Patent Publication Number: US-7212859-B1

Title: Dual-chamber implantable cardiac stimulation system and device with selectable arrhythmia termination electrode configurations and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 09/514,737, filed Feb. 28, 2000, now issued as U.S. Pat. No. 6,456,876. 

   FIELD OF THE INVENTION 
   The present invention is generally directed to a dual-chamber implantable cardiac defibrillator (ICD), system and method. The present invention is more particularly directed to a dual-chamber ICD, system and method wherein electrode configurations for ventricular arrhythmia termination and atrial arrhythmia termination are selectable from among a plurality of electrode configurations. 
   BACKGROUND OF THE INVENTION 
   Implantable cardiac defibrillators (ICDs) are well known in the art. These devices, encapsulated in a conductive housing or enclosure, are generally implanted in the left pectoral region of a patient and electrically connected to the heart with one or more electrode-carrying leads. One lead includes at least one defibrillation electrode arranged to be positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillating shock from the defibrillation electrode in the right ventricle to the conductive housing to terminate the arrhythmia. Alternatively, such arrhythmia terminating systems may further include another defibrillation electrode positioned in the right atrium and electrically connected to the conductive housing. In this arrangement, the defibrillating shock is delivered from the parallel connected right atrial electrode and the conductive housing to the right ventricular electrode. 
   Implantable atrial defibrillators are also known. These devices are also encapsulated in a conductive housing or enclosure and are electrically coupled to the heart by one or more electrode-carrying leads. The leads are known to include a defibrillation electrode positioned in the right atrium of the heart. When an arrhythmia detector detects an atrial arrhythmia, such as atrial fibrillation, an atrial defibrillating shock is then applied from the right atrial defibrillation electrode to the conductive housing. 
   Although not presently commercially available, combined implantable dual-chamber (atrial and ventricular) defibrillators continue to be investigated and under development. Because of the vast differences between ventricular and atrial arrhythmias, dual-chamber devices remain a challenge. For example, ventricular fibrillation is an immediately life threatening condition while atrial fibrillation, although uncomfortable and debilitating, is not a life threatening condition. Hence, there must be a preference for effective ventricular fibrillation treatment over atrial fibrillation treatment. Further, unless safety measures are taken in delivering atrial fibrillation terminating shocks, there is a potential for atrial fibrillation terminating shocks inducing ventricular fibrillation. Still further, by the time a ventricular fibrillation terminating shock is delivered, the patients are, in most occurrences, unconscious while, when atrial fibrillation terminating shocks are delivered, the patients are conscious and able to perceive discomfort from the atrial fibrillation termination shocks. Lastly, because of the relative locations of the atria and ventricles, the most effective ventricular fibrillation termination electrode configurations are different from the most effective atrial fibrillation termination electrode configurations. 
   Hence, in terms of arrhythmia termination, ventricular fibrillation termination electrode configurations must be those which provide the greatest assurance of successful arrhythmia termination. These configurations are those which exhibit the lowest ventricular defibrillation thresholds. In contrast, in arriving at an atrial fibrillation termination electrode configuration, consideration must be given to both atrial termination effectiveness and the degree of perceived discomfort by the patient to the delivered atrial arrhythmia termination shocks. The present invention addresses these issues. 
   SUMMARY OF THE INVENTION 
   The present invention provides an implantable cardiac stimulation system, device and method wherein atrial arrhythmia terminating pulses are confined to within the heart while ventricular arrhythmia terminating pulses may be delivered between a defibrillation electrode within the heart and another defibrillation electrode outside of the heart. With this arrangement, effectiveness of ventricular arrhythmia termination may be preserved while atrial arrhythmia termination may be achieved with minimized patient discomfort because the atrial termination pulse currents are confined within the heart and are precluded from recruiting the many pain neurons and skeletal muscle cells of the pectoral region. 
   In accordance with one embodiment of the present invention, the device and system delivers ventricular arrhythmia terminating pulses with an electrode configuration which includes the conductive housing encapsulating the device and delivers atrial arrhythmia termination pulses with an electrode configuration electrically independent of the conductive housing and thus confined to within the heart. 
