Patent Publication Number: US-2010114235-A1

Title: Hybrid battery system for implantable cardiac therapy device

Description:
RELATED APPLICATIONS 
     This application is related to co-pending and commonly-owned U.S. patent applicaton Ser. No. ______, filed on even date herewith, entitled “Hybrid Battery System With Bioelectric Cell For Implantable Cardiac Therapy Device”, (attorney docket number A07E3046 [1587.1870000]), which is incorporated by reference herein in its entirety as if reproduced in full below. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to implantable cardiac therapy devices, and to power sources for the same. More particularly, the invention relates to a hybrid battery system for use in an implantable cardiac therapy device. 
     2. Background Art 
     Implantable cardiac therapy devices (ICTDs) enjoy widespread use for providing convenient, portable, sustained therapy for cardiac patients with a variety of cardiac arrhythmias. ICTDs may combine a pacemaker and defibrillator in a single implantable device. Such devices may be configured to provide ongoing cardiac pacing in order to maintain an appropriate cardiac rhythm. In addition, should the ICTD detect that the patient is experiencing an episode of ventricular fibrillation (or an episode of ventricular tachycardia), the ICTD can deliver appropriate defibrillation therapy. 
     An ICTD requires a portable power supply in the form of a battery. The battery has several inherent requirements including safety and also the ability to provide power to the ICTD for an extended period of time, thereby minimizing the frequency of invasive procedures to replace the battery. 
     However, ICTDs have additional, specialized power requirements due to the specific nature of their function. Long-term cardiac pacing can be supported by a low voltage, low current power source. Defibrillation therapy, however, requires rapid, high voltage, high current delivery to the heart. There does not exist a single battery which is optimized to effectively provide both types of electrical sourcing. 
     Presently, the lithium/silver vanadium oxide battery (LiSVO battery) is a common power source for ICTDs. The LiSVO battery is capable of producing high power pulses and charging the capacitors of the device in a timely manner. Further, the LiSVO battery has a high energy density (which, in theory, provides long battery life), and its self-discharge rate is low. 
     However, the LiSVO battery suffers from disadvantages as well. Its internal resistances from both the anode and cathode tend to increase as the battery discharges over time, particularly during midlife. As a result, over time, the loaded voltage will be lower and the time for charging the shocking capacitors will be longer. In some cases, the time to charge the shocking capacitors could be doubled, which may render the battery unacceptable for defibrillation. This may result in a medical decision to replace the device, which in turn means the patient may have to accept a premature surgery. In the past, the increased battery charge time has been a major issue for ICTDs. 
     A recent improvement has been the use of a hybrid battery source. A hybrid battery system combines two different physical batteries, with different but complementary electrical properties, into a single functional package. The single functional package effectively serves as the battery for the ICTD. A first physical battery (which may also be referred to as a cell) of the hybrid battery typically has a high energy density for long battery life, but may have a relatively low voltage and/or current output. A second physical battery (or cell) has higher peak current delivery capability (typically a result of lower internal resistance), and may have a higher voltage output, and superior recharging time and recharging properties. However, the second cell typically has lower energy density that the first battery. The two cells are coupled in the hybrid battery, with the first cell providing charging to the second cell. 
     A hybrid battery with the indicated architecture has been described, for example, by Greatbatch (see U.S. Pat. No. 7,079,893 B2, issued Jul. 18, 2006). However, existing hybrid batteries may still not be optimally tuned for application in an ICTD. For example, the voltage output or output current of the second cell may not be as high as desirable. The second cell may also have undesirable properties associated with recharging (for example, it may not be safe to charge the second cell too quickly), requiring complex regulation circuitry. (Section 6 of this document, “System and Method For Hybrid Battery Optimized for ICTD,” provides a discussion and characterization of a “regulated charging process” and an “unregulated charging process.”) 
     In addition, full advantage may not be taken of the electrical properties of the primary cell. Furthermore, existing hybrid batteries may not have an optimized energy density distribution (that is, an optimized distribution of storage capacity) between the primary and secondary cells. Finally, the secondary cell may introduce an undesirable degree of bulk or weight in the design of the ICTD. 
     What is needed, then, is a hybrid battery design which is optimized in terms of electrical properties, structural properties, and operational properties, for use in an implantable cardiac therapy device. 
     BRIEF SUMMARY 
     The present system and method employs a hybrid battery comprised of at least two types of cells to power an implantable cardiac therapy device (ICTD). A first type of cell provides low voltage but high energy density. The first type of cell directly provides power to the ICTD for purposes of routine cardiac monitoring, pacing, and general low current ICTD operations (including, for example, communications). The first type of cell is also coupled to a second cell via a simple DC-to-DC converter. The second type of cell is maintained at full or nearly full charge by the energy provided by the first type of cell. The second type of cell has low internal resistance and high voltage, making it suitable to rapidly charge ICTD capacitors for cardiac shocking (that is, for defibrillation). The second type of cell also has other properties optimizing it for usage in an ICTD. 
     An optimized energy density distribution may be implemented between the first type of cell and the second type of cell. In one embodiment, the first type of cell is a LiMnO 2  battery, while the second type of cell is a Li ion polymer battery. Each type of cell may be implemented as a single physical cell, or alternatively as two or more physical cells of the same type. 
     Further embodiments, features, and advantages of the present system and method, as well as the structure and operation of the various embodiments of the present system and method, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems presented herein for a hybrid battery optimized for an implantable cardiac therapy device. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. 
       In the drawings, like reference numbers indicate identical or functionally similar elements. Further, the drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number (e.g., an element numbered  302  first appears in  FIG. 3 ). 
       Additionally, some elements may be labeled with only a number to indicate a generic form of the element, while other elements labeled with the same number followed by another number or a letter (or a letter/number combination) may indicate a species of the element. A period or underscore may be introduced in the label for clarity of reading, and has no other significance. 
         FIG. 1  is a simplified diagram illustrating an exemplary implantable cardiac therapy device (ICTD) in electrical communication with a patient&#39;s heart by means of leads suitable for delivering multi-chamber stimulation and pacing therapy, and for detecting cardiac electrical activity. 
         FIG. 2  is a functional block diagram of an exemplary ICTD that can detect cardiac electrical activity and analyze cardiac electrical activity, as well as provide cardioversion, defibrillation, and pacing stimulation in four chambers of a heart. 
         FIG. 3  is a functional block diagram of the internal architecture and principle external connections of an exemplary external programming device which may be used by a human programmer to monitor or program an ICTD. 
         FIG. 4  is functional block diagram of an exemplary hybrid battery system, along with interconnections to some elements of an exemplary ICTD, according to an embodiment of the present system and method. 
         FIG. 5  is functional block diagram of an exemplary hybrid battery system, along with interconnections to some elements of an exemplary ICTD, according to an embodiment of the present system and method. 
         FIG. 6  is an exploded view of an exemplary hybrid battery system according to an embodiment of the present system and method. 
         FIG. 7  shows a set of experimentally measured plots of the time required for various Li ion polymer cells to charge shocking capacitors in a representative ICTD. 
         FIG. 8  shows a set of experimentally measured plots of the time required for various Li ion polymer cells to charge shocking capacitors in a representative ICTD. 
         FIG. 9  shows a set of experimentally measured plots of the time required for a representative Li ion polymer cell to charge the shocking capacitors of a representative ICTD at different current levels. 
     
    
    
     DETAILED DESCRIPTION 
     
         
         1. Overview 
         2. Exemplary Environment—Overview 
         3. Exemplary ICTD in Electrical Communication with a Patient&#39;s Heart 
         4. Functional Elements of an Exemplary ICTD 
         5. ICTD Programmer 
         6. System and Method For Hybrid Battery Optimized for ICTD 
         7. Choice of Power Cells 
         8. Lithium Ion Polymer Cell vs. Lithium/Silver Vanadium Oxide Cell 
         9. Lithium Ion Polymer Cell vs. Standard Lithium Ion Cell 
         10. Charging of Lithium Ion Polymer Cell from Primary Cell 
         11. Relative Storage Capacities of Different Types of Cells 
         12. Alternative Embodiments 
         13. Conclusion 
       
    
     1. Overview 
     The following detailed description of systems and methods for a hybrid battery optimized for an implantable cardiac therapy device refers to the accompanying drawings that illustrate exemplary embodiments consistent with these systems and methods. Other embodiments are possible, and modifications may be made to the embodiments within the spirit and scope of the methods and systems presented herein. Therefore, the following detailed description is not meant to limit the methods and systems described herein. Rather, the scope of these methods and systems is defined by the appended claims. 
     It would be apparent to one of skill in the art that the systems and methods for a hybrid battery optimized for an implantable cardiac therapy device, as described below, may be implemented in many different embodiments of hardware, software, firmware, and/or the entities illustrated in the figures. Any actual hardware and/or software described herein is not limiting of these methods and systems. In addition, more than one embodiment of the present system and method may be presented below, and it will be understood that not all embodiments necessarily exhibit all elements, that some elements may be combined or connected in a manner different than that specifically described herein, and that some differing elements from the different embodiments presented herein may be functionally and structurally combined to achieve still further embodiments of the present system and method. 
     Thus, the operation and behavior of the methods and systems will be described with the understanding that modifications and variations of the embodiments are possible, given the level of detail presented herein. 
     2. Exemplary Environment—Overview 
     Before describing in detail the methods and systems for a hybrid battery optimized for an implantable cardiac therapy device, it is helpful to describe an example environment in which these methods and systems may be implemented. The methods and systems described herein may be particularly useful in the environment of an implantable cardiac therapy device (ICTD). 
     An ICTD may also be referred to synonymously herein as a “stimulation device”, emphasizing the role of the ICTD in providing pacing and shocking to a human heart. However, an ICTD may provide operations or services in addition to stimulation, including but not limited to cardiac monitoring. 
     An ICTD is a physiologic measuring device and therapeutic device that is implanted in a patient to monitor cardiac function and to deliver appropriate electrical therapy, for example, pacing pulses, cardioverting and defibrillator pulses, and drug therapy, as required. ICTDs include, for example and without limitation, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators, implantable cardiac rhythm management devices, and the like. Such devices may also be used in particular to monitor cardiac electrical activity and to analyze cardiac electrical activity. The term “implantable cardiac therapy device” or simply “ICTD” is used herein to refer to any such implantable cardiac therapy device. 
       FIGS. 1 and 2  illustrate such an environment. 
       FIG. 3  illustrates the architecture of an external programming device which may be used to monitor, program, or interact with an ICTD. 
