging for one or more therapies and enable delayed capacitor charging for one or more therapies. 
       FIG.  14    is a diagram of an IMD system  500  that may be configured to control delayed capacitor charging for pacing therapy delivery according to another example. 
     IMD system  500  may include ICD  514  and intra-cardiac pacemaker  550 . ICD  514  is shown coupled to transvenous leads  510  and  520  in communication with the right atrium (RA)  502  and right ventricle (RV)  504 , respectively, of heart  508 . ICD  514  is shown as a dual-chamber pacemaker and cardioverter/defibrillator configured to sense cardiac signals and deliver electrical stimulation pulses in RA  502  and RV  504 . ICD  514  includes a housing  515  enclosing electronic circuitry, e.g., a sensing circuit, therapy delivery circuit, control circuit, memory, telemetry circuit, other optional sensors, and a power source as generally described in conjunction with  FIG.  3    above. ICD  514  is shown implanted in a right pectoral position in  FIG.  14   , however it is recognized that ICD  514  may be implanted in other locations, e.g., in a left pectoral position, particularly when ICD  514  includes cardioversion and defibrillation capabilities using housing  515  as an active electrode. 
     ICD  514  has a connector assembly  517  for receiving proximal connectors of RA lead  510  and RV lead  520 . RA lead  510  may carry a distal tip electrode  512  and ring electrode  514  for sensing atrial signals, e.g., P-waves attendant to atrial depolarization, and delivering RA pacing pulses. RV lead  520  may carry pacing and sensing electrodes  522  and  524  for sensing ventricular signals, e.g., R-waves attendant to RV depolarization, and for delivering RV pacing pulses. RV lead  520  may also carry RV defibrillation electrode  526  and a superior vena cava (SVC) defibrillation electrode  528 . Defibrillation electrodes  526  and  528  are shown as coil electrodes spaced apart proximally from the distal pacing and sensing electrodes  522  and  524  and may be used for delivering high voltage CV/DF shock pulses. 
     ICD  514  may be configured to provide dual chamber pacing in RA  502  and RV  504 . In some examples, IMD system  500  may include an intracardiac pacemaker  550  positioned in left ventricle  506  for sensing left ventricular signals, e.g., R-waves attendant to left ventricular depolarizations, and for delivering pacing pulses to left ventricle  506 . IMD system  500  may be configured to deliver multi-chamber pacing therapies such as cardiac resynchronization therapy (CRT). Intra-cardiac pacemaker  550  may be configured to deliver left ventricular pacing pulses to synchronize left ventricular contraction with RA and RV contractions to promote a normal atrio-ventricular interval and coordinated ventricular contractions. ICD  514  may be configured to deliver RA pacing pulses and RV pacing pulses as needed to prevent the heart rate from falling below a programmed lower pacing rate. In some patients, occasional atrial bradycardia or AV conduction block may cause slowing of the intrinsic rate requiring pacing of the RA  502  and/or RV  504 . During CRT, however, left ventricular pacing by intra-cardiac pacemaker  550  may occur on a beat-by-beat basis, whether RA  502  and RV  504  are being paced or sensed, for promoting optimal heart chamber synchrony. 
     Intra-cardiac pacemaker  500  may include housing based electrodes  552  and  554  for sensing cardiac signals in the left ventricle  506  and delivering left ventricular pacing pulses. Pacemaker  500  may include a sensing circuit and therapy delivery circuit that includes at least one pacing channel in a low voltage therapy circuit including a low voltage charging circuit, a holding capacitor, and an output capacitor, e.g., as generally described in conjunction with  FIG.  5   , for generating and delivering pacing pulses to the left ventricle. In some examples, intra-cardiac pacemaker  550  includes a control circuit configured to perform the methods disclosed herein in conjunction with the accompanying flow charts for controlling holding capacitor charging. For example, the control circuit of intra-cardiac pacemaker  550  may switch between delayed holding capacitor charging following detection of an increased intrinsic heart rate and charging without delay during a pacing interval following detection of a decreased intrinsic heart rate, respectively, as described above. Some patients may require sustained or prolonged episodes of left ventricular pacing in order to promote heart chamber synchrony. In this case, intra-cardiac pacemaker  550  may be configured to perform capacitor charging during each pacing interval without delay. 
