Patent Publication Number: US-6658289-B2

Title: Universal pacing and defibrillation system

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an implantable cardiac stimulation system and more particularly to a universal pacing and defibrillation system capable of sensing cardiac activity, pacing, and defibrillating in all four chambers of the heart. The present invention is further directed to a lead implantable in the coronary sinus region of the heart which permits implementation of a universal pacing and defibrillation system with three leads. 
     BACKGROUND OF THE INVENTION 
     Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are usually positioned within the right side of the heart, either within the right ventricle or right atrium, or both, for making electrical contact with their respective heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to sense cardiac electrical activity and deliver the desired therapy. 
     Traditionally, therapy delivery had been limited to the venous, or right side of the heart. The reason for this is that implanted electrodes can cause blood clot formation in some patients. If a blood clot were released arterially from the heart left side, as for example the left ventricle, it could pass directly to the brain potentially resulting in a paralyzing or fatal stroke. However, a blood clot released from the right heart, as from the right ventricle, would pass into the lungs where the filtering action of the lungs would prevent a fatal or debilitating embolism in the brain. 
     Recently, new lead structures and methods have been proposed and even practiced for delivering cardiac rhythm management therapy to the left heart. These lead structures and methods avoid direct electrode placement within the left atrium and left ventricle of the heart by lead implantation within the coronary sinus region of the heart. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portions of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus. 
     It has been demonstrated that electrodes placed in the coronary sinus region of the heart may be used for left atrial pacing, left ventricular pacing, or cardioversion and defibrillation. These advancements enable implantable cardiac stimulation devices to address the needs of a patient population with left ventricular dysfunction and/or congestive heart failure which would benefit from left heart side pacing, either alone or in conjunction with right heart side pacing (bi-chamber pacing), and/or defibrillation. 
     Cardiac leads intended for use in the left heart via the coronary sinus region are difficult to position due to the tortuous venous routes of the human anatomy. Moreover, to provide both pacing and defibrillation of both the left atrium and the left ventricle from the coronary sinus region with multiple leads employing the appropriate types of electrodes is extremely difficult given the space constrains to accommodate multiple leads in the coronary sinus region. Hence, such implants are too cumbersome, difficult, and time consuming to perform and would likely result in compromised performance or system malfunction. 
     Universal pacing and defibrillation systems, capable of pacing and defibrillating all four heart chambers of the heart would require numerous pacing and defibrillation electrodes to be employed within the heart and its coronary venous system. To implement such a universal pacing and defibrillation system utilizing current state of the art lead configuration approaches, an inordinate number of leads would be required. This would result in lengthy implant procedures and possibly more leads than the human anatomy is able to accommodate. An inordinate number of leads may also make it difficult to accurately locate each electrode at a most efficacious position within the heart. 
     Efforts to minimize the number of required leads could also be fraught with potential obstacles. Such an effort would most likely include loading a lead up with too many electrodes. While this would reduce the number of required leads, such a lead would be difficult to implant. More importantly, owing to the differences in physiology from one patient to another, such a lead would most likely not “fit” a large number of patients in terms of resulting efficacious electrode positioning. 
     Efforts to achieve a universal pacing and defibrillation system, if successful, could provide significant improved therapies. Coordinated right heart and left heart pacing therapies would be made possible. Further, improved defibrillation therapies would also be made possible. Such therapies could include improved defibrillation energy distribution within the heart and/or new and improved sequential defibrillation pulse techniques. 
     SUMMARY OF THE INVENTION 
     The present invention provides an implantable system for the defibrillation of the ventricles of a heart wherein a defibrillation pulse is delivered between first and second pluralities of electrodes, at least one of the electrodes being a left ventricular defibrillation electrode. 
     The implantable system includes a first lead configured for implant in the right ventricle of the heart and including a right ventricular defibrillation electrode, a second lead configured for implant in the right atrium of the heart, a right atrial defibrillation electrode carried by one of the first and second leads, and a third lead configured for implant in the coronary sinus region of the heart. The third lead includes a left ventricular defibrillation electrode and a left atrial defibrillation electrode, the left ventricular defibrillation electrode and the left atrial defibrillation electrode being spaced apart so that when the third lead is implanted in the coronary sinus region with the left ventricular defibrillation electrode adjacent the left ventricle, the left atrial defibrillation electrode is within the coronary sinus adjacent the left atrium. 