   In accordance with another embodiment, a plurality of atrial arrhythmia terminating electrode configurations are available to the device, wherein each available atrial arrhythmia termination electrode configuration is electrically isolated from the conductive housing and thus confined to within the heart. One of the electrode configurations for atrial arrhythmia termination may be selected for use based upon a common arrhythmia termination characteristic, such as defibrillation energy threshold or perceived discomfort by the patient. The ventricular arrhythmia termination electrode configuration includes an electrode outside of the heart, such as the conductive device housing, to maintain ventricular arrhythmia termination effectiveness. 
   In accordance with a further aspect of the present invention, a preferred embodiment delivers a rounded, reduced-pain waveform for the atrial therapy as described in U.S. Pat. Nos. 5,906,633 and 5,830,236 which are incorporated herein by reference. 
   In accordance with a still further aspect of the present invention, a first plurality of electrode configurations are provided for ventricular arrhythmia termination, wherein at least one of the first plurality of electrode configurations includes the conductive device housing, and a second plurality of electrode configurations are provided for atrial arrhythmia termination, wherein each of the second plurality of electrode configurations is electrically isolated from the conductive device housing. A first electrode configuration from among the first plurality of electrode configurations may be selected for use based upon a common arrhythmia terminating characteristic, such as arrhythmia termination energy threshold. A second electrode configuration from among the second plurality of electrode configurations may be selected for use based upon a common arrhythmia termination characteristic such as arrhythmia termination energy threshold or perceived discomfort by the patient to the atrial arrhythmia termination pulses. 
   The cardiac stimulation device includes an electrode configuration selector, such as a switch, which selectively couples an atrial arrhythmia pulse generator to a pair of terminals which are coupled to the selected atrial arrhythmia termination electrode configuration and which selectively couples a ventricular arrhythmia termination pulse generator to a pair of terminals coupled to the selected ventricular arrhythmia termination electrode configuration. 
   As a result, by virtue of the present invention, ventricular arrhythmia termination effectiveness may be maintained while permitting flexibility in the selection of an atrial arrhythmia termination electrode configuration to achieve effective atrial arrhythmia termination with a minimum of perceived discomfort by the patient. 

   
     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  is a functional block diagram of a dual-chamber implantable stimulation device illustrating the basic elements of a stimulation device which can provide cardioversion, defibrillation and pacing stimulation; and 
       FIG. 2  is a flow chart describing a method of using the present invention to advantage. 
   

   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. 
   In  FIG. 1 , a simplified block diagram is shown of an implantable cardiac stimulation system  200  including a dual-chamber implantable stimulation device  10  and a lead system  11 . The system  200 , as will be seen hereinafter, is capable of treating both fast and slow arrhythmias with stimulation therapy, including atrial and ventricular cardioversion, defibrillation, and pacing stimulation. While a combined pacer and defibrillator device is shown, this is for illustration purposes only, and one of skill in the art could readily eliminate or disable the pacing circuitry to provide a single or dual-chamber defibrillation device or add circuitry to provide a device capable of providing stimulation or defibrillation to three or four cardiac chambers without departing from the present invention. 
   To provide atrial chamber pacing stimulation and sensing, the implantable stimulation device  10  is shown in electrical communication with a patient&#39;s heart  12  by way of an implantable atrial lead  20  of lead system  11  having an atrial tip electrode  22  and an atrial ring electrode  24  which typically is implanted in the patient&#39;s atrial appendage. 
   The implantable stimulation device  10  is also shown in electrical communication with the patient&#39;s heart  12  by way of an implantable ventricular lead  30  of lead system  11  having, in this embodiment, a ventricular tip electrode  32 , a ventricular ring electrode  34 , a right ventricular (RV) coil defibrillation electrode  36 , and a coil defibrillation electrode  38 . Typically, the ventricular lead  30  is transvenously inserted into the heart  12  so as to place the RV coil electrode  36  in the right ventricular apex, and the coil electrode  38  in the superior vena cava or the right atrium. Accordingly, the ventricular lead  30  is capable of receiving cardiac signals and delivering stimulation in the form of pacing and shock therapy to the right ventricle and right atrium. 
   Further, in accordance with this preferred embodiment, the lead system preferably includes a coronary sinus (CS) lead  46  having a coil defibrillation electrode  48 . The CS lead  46  may be advanced through the SVC, into the right atrium, through the os or ostium of the coronary sinus, and into the coronary sinus for placing the electrode  48  adjacent the left atrium and the left ventricle. Alternatively, the lead  46  may be advanced into any of the left ventricular veins, such as the left cardiac vein. Although not illustrated in  FIG. 1 , the lead  46  may further include a distal pacing electrode to provide pacing stimulation to the left side of the heart along with atrial and ventricular cardioversion and/or defibrillation by electrode  48 . For example, a lead designed for placement in the coronary sinus region could be implanted to deliver left atrial pacing, atrial shocking therapy, left ventricular pacing and left ventricular shocking stimulation. 