     3. Exemplary ICTD in Electrical Communication with a Patient&#39;s Heart 
     The techniques described below are intended to be implemented in connection with any ICTD or any similar stimulation device that is configured or configurable to stimulate nerves and/or stimulate and/or shock a patient&#39;s heart. 
       FIG. 1  shows an exemplary stimulation device  100  in electrical communication with a patient&#39;s heart  102  by way of three leads  104 ,  106 ,  108 , suitable for delivering multi-chamber stimulation and shock therapy. The leads  104 ,  106 ,  108  are optionally configurable for delivery of stimulation pulses suitable for stimulation of autonomic nerves. In addition, the device  100  includes a fourth lead  110  having, in this implementation, three electrodes  144 ,  144 ′,  144 ″ suitable for stimulation of autonomic nerves. This lead may be positioned in and/or near a patient&#39;s heart or near an autonomic nerve within a patient&#39;s body and remote from the heart. Of course, such a lead may be positioned epicardially or at some other location to stimulate other tissue. 
     The right atrial lead  104 , as the name implies, is positioned in and/or passes through a patient&#39;s right atrium. The right atrial lead  104  optionally senses atrial cardiac signals and/or provide right atrial chamber stimulation therapy. As shown in  FIG. 1 , the stimulation device  100  is coupled to an implantable right atrial lead  104  having, for example, an atrial tip electrode  120 , which typically is implanted in the patient&#39;s right atrial appendage. The lead  104 , as shown in  FIG. 1 , also includes an atrial ring electrode  121 . Of course, the lead  104  may have other electrodes as well. For example, the right atrial lead optionally includes a distal bifurcation having electrodes suitable for stimulation of autonomic nerves. 
     To sense atrial cardiac signals, ventricular cardiac signals and/or to provide chamber pacing therapy, particularly on the left side of a patient&#39;s heart, the stimulation device  100  is coupled to a coronary sinus lead  106  designed for placement in the coronary sinus and/or tributary veins of the coronary sinus. Thus, the coronary sinus lead  106  is optionally suitable for positioning at least one distal electrode adjacent to the left ventricle and/or additional electrode(s) adjacent to the left atrium. In a normal heart, tributary veins of the coronary sinus include, but may not be limited to, the great cardiac vein, the left marginal vein, the left posterior ventricular vein, the middle cardiac vein, and the small cardiac vein. 
     Accordingly, an exemplary coronary sinus lead  106  is optionally designed to receive atrial and ventricular cardiac signals and to deliver left ventricular pacing therapy using, for example, at least a left ventricular tip electrode  122 , left atrial pacing therapy using at least a left atrial ring electrode  124 , and shocking therapy using at least a left atrial coil electrode  126 . For a complete description of a coronary sinus lead, the reader is directed to U.S. Pat. No. 5,466,254, “Coronary Sinus Lead with Atrial Sensing Capability” (Helland), which is incorporated herein by reference. The coronary sinus lead  106  further optionally includes electrodes for stimulation of autonomic nerves. Such a lead may include pacing and autonomic nerve stimulation functionality and may further include bifurcations or legs. For example, an exemplary coronary sinus lead includes pacing electrodes capable of delivering pacing pulses to a patient&#39;s left ventricle and at least one electrode capable of stimulating an autonomic nerve. An exemplary coronary sinus lead (or left ventricular lead or left atrial lead) may also include at least one electrode capable of stimulating an autonomic nerve, such an electrode may be positioned on the lead or a bifurcation or leg of the lead. 
     Stimulation device  100  is also shown in electrical communication with the patient&#39;s heart  102  by way of an implantable right ventricular lead  108  having, in this exemplary implementation, a right ventricular tip electrode  128 , a right ventricular ring electrode  130 , a right ventricular (RV) coil electrode  132 , and an SVC coil electrode  134 . Typically, the right ventricular lead  108  is transvenously inserted into the heart  102  to place the right ventricular tip electrode  128  in the right ventricular apex so that the RV coil electrode  132  will be positioned in the right ventricle and the SVC coil electrode  134  will be positioned in the superior vena cava. Accordingly, the right ventricular lead  108  is capable of sensing or receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle. An exemplary right ventricular lead may also include at least one electrode capable of stimulating an autonomic nerve, such an electrode may be positioned on the lead or a bifurcation or leg of the lead. 
     4. Functional Elements of an Exemplary ICTD 
     An implantable cardiac therapy device may be referred to variously, and equivalently, throughout this document as an “implantable cardiac therapy device”, an “ICTD”, an “implantable device”, a “stimulation device”, and the respective plurals thereof. 
       FIG. 2  shows an exemplary, simplified block diagram depicting various components of stimulation device  100 . The stimulation device  100  can be capable of treating both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. The stimulation device can be solely or further capable of delivering stimuli to autonomic nerves. While a particular multi-chamber device is shown, it is to be appreciated and understood that this is done for illustration purposes only. For example, various methods may be implemented on a pacing device suited for single ventricular stimulation and not bi-ventricular stimulation. Thus, the techniques and methods described below can be implemented in connection with any suitably configured or configurable stimulation device. Accordingly, one of skill in the art could readily duplicate, eliminate, or disable the appropriate circuitry in any desired combination to provide a device capable of treating the appropriate chamber(s) or regions of a patient&#39;s heart with cardioversion, defibrillation, pacing stimulation, and/or autonomic nerve stimulation. 
     Housing  200  for stimulation device  100  is often referred to as the “can”, “case” or “case electrode”, and may be programmably selected to act as the return electrode for all “unipolar” modes. Housing  200  may further be used as a return electrode alone or in combination with one or more of the coil electrodes  126 ,  132  and  134  (see  FIG. 1 ) for shocking purposes. Housing  200  further includes a connector (not shown) having a plurality of terminals  201 ,  202 ,  204 ,  206 ,  208 ,  212 ,  214 ,  216 ,  218 ,  221  (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the terminals). 
     To achieve right atrial sensing, pacing and/or autonomic stimulation, the connector includes at least a right atrial tip terminal (AR TIP)  202  adapted for connection to the atrial tip electrode  120 . A right atrial ring terminal (AR RING)  201  is also shown, which is adapted for connection to the atrial ring electrode  121 . To achieve left chamber sensing, pacing, shocking, and/or autonomic stimulation, the connector includes at least a left ventricular tip terminal (VL TIP)  204 , a left atrial ring terminal (AL RING)  206 , and a left atrial shocking terminal (AL COIL)  208 , which are adapted for connection to the left ventricular tip electrode  122 , the left atrial ring electrode  124 , and the left atrial coil electrode  126 , respectively. Connection to suitable autonomic nerve stimulation electrodes is also possible via these and/or other terminals (e.g., via a nerve stimulation terminal S ELEC  221 ). 
     To support right chamber sensing, pacing, shocking, and/or autonomic nerve stimulation, the connector further includes a right ventricular tip terminal (VR TIP)  212 , a right ventricular ring terminal (VR RING)  214 , a right ventricular shocking terminal (RV COIL)  216 , and a superior vena cava shocking terminal (SVC COIL)  218 , which are adapted for connection to the right ventricular tip electrode  128 , right ventricular ring electrode  130 , the RV coil electrode  132 , and the SVC coil electrode  134 , respectively. Connection to suitable autonomic nerve stimulation electrodes is also possible via these and/or other terminals (e.g., via the nerve stimulation terminal S ELEC  221 ). 
     At the core of the stimulation device  100  is a programmable microcontroller  220  that controls the various modes of stimulation therapy. As is well known in the art, microcontroller  220  typically includes a processor or microprocessor  231 , or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy, and may further include onboard memory  232  (which may be, for example and without limitation, RAM, ROM, PROM, one or more internal registers, etc.), logic and timing circuitry, state machine circuitry, and I/O circuitry. 
     Typically, microcontroller  220  includes the ability to process or monitor input signals (data or information) as controlled by a program code stored in a designated block of memory. The type of microcontroller is not critical to the described implementations. Rather, any suitable microcontroller  220  may be used that carries out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions are well known in the art. 
     Representative types of control circuitry that may be used in connection with the described embodiments can include the microprocessor-based control system of U.S. Pat. No. 4,940,052 (Mann et al.), the state-machine of U.S. Pat. No. 4,712,555 (Thornander) and U.S. Pat. No. 4,944,298 (Sholder), all of which are incorporated by reference herein. For a more detailed description of the various timing intervals used within the stimulation device and their inter-relationship, see U.S. Pat. No. 4,788,980 (Mann et al.), also incorporated herein by reference. 
       FIG. 2  also shows an atrial pulse generator  222  and a ventricular pulse generator  224  that generate pacing stimulation pulses for delivery by the right atrial lead  104 , the coronary sinus lead  106 , and/or the right ventricular lead  108  via an electrode configuration switch  226 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart (or to autonomic nerves or other tissue) the atrial and ventricular pulse generators,  222  and  224 , may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators  222  and  224  are controlled by the microcontroller  220  via appropriate control signals  228  and  230 , respectively, to trigger or inhibit the stimulation pulses. 
     Microcontroller  220  further includes timing control circuitry  233  to control the timing of the stimulation pulses (e.g., pacing rate, atrio-ventricular (e.g., AV) delay, atrial interconduction (AA) delay, or ventricular interconduction (VV) delay, etc.) as well as to keep track of the timing of refractory periods, blanking intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., which is well known in the art. 
     Microcontroller  220  further includes an arrhythmia detector  234 , a morphology detector  236 , and optionally an orthostatic compensator and a minute ventilation (MV) response module (the latter two are not shown in  FIG. 2 ). These components can be utilized by the stimulation device  100  for determining desirable times to administer various therapies, including those to reduce the effects of orthostatic hypotension. The aforementioned components may be implemented in hardware as part of the microcontroller  220 , or as software/firmware instructions programmed into the device and executed on the microcontroller  220  during certain modes of operation. 
     Microcontroller  220  further includes an AA delay, AV delay and/or VV delay module  238  for performing a variety of tasks related to AA delay, AV delay and/or VV delay. This component can be utilized by the stimulation device  100  for determining desirable times to administer various therapies, including, but not limited to, ventricular stimulation therapy, bi-ventricular stimulation therapy, resynchronization therapy, atrial stimulation therapy, etc. The AA/AV/VV module  238  may be implemented in hardware as part of the microcontroller  220 , or as software/firmware instructions programmed into the device and executed on the microcontroller  220  during certain modes of operation. Of course, such a module may be limited to one or more of the particular functions of AA delay, AV delay and/or W delay. Such a module may include other capabilities related to other functions that may be germane to the delays. Such a module may help make determinations as to fusion. 