     The system further includes a power supply and a control circuit operatively associated with the power supply and the defibrillation electrodes and configured for delivering a ventricular defibrillation pulse between a first plurality of the defibrillation electrodes and a second plurality of the defibrillation electrodes. The second plurality of defibrillation electrodes includes the left ventricular defibrillation electrode. 
     In accordance with a further aspect of the present invention, the power supply and control circuit are contained within an electrically conductive cage which may serve as an additional electrode for delivering the defibrillation pulse. 
     The present invention further provides an implantable cardiac lead. The lead includes an elongated lead body, a first defibrillation electrode carried by the lead body, and an electrode assembly carried by the lead body proximal to the first defibrillation electrode, the electrode assembly including a second defibrillation electrode and at least the pacing electrode. The lead is configured for implant in the coronary sinus region of the heart and the first defibrillation electrode and the electrode assembly are spaced apart on the lead body so that when the lead is implanted in the coronary sinus region with the first defibrillation electrode adjacent the left ventricle, the second defibrillation electrode and the at least one pacing electrode of the electrode assembly are within the coronary sinus adjacent to and in electrical contact with the left atrium. The lead permits a system capable of defibrillating the ventricles from all four chambers of the heart to be implemented with three leads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention may be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a universal pacing and defibrillating system embodying the present invention including an implantable stimulation device in electrical communication with three leads implanted into a patient&#39;s heart for delivering multi-chamber pacing stimulation and defibrillation therapy; 
     FIG. 2 illustrates another universal pacing and defibrillating system embodying the present invention including an implantable stimulation device in electrical communication with three leads implanted into a patient&#39;s heart for delivering multi-chamber pacing and defibrillation therapy; 
     FIG. 3 is a functional block diagram of a multi-chamber implantable stimulation device which may be employed in the systems of FIGS. 1 and 2 and illustrating the basic elements of the stimulation device to provide cardioversion, defibrillation and pacing stimulation in four chambers of the heart; 
     FIG. 4 is a simplified diagram of a first electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with one embodiment of the present invention; 
     FIG. 5 is a simplified diagram of a second electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with another embodiment of the present invention; 
     FIG. 6 is a simplified diagram of still another electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with a further embodiment of the present invention; 
     FIG. 7 is a simplified diagram of a further electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with a still further embodiment of the present invention; 
     FIG. 8 is a simplified diagram of a still another electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with a still another embodiment of the present invention; 
     FIG. 9 is a simplified diagram of a further electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with a further embodiment of the present invention; 
     FIG. 10 is a simplified diagram of a further electrode configuration obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the ventricles in accordance with a further embodiment of the present invention; 
     FIG. 11 is a simplified diagram of electrode configurations obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the atria in accordance with the present invention; 
     FIG. 12 is a simplified diagram of further electrode configurations obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the atria in accordance with another embodiment of the present invention; and 
     FIG. 13 is a simplified diagram of the still further electrode configurations obtainable with the lead systems of FIGS. 1 and 2 for defibrillating the atria in accordance with a still another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is of the best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be ascertained with reference to the issued claims. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. 
     As shown in FIG. 1, there is a universal pacing and defibrillation system  10  embodying the present invention including a stimulation device  19  in electrical communication with a patient&#39;s heart  11  by way of three leads,  20 ,  26  and  30 , suitable for delivering multi-chamber stimulation and shock therapy. To sense atrial cardiac signals and to provide right atrial chamber pacing stimulation therapy, the stimulation device  19  is coupled to an implantable right atrial lead  26  having an electrode pair  27  including a right atrial tip electrode  28  and a right atrial ring electrode  29 . Typically, the electrodes  28  and  29  are implanted in the appendage of the patient&#39;s right atrium. 
     To sense left atrial and ventricular cardiac signals, to provide left atrial and ventricular pacing therapy, and to provide left atrial and ventricular defibrillation shocks, the stimulation device  19  is coupled to a “coronary sinus” lead  30  designed for placement in the “coronary sinus region” via the ostium of the coronary sinus  15  for positioning a defibrillation electrode and at least one pacing electrode adjacent to the left ventricle  16  and a defibrillation electrode and at least one pacing electrode adjacent to the left atrium  18 . As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus. 