     For example, patient  512  may be dependent on LV pacing by intra-cardiac pacemaker  550  for promoting heart chamber synchrony, but RA pacing and RV pacing may be seldom required. In this situation, ICD  514  may be configured to switch between delayed capacitor charging and charging without delay modes in one or both of the RA and RV pacing channels of ICD  514  to conserve battery charge. For example, when increased intrinsic rate criteria are satisfied by sensed events (P-waves) in the RA and/or sensed events (R-waves) in the RV, the holding capacitors corresponding to the RA pacing channel and the RV pacing channel may be charged according to the delayed charging mode. 
     ICD  514  may include at least two pacing channels of a low voltage therapy circuit, e.g., any two of channels  342 ,  344  and  346  of low voltage therapy module  85  as shown in  FIG.  5   , for providing pacing to RA  502  and RV  504 . For example, RA electrodes  516  and  518  may be coupled to pacing channel  346  of low voltage therapy circuit  85  for delivering RA pacing pulses. RV electrode  522  and  524  may be coupled to pacing channel  344  of low voltage therapy circuit  85  for delivering RV pacing pulses. The control circuit of ICD  514  may be configured to delay capacitor charging in one or both of the RA pacing channel and the RV pacing channel based on increased intrinsic rate criteria being satisfied in the respective heart chamber. 
     For instance, with reference to the low voltage therapy circuit  85  of  FIG.  5   , charging of low voltage holding capacitor  358  may be delayed in response to detecting an increased intrinsic atrial rate based on the rate of sensed P-waves by the sensing circuit of ICD  514 . A hysteresis interval may be set after each sensed P-wave for detecting an increased intrinsic atrial rate in response to a threshold number of cardiac cycles in which a sensed P-wave occurs during the hysteresis interval. ICD  514  may switch to charging the holding capacitor  358  of the pacing channel  346  being used as the atrial pacing channel to the charging without delay mode in response to detecting a decreased intrinsic atrial rate based on one or more expired atrial pacing intervals. 
     The control circuit of ICD  514  may set an AV pacing interval following each atrial pacing pulse and sensed P-wave in RA  502  for controlling the timing of pacing pulses delivered to RV  504  by pacing channel  344  used as an RV pacing channel coupled to electrodes  522  and  524 . Additionally or alternatively, the control circuit of ICD  514  may set a VV pacing interval following each RV pacing pulse and each R-wave sensed in RV  504  for controlling the timing of RV pacing pulses delivered by pacing channel  344 . The control circuit of ICD  514  may delay charging of low voltage holding capacitor  356  until after a capacitor charging delay interval or after expiration of an expired AV or VV pacing interval in response to detecting an increased intrinsic ventricular rate based on a threshold number of cardiac cycles having an R-wave sensed in the RV  504  during a hysteresis interval. The control circuit of ICD  514  may switch to charging without delay in response to the expiration of a threshold number of pacing intervals and/or charging delay intervals. In some examples, control of capacitor charging using delayed charging after detecting an increased intrinsic rate is used only for controlling charging during AA or VV pacing intervals. Delayed capacitor charging may not be used for controlling capacitor charging associated with AV pacing intervals since AV pacing intervals are relatively shorter than AA and VV pacing intervals and a ventricular pacing pulse delivered at a long AV interval due to delayed capacitor charging may be undesirable. 