     The microcontroller  220  of  FIG. 2  also includes an activity module  239 . This module may include control logic for one or more activity related features. For example, the module  239  may include an algorithm for determining patient activity level, calling for an activity test, calling for a change in one or more pacing parameters, etc. These algorithms are described in more detail with respect to the figures. The module  239  may be implemented in hardware as part of the microcontroller  220 , or as software/firmware instructions programmed into the device and executed on the microcontroller  220  during certain modes of operation. The module  239  may act cooperatively with the AA/AV/VV module  238 . 
     Microcontroller  220  may also include a battery control module  286 . Battery control module  286  may be used, for example, to control a battery  276  (which may be a hybrid battery  276 .H, illustrated in  FIGS. 4 ,  5 , and  6 ) as discussed in further detail below in this document. Battery control  286  may be hardwired circuitry, or may be implemented as software or firmware running on microcontroller  220 . Battery control  286  may be coupled to battery  276  via battery signal line  290  and battery control line  292 . Battery signal line  290  may deliver to battery control  286  status or operational information regarding battery  276 . Battery control line  292  may be used to change an operational state of battery  276 . For example, battery control line  292  may deliver control signals from battery control  286  to battery  276 . For example, in an embodiment where battery  276  is a hybrid battery, battery control  286  may send control signals to determine if a second cell is connected to a first cell for recharging of the second cell. The details of this are further discussed below. 
     In an alternative embodiment, battery control  286  may be a separate module from microcontroller  220 , but may be coupled to microcontroller  220 . For example, separate module battery control  286  may obtain required ICTD operational status information from microcontroller  220 . Or, for example, separate module battery control  286  may report battery status or battery operational information to microcontroller  220 . In addition, separate module battery control  286  may also be coupled to battery  276 . 
     In an alternative embodiment, battery control  286  may be implemented as an internal physical module of battery  276  (for example, battery control  286  may be implemented as a microchip which is situated internally to the exterior housing of battery  276 ). However, battery control  286  may still be coupled to microcontroller  220  via battery signal line  290  and battery control line  292 . In an alternative embodiment, battery control functions of battery control  286  may be distributed across a first module which is part of battery  276 , and one or more additional modules which are external to battery  276 . The battery control module(s) external to battery  276  may for example be part of microcontroller  220 . 
     Battery  276  is discussed in more detail below in this document. 
     The electrode configuration switch  226  includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, switch  226 , in response to a control signal  242  from the microcontroller  220 , determines the polarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art. 
     Atrial sensing circuits  244  and ventricular sensing circuits  246  may also be selectively coupled to the right atrial lead  104 , coronary sinus lead  106 , and the right ventricular lead  108 , through the switch  226  for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR. SENSE) sensing circuits,  244  and  246 , may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. Switch  226  determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. The sensing circuits (e.g.,  244  and  246 ) are optionally capable of obtaining information indicative of tissue capture. 
     Each sensing circuit  244  and  246  preferably employs one or more low power, precision amplifiers with programmable gain and/or automatic gain control, bandpass filtering, and a threshold detection circuit, as known in the art, to selectively sense the cardiac signal of interest. The automatic gain control enables the device  100  to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation. 
     The outputs of the atrial and ventricular sensing circuits  244  and  246  are connected to the microcontroller  220 , which, in turn, is able to trigger or inhibit the atrial and ventricular pulse generators  222  and  224 , respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart. Furthermore, as described herein, the microcontroller  220  is also capable of analyzing information output from the sensing circuits  244  and  246  and/or the analog-to-digital (A/D) data acquisition system  252  to determine or detect whether and to what degree tissue capture has occurred and to program a pulse, or pulses, in response to such determinations. The sensing circuits  244  and  246 , in turn, receive control signals over signal lines  248  and  250  from the microcontroller  220  for purposes of controlling the gain, threshold, polarization charge removal circuitry (not shown), and the timing of any blocking circuitry (not shown) coupled to the inputs of the sensing circuits,  244  and  246 , as is known in the art. 
     For arrhythmia detection, the device  100  utilizes the atrial and ventricular sensing circuits,  244  and  246 , to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. In reference to arrhythmias, as used herein, “sensing” is reserved for the noting of an electrical signal or obtaining data (information), and “detection” is the processing (analysis) of these sensed signals and noting the presence of an arrhythmia. In some instances, detection or detecting includes sensing and in some instances sensing of a particular signal alone is sufficient for detection (e.g., presence/absence, etc.). 
     The timing intervals between sensed events (e.g., P-waves, R-waves, and depolarization signals associated with fibrillation which are sometimes referred to as “F-waves” or “Fib-waves”) are then classified by the arrhythmia detector  234  of the microcontroller  220  by comparing them to a predefined rate zone limit (i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of remedial therapy that is needed (e.g., bradycardia pacing, anti-tachycardia pacing, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”). 
     Cardiac signals are also applied to inputs of an analog-to-digital (A/D) data acquisition system  252 . The data acquisition system  252  is configured to acquire intracardiac electrogram (EGM) signals, convert the raw analog data into a digital signal, and store the digital signals for later processing and/or telemetric transmission to an external device  254 . Data acquisition system  252  may be configured by microcontroller  220  via control signals  256 . The data acquisition system  252  is coupled to the right atrial lead  104 , the coronary sinus lead  106 , the right ventricular lead  108  and/or the nerve stimulation lead  110  through the switch  226  to sample cardiac signals across any pair of desired electrodes. 
     The microcontroller  220  is further coupled to a memory  260  by a suitable data/address bus  262 , wherein the programmable operating parameters used by the microcontroller  220  are stored and modified, as required, in order to customize the operation of the stimulation device  100  to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape, number of pulses, and vector of each shocking pulse to be delivered to the patient&#39;s heart  102  within each respective tier of therapy. One feature may be the ability to sense and store a relatively large amount of data (e.g., from the data acquisition system  252 ), which data may then be used for subsequent analysis to guide the programming of the device. 
     Essentially, the operation of the ICTD control circuitry, including but not limited to pulse generators, timing control circuitry, delay modules, the activity module, battery utilization and related voltage and current control, and sensing and detection circuits, may be controlled, partly controlled, or fine-tuned by a variety of parameters, such as those indicated above which may be stored and modified, and may be set via an external ICTD programming device. 
     Advantageously, the operating parameters of the implantable device  100  may be non-invasively programmed into the memory  260  through a telemetry circuit  264  in telemetric communication via communication link  266  with the external device  254 , such as a general purpose computer, a dedicated ICTD programmer, a transtelephonic transceiver, or a diagnostic system analyzer. The microcontroller  220  activates the telemetry circuit  264  with a control signal  268 . The telemetry circuit  264  advantageously allows intracardiac electrograms and status information relating to the operation of the device  100  (as contained in the microcontroller  220  or memory  260 ) to be sent to the external device  254  through an established communication link  266 . The ICTD  100  may also receive human programmer instructions via the external device  254 . 
     The stimulation device  100  can further include a physiologic sensor  270 , commonly referred to as a “rate-responsive” sensor because it is typically used to adjust pacing stimulation rate according to the exercise state of the patient. However, the physiological sensor  270  may further be used to detect changes in cardiac output (see, e.g., U.S. Pat. No. 6,314,323, entitled “Heart stimulator determining cardiac output, by measuring the systolic pressure, for controlling the stimulation”, to Ekwall, issued Nov. 6, 2001, which discusses a pressure sensor adapted to sense pressure in a right ventricle and to generate an electrical pressure signal corresponding to the sensed pressure, an integrator supplied with the pressure signal which integrates the pressure signal between a start time and a stop time to produce an integration result that corresponds to cardiac output), changes in the physiological condition of the heart, or diurnal changes in activity (e.g., detecting sleep and wake states). Accordingly, the microcontroller  220  may respond by adjusting the various pacing parameters (such as rate, AA delay, AV delay, VV delay, etc.) at which the atrial and ventricular pulse generators,  222  and  224 , generate stimulation pulses. 
     While shown as being included within the stimulation device  100 , it is to be understood that the physiologic sensor  270  may also be external to the stimulation device  100 , yet still be implanted within or carried by the patient. Examples of physiologic sensors that may be implemented in device  100  include known sensors that, for example, sense respiration rate, pH of blood, ventricular gradient, cardiac output, preload, afterload, contractility, hemodynamics, pressure, and so forth. Another sensor that may be used is one that detects activity variance, wherein an activity sensor is monitored diurnally to detect the low variance in the measurement corresponding to the sleep state. For a complete description of an example activity variance sensor, the reader is directed to U.S. Pat. No. 5,476,483 (Bornzin et al.), issued Dec. 19, 1995, which patent is hereby incorporated by reference. 
     More specifically, the physiological sensors  270  optionally include sensors for detecting movement and minute ventilation in the patient. The physiological sensors  270  may include a position sensor and/or a minute ventilation (MV) sensor to sense minute ventilation, which is defined as the total volume of air that moves in and out of a patient&#39;s lungs in a minute. Signals generated by the position sensor and MV sensor are passed to the microcontroller  220  for analysis in determining whether to adjust the pacing rate, etc. The microcontroller  220  monitors the signals for indications of the patient&#39;s position and activity status, such as whether the patient is climbing upstairs or descending downstairs or whether the patient is sifting up after lying down. 
     The stimulation device additionally includes a battery  276  that provides operating power to all of the circuits shown in  FIG. 2 , as well as to any additional circuits which may be present in alternative embodiments. Operating power in the form of electrical current and/or voltage may be provided via a power bus or power buses  294 , depicted in  FIG. 2  as a first power bus  294 . 1  and a second power bus  294 . 2 . In  FIG. 2 , the connection(s) of power bus(es)  294  to other elements of ICTD  100  for purposes of powering those elements is not illustrated, but is implied by the dotted end-lines of bus(es)  294 . 
     For the stimulation device  100 , which employs shocking therapy, the battery  276  is capable of operating at low current drains for long periods of time (e.g., preferably less than 10 μA), and is capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse (e.g., preferably, in excess of 2 Amps, at voltages above 2 volts, for periods of 10 seconds or more). In an embodiment, discussed in detail later in this document, battery  276  may be configured to provide a current as high as 3.5 to 4.5 Amps and/or unloaded voltages in excess of 4 volts, for rapid charging of shocking circuitry. Battery  276  also desirably has a predictable discharge characteristic so that elective replacement time can be determined. 