     Accordingly, in accordance with one aspect of the present invention, the coronary sinus lead  30  is designed to receive left ventricular cardiac signals and to deliver left ventricular pacing therapy using an electrode pair  31  including a left ventricular tip electrode  32  and a left ventricular ring electrode  33 . For shocking from or to the left ventricle  16 , the lead  30  includes a left ventricular coil electrode  34 . The coronary sinus lead  30  is further designed to receive left atrial cardiac signals and to deliver left atrial pacing therapy using an electrode pair  36  including electrodes  37  and  38 . For shocking from or to the left atrium, the lead  30  further includes a left atrial coil electrode  35 . The left atrial electrodes  35 ,  37  and  38  comprise an electrode assembly  39 . In order to provide this functionality, the left ventricular coil electrode  34  and the electrode assembly  39  are spaced apart on the lead  30  so that when electrodes  32 ,  33  and  34  are adjacent to and in electrical contact with the left ventricle  16 , the electrodes  35 ,  37  and  38  of electrode assembly  39  are within the coronary sinus  15  adjacent to and in electrical contact with the left atrium  18 . 
     The stimulation device  19  is also shown in electrical communication with the patient&#39;s heart  11  by way of an implantable right ventricular lead  20  having, in this embodiment, a pacing and sensing electrode pair  21  including a right ventricular tip electrode  22  and a right ventricular ring electrode  23 . The lead  20  further includes defibrillation electrodes including a right ventricular (RV) coil electrode  24 , and an SVC/RA coil electrode  25 . Typically, the right ventricular lead  20  is transvenously inserted into the heart  11  so as to place the right ventricular tip electrode  22  in the right ventricular apex so that the RV coil electrode  24  will be positioned in the right ventricle  12  and the RA coil electrode  25  will be positioned in the right atrium  14  or superior vena cava. As is well known in the art, the RA coil electrode  25  may be positioned in either the right atrium or superior vena cava. Hence, for purposes of brevity, the electrode  25  will be referred to herein as the RA coil electrode  25  and should be understood to also include placement of the electrode  25  in either the right atrium or superior vena cava with equal effect. Accordingly, the right ventricular lead  30  is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle. 
     FIG. 2 shows another universal pacing and defibrillation system  17  embodying the present invention. The system  17  is similar to the system  10  of FIG. 1 except for a few differences. Hence, only the differences will be described herein. 
     One difference relates to the RA coil electrode  25 . Here, instead of being carried by the RV lead  20 , it is carried by the RA lead  26 . As with the system  10  of FIG. 1, the RA coil electrode  25  of FIG. 2 may be positioned in either the superior vena cava or the right atrium. 
     Another difference relates to the electrode pair  36  of the CS lead  30 . Here, instead of electrodes  37  and  38  being distal to the LA coil electrode  35 , the electrodes  37  and  38  of the electrode pair  36  are proximal to the LA coil electrode  35 . 
     In all other respects, the systems of FIGS. 1 and 2 are identical. Both systems provide for the same therapy delivery electrode configurations which will be described subsequently with respect to FIGS. 4-13. 
     As illustrated in FIG. 3, a simplified block diagram is shown of the multi-chamber implantable stimulation device  19 , which is capable of treating both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a particular multi-chamber device is shown, this is for illustration purposes only, and 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) with cardioversion, defibrillation and pacing stimulation. 
     The housing  40  for the stimulation device  19 , shown schematically in FIG. 3, is often referred to as the “can”, “case” or “case electrode” and may be programmably selected to act as an electrode for some of the possible therapy delivery electrode configurations. Hence, the housing  40  may be used in combination with one or more of the coil electrodes,  24 ,  25 ,  34  and  35 , for defibrillation purposes. The housing  40  further includes a connector (not shown) having a plurality of terminals,  42 ,  43 ,  44 ,  45 ,  48 ,  49 ,  52 ,  53 ,  54 ,  55 ,  57 , and  58  (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the terminals). As such, to achieve right atrial sensing and pacing, the connector includes a right atrial tip terminal (A R  TIP)  48  and a right atrial ring terminal (A R  RING)  49  adapted for connection to the right atrial tip electrode  28  and the right atrial ring electrode  29 , respectively. 