   The housing  40  (shown schematically) for the implantable stimulation device  10  includes a connector (not shown) having an atrial tip terminal  42  and an atrial ring terminal  44 , which are adapted for connection to the atrial tip electrode  22  and the atrial ring electrode  24 , respectively. The housing  40  further includes a ventricular tip terminal  52 , a ventricular ring terminal  54 , a ventricular shocking terminal  56 , and an SVC shocking terminal  58 , which are adapted for connection to the ventricular tip electrode  32 , the ventricular ring electrode  34 , the RV coil electrode  36 , and the SVC coil electrode  38 , respectively. The housing  40  still further includes a CS pin terminal  50  adapted for connection to the CS coil electrode  48 . The housing  40  (often referred to as the “enclosure”, “can”, “case” or “case electrode”) encapsulates the circuitry of the implantable stimulation device  10  and is formed of electrically conductive material. It may be programmably selected to serve as a return defibrillation electrode, alone or in combination with one of the coil electrodes. 
   In accordance with the present invention, the case  40  and coil defibrillation electrodes  36 ,  38 , and  48  provide a plurality of selectable ventricular arrhythmia termination electrode configurations and a plurality of atrial arrhythmia termination electrode configurations. Further, in accordance with the present invention, at least one ventricular arrhythmia termination electrode configuration employs the case  40 , which has proven to provide effective ventricular arrhythmia termination, and each one of the atrial arrhythmia termination electrode configurations is electrically isolated or independent of the case  40  to confine the atrial arrhythmia termination shock currents to within the heart to minimize the perception of discomfort by the patient to the atrial arrhythmia terminating shocks. 
   For example, in accordance with this embodiment, the ventricular arrhythmia termination electrode configurations may include the case  40  as a cathode and the RV coil electrode  36  as an anode. Another electrode configuration may include the CS coil electrode  48  as a cathode and the RV coil electrode  36  as an anode. Lastly, another configuration may include the RA coil electrode  38  and the CS coil electrode  48  coupled together as a cathode and the RV coil electrode  36  as an anode. 
   With respect to atrial arrhythmia termination, the electrode configurations may include the CS coil electrode  48  as an anode and the RA coil electrode  38  as a cathode. Another configuration may include the RV coil electrode  36  as an anode and the RA coil electrode  38  as a cathode. Lastly, the CS coil electrode  48  and RV coil electrode  36  may be connected together as an anode and the RA coil electrode  48  employed as a cathode. Each one of the above atrial arrhythmia termination electrode configurations is electrically independent or isolated from the case  40  and confined within the heart  12 . 
   At the core of the implantable stimulation device  10  is a programmable microcontroller  60  which controls the various modes of stimulation therapy. As is well known in the art, the microcontroller  60  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 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 is well known in the art. Representative types of control circuitry that may be used with the invention include the microprocessor-based control system of U.S. Pat. No. 4,940,052 (Mann et al.) and the state-machines of U.S. Pat. No. 4,712,555 (Thornander et al.) and U.S. Pat. No. 4,944,298 (Sholder). 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.). The &#39;052, &#39;555, &#39;298 and &#39;980 patents are incorporated herein by reference. 
   As shown in  FIG. 1 , an atrial pulse generator  70  and a ventricular pulse generator  72  generate pacing stimulation pulses for delivery by the atrial lead  20  and the ventricular lead  30 , respectively, via a switch bank  74 . 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 circuitry that controls the timing of such stimulation pulses (e.g., pacing rate and atrio-ventricular (AV) delay), as well as keeping track of the timing of any refractory periods, PVARP intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., that are well known in the art. 
   The switch bank  74  includes a plurality of switches for switchably connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch bank  74 , in response to a control signal  80  from the microcontroller  60 , determines the polarity of the stimulation pulses (e.g., unipolar or bipolar) by selectively closing the appropriate combination of switches as is known in the art. 