     In an embodiment, battery  276  may be a hybrid battery comprised of dual types of cells, as described further below. Such a hybrid battery may provide power via a plurality of power buses, such as buses  249 . 1  and  294 . 2  of  FIG. 2 . In an embodiment, each power bus may be configured to deliver different voltages, different currents, and/or different power levels. Battery  276  may be monitored and/or controlled via battery control  286 , as discussed in part above, and as also discussed further below. 
     The stimulation device  100  can further include magnet detection circuitry (not shown), coupled to the microcontroller  220 , to detect when a magnet is placed over the stimulation device  100 . A magnet may be used by a clinician to perform various test functions of the stimulation device  100  and/or to signal the microcontroller  220  that the external programmer  254  is in place to receive or transmit data to the microcontroller  220  through the telemetry circuit  264 . 
     The stimulation device  100  further includes an impedance measuring circuit  278  that is enabled by the microcontroller  220  via a control signal  280 . The known uses for an impedance measuring circuit  278  include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper lead positioning or dislodgement; detecting operable electrodes and automatically switching to an operable pair if dislodgement occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device has been implanted; measuring stroke volume; and detecting the opening of heart valves, etc. The impedance measuring circuit  278  is advantageously coupled to the switch  226  so that any desired electrode may be used. 
     In the case where the stimulation device  100  is intended to operate as an implantable cardioverter/defibrillator (ICTD) device, it detects the occurrence of an arrhythmia, and automatically applies an appropriate therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller  220  further controls a shocking circuit  282  by way of a control signal  284 . The shocking circuit  282  generates shocking pulses of low (e.g., up to approximately 0.5 J), moderate (e.g., approximately 0.5 J to approximately 10 J), or high energy (e.g., approximately 11 J to approximately 40 J), as controlled by the microcontroller  220 . Such shocking pulses are applied to the patient&#39;s heart  102  through at least two shocking electrodes, and as shown in this embodiment, selected from the left atrial coil electrode  126 , the RV coil electrode  132 , and/or the SVC coil electrode  134 . As noted above, the housing  200  may act as an active electrode in combination with the RV coil electrode  132 , or as part of a split electrical vector using the SVC coil electrode  134  or the left atrial coil electrode  126  (i.e., using the RV electrode as a common electrode). Other exemplary devices may include one or more other coil electrodes or suitable shock electrodes (e.g., a LV coil, etc.). 
     Shocking circuit  282  either has within it, or is coupled to, one or more shocking capacitors (not shown in  FIG. 2 , but see for example element  424  of  FIGS. 4 and 5 ). The shocking capacitor(s)  424  may be used to store up energy, and then release that energy, during the generation of shocking pulses. 
     Cardioversion level shocks are generally considered to be of low to moderate energy level (where possible, so as to minimize pain felt by the patient), and/or synchronized with an R-wave and/or pertaining to the treatment of tachycardia. Defibrillation shocks are generally of moderate to high energy level (i.e., corresponding to thresholds in the range of approximately 5 J to approximately 40 J), delivered asynchronously (since R-waves may be too disorganized), and pertaining exclusively to the treatment of fibrillation. Accordingly, microcontroller  220  is capable of controlling the synchronous or asynchronous delivery of the shocking pulses. 
     5. ICTD Programmer 
     As indicated above, the operating parameters of the implantable device  100  may be non-invasively programmed into the memory  260  through a telemetry circuit  264  in telemetric communication via communication link  266  with the external device  254 . The external device  254  may be a general purpose computer running custom software for programming the ICTD  100 , a dedicated external programmer device of ICTD  100 , a transtelephonic transceiver, or a diagnostic system analyzer. Generically, all such devices may be understood as embodying computers, computational devices, or computational systems with supporting hardware or software which enable interaction with, data reception from, and programming of ICTD  100 . 
     Throughout this document, where a person is intended to program or monitor ICTD  100  (where such person is typically a physician or other medical professional or clinician), the person is always referred to as a “human programmer” or as a “user”. The term “human programmer” may be viewed as synonymous with “a person who is a user of an ICTD programming device”, or simply with a “user”. Any other reference to “programmer” or similar terms, such as “ICTD programmer”, “external programmer”, “programming device”, etc., refers specifically to the hardware, firmware, software, and/or physical communications links used to interface with and program ICTD  100 . 
     The terms “computer program”, “computer code”, and “computer control logic” are generally used synonymously and interchangeably in this document to refer to the instructions or code which control the behavior of a computational system. The term “software” may be employed as well, it being understood however that the associated code may in some embodiments be implemented via firmware or hardware, rather than as software in the strict sense of the term (e.g., as computer code stored on a removable medium, or transferred via a network connection, etc.). 
     A “computer program product” or “computational system program product” is a medium (for example, a magnetic disk drive, magnetic tape, optical disk (e.g., CD, DVD), firmware, ROM, PROM, flash memory, a network connection to a server from which software may be downloaded, etc) which is suitable for use in a computer or computation system, or suitable for input into a computer or computational system, where the medium has control logic stored therein for causing a processor of the computational system to execute computer code or a computer program. Such medium, also referred to as “computer program medium”, “computer usable medium”, and “computational system usable medium”, are discussed further below. 
       FIG. 3  presents a system diagram representing an exemplary computer, computational system, or other programming device, which will be referred to for convenience as ICTD programmer  254 . It will be understood that while the device is referred to an “ICTD programmer”, indicating that the device may send programming data, programming instructions, programming code, and/or programming parameters to ICTD  100 , the ICTD programmer  254  may receive data from ICTD  100  as well, and may display the received data in a variety of formats, analyze the received data, store the received data in a variety of formats, transmit the received data to other computer systems or technologies, and perform other tasks related to operational and/or physiologic data received from ICTD  100 . 
     ICTD programmer  254  includes one or more processors, such as processor  304 . Processor  304  is used for standard computational tasks well known in the art, such as retrieving instructions from a memory, processing the instructions, receiving data from memory, performing calculations and analyses on the data in accordance with the previously indicated instructions, storing the results of calculations back to memory, programming other internal devices within ICTD programmer  254 , and transmitting data to and receiving data from various external devices such as ICTD  100 . 
     Processor  304  is connected to a communication infrastructure  306  which is typically an internal communications bus of ICTD programmer  254 ; however, if ICTD programmer  254  is implemented in whole or in part as a distributed system, communication infrastructure  306  may further include or may be a network connection. 
     ICTD programmer  254  may include a display interface  302  that forwards graphics, text, and other data from the communication infrastructure  306  (or from a frame buffer not shown) for display on a display unit  330 . The display unit may be, for example, a CRT, an LCD, or some other display device. Display unit  330  may also be more generally understood as any device which may convey data to a human programmer. 
     Display unit  330  may also be used to present a user interface which displays internal features of, operating modes or parameters of, or data from ICTD  100 . The user interface presented via display unit  330  of ICTD programmer  254  may include various options that may be selected, deselected, or otherwise changed or modified by a human programmer of ICTD  100 . The options for programming the ICTD  100  may be presented to the human programmer via the user interface in the form of buttons, check boxes, menu options, dialog boxes, text entry fields, or other icons or means of visual display well known in the art. 
     ICTD programmer  254  may include a data entry interface  342  that accepts data entry from a human programmer via data entry devices  340 . Such data entry devices  340  may include, for example and without limitation, a keyboard, a mouse, a touchpad, a touch-sensitive screen, a microphone for voice input, or other means of data entry, which the human programmer uses in conjunction with display unit  330  in a manner well known in the art. For example, either a mouse or keystrokes entered on a keyboard may be used to select check boxes, option buttons, menu items, or other display elements indicating human programmer choices for programming ICTD  100 . Direct text entry may be employed as well. Data entry device  340  may also take other forms, such as a dedicated control panel with specialized buttons and/or other mechanical elements or tactile sensitive elements for programming ICTD  100 . 
     In the context of the present system and method, display interface  302  may present on display unit  330  a variety of data related to patient cardiac function and performance, and also data related to the current operating mode, operational state, or operating parameters of ICTD  100 . Modifications to ICTD  100  operational state(s) may be accepted via data entry interface  342  and data entry device  340 . In general, any interface means which enables a human programmer to interact with and program ICTD  100  may be employed. In one embodiment, for example, a visual data display may be combined with tactile data entry via a touch-screen display. 
     In another embodiment, a system of auditory output (such as a speaker or headset and suitable output port for same, not shown) may be employed to output data relayed from ICTD  100 , and a system of verbal input (such as a microphone and suitable microphone port, not shown) may be employed to program ICTD  100 . Other modes of input and output means may be employed as well including, for example and without limitation, a remote interaction with ICTD  100 , viewing printed data which has been downloaded from ICTD  100 , or the programming of ICTD  100  via a previously coded program script. 
     All such means of receiving data from ICTD  100  and/or programming ICTD  100  constitute an interface  302 ,  330 ,  342 ,  340  between ICTD  100  and a human programmer of ICTD  100 , where the interface is enabled via both the input/output hardware (e.g., display screen, mouse, keyboard, touchscreen, speakers, microphone, input/output ports, etc.) and the hardware, firmware, and/or software of ICTD programmer  254 . 
     ICTD programmer  254  also includes a main memory  308 , preferably random access memory (RAM), and may also include a secondary memory  310 . The secondary memory  310  may include, for example, a hard disk drive  312  and/or a removable storage drive  314 , representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive  314  reads from and/or writes to a removable storage unit  318  in a well known manner. Removable storage unit  318  represents a magnetic disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive  314 . As will be appreciated, the removable storage unit  318  includes a computer usable storage medium having stored therein computer software and/or data. 
     In alternative embodiments, secondary memory  310  may include other similar devices for allowing computer programs or other instructions to be loaded into ICTD programmer  254 . Such devices may include, for example, a removable storage unit  322  and an interface  320 . Examples of such may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an erasable programmable read only memory (EPROM), programmable read only memory (PROM), or flash memory) and associated socket, and other removable storage units  322  and interfaces  320 , which allow software and data to be transferred from the removable storage unit  322  to ICTD programmer  254 . 
     ICTD programmer  254  also contains a communications link  266  to ICTD  100 , which may be comprised in part of a dedicated port of ICTD programmer  254 . From the perspective of ICTD programmer  254 , communications link  266  may also be viewed as an ICTD interface. Communications link  266  enables two-way communications of data between ICTD programmer  254  and ICTD  100 . Communications link  266  has been discussed above (see the discussion of  FIG. 2 ). 
