Abstract:
An implantable cardiac stimulation device delivers successive defibrillation waveforms using alternating counter electrodes selected from a case electrode and an electrode located on a coronary sinus lead. The case electrode and coronary sinus electrode may be selected individually in an alternating fashion such that the defibrillation pathway alternates between two single pathways during a defibrillation regimen. The case electrode and the coronary sinus electrode may also be used simultaneously in parallel to create a dual pathway. Defibrillation waveforms may then be delivered in a cyclical fashion between the individual counter electrodes and the combined counter electrodes such that alternation between single pathways and a dual pathway is achieved.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to implantable medical devices and more specifically to an implantable defibrillating device equipped with more than one counter electrode and an associated method for automatically alternating defibrillation waveform pathways during a defibrillation regimen. 
   BACKGROUND OF THE INVENTION 
   An implantable cardioverter-defibrillator, commonly referred to as an “ICD,” is capable of recognizing tachycardia or fibrillation and delivering electrical therapy to terminate such arrhythmias. ICDs are often configured to perform pacemaking functions as well. A pacemaker generally delivers rhythmic electrical pulses to the heart to maintain a normal rhythm in patients having conduction abnormalities or bradycardia, which is too slow of heart rate. Pathologic tachycardia, which is a rapid heart rate not associated with a normal physiologic response such as a response to exercise, is typically treated with low to moderate-energy shocking pulses. The treatment of tachycardia is often referred to as “cardioversion.” Fibrillation is characterized by rapid, unsynchronized depolarizations of the myocardial tissue. Ventricular fibrillation is most often fatal if not treated within a few minutes of its onset. The termination of fibrillation, referred to as “defibrillation,” is accomplished by delivering high-energy shocking pulses. 
   Upon detection of fibrillation, a defibrillation therapy, referred to herein as a “regimen,” delivered by an implantable defibrillator may include delivery of multiple defibrillation waveforms. Each waveform is defined by a number of parameters including the shape and energy of each pulse. A conventional wave shape is a biphasic waveform in which two pulses that have opposite polarity are generated on the order of 100 microseconds apart. Each waveform within a regimen is delivered on the order of 10 seconds apart. During the time between each defibrillation waveform, the capacitor used for delivering the next waveform is charged, and the defibrillator re-determines if fibrillation is still present. If fibrillation is no longer detected, the regimen is terminated prior to delivering another shock. 
   Early implantable defibrillation systems required a thoracotomy to allow placement of electrode patches on the epicardial surface of the heart. The risk of morbidity and mortality associated with an open thoracic approach led to the development of transvenous systems that are available today. Transvenous systems include placement of a lead in the right side of the heart with an electrode in the right ventricle, typically near the apex, and a second proximal electrode, typically in the superior vena cava. However, defibrillation using a single lead in the right side of the heart is not successful in all patients and implantation of an epicardial patch is commonly indicated. 
   The relatively large physical size of early implantable defibrillators, due to large capacitors needed for delivering the high-energy shocks, restricted the implantation of the device to the abdominal region. As capacitor technology has improved, the size of the defibrillators has decreased making pectoral implantation feasible. With the ability to implant the device in the pectoral region, the housing of the device becomes available as an active electrode, sometimes referred to as an “active can,” in combination with the right ventricular lead eliminating the need for an epicardial patch electrode in most patients. Thus, the pectoral implantation of the device overcame the need for a thoracic approach. 
   Implantable defibrillation systems have been described that use either single or dual defibrillation pathways utilizing combinations of two or three electrodes, selected from a right ventricular lead and the active can. Investigations have been made to determine the optimal defibrillation electrode configuration and results show improved effectiveness of active can configurations, particularly with dual pathway defibrillation using three electrodes. 
   As the device size continues to be reduced, however, the effectiveness of active can configurations comes into question. Development of coronary sinus electrodes, implanted endovascularly in the area of the left heart, provides additional electrode configurations available for defibrillation. With new configurations available between electrodes implanted in the right ventricle and endovascular electrodes on the left side of the heart, investigation continues for determining the optimal electrode configuration for achieving successful defibrillation at the lowest energy requirement. 
   However, no single defibrillation electrode configuration will be optimal for all patients. Differences in implant location, patient anatomy and disease state, which can change overtime, will result in different optimal electrode configurations between patients and perhaps within the same patient over time. A given defibrillation pathway selected as the primary pathway based on clinical testing may not continue to be the optimal defibrillation pathway. Therefore, a final determination of an optimal electrode configuration remains elusive. 
   The use of multiple single or dual pathways during a defibrillation regimen, therefore, would be advantageous in patients who are not successfully treated by the first defibrillation shock waveform delivered along a primary pathway. An implantable defibrillation device is needed, therefore, which allows automatic switching between defibrillation pathways during a defibrillation regimen. 
   SUMMARY OF THE INVENTION 
   What is described herein is an implantable cardiac stimulation device equipped with a right ventricular lead, a coronary sinus lead, and a case electrode for delivering defibrillation therapy. The size of the device is such that it is suitable for implantation in the pectoral region. Thus, either the case electrode, provided by the device housing, or an electrode located on the coronary sinus lead, positioned in the vicinity of the left side of the heart, may function as the counter electrode during high voltage shock delivery for the purpose of defibrillation. 
   Each of the case electrode and the coronary sinus electrode provides a different defibrillation pathway through the heart tissue when paired with an electrode located on the right ventricular lead. The case electrode and a coronary sinus electrode may also be selected concurrently as parallel counter electrodes, creating a dual pathway from an electrode located on the right ventricular lead. 