     To achieve left chamber sensing, pacing and shocking, the connector includes a left ventricular tip terminal (V L  TIP)  52 , a left ventricular ring terminal (V L  RING)  53 , a left atrial ring terminal (A L  RING)  58 , a left atrial tip terminal (A L  TIP)  57 , a left atrial shocking terminal (A L  COIL)  55 , and a left ventricular shocking terminal (V L  COIL)  54  which are adapted for connection to the left ventricular tip electrode  32 , the left ventricular ring electrode  33 , the left atrial ring electrode  38 , the left atrial tip electrode  37 , the left atrial coil electrode  35 , and the left ventricular coil electrode  34 , respectively. 
     To support right chamber sensing, pacing and shocking, the connector further includes a right ventricular tip terminal (V R  TIP)  42 , a right ventricular ring terminal (V R  RING)  43 , a right ventricular shocking terminal (V R  COIL)  44 , and a right atrial shocking terminal (A R  COIL)  45 , which are adapted for connection to the right ventricular tip electrode  22 , right ventricular ring electrode  23 , the RV coil electrode  24 , and the RA coil electrode  25 , respectively. 
     At the core of the stimulation device  19  is a programmable microcontroller  60  which controls the various modes of stimulation therapy. As is well known in the art, the microcontroller  60  typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the microcontroller  60  includes the ability to process or monitor input signals (data) as controlled by a program code stored in a designated block of memory. The details of the design and operation of the microcontroller  60  are not critical to the present invention. Rather, any suitable microcontroller  60  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. 
     As shown in FIG. 3, an atrial pulse generator  70  and a ventricular pulse generator  72  generate pacing stimulation pulses for delivery by the right atrial lead  26 , the right ventricular lead  20 , and/or the coronary sinus lead  30  via an electrode configuration switch  74 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial and ventricular pulse generators,  70  and  72 , may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators,  70  and  72 , are controlled by the microcontroller  60  via appropriate control signals,  76  and  78 , respectively, to trigger or inhibit the stimulation pulses. 
     The microcontroller  60  further includes timing control circuitry  79  which is used to control the timing of such stimulation pulses (e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A—A) delay, secondary defibrillation shock delay, or ventricular interconduction (V—V) 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. 
     The switch  74  includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch  74 , in response to a control signal  80  from the microcontroller  60 , 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  82  and ventricular sensing circuits  84  may also be selectively coupled to the right atrial lead  26 , coronary sinus lead  30 , and the right ventricular lead  20 , through the switch  74  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,  82  and  84 , may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch  74  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. 
     Each sensing circuit,  82  and  84 , 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  10  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,  82  and  84 , are connected to the microcontroller  60  which, in turn, are able to trigger or inhibit the atrial and ventricular pulse generators,  70  and  72 , respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart. 
     For arrhythmia detection, the device  19  utilizes the atrial and ventricular sensing circuits,  82  and  84 , to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. As used herein “sensing” is reserved for the noting of an electrical signal, and “detection” is the processing of these sensed signals and noting the presence of an arrhythmia. 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 microcontroller  60  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 the inputs of an analog-to-digital (A/D) data acquisition system  90 . The data acquisition system  90  is configured to acquire intracardiac electrogram 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  102 . The data acquisition system  90  is coupled to the right atrial lead  26 , the coronary sinus lead  30 , and the right ventricular lead  20  through the switch  74  to sample cardiac signals across any pair of desired electrodes. 
     The microcontroller  60  is further coupled to a memory  94  by a suitable data/address bus  96 , wherein the programmable operating parameters used by the microcontroller  60  are stored and modified, as required, in order to customize the operation of the stimulation device  19  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 and vector of each shocking pulse to be delivered to the patient&#39;s heart  12  within each respective tier of therapy. 
     Advantageously, the operating parameters of the implantable device  19  may be non-invasively programmed into the memory  94  through a telemetry circuit  100  in telemetric communication with the external device  102 , such as a programmer, transtelephonic transceiver, or a diagnostic system analyzer. The telemetry circuit  100  is activated by the microcontroller by a control signal  106 . The telemetry circuit  100  advantageously allows intracardiac electrograms and status information relating to the operation of the device  19  (as contained in the microcontroller  60  or memory  94 ) to be sent to the external device  102  through an established communication link  104 . 