     ICTD programmer  254  may also include a communications interface  324 . Communications interface  324  allows software and data to be transferred between ICTD programmer  254  and other external devices (apart from ICTD  100 ). Examples of communications interface  324  may include a modem, a network interface (such as an Ethernet card), a communications port, a Personal Computer Memory Card International Association (PCMCIA) slot and card, etc. Software and data transferred via communications interface  324  are in the form of signals  328  which may be electronic, electromagnetic, optical or other signals capable of being received by communications interface  324 . These signals  328  are provided to communications interface  324  via a communications path (e.g., channel)  326 . This channel  326  carries signals  328  and may be implemented using wire or cable, fiber optics, a telephone line, a cellular link, an radio frequency (RF) link and other communications channels. 
     The terms “computer program medium”, “computer usable medium”, and “computational system usable medium” are used, synonymously, to generally refer to media such as removable storage drive  314 , a hard disk installed in hard disk drive  312 , and removable storage units  318  and  322 . These computer program products or computational system program products provide software to ICTD programmer  254 . 
     It should be noted, however, that it is not necessarily the case that the necessary software, computer code, or computer program (any of which may also referred to as computer control logic) be loaded into ICTD programmer  254  via a removable storage medium. Such computer program may be loaded into ICTD programmer  254  via communications link  328 , or may be stored in memory  308  of ICTD programmer  254 . Computer programs are stored in main memory  308  and/or secondary memory  310 . Computer programs may also be received via communications interface  324 . 
     Accordingly, such computer programs represent controllers of ICTD programmer  254 , and thereby controllers of ICTD  100 . Software may be stored in a computer program product and loaded into ICTD programmer  254  using removable storage drive  314 , interface  320 , hard drive  312  or communications interface  324 . 
     In an embodiment of the present system and method, ICTD programmer  254  may be used to modify ICTD operating parameters of battery control  286 . In this way, ICTD programmer  254  may be used to modify the operations of a battery  276 , such as a hybrid battery discussed in further detail below. 
     6. System and Method For Hybrid Battery Optimized for ICTD 
       FIG. 4  is a schematic diagram of an exemplary hybrid battery system  276 .H according to the present system and method.  FIG. 4  also includes some elements of exemplary connections between exemplary hybrid battery system  276 .H and other elements of ICTD  100 . 
     In an embodiment, an exemplary hybrid battery system  276 .H may be comprised of an exemplary primary cell  402  and an exemplary secondary cell  404 . In an alternative embodiment, two or more primary cells  402  may be employed. In an alternative embodiment, two or more secondary cells  404  may be employed. 
     In an embodiment, primary cell  402  may be a lithium-magnesium oxide (Li/MnO 2 ) cell. In an alternative embodiment, primary cell  402  may be a lithium carbon monofluoride (LiCF x ) cell. In an embodiment, secondary cell  404  may be a lithium ion polymer cell. The present system and method may enjoy several advantages due to the specific selections of power cells. These advantages are discussed in detail below in this document in the section entitled “Choice of Power Cells”. 
     Primary cell  402  and secondary cell  404  may be coupled by charging means  406 . Further coupled between charging means  406  and secondary cell  404  may be a charging control switch  408  and a variable resistor  412 . In an embodiment, and as shown in  FIG. 4 , primary cell  402  and secondary cell  404  may be coupled in parallel. Secondary cell  404  may also be coupled to a secondary cell charging control circuit  410 , which may also be known as a charging control circuit  410 . Secondary cell charging control circuit  410  may be, for example, a programmable logic control (PLC) circuit. 
     Secondary cell charging control circuit  410  may further be coupled to shocking circuit  282  of ICTD  100 . Secondary cell charging control circuit  410  may also be coupled to charging control switch  408  via charging control line  414 . 
     In an embodiment, secondary cell charging control circuit  410  is internal to hybrid battery system  276 .H, and therefore contained within exterior casing  428 . In an alternative embodiment (not illustrated in  FIG. 4 ), secondary cell charging control circuit  410  may be external to hybrid battery system  276 .H, and may for example comprise or be part of battery control module  286  of ICTD  100  (discussed above in conjunction with  FIG. 2 ). In the latter embodiment, secondary cell charging control circuit  410  may be coupled to hybrid battery system  276 .H, and in particular to charging control switch  408 , via suitable control lines such as battery control line  292  (see  FIG. 2 ). By way of exemplary embodiments, the discussion below assumes that secondary cell charging control circuit  410  is internal to hybrid battery system  276 .H unless otherwise indicated. 
     An additional element of hybrid battery system  276 .H may be a first internal bus  416  which is coupled to primary cell  402 . First internal bus  416  is configured to be coupled to first power bus  294 . 1  of ICTD  100 . In turn, first power bus  294 . 1  may be connected to numerous elements of ICTD  100  already discussed above. These elements may include, for example and without limitation, memory  260 , telemetry circuit  264 , physiologic sensor  270 , impedance measuring circuit  278 , microcontroller  220 , atrial pulse generator  222 , atrial sensing circuits  244 , ventricular sensing circuits  246 , analog-to-digital converter  252 , and electrode configuration switch  226 . Collectively, these elements and similar elements of ICTD  100  may be referred to as background operation circuitry  430 . 
     It is an advantage of the present system and method that background operation circuitry  430  is powered via the lower voltage primary cell  402  rather than the higher voltage secondary cell  404 . Background operations, such as cardiac pacing and monitoring, can typically be powered at lower currents and voltages than cardiac shocking (for example, at approximately 2 volts for pacing, as opposed to approximately 4 volts for shocking). If low voltage background circuits  430  are run off a high voltage cell (for example, if background activities are run off a high voltage secondary cell  404 ), then energy is wasted, reducing overall battery life. In the alternative, the voltage from a high powered cell could be stepped down to run lower voltage background operations, but power would be lost here as well. Running low voltage background circuits  430  off the low voltage primary cell  402  ensures overall longer life of hybrid battery system  276 .H. 
     Hybrid battery system  276 .H may also include a second internal bus  420  which is coupled to secondary cell  404 . Second internal bus  420  may be configured to be coupled to a second power bus  294 . 2  of ICTD  100 . Second power bus  294 . 2  may be coupled to shocking circuit  282  of ICTD  100 . Shocking circuit  282  may include, among other elements, one or more shocking capacitor(s)  424 . Shocking capacitor(s)  424  may be charged via the power provided from secondary cell  404  via second internal bus  420  and second power bus  294 . 2 . 
     Shocking circuit  282  typically also includes means of high voltage step-up charging  434 , such as a flyback charging circuit  434 . Charging circuit  434  accepts current from secondary cell  404 , and charges shocking capacitor(s)  424  to high voltages (typically in excess of 800 volts). This ensures that a high current necessary for cardiac shocking can be supplied from shocking capacitor(s)  424 . 
     In an embodiment, shocking circuit  282  may include a discharge control circuit  436 , which is configured to control and/or regulate the discharge of shocking capacitor(s)  424  for cardiac shocking. Discharge control circuit  436  may in turn be programmed by, controlled partly or wholly by, or work in conjunction with control signals from microcontroller  220  of ICTD  100 . Discharge control circuit  436  may for control purposes be coupled to high voltage step-up charging  434 , to shocking capacitors  424 , and/or to microcontroller  220  in ways which will be apparent to persons skilled in the relevant arts (coupling not illustrated in  FIG. 4 ). In an alternative embodiment, the functions of discharge control circuit  436  may be provided entirely by microcontroller  220  of ICTD  100 , with microcontroller  220  being suitably coupled to high voltage step-up charging  434  and/or shocking capacitor  424 . 
     Additional elements of hybrid battery system  276 .H may include connections or leads to grounding elements  426 . Grounding elements  426  may, for example, be the exterior housing or “can”  200  of ICTD  100 . 
     In operational use when hybrid battery system  276 .H is installed in an operational ICTD  100 , primary cell  402  may provide continuous low voltage, low current power to background operation circuitry  430  of ICTD  100 . 
     Primary cell  402 , as already noted, may be coupled to background operation circuitry  430  via first internal bus  416  of battery  276 .H and first power bus  294 . 1  of ICTD  100 . 
     Primary cell  402  may also be coupled via charging means  406  to secondary cell  404 . When secondary cell  404  is not in use for shocking a patient, then in typical operation charging control switch  408  is closed. With charging control switch  408  closed, primary cell  402  and secondary cell  404  are coupled in parallel. Further, with charging control switch  408  closed, primary cell  402  and secondary cell  404  are configured so that secondary cell  404  may be continuously charged via charging means  406 . 
     It is an advantage of the present system and method that because secondary cell  404  is a lithium ion polymer cell, it may be possible to continually charge secondary cell  404 . Other possible types of secondary cells, such as, for example, a standard lithium ion cell (which is not a lithium ion polymer cell), may require careful regulation of the charging process. For example, regulation may be required to ensure that the other types of secondary cells do not charge too rapidly, or do not overcharge. Charging too rapidly or overcharging may damage these other types of secondary cells and may even result in rupture or burning of the secondary cell. 
     Regulation of the charging process may take the form of monitoring the charge on the secondary cell  404 , and stopping the charging process when secondary cell  404  is fully charged. Continued monitoring may be required to determine when secondary cell  404  has lost charge (for example, due to self-discharge over time), requiring that the charging process be started again. Alternatively, the rate of charging, for example, the rate of current flow from primary cell  402  to secondary cell  404 , may need frequent adjustment to prevent overcharging of secondary cell  404 . Additional circuitry and cost may be entailed to provide for such monitoring and regulation of the charging process. 
     However, in an embodiment of the present system and method, when a secondary cell  404  is a lithium ion polymer cell, it may be possible to charge secondary cell  404  from primary cell  402  at a steady, continuous rate. Put another way, primary cell  402  may transfer power to secondary cell  404  via a continuous charging. As a result, there may be no requirement for complex regulation circuitry to turn the charging process on or off, or to reduce the rate of the charging process. Secondary cell  404  may be continuously charged from primary cell  402 , or put another way, secondary cell  404  may be charged from primary cell  402  via an unregulated charging process. 
     As used herein, an “unregulated charging process” is a charging process where there is no requirement for circuitry or for a method to monitor and adjust the charging process on account of the possibility of overcharge of secondary cell  404 . The term “unregulated charging process” is not intended to refer to the operation of charging means  406  but only to the regulation of the charging of secondary cell  404  to prevent an overcharging condition. For example, a person skilled in the art will recognize that charging means  406  may be implemented as a regulated DC-to-DC converter which will use feedback to regulate the output voltage at a desired voltage level. Such regulation is separate and apart from overcharge regulation. 