   An atrial sense amplifier  82  and a ventricular sense amplifier  84  are also coupled to the atrial and ventricular leads  20  and  30 , respectively, through the switch bank  74  for detecting the presence of cardiac activity. The switch bank  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 sense amplifier,  82  and  84 , preferably employs a low power, precision amplifier with programmable gain and/or automatic gain control, bandpass filtering, and a threshold detection circuit, known in the art, to selectively sense the cardiac signal of interest. The automatic gain control enables the implantable stimulation device  10  to deal effectively with sensing the low frequency, low amplitude signal characteristics of atrial and ventricular fibrillation. 
   The outputs of the atrial and ventricular sense amplifiers,  82  and  84 , are connected to the microcontroller  60  which, in turn, inhibits the atrial and ventricular pulse generators,  70  and  72 , respectively, in a demand fashion whenever cardiac activity is sensed in the respective chambers. The sense amplifiers,  82  and  84 , in turn, receive control signals over signal lines,  86  and  88 , from the microcontroller  60  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 sense amplifiers,  82  and  84 , as is known in the art. 
   For arrhythmia detection, the device includes an arrhythmia detector  62  which utilizes the atrial and ventricular sense amplifiers,  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 depolarization, and “detection” is the processing of these sensed depolarization signals and noting the presence of an arrhythmia. The timing intervals between sensed events (e.g., the P—P and R—R intervals) 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 atrial and ventricular fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of arrhythmia detected (e.g., bradycardia, tachycardia, and atrial or ventricular fibrillation), to employ a corresponding arrhythmia terminating therapy, also known 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 atrial and ventricular leads,  20  and  30 , through the switch bank  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 implantable stimulation device  10  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 stimulation device  10  may be non-invasively programmed into the memory  94  through a telemetry circuit  100  in telemetric communication with an 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 implantable stimulation device  10  (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 implantable stimulation device  10  further includes a physiologic sensor  110 . Such sensors are commonly called “rate-responsive” sensors. The physiological sensor  110  is used to detect the exercise state of the patient, to which the microcontroller  60  responds by adjusting the rate and AV Delay at which the atrial and ventricular pulse generators,  70  and  72 , generate stimulation pulses. The type of sensor used is not critical to the present invention and is shown only for completeness. 
   The stimulation device additionally includes a battery  114  which provides operating power to all of the circuits shown in  FIG. 1 . For the implantable stimulation device  10 , which employs shocking therapy, the battery  114  must be capable of operating at low current drains for long periods of time (preferably less than 10 μA), and then be capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse (preferably, in excess of 2 A, at voltages above 2 V, for periods of 10 seconds or more). The battery  114  preferably has a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the implantable stimulation device  10  preferably employs lithium/silver vanadium oxide batteries, as is true for most (if not all) current devices. 
   The implantable stimulation device  10  further includes a magnet detection circuitry (not shown), coupled to the microcontroller  60 . It is the purpose of the magnet detection circuitry to detect when a magnet is placed over the implantable stimulation device  10 , which magnet may be used by a clinician to perform various test functions of the implantable stimulation device  10 , such as defibrillation threshold tests, and/or to signal the microcontroller  60  that an external programmer  102  is in place to receive or transmit data to the microcontroller  60  through the telemetry circuits  100 . 
   It is the primary function of the implantable stimulation device  10  to function as an implantable dual-chamber cardioverter/defibrillator (ICD) device. That is, it must detect the occurrence of an atrial or ventricular arrhythmia, such as atrial fibrillation or ventricular fibrillation, and automatically apply an appropriate electrical shock therapy to the heart  12  with an appropriate electrode configuration for terminating the detected arrhythmia. To this end, the microcontroller  60  further controls a shocking circuit  130 . The shocking circuit  130  has a first pair of outputs  131  for delivering ventricular arrhythmia (such as ventricular fibrillation) terminating pulses and a second pair of outputs  133  for delivering atrial arrhythmia (such as atrial fibrillation) terminating pulses. The magnitude of the pulse energies and the outputs to be used (atrial or ventricular) are controlled by a control signal  132 . 
   To that end, the shocking circuit  130  is capable of generating shocking pulses of low (up to 0.5 joules), moderate (0.5–10 joules), or high energy (11–40 joules), as controlled by the microcontroller  60 . Such shocking pulses are applied to the patient&#39;s heart through an electrode configuration from among the plurality of electrode configurations previously described and selected by the microcontroller  60 . For example, the microcontroller may select the RV and case (electrodes,  36  and  40 ) for ventricular defibrillation and the RA and CS (electrodes,  38  and  48 ) for atrial defibrillation. The selection by the microcontroller  60  is implemented by the switch bank  74  for connecting the appropriate output pair,  131  or  133 , to the appropriate device terminals ( 50 ,  40 ,  56 ,  58 ). 