     In other examples, a pacemaker or ICD incorporating the techniques disclosed herein may be a single chamber pacemaker or ICD coupled to a single transvenous lead or a multi-chamber pacemaker or ICD coupled to three transvenous leads including a RA lead, RV lead and a coronary sinus lead for sensing and stimulating in RA  502 , RV  504  and LV  506 , respectively. In other examples of an IMD system performing the techniques disclosed herein, the intra-cardiac pacemaker  550  may be included in an implantable system with ICD  14  and extra-cardiovascular lead  16  shown in  FIG.  1 A . Intra-cardiac pacemaker  550  may be placed in any atrial or ventricular chamber and control capacitor charging using the methods described above for delaying capacitor charging in response to increased intrinsic heart rate criteria being satisfied. 
       FIG.  15    is a flow chart  600  of a method for controlling holding capacitor charging according to yet another example. Control circuit  80  may control the therapy delivery circuit  84  to charge the holding capacitor(s) used for pacing pulse delivery in response to a pacing interval expiring at block  602  during a delayed capacitor charging mode. Capacitor charging is withheld for the pacing interval by delaying charging until after the pacing interval expires. 
     If decreased intrinsic heart rate criteria are satisfied at block  604 , e.g., using any of the decreased intrinsic heart rate criteria described above such as a threshold number of expired pacing intervals or a slope of the heart rate change being less than (more negative than) a drop threshold, control circuit  80  may switch to charging the holding capacitor(s) during the pacing interval but after expiration of the capacitor charging delay interval at block  606 . In this way, charging is only performed when the intrinsic heart rate is slower than the rate corresponding to the capacitor charging delay interval. A sensed event during the capacitor charging delay interval causes pacing pulse inhibition and charging is withheld. When increased intrinsic heart rate detection criteria are met, as determined at block  608 , control circuit  80  switches back to delayed charging at block  602  and withholds capacitor charging until a pacing interval expires. Charging occurs when the heart rate is slower than the pacing rate. 
     This method of charging only when the intrinsic rate is less than the rate corresponding to capacitor charging delay interval may be used in an IMD and electrode system having relatively low pacing capture thresholds. For example, intracardiac pacemaker  550  having housing-based electrodes in close proximity or in intimate contact with the endocardium or ICD  514  having transvenous leads with endocardial electrodes are expected to have relatively low pacing capture thresholds. The time required to charge a holding capacitor to the programmed pacing voltage amplitude may be relatively short such that charging may occur after a charging delay interval, even when the likelihood of a pacing interval expiring is increased based on decreased intrinsic rate criteria being met. When the likelihood of a pacing interval expiring is relatively lower, based on increased intrinsic heart rate criteria being met, charging may occur after the pacing interval expires without resulting in a clinically significant delay to pacing pulse delivery. In this example, charging after the capacitor charging delay interval may be considered the “charging without delay mode” since charging is still performed during the pacing interval. Charging after the pacing interval expires may be considered the “delayed charging mode” since charging is withheld and delayed until after the pacing interval expires. 
     Methods described in conjunction with flow diagrams presented herein may be implemented in a non-transitory computer-readable medium that includes instructions for causing a programmable processor to carry out the methods described. A non-transitory computer-readable medium includes but is not limited to any volatile or non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flash memory, or other computer-readable media, with the sole exception being a transitory, propagating signal. The instructions may be implemented by processing circuitry hardware as execution of one or more software modules, which may be executed by themselves or in combination with other software. 
     Various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, electrical stimulators, or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     In one or more examples, the functions described in this disclosure may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on, as one or more instructions or code, a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media forming a tangible, non-transitory medium. Instructions may be executed by one or more processors, such as one or more DSPs, ASICs, FPGAs, general purpose microprocessors, or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to one or more of any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. 
     In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. Also, the techniques could be fully implemented in one or more circuits or logic elements. The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including an IMD, an external programmer, a combination of an IMD and external programmer, an integrated circuit (IC) or a set of ICs, and/or discrete electrical circuitry, residing in an IMD and/or external programmer. 
     Thus, IMD systems and methods for controlling holding capacitor charging for pacing therapy delivery have been presented in the foregoing description with reference to specific embodiments. In other examples, various methods described herein may include steps performed in a different order or different combination than the illustrative examples shown and described herein. It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the disclosure and the following claims.