     Even with an unregulated charging process, it may be desirable to establish a rate of current flow, for example, to set a maximum limit to the current from primary cell  402  to secondary cell  404 . This limit may be set, for example, via variable resistor  412 . The phrase “unregulated charging process” may be further understood to mean that such a maximum limit to the current flow, once set, does not need to be further regulated or controlled over time in order to prevent overcharge or damage to secondary cell  404 . The maximum permitted current flow from primary cell  402  to secondary cell  404  may be set, for example, as part of a fixed design element of hybrid battery system  276 .H. Or, for example, the maximum permitted current flow from primary cell  402  to secondary cell  404  may be set on a per unit basis (that is, per individual specimen of hybrid battery system  276 .H) during an initial configuration or set up of hybrid battery system  276 .H. 
     Charging means  406  may be, for example, a DC-to-DC converter  406 . In an embodiment of the present system and method, DC-to-DC converter  406  may be a precision converter, meaning that the converter is configured to deliver a specific voltage level to a high degree of precision. No other charging circuitry may be required to charge secondary cell  404  from primary cell  402 . In an embodiment of the present system and method, primary cell  402  may put out a voltage on the order of two volts. DC-to-DC converter  406  steps up this voltage to a voltage above four volts, such as to a voltage of 4.1 volts or 4.2 volts. In this way secondary cell  404  is maintained at a voltage, such as, for example, approximately 4.1 to 4.2 volts, which is substantially above the voltage of primary cell  402 . In an embodiment of the present system and method, DC-to-DC converter  406  is configured for high-efficiency voltage conversion, resulting in minimal energy loss. DC-to-DC converters are well known in the art. For example, the DC-to-DC converter may be a capacitive or inductive, switch-mode power converter. Selection and implementation of an appropriate DC-to-DC converter would be apparent to a person skilled in the relevant arts. 
     Because secondary cell  404  may have a self-discharge process, secondary cell  404  may never reach exactly the voltage level put out by DC-to-DC converter  406 . Therefore, even when secondary cell  404  is substantially fully charged, a small charging current may continue to flow from DC-to-DC converter  406  to secondary cell  404 . This small current, which may be on the order of 100 μAmps, may be referred to as a “trickle charge”. In some embodiments of the present system and method, battery life of secondary cell  404  may be preserved by preventing the trickle charge. Therefore, in some embodiments of the present system and method, a regulated charging process may be employed. In some embodiments, hybrid battery system  276 .H may have charging regulation circuitry (not shown in  FIGS. 4  or  5 ). The charging regulation circuitry may be configured to stop the charging process when the charging current falls below a certain threshold value. The charging regulation circuitry may further be configured to restart the charging process when the voltage on secondary cell  404  falls below a designated voltage level, for example, 4 volts. The charging regulation circuitry may stop or start the charging process by any of several means, such as for example by opening or closing charging control switch  408 . 
     In an embodiment of the present system and method, the choice of whether to configure hybrid battery system  276 .H for an unregulated charging process or a regulated charging process may depend on the particular choice of secondary cell  404 . For example, the choice may depend on a particular brand of secondary cell  404  which may be employed. 
     Variable resistor  412  may also be coupled between charging means  416  and secondary cell  404 . As discussed above, secondary cell  404  may be charged from primary cell  402  via an unregulated charging process without risk of damage to secondary cell  404 , and without risk of harm to the patient in whom ICTD  100  is implanted. However, there may still be a maximum safe current from primary cell  402  to secondary cell  404 . Further, primary cell(s)  402  may only be able to safely discharge current provided the current flow from primary cell(s)  402  is below a certain rate, for example, typically on the order of a few milliamperes. Variable resistor  412  may therefore serve the purpose of limiting a rate at which current is drawn from primary cell  402  when primary cell  402  is charging secondary cell  404 . The exact resistance of variable resistor  412  may be slowly varied over an extended period of time (such as over periods of several months) via control circuitry (not illustrated) in response to the fact that primary cell  402  slowly loses power over the extended period of time. 
     In an embodiment of the present system and method, exemplary charging control switch  408  is coupled to secondary cell charging control circuit  410  via exemplary charging control line  414 . Charging control switch  408  may, for example, be a transistor such as a field effect transistor (FET), or other switching element well known in the art. In normal operation, when secondary cell  404  is not charging shocking capacitor  424  for shocking purposes, secondary cell charging control circuit  410  maintains charging control switch  408  in a closed state. This enables primary cell  402  to be coupled to secondary cell  404 , allowing secondary cell  404  to be charged as already described above. 
     When secondary cell charging control circuit  410  determines that a shocking process is occurring or is about to occur, secondary cell charging control circuit  410  sends a signal via charging control line  414  to charging control switch  408 . The signal causes charging control switch  408  to enter an open state. When charging control switch  408  is open, secondary cell  404  is no longer charged by primary cell  402 . Further, with charging control switch  408  open, primary cell  402  is no longer coupled even indirectly to shocking circuit  282  or shocking capacitor  404  of shocking circuit  282 . 
     As a result, during a shocking process, all energy for the shocking process is provided by secondary cell  404 , which is optimized to provide power for the shocking process. With charging control switch  408  open, primary cell  402  is electrically isolated from shocking capacitor  404 . Therefore, none of the power to shocking capacitor  404  is provided by primary cell  402 , which conserves the energy storage of primary cell  402 . In this way, the power of primary cell  402  is preserved for those applications for which primary cell  402  is optimized, thereby extending the overall life of hybrid battery system  276 .H. 
     In an embodiment, secondary cell charging control circuit  410  detects that a shocking process is in progress by detecting a power discharge from secondary cell  404  or by detecting a load from shocking circuit  282  via second internal bus  420  and second power bus  294 . 2 . 
     In an alternative embodiment of the present system and method, not illustrated in the figure, secondary cell charging control circuit  410  may be coupled to discharge control circuit  436  or to microcontroller  220  of ICTD  100  (for example, to battery control element  286  of microcontroller  220 ). Discharge control circuit  436  or microcontroller  220  (or, specifically, battery control element  286 ) may send a signal to secondary cell charging control circuit  410  indicating that a shocking process in is progress or is about to commence. 
     A shocking process may be a single shock or a series of shocks closely spaced in time. For example, a series of shocks may be spaced 5 to 10 seconds apart, though shorter or longer intervals are possible. The exact shocking process, including voltage(s) employed, the number of shocks, and timing of the shocks, may be determined by discharge control circuit  436  of shocking circuit  282 , or by microcontroller  220 , or by a combination of discharge control circuit  436  and microcontroller  220 . Secondary cell charging control circuit  410  may determine that a shocking process has concluded, for example by monitoring the discharge activity of secondary cell  404  and/or by monitoring a power drain of shocking circuit  282  and/or shocking capacitor  424 . In an alternative embodiment, secondary cell charging control circuit  410  may determine that a shocking process has concluded by receiving an appropriate signal from discharge control circuit  436  or from microcontroller  220  (for example, from battery control element  286  of microcontroller  220 ). 
     When secondary cell charging control circuit  410  has determined that the shocking process is concluded, secondary cell charging control circuit  410  may send a signal via charging control line  414  to charging control switch  408 . The signal closes charging control switch  408 . This recouples primary cell  402  with secondary cell  404 , so that secondary cell  404  may be recharged for future shocking. 
       FIG. 5  represents an exemplary hybrid battery system  276 .H and elements of an associated exemplary ICTD  100  according to another embodiment of the present system and method. Many elements shown in  FIG. 5  are the same as elements shown in  FIG. 4  and a detailed discussion of them will not be repeated here. 
     Note that In  FIG. 5 , and strictly due to considerations of clarity of illustration, elements of shocking circuit  282  are not all shown in immediate proximity to each other as in  FIG. 4 . Those elements which may be considered part of shocking circuit  282  still include shocking capacitor  424 , high voltage step-up charging  434 , and discharge control circuit  436 . All three elements  424 ,  434 ,  436  are also labelledlabeled parenthetically as “( 282 )” to indicate their inclusion with shocking circuit  282 . Persons skilled in the relevant arts will appreciate that the layouts shown in both  FIGS. 4 and 5  are schematic in nature, and the inclusion of elements  424 ,  434 , and  436  as part of shocking circuit  282  is dependent on the functional roles, interconnections, and/or interactions of these elements, as opposed to any particular schematic layout selected for purposes of clarity of illustration. 
     In  FIG. 5 , ICTD  100  has been configured so that discharge control circuit  436  is now powered via primary cell  402 . Discharge control circuit  436  is coupled to primary cell  402  via first internal bus  416  and first power bus  294 . 1 . In general, the elements of shocking circuit  282  which may be powered via primary cell  402  may include elements which pertain to regulation, control, monitoring, activation, termination, and/or timing of the shocking process. 
     Shocking capacitor  424  is still powered via secondary cell  404 . Shocking capacitor  424  may still be coupled to secondary cell  404  via second internal bus  420  and second power bus  294 . 2 . In  FIG. 5 , the charging of shocking capacitor  424  is controlled by discharge control circuit  436  of shocking circuit  282  (discussed above in more detail in conjunction with  FIG. 2  and  FIG. 4 ). 
     For example, charging of shocking capacitor  424  may be controlled by discharge control circuit  436  via an exemplary shocking capacitor control line  514  which may open or close a shocking capacitor control switch  508 . Shocking capacitor control switch  508  may determine whether secondary cell  404  is coupled to shocking capacitor  424 . In this way, even though discharge control circuit  436  is powered by primary cell  402 , the high voltage and high current shocking capacitor  424  continues to be charged as necessary via secondary cell  404 . 
     Note that in  FIG. 5 , charging of shocking capacitor  424  from secondary cell  404  is still via high voltage step-up charging means  434 . In  FIG. 5 , high voltage step-up charging  434  is illustrated as being configured between shocking capacitor control switch  508  and shocking capacitor  424 . In an alternative embodiment, shocking capacitor control switch  508  may be configured between high voltage step-up charging  434  and shocking capacitor  424 , so that shocking capacitor control switch  508  controls the flow of charging current from step-up charging circuit  434  to shocking capacitor  424 . Persons skilled in the relevant arts will appreciate that in such an embodiment, suitable changes would be made in the circuit connections to ensure that secondary cell  404  is coupled to charging circuit  434 . 
     Charging control circuit  410  of hybrid battery  276 .H still controls whether primary cell  402  is coupled to secondary cell  404 . This control is via charging control line  414  and charging control switch  408  as before. Secondary cell charging control circuit  410  may determine if shocking capacitor  424  is being charged by monitoring the activity of shocking capacitor control switch  508  via second internal bus  420  and second power bus  294 . 2 , or via some other control line or monitoring line (not illustrated in  FIG. 5 ). 