     In the preferred embodiment, the stimulation device  19  further includes a physiologic sensor  108 , 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  108  may further be used to detect changes in 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  60  responds by adjusting the various pacing parameters (such as rate, AV Delay, V—V Delay, etc.) at which the atrial and ventricular pulse generators,  70  and  72 , generate stimulation pulses. 
     The stimulation device additionally includes a battery  110  which provides operating power to all of the circuits shown in FIG.  3 . For the stimulation device  19 , which employs shocking therapy, the battery  110  must be capable of operating at low current drains for long periods of time, and then be capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse The battery  110  must also have a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the device  19  preferably employs lithium/silver vanadium oxide batteries, as is true for most (if not all) current devices. 
     As further shown in FIG. 3, the device  19  is shown as having an impedance measuring circuit  112  which is enabled by the microcontroller  60  via a control signal  114 . The impedance measuring circuit  112  is not critical to the present invention and is shown for only completeness. 
     In the case where the stimulation device  19  is intended to operate as an implantable cardioverter/defibrillator (ICD) device, it must detect the occurrence of an arrhythmia, and automatically apply an appropriate electrical shock therapy to the heart aimed at terminating the detected arrhythmia. As will be seen hereinafter, the shocking therapy may include a single shock to an electrode configuration or a primary shock to one electrode configuration followed, after a secondary shock delay, by a secondary shock to another and different electrode configuration. To this end, the microcontroller  60  further controls a shocking circuit  116  by way of a control signal  118 . The shocking circuit  116  generates shocking pulses of low (up to 0.5 Joules), moderate (0.5-10 Joules), or high energy (11 to 40 Joules), as controlled by the microcontroller  60 . Such shocking pulses are applied to the patient&#39;s heart  12  through efficacious combinations of the shocking electrodes including the case electrode  40 , and as shown in FIGS. 1 and 2, selected from the left atrial coil electrode  35 , the RV coil electrode  24 , the RA coil electrode  25  and the left ventricular coil electrode  34 . As noted above, the housing  40  may also act as an active electrode. 
     Cardioversion shocks are generally considered to be of low to moderate energy level (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 5-40 Joules), delivered asynchronously (since R-waves may be too disorganized), and pertaining exclusively to the treatment of fibrillation. Accordingly, the microcontroller  60  is capable of controlling the synchronous or asynchronous delivery of the shocking pulses. 
     FIGS. 4-10 show simplified representations of electrode configurations for providing ventricular defibrillation shocks to the heart  11  in accordance with the present invention. The reference numerals identifying the various elements shown in FIGS. 4-10 are identical to those used for corresponding elements shown in FIGS. 1 and 2 for clarity and to illustrate the applicability of the systems of FIGS. 1 and 2 for achieving the electrode configurations shown in each of FIGS. 4-10. 
     In FIG. 4, two electrode configurations are shown to permit a defibrillation shock to be applied simultaneously to the electrode configurations and between the right heart and the left heart. More specifically, the switch  74  of FIG. 3 may be set to connect the right atrial coil electrode  25  to the right ventricular coil electrode  24  to form a first pole or joint connection and to connect the left atrial coil electrode  35  and the left ventricular coil electrode  34  together to form a second pole or joint connection. After the electrodes are connected in this manner by switch  74 , the defibrillation shock is then applied between the first and second poles. This causes the defibrillation shock to be applied simultaneously with the first electrode configuration between the right atrium  14 , with the right atrial coil electrode  25 , and the left atrium  18  and left ventricle  16 , with the left atrial and left ventricular coil electrodes  35  and  34 , respectively and with the second electrode configuration between the right ventricle  12 , with the right ventricular coil electrode  24 , and the left atrium  18  and left ventricle  16 , with the left atrial and left ventricular coil electrodes  35  and  34 , respectively. 
     The arrows in FIG. 4 illustrate the direction of defibrillation shock current flow from the base of the arrow (the cathode) to the tip of the arrow (the anode). As is well known in the art, the direction of each of the arrows, and thus the polarity of the shock, may be reversed. In either case, the defibrillation shock is applied across the heart in a right to left direction as illustrated or in a left to right direction if polarity is reversed. This permits a large portion of the ventricular myocardium to be within the shock field for depolarizing the ventricular myocardium. 