   Ventricular cardioversion and atrial defibrillation 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 atrial fibrillation or ventricular tachycardia. Ventricular 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 ventricular fibrillation. Accordingly, the microcontroller  60  is capable of controlling the energy magnitude of and the electrode configuration used to apply the shocking pulses. Preferably, the device has the option of delivering a rounded, reduced-pain waveform for the atrial therapy as described in U.S. Pat. Nos. 5,906,633 and 5,830,236 which are incorporated herein by reference. 
   The electrode configurations from among the plurality of electrode configurations for atrial and ventricular defibrillation may be preprogrammed by the physician. The selection of the particular electrode configuration to be used may be based upon a common arrhythmia terminating characteristic. The evaluation may be performed by the physician, using the magnet and external device as previously described, or it may be performed by the device itself. For example, for ventricular defibrillation, the electrode configuration selection may be based upon a ventricular defibrillation threshold. In this case, the minimum energy required to defibrillate the ventricles is determined for each available ventricular defibrillation electrode configuration and the one exhibiting the lowest energy threshold is selected. Similarly, for atrial defibrillation, the common arrhythmia terminating characteristic may be based upon an atrial defibrillation threshold and/or the degree of perceived pain by the patient. Each available atrial defibrillation electrode configuration is evaluated with respect to these characteristics. The electrode configuration yielding the lowest values for these characteristics may then be selected. It is possible, however, that the electrode configuration yielding the lowest perceived pain is not the one which yields the lowest defibrillation threshold. In this case, the physician must make a choice having in mind the pain tolerance of the patient. 
   Once the evaluations are made, the selected electrode configurations are stored in memory  94 . Thereafter, a first electrode configuration, from among the plurality of ventricular defibrillation electrode configurations, will be selected by the microcontroller  60  and switch bank  74  for delivering ventricular defibrillation pulses from the shocking circuit  130  at outputs  131  to the heart  12  when ventricular fibrillation is detected. Similarly, a second electrode configuration, from among the plurality of atrial defibrillation electrode configurations, will be selected by the microcontroller  60  and switch bank  74  for delivering atrial defibrillation pulses from the shocking circuit  130  at outputs  133  to the heart  12  when atrial fibrillation is detected. 
   Because there are many electrode configurations available for ventricular and atrial fibrillation, ventricular defibrillation effectiveness will be assured while permitting effective atrial fibrillation with a low perception of pain on the part of the patient. 
     FIG. 2 , a flow chart describing a method of using the novel features of the present invention to advantage after the ventricular and atrial defibrillation electrode configurations to be used are selected. In this flow chart, and the other flow charts described herein, the various algorithmic steps are summarized in individual “blocks”. Such blocks describe specific actions or decisions that must be made or carried out as the algorithm proceeds. Where a microcontroller (or equivalent) is employed, the flow charts presented herein provide the basis for a “control program” that may be used by such a microcontroller (or equivalent) to effectuate the desired control of the stimulation device. Those skilled in the art may readily write such a control program based on the flow charts and other descriptions presented herein. 
   The method of  FIG. 2  initiates at a decision block  140  wherein the arrhythmia detector  62  determines if ventricular fibrillation (VF) is present. If ventricular fibrillation is not detected by the arrhythmia detector  62 , the method advances to a decision block  150  wherein the arrhythmia detector  62  determines if atrial fibrillation (AF) is present. If the heart  12  is not experiencing an episode of atrial fibrillation, the process returns, i.e., terminates. However, if the heart  12  is experiencing an episode of atrial fibrillation, the implantable stimulation device  10  proceeds to provide atrial defibrillation therapy in a manner to be described hereinafter. 
   Returning now to decision block  140 , if it is determined in decision block  140  that the heart  12  is experiencing ventricular fibrillation, the process immediately advances to an activity step  142  wherein the selected ventricular defibrillation electrode configuration is selected by the microcontroller  60  and switch bank  74 . For example, if the selected electrode configuration for ventricular defibrillation is the electrode configuration wherein the device case  40  is used as a cathode and the RV coil electrode  36  is used as an anode, the switch bank  74  will couple the shocking circuit output pair  131  to terminals  40  and  56  of the device. Following activity block  142 , the method immediately advances to activity block  144  wherein the output capacitor of the shocking circuit  130  is charged to a preprogrammed ventricular defibrillation voltage level. 