     In an alternative embodiment, secondary cell charging control circuit  410  monitors the shocking activity of discharge control circuit  436  and shocking capacitor  424  via a shocking circuit monitoring line  592  which couples discharge control circuit  436  to secondary cell charging control circuit  410 . In an embodiment, shocking circuit monitoring line  592  may be an element of or may be the same as battery control line  292  (discussed above in conjunction with  FIG. 2 ). In an alternative embodiment, shocking circuit monitoring line  592  may be an additional control line apart from battery control line  292 . 
     In summary, when ICTD  100  starts a high current pulse discharge (or series of discharges) for cardiac shocking, secondary cell  404  is disconnected from charging means  406  by secondary cell charging control circuit  410 . After the high current pulse discharge or series of charges is over, secondary cell charging control circuit  410  automatically switches secondary cell  404  to be recoupled with primary cell  402 , so that secondary cell  404  is charged again by primary cell  402 . As discussed further below, the output voltage of charging means  406  is set at approximately 4.1 to 4.2 volts, so that secondary cell  404  can be maintained at a voltage level higher than 4.0 volts. 
     Persons skilled in the relevant arts will appreciate that while  FIGS. 4 and 5  illustrate a single shocking capacitor  424 , in embodiments of the present system and method two or more shocking capacitors  424  may be charged via secondary cell  404 . As noted above, the charging of shocking capacitors  424  from secondary cell  404  is done via a high voltage step-up converter  434 , in order to charge capacitors  424  to hundreds of volts from the approximately 4 volts of secondary cell  404 . Implementation of high voltage step-up converter  434  will be apparent to a person skilled in the art and may be, for example, a flyback (buck boost) converter or other topology converter or current source. Additional shocking capacitor control switches  508  and other elements (e.g., control lines, additional power buses, etc.) may be included to support additional capacitors. 
     Persons skilled in the relevant arts will further appreciate that the exact configurations, connections, and arrangements of electrical components shown in  FIG. 4  and  FIG. 5  are exemplary only. Additional components, fewer components, alternative components, and variations in the connections may be employed consistent with the system and method for a hybrid battery system described herein. 
       FIG. 6  presents an exploded view of an exemplary hybrid battery system  276 .H according to an embodiment of the present system and method. As can be seen from the figure, hybrid battery system  276 .H may include an exterior casing  428  which may include a first part  428 . 1  and a second part  428 . 2 . First casing part  428 . 1  and a second casing part  428 . 2  may be configured to be coupled to each other, and to enclose the other elements of hybrid battery system  276 .H, when hybrid battery system  276 .H is fully assembled. Exterior casing  428  may also have openings for ports (not labeled) for power and data couplings. 
     Hybrid battery system  276 .H may also include a primary cell  402 , or in an alternative embodiment a plurality of primary cells  402 . For example, shown in the figure are two primary cells  402 . Having more than one primary cell provides additional storage capacity for longer life. Hybrid battery system  276 .H may also include a secondary cell  404 , or in an alternative embodiment a plurality of secondary cells  404 . 
     In an embodiment, secondary cell  404  may be a lithium ion polymer cell. A lithium ion polymer cell uses an internal gel as an electrolyte, and may therefore be flat or configured in other shapes which lend themselves to a compact configuration for hybrid battery system  276 .H. This is an advantage of the lithium ion polymer cell compared to other types of cells. For example, a standard lithium ion cell uses a liquid electrolyte, and so cannot readily be configured in a flat shape or other compact shapes. 
     Finally, hybrid battery system  276 .H may include a circuit assembly  624 . Circuit assembly  624  may include a number of elements already discussed above including, for example, and without limitation, charging means  406 , charging control switch  408 , secondary cell charging control circuit  410 , variable resistor  412 , and various buses and control lines already discussed above. 
     The present system and method pertains to a hybrid battery system which is substantially optimized for use with an ICTD  100 . Exemplary embodiments of the present system and method have been described above in conjunction with  FIGS. 4 ,  5 , and  6 . 
     7. Choice of Power Cells 
     Several elements distinguish the present system and method with respect to both prior batteries employed for use in ICTDs and to prior hybrid battery systems. Among these elements are the choices of power cells employed with the present system and method. 
     Persons skilled in the relevant arts will recognize that the term “battery” is sometimes employed in place of the word “cell” so that, for example, a “lithium ion polymer cell” may also be described, equivalently, as a “lithium ion polymer battery”. Within this document, individual batteries (lithium ion polymer, lithium/silver vanadium oxide, lithium magnesium oxide, etc.) are generally referred to as “cells” rather than batteries. This usage is strictly to help distinguish these cells from the overall hybrid battery system of the present system and method, which is comprised of multiple cells, and the usage (“cell” vs. “battery”) has no further significance. 
     The inventors have investigated the performance properties of the Li ion polymer cell for use as the secondary cell in the context of charging shocking capacitors within an ICTD.  FIG. 7  shows a set of plots  710  of the measured time required, in seconds, for various Li ion polymer cells (listed in legend  715  at right) to charge the shocking capacitors to approximately 750 to 800 volts in a representative ICTD (the Epic II ICD, manufactured by St. Jude Medical, Inc., of St. Paul, Minn.). The discharge current of the Li ion polymer cells was set at approximately 3 Amperes. As can be seen from plots  710 , charging times were consistently at or below approximately 5 seconds, with only a slight increase in charging times over a series of shocks. 
     As discussed further below in conjunction with  FIG. 9 , charging times of approximately 5 seconds were specifically associated with a discharge current of approximately 3 Amperes. Emerging Li ion polymer cells are capable of significantly higher currents, of approximately 4 to 4.5 Amperes, which may result in charging times of approximately 2.5 to 3 seconds, or even less. 
       FIG. 8  shows a set of plots  810  of the time required, in seconds, for various Li ion polymer cells (listed in legend  815  at right) to charge the shocking capacitors to approximately 750 to 800 volts in another representative ICTD (the Atlas +HF ICD, manufactured by St. Jude Medical, Inc., of St. Paul, Minn.). Again, a current of approximately 3 Amperes from the Li ion polymer cells was employed. As can be seen from plots  810 , charging times were consistently in the neighborhood of 5 seconds, and in many cases below 5 seconds with some of the cells tested. 
       FIG. 9  shows a set of plots  910  of the time required, in seconds, for a representative Li ion polymer cell (the DLG 303448H, manufacturer DLG Battery (Shanghai) Co., Ltd., Fengxian District, Shanghai, China) to charge the shocking capacitors to approximately 750 to 800 volts in a representative ICTD (the Epic II ICD, manufactured by St. Jude Medical, Inc., of St. Paul, Minn.). Different current levels (listed in legend  915 ) were employed, ranging from 3 Amps to 4.5 Amps. As can be seen from plots  910 , charging times of well under 5 seconds could be achieved, in some cases being lower than 2.5 seconds. 
     As discussed in further detail below, a charge time of 5 seconds or less represents a significant improvement over charge times available with present systems using Li/SVO cells. As also discussed in further detail below, the Li ion polymer cell can provide current levels on the order of several Amps (for example, 3 to 5 Amps), thereby enabling the charge times on the order of 5 seconds or less, in some cases even less than 3.5 seconds, or even less than 3 seconds. Using a standard Li ion cell, current levels of 3 to 5 Amps could only be provided by a standard cell of undesirable size and weight, or a combination of multiple standard Li ion cells of undesirable size and weight, for the present application. Therefore, and as also discussed in further detail below, a Li ion polymer cell is to be preferred over a standard Li ion cell for the present system and method. 
     8. Lithium Ion Polymer Cell vs. Lithium/Silver Vanadium Oxide Cell 
     As already noted, the lithium/silver vanadium oxide (Li/SVO) cell has been used as a power source of ICTDs  100  for many years. While it has some desirable electrical properties, the internal resistance for both the anode and cathode increase as a result of the discharging process, particularly during midlife. This may ultimately result in premature battery replacement. 
     The Lithium ion polymer (Li ion polymer) cell, already described above as being used as the secondary cell  404  in exemplary embodiments of the present system and method, has both a higher voltage and lower internal resistance compared to the Li/SVO cell, making it desirable for use as the cell which charges shocking capacitor(s)  424  of ICTD  100 . 
     In particular, the Li ion polymer cell has a higher current output than the Li/SVO cell. The discharge current of a typical Li/SVO battery used in an ICTD is approximately 3 Amps. A Li ion polymer battery may be discharged with a higher current, such as 3.5 to 4.5 Amps. Therefore, using the Li ion polymer cell as the power source  404  for the shocking capacitors  424 , the discharge time, or equivalently, the time to charge the shocking capacitors  424 , may be less than with the Li/SVO cell. For example, while it typically requires 10 to 18 seconds for a Li/SVO cell to charge the shocking capacitors  424  to approximately 750 volts to 800 volts, a Li ion polymer cell may charge the shocking capacitors to the same voltage (approximately 750 volts to 800 volts) in approximately 5 seconds, or even less time. 
     As described above, the present system and method employs a hybrid battery system  276 .H utilizing two different types of power cells, a primary cell  402  and a secondary cell  404 , in one package. In an embodiment, a secondary cell  404  which may be a Li ion polymer cell is continuously charged by one or more physically small primary battery cells  402 , which may be Lithium Magnesium Oxide (Li/MnO 2 ) cells or Lithium Carbon Monoflouride (LiCF x ) cells. The discussion below generally refers to the Li/MnO 2  cell as the primary cell, it being understood that in some embodiments of the present system and method, the LiCF x  or other cells may be employed instead as primary cell  402 . 
     Charging means  406  is employed to charge the secondary cell from the primary cell. In an embodiment, the charging means  406  is a DC-to-DC converter  406 , and the secondary cell  404  is charged by the primary cell  402  via DC-to-DC converter  406 . DC-to-DC converter  406  steps up the voltage going from the primary cell  402  to the secondary cell  404 . 
     A Li ion polymer battery with, for example, LiCoO 2  cathode material, may be recharged up to 4.23V. This is about one volt higher than a new Li/SVO battery. The internal resistance of a Li ion polymer battery may be lower than 0.1 Ω. In an embodiment, the output voltage of DC-to-DC converter  406  is set at approximately 4.2 volts. In this way, Li ion polymer cell  404  can be maintained at an unloaded voltage higher than 4.0 volts. A further advantage of the Li ion polymer cell is that, unlike with the Li/SVO cell, there is no significant increase in internal resistance over the life of the Li ion polymer cell. Therefore, in the capacitor charging process the voltage drop will be less, and the loaded voltage remains higher over the life of the Li ion polymer cell, as compared with the Li/SVO cell. The unloaded voltage on the Li ion polymer cell can be maintained at approximately 4.1 to 4.2 volts, while the loaded voltage, during charging of shocking capacitor(s)  424 , may be maintained at approximately 3.5 volts. 