     FIG. 5 illustrates a third electrode configuration in addition to the first and second electrode configurations of FIG.  4 . Here, the case  40  of the device  19  is used as an electrode. The third electrode configuration is formed by the switch  74  of FIG. 3 connecting the case  40  to the second pole formed by the joint connection of the left ventricular and left atrial coil electrodes  34  and  35 . Consistent with the first and second configurations, the third electrode configuration permits the defibrillation shock to be applied between the right heart and the left heart. 
     FIG. 6 also illustrates two electrode configurations across which a defibrillation shock may be simultaneously applied. Here, the defibrillation shock is applied between the apex of the heart and the base of the heart in a bottom to top direction. 
     More specifically, the switch  74  of FIG. 3 may be set to connect the right atrial coil electrode  25  to the left atrial coil electrode  35  to form a first pole or joint connection and to connect the right ventricular coil electrode  24  and the left ventricular coil electrode  34  together to form a second pole or joint connection. After the electrodes are connected in this manner by switch  74 , the defibrillation shock is then applied between the first and second poles. This causes the defibrillation shock to be applied simultaneously with the first electrode configuration between the right atrium  14 , with right atrial coil electrode  25 , and the right ventricle  12  and left ventricle with the right ventricular and the left ventricular coil electrodes  24  and  34 , respectively and with the second electrode configuration between the left atrium  18 , with left atrial coil electrode  35 , and the right ventricle  12  and left ventricle  16 , with the right ventricular and left ventricular coil electrodes  24  and  35 , respectively. 
     As with the previous embodiments, the arrows in FIG. 6 illustrate the direction of defibrillation shock current from the base of the arrow (the cathode) to the tip of the arrow (the anode). As previously described, the directions of each of the arrows, and thus the polarity of the shock, may be reversed. In either case, the defibrillation shock is across the heart in a top to bottom direction as illustrated or in a bottom to top direction if polarity is reversed. This also permits a large portion of the ventricular myocardium to be within the shock field for depolarizing the ventricular myocardium. 
     FIG. 7 illustrates a third electrode configuration which may be added to the first and second electrode configurations of FIG.  6 . Here, the case  40  of device  19  is once again used as an electrode. The third electrode configuration is formed by the switch  74  of FIG. 3 connecting the case  40  to the first pole formed by the joint connection of the right atrial and left atrial coil electrodes  25  and  35 , respectively. This permits the defibrillation shock to also be applied between the case  40  of device  19  and the right ventricle  12  and left ventricle  16 . Consistent with the first and second electrode configurations, the third electrode configuration permits the defibrillation shock to be applied between the base and apex of the heart in a top to bottom direction. 
     FIG. 8 illustrates the first and second electrode configurations of FIG. 6 with the defibrillation shock pulse polarity reversed. Here, the switch  74  of FIG. 3 is set to connect the right ventricular coil electrode  24  and the left ventricular coil electrode  34  together to form a first pole and the right atrial coil electrode  25  and left atrial coil electrode  35  together to form a second pole. When the defibrillation shock is provided by the device  19 , the shock is applied by the first electrode configuration between the right ventricle  12 , with the right ventricular coil electrode  24 , and the right atrium  14 , and left atrium  18 , with the right atrial and left atrial coil electrodes  25  and  35 , respectively. The shock is simultaneously applied by the second electrode configuration between the left ventricle  16 , with the left ventricular coil electrode  34 , and the right atrium  14  and left atrium  18 , with the right atrial and left atrial coil electrodes  25  and  35 , respectively. As a result, the defibrillation shock is applied between the apex and base of the heart in a bottom to top direction. Again, this results in a large portion of the ventricular myocardium to be within the defibrillation field for defibrillating the ventricular myocardium. 
     FIG. 9 illustrates a still further electrode configuration obtainable with the systems of FIGS. 1 and 2 for defibrillating the ventricles  12  and  16  of the heart  11 . Here, the heart  11  is defibrillated in a bottom to top direction by making use of the case  40  of the device  19  as an electrode. More specifically, the right ventricular coil electrode  24  and left ventricular coil electrode  34  are connected together by the switch  74  to form a first pole while the case  40  of the device  19  forms the second pole. When the defibrillation shock is provided by the device  19 , the defibrillation shock propagates from the right and left ventricles  12  and  16 , respectively, to the device case  40 . This results in defibrillation of the heart  11  from the apex to the base in a bottom to top direction. 