   After the output capacitor is charged in accordance with activity block  144 , the process then advances to step  146  wherein the ventricular defibrillation energy from the shocking circuit  130  is delivered through the switch bank  74  to the selected electrode configuration to defibrillate the ventricles. Once the defibrillating energy is applied to the ventricles, the method then advances to decision block  148  wherein it is determined if the ventricular fibrillation has been terminated. If the ventricular fibrillation has not been terminated, the process then returns to step  142  to once again initiate the ventricular fibrillation therapy. Alternatively, the method could return to step  144  if the switch bank  74  holds the selected electrode configuration in electrical connection with the shocking circuit outputs  131 . 
   If, in decision block  148 , it is determined that ventricular fibrillation is no longer present and hence has been terminated, the process returns, i.e., terminates. Decision block  140  will thereafter be repeated a preprogrammed time later to once again detect for ventricular fibrillation. 
   If, it is determined in decision block  140  that ventricular fibrillation is not present, the process advances to decision block  150  as previously described. If it is determined in decision block  150  that the heart  12  is experiencing an episode of atrial fibrillation, the method advances to activity block  152  wherein the microcontroller  60  and switch bank  74  selects and couples the selected atrial defibrillation electrode configuration to the outputs  133  of the shocking circuit  130 . For example, the selected electrode configuration for atrial defibrillation may include the CS coil electrode  48  as an anode and the RA coil electrode  38  as a cathode. Once the switch bank  74  has selectively coupled the selected atrial defibrillation electrode configuration to outputs  133  of shocking circuit  130 , the method advances to step  154  wherein the output capacitor for atrial defibrillation of the shocking circuit  130  is charged to a preprogrammed atrial defibrillation voltage level. When the capacitor is fully charged in accordance with activity block  154 , the method proceeds to step  156  wherein the atrial defibrillation energy is applied from outputs  133 , through the switch bank  74 , and to the selected electrode configuration. Activity block  156  may further include a safety protocol for timing cardiac intervals and to ensure reliable R-wave detection. Preferably, the atrial defibrillation energy applied is synchronized to a first detected R-wave following a cardiac cycle having a cycle length greater than a minimum interval. This serves to minimize the probability of the atrial defibrillation shock inducing ventricular fibrillation. 
   Following activity block  156 , the method advances to decision block  158  wherein the arrhythmia detector  62  determines if ventricular fibrillation is present. This step is implemented in the event that the atrial defibrillation shock induced ventricular fibrillation. If the heart  12  is determined in decision block  158  to be experiencing ventricular fibrillation, the method returns to step  142  to begin the process of delivering ventricular defibrillation therapy. However, if ventricular fibrillation is found to not be present in accordance with decision block  158 , the method advances to step  160  wherein the arrhythmia detector  62  determines if the heart  12  is still experiencing an episode of atrial fibrillation. If atrial fibrillation is still present, the method then returns to step  152  to begin the process of delivering atrial defibrillation therapy. However, if in decision block  160  it is determined that the heart is no longer experiencing an episode of atrial fibrillation, and thus indicating that the atrial fibrillation has been terminated, the method then returns, i.e., terminates. 
   Alternatively, if in decision block  160 , it is determined that atrial fibrillation is still present, the process could return to activity block  154 . This would be advantageous if the selected atrial defibrillation electrode configuration is held in connection with the atrial defibrillation outputs until the atrial fibrillation is terminated. 
   As can be seen from the foregoing, the present invention provides an implantable cardiac stimulation system, device, and method wherein a plurality of electrode configurations are provided for both ventricular defibrillation and atrial defibrillation. At least one electrode configuration for ventricular defibrillation includes the device case to ensure effective ventricular fibrillation therapy. However, each of the electrode configurations of the atrial defibrillation electrode configurations is electrically isolated from the case and hence confined within the heart. This allows the selection of the best atrial defibrillation electrode configuration which exhibits the lowest atrial defibrillation threshold and/or which minimizes perception of discomfort to the patient from the atrial defibrillation shocks. Hence, the present invention affords effective ventricular defibrillation therapy while also providing effective atrial defibrillation therapy with minimized discomfort to the patient. 
   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.