     As a result of all these combined advantages of the Li ion polymer cell, the discharge time for high voltage shocking (that is, the time to charge shocking capacitor(s)  424 ) will be significantly less compared to the discharge time using a Li/SVO battery. The time to charge the shocking capacitors is approximately 10 to 20 seconds for the Li/SVO cells presently in use. Charging times of approximately 5 seconds or even less, such as less than 4 seconds or less than 3.5 seconds, may be achieved with the Li ion polymer cell. 
     As described above, secondary cell  404  is used to charge the shocking capacitor(s)  424  when cardiac shocking is required. The primary cell  402 , in addition to continuously charging the secondary cell, is also used to power background operations of ICTD  100 . Such background operations may include cardiac monitoring, cardiac pacing, and various communications, data processing, and other maintenance activities of the ICTD  100 . 
     In an embodiment of the present system and method, any control processing related to cardiac shocking may be powered by secondary cell  404 . In an alternative embodiment, control processing related to cardiac shocking may be powered in part or in whole by primary cell  402 , but charging of shocking capacitor(s)  424  is still performed by secondary cell  404 . 
     In summary, compared to the Li/SVO battery which has been used to charge shocking capacitor(s)  424  in the past, the Lithium ion polymer cell has the following advantages as the secondary cell  404 : (i) higher loaded voltage compared to the Li/SVO battery; (ii) lower internal resistance compared to Li/SVO battery; (iii) higher discharge current during capacitor charging; (iv) reduced discharge time during capacitor charging; (v) faster voltage recovery (faster charging time); and (vi) lower cost. 
     9. Lithium Ion Polymer Cell vs. Standard Lithium Ion Cell 
     Li ion polymer cells also offer advantages as a secondary cell  404 , as compared with standard Li ion cells that might be considered for use in the same capacity (that is, as a candidate for secondary cell  404 ). 
     Because Li ion polymer cells use gelatinous electrolyte, their self-discharge rate is relatively lower than that of a regular Li ion battery. (The self-discharge rate reflects the rate at which a cell spontaneously loses power, even with no external load or usage, due to internal chemical reactions.) The self-discharge rate of the Li ion polymer cell is in the range from 2% to 5% per month. The self-discharge rate of the standard Li ion cell is in the range of 5% to 10% per month. Because the Li ion polymer cell has a lower self-discharge rate, it will require less electrical charge from primary cell  402  (as compared with the charge that would be required if the standard Li ion cell were employed as secondary cell  404 ). Since less charge is required from primary cell  402 , more power is preserved in primary cell  402 . This enhances the overall functional lifetime of hybrid battery system  276 .H. 
     Also, and as noted above, Li ion polymer cells can be manufactured in thin, pliable shapes that offer advantages in device packaging compared with standard Li ion batteries, which have more bulk and are generally of rigid construction. 
     For typical shocking purposes, a desired storage of a secondary cell might be 250 milliampere-hours. This is more than sufficient to provide power for a series of six shocks during a defibrillation process. A standard lithium ion cell might have a discharge current capacity of 1 C to 2 C, meaning that it can only provide current at a rate equivalent to its storage capacity, or at most twice its storage capacity. For example, a standard Li ion cell with a storage capacity of 250 milliampere-hours and a discharge current of 2 C can provide at most 500 milliamps of current. At such a current flow, it may take a minute or several minutes to charge the shocking capacitors. This is insufficient for real-world applications, so a larger cell (or additional cells) would be required. 
     By contrast, an exemplary Li ion polymer cell may have a discharge current capacity of anywhere from 5 C to 20 C, or even higher. At this discharge current capacity, the Li ion polymer cell may be able to discharge at a rate from 5 times to 20 times its storage capacity. Again assuming a total cell power storage of 250 milliampere-hours, an exemplary Li ion polymer cell can deliver a current from 1.25 amps (for a 5 C cell) to 5 amps (for a 20 C cell). It may be possible to achieve a shocking capacitor charge time of as short as 5 seconds or even less, such as approximately 3.5 seconds, 3 seconds, or even less. This is a dramatic improvement over the charge times of approximately 10 to 20 seconds achieved with presently used Li/SVO batteries. As shown in  FIG. 9  (already discussed above), with some Li ion polymer cells it may be possible to charge shocking capacitor(s)  424  in times under 3 seconds, and possibly even under 2.5 seconds, which is much less than the charge times available with present devices. 
     10. Charging of Lithium Ion Polymer Cell from Primary Cell 
     In an embodiment, one or more primary Li/MnO2 button cell(s)  402  is (are) connected with a Li ion polymer cell  404  in parallel through a DC-to-DC converter  406  (see  FIGS. 4 and 5 ). Both cells, along with the DC-to-DC converter, are packaged in one device  276 .H comprising the hybrid battery system  276 .H. The entire system is enclosed in exterior casing  428 . (See  FIGS. 4 ,  5 , and  6 .) Except during the brief time periods when cardiac shocking may be in progress, the Li ion polymer cell  404  is continuously charged by the small Li/MnO2 cell(s)  402  with a low current flow, typically at milliampere levels. 
     An advantage of the present system and method is that the Li ion polymer cell  404  can be continuously charged, meaning that little or no additional circuitry is required to regulate the rate of charging. A secondary cell charging control circuit  410  may be present to ensure that the Li ion polymer cell  404  is disconnected from primary cell  402  when cardiac shocking is in progress. However, in normal usage of an ICTD  100 , cardiac shocking is not in progress the great majority of the time. 
     By default, primary cell  402  is coupled to secondary cell  404 , and during those intervals when cardiac shocking is not in progress, secondary cell charging control circuit  410  is configured to automatically enable the default coupling between the primary cell  402  and the secondary Li ion polymer cell  404 . This ensures that the secondary cell  404  is charged by the primary cell  402 . Variable resistor  412  may limit the current flow from primary cell  402  to secondary cell  404 . When using a Li ion polymer cell as the secondary cell  404 , no other control, regulation, or rate monitoring of the charging process is required to ensure the safe charging of secondary cell  404 . This greatly simplifies the design of hybrid battery  276 .H in terms of both design complexity and cost. 
     11. Relative Storage Capacities of Different Types of Cells 
     In an embodiment of the present system and method, the size and capacity of the two different types of cells (primary and secondary) are appropriately selected. 
     Over the life of a typical ICTD  100 , about 25% to 30% of total ICTD battery capacity is used for high voltage shocking; the other 70% to 75% of capacity is used for pacing and background operation and reforming the electrolytic capacitors. It is desirable, over the life of the ICTD, to maintain the available voltages from the hybrid battery system, at suitably high respective levels for both background operation and cardiac shocking. 
     Regarding the secondary cell  404 , it is an advantage of the present system and method to select the size of the Li ion polymer cell such that the cell provides approximately 25-30% of the total initial power storage capacity of the hybrid battery, for example, around 400 milliampere-hours. Other capacity Li ion batteries may be used, ranging from about 150 milliampere-hours up to 600 milliampere-hours, depending upon the desired tradeoffs in device volume vs. charge time, and the total number of sequential high voltage charge capabilities needed. 
     In general, however, it is desirable to avoid a Li ion polymer battery with too small a capacity. If the size is too small, the internal resistance will be higher, and that will negatively impact the discharge rate. In addition, if a patient requires a large number of shocks in a short time, a secondary cell  404  which is too small will be unable to provide the required number of shocks. 
     The primary cell is selected to provide approximately 70% to 75% of the total initial power storage of the hybrid battery system. The use of the Li/MnO 2  cell as primary cell  402  also has advantages. Its voltage is high, with a nominal voltage of about 3.0 volts. The energy density is also relatively high. It has long storage life. Its cost is low. A Li/MnO 2  button cell with 1000 milliampere-hours or two button cells with 550 milliampere-hours each are selected to charge the Li ion polymer battery. The Li/MnO 2  button cells can be discharged with current at milliampere level, which is appropriate for charging purpose. Other capacity primary cells may be used to obtain the desired device longevity, depending upon expected usage and average current consumption by the device. 
     12. Alternative Embodiments 
     In an embodiment of the present system and method, each primary cell  402  (for example, lithium magnesium oxide cell(s), etc.) and each secondary cell  404  (for example, lithium ion polymer cell(s)) is a self-contained, sealed battery unit, of a kind which may be purchased off-the-shelf and readily coupled to conventional electrical contacts in a larger system. In an alternative embodiment, either or both of the primary cell or the secondary cell may be specially constructed from custom parts or elements, specifically tailored for integration into the hybrid battery system of the present system and method. The details of such construction, if any, are beyond the scope of this document. 
     In embodiments described above, the hybrid battery system employs a single type of primary cell  402  for powering background operations and for charging secondary cell  404 , and also employs a single type of secondary cell  404  for charging shocking capacitor(s)  424 . In an alternative embodiment, more than one type of primary cell may be employed for powering different types of background operation circuitry  430  or for charging different types of secondary cells  404 . 
     In an alternative embodiment, a first type of primary cell  402  may be employed to provide power to background operation circuitry  430 , and a second type of primary cell  402  may be employed to charge secondary cell  404 . 
     In an alternative embodiment, different types of secondary cells  404  may be employed, which may be suitable for different types, patterns, time durations, or required power levels of shocking activity. Suitable switching and/or coupling circuitry may be employed to select and support the additional types of power cells as appropriate. 
     13. Conclusion 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present system and method as contemplated by the inventor(s), and thus, are not intended to limit the present method and system and the appended claims in any way. 
     Moreover, while various embodiments of the present system and method have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the present system and method. Thus, the present system and method should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     In addition, it should be understood that the figures and screen shots illustrated in the attachments, which highlight the functionality and advantages of the present system and method, are presented for example purposes only. The architecture of the present system and method is sufficiently flexible and configurable, such that it may be utilized (and navigated) in ways other than that shown in the accompanying figures. Moreover, the steps indicated in the exemplary system(s) and method(s) described above may in some cases be performed in a different order than the order described, and some steps may be added, modified, or removed, without departing from the spirit and scope of the present system and method. 
     Further, the purpose of the foregoing Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is not intended to be limiting as to the scope of the present system and method in any way.