     FIG. 10 illustrates a last example of an electrode configuration obtainable with the system of FIGS. 1 and 2 for defibrillating the ventricles in accordance with the present invention. Here, the heart  11  is defibrillated in a right to left direction by also making use of the case  40  of the device  19  as an electrode. More specifically, the right ventricular coil electrode  24  and the right atrial coil electrode  25  are connected together by the switch  74  of FIG. 3 to form a first pole while the case  40  forms the second pole. When the defibrillation shock is provided by the device  19 , the defibrillation shock propagates from the right atrium and right ventricle  14  and  12 , respectively, to the device case  40 . This results in defibrillation of the heart  11  from the right heart to the left heart for defibrillating the ventricles. 
     The systems of FIGS. 1 and 2 may also be used to provide sequential defibrillation shocks to the heart for defibrillating the ventricles. As will be appreciated by those skilled in the art, no defibrillation shock alone, applied with implanted electrodes, will depolarize all of the ventricular myocardium. While a large percentage of the ventricular myocardium may be depolarized by utilizing the electrode configurations and methods previously described with reference to FIGS. 4-10, some ventricular myocardium will still not be depolarized. To increase the probability of depolarizing all of the ventricular myocardium, in accordance with the present invention, a secondary defibrillation pulse may be applied using a different electrode configuration, a secondary shock delay after the primary shock. The delay is maintained to be relatively short so that when the secondary shock is delivered, the first portion of the ventricular myocardium depolarized with the primary shock is still depolarized. The secondary shock delay may be, for example, on the order of 10 milliseconds. 
     Further, in accordance with the present invention, the secondary defibrillation shock is applied with an electrode configuration which results in the secondary shock being applied to the heart substantially orthogonally to the primary defibrillation shock. This may be achieved, for example, by applying the primary shock as illustrated in FIG. 4 (substantially right to left) and then applying the secondary shock as illustrated in FIG. 6 (top to bottom) or FIG. 9 (bottom to top). As a further example, this may be achieved by applying the primary shock as illustrated in FIG. 6 (top to bottom) and then applying the secondary shock as illustrated in FIG. 10 (right to left). As a still further example, the primary shock may be applied as illustrated in FIG. 5 (right to left) and then the secondary pulse applied as illustrated in FIG. 8 (bottom to top). As a last example, the primary shock may be applied as illustrated in FIG. 7 (top to bottom) and then the secondary shock may be applied as illustrated in FIG. 5 (right to left). 
     In each of the above examples, the first and second defibrillation shocks are applied substantially orthogonally to each other with the secondary shock occurring while the first portion of the heart, depolarized by the primary shock, is still depolarized. In this manner, the secondary shock is available to depolarize what ventricular myocardium remains to be depolarized following the primary shock. Because the shocks are delivered substantially orthogonally to each other, the probability of the secondary shock capturing any ventricular myocardium remaining to be depolarized is substantially enhanced. 
     Similarly, in accordance with further aspects of the present invention, sequential shocks may also be employed for defibrillating the atria. FIGS. 11-13 illustrate examples. 
     As shown in FIG. 11, a first shock may be applied between the right atrium  14  and left atrium  18  using the right atrial coil electrode  25  and the left atrial coil electrode  35 . While the atrial myocardium depolarized by the primary shock is still depolarized, a secondary atrial defibrillation shock is applied between the right atrium  14 , with right atrium coil electrode  25 , and the case  40  of device  19 . 
     In FIG. 12 a first atrial defibrillation shock is applied between the right atrium  14 , using right atrial coil electrode  25 , and the combination of the case  40  of device  19  and the left atrium  18 , using the left atrial coil electrode  35 . The primary shock is then followed by a secondary shock applied between the right atrium  14 , using right atrial coil electrode  25 , and the case  40  of device  19 . 
     Lastly, FIG. 13 illustrates an example similar to FIG.  11 . Here, the primary shock is applied between the right atrium  14 , using right atrial coil electrode  25 , and the case  40  of device  19 . The secondary shock then follows between the left atrium  18 , using left atrial coil electrode  35  and the right atrium  14 , using right atrial coil electrode  25 . 
     The electrode configurations and methods illustrated in FIGS. 11-13 provide efficacious defibrillation of the atria. The secondary shocks are available to depolarize remaining atrial myocardium not depolarized by the primary shocks. The probability of complete depolarization is enhanced because the primary and secondary shocks are applied in different directions. 
     While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.