   A method is provided for automatically alternating the counter electrode assignment between the single case electrode, the single coronary sinus electrode and the parallel combination of the case and coronary sinus electrodes during a defibrillation regimen. By alternating the defibrillation pathway, defibrillation success may be improved when the first waveform of a defibrillation regimen fails to terminate fibrillation. 
   When operating according to a preferred embodiment, a defibrillation waveform is first delivered upon detection of ventricular fibrillation by selecting an electrode located on a coronary sinus lead as the counter electrode paired with a right ventricular electrode. If the ventricular fibrillation is not terminated, a second defibrillation waveform is delivered by selecting the case electrode as the counter electrode paired with the right ventricular electrode. If the ventricular fibrillation is still not terminated, a third defibrillation waveform is delivered by selecting a coronary sinus electrode and the case electrode together as counter electrodes in parallel, creating a dual pathway for defibrillation. 
   A physician may determine the preferred counter electrode to be used as the primary pathway based on clinical testing. If fibrillation is not terminated after delivering a defibrillation waveform via this primary path, alternative single or dual pathways are selected by selecting a different single or combined counter electrode. The method provided herein, therefore, allows automatic cycling between a coronary sinus electrode and a case electrode as the counter electrode during a defibrillation regimen. The method further allows automatic cycling between a parallel combination of counter electrodes for a dual pathway and a single counter electrode for a single pathway. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein: 
       FIG. 1  is a simplified, partly cutaway view illustrating an implantable stimulation device in electrical communication with at least three leads implanted into a patient&#39;s heart for delivering multi-chamber stimulation and shock therapy; 
       FIG. 2  is a functional block diagram of the multi-chamber implantable stimulation device of  FIG. 1 , illustrating the basic elements that provide pacing stimulation, cardioversion, and defibrillation in four chambers of the heart; 
       FIG. 3  is a process flow chart illustrating a method used by the implantable stimulation device of  FIG. 2 , for automatically cycling between counter electrodes on successive defibrillation waveforms during a defibrillation regimen; and 
       FIG. 4  is a process flow chart illustrating an alternative method used by the implantable stimulation device of  FIG. 2 , for automatically cycling between counter electrodes on successive defibrillation waveforms during a defibrillation regimen. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The following description is of a 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. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. The present invention is directed at providing automatic cycling between defibrillation pathways during a defibrillation regimen. A general cardiac stimulation device will be described in conjunction with  FIGS. 1 and 2 , in which the features included in the present invention could be implemented. It is recognized, however, that numerous variations of such a device exist in which the methods included in the present invention could be implemented without deviating from the scope of the present invention. 
     FIG. 1  illustrates a stimulation device  10  in electrical communication with a patient&#39;s heart  12  by way of three leads  20 ,  24  and  30  suitable for delivering multi-chamber stimulation and shock therapy. To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the stimulation device  10  is coupled to an implantable right atrial lead  20  having at least an atrial tip electrode  22 , which typically is implanted in the patient&#39;s right atrial appendage. The right atrial lead  20  may also have an atrial ring electrode  23  to allow bipolar stimulation or sensing in combination with the atrial tip electrode  22 . 
   To sense the left atrial and ventricular cardiac signals and to provide left-chamber stimulation therapy, the stimulation device  10  is coupled to a “coronary sinus” lead  24  designed for placement in the “coronary sinus region” via the coronary sinus ostium so as to place a distal electrode adjacent to the left ventricle and additional electrode(s) adjacent to the left atrium. 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, the coronary sinus lead  24  is designed to: receive atrial and ventricular cardiac signals; deliver left ventricular pacing therapy using at least a left ventricular tip electrode  26  for unipolar configurations or in combination with left ventricular ring electrode  25  for bipolar configurations; deliver left atrial pacing therapy using at least a left atrial ring electrode  27 , and deliver shocking therapy using at least a left atrial coil electrode  28 . 
   The stimulation device  10  is also shown in electrical communication with the patient&#39;s heart  12  by way of an implantable right ventricular lead  30  having, in this embodiment, a right ventricular tip electrode  32 , a right ventricular ring electrode  34 , a right ventricular (RV) coil electrode  36 , and a superior vena cava (SVC) coil electrode  38 . Typically, the right ventricular lead  30  is transvenously inserted into the heart  12  so as to place the right ventricular tip electrode  32  in the right ventricular apex so that the RV coil electrode  36  will be positioned in the right ventricle and the SVC coil electrode  38  will be positioned in the right atrium and/or superior vena cava. 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  illustrates a simplified block diagram of the multi-chamber implantable stimulation device  10 , 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 stimulation device  10  includes a housing  40  which is often referred to as “can,” “case,” or “case electrode”, and which may be programmably selected to act as the return electrode for all “unipolar” modes. The housing  40  may further be used as a return electrode alone or in combination with one or more of the coil electrodes  28 ,  36 , or  38 , for defibrillation shocking purposes. The housing  40  further includes a connector having a plurality of terminals  42 ,  43 ,  44 ,  45 ,  46 ,  48 ,  52 ,  54 ,  56 , and  58  (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the corresponding terminals). As such, to achieve right atrial sensing and stimulation, the connector includes at least a right atrial tip terminal (A R  TIP)  42  adapted for connection to the atrial tip electrode  22 . The connector may also include a right atrial ring terminal (A R  RING)  43  for connection to the right atrial ring electrode  23 . 
   To achieve left chamber sensing, pacing, and shocking, the connector includes at least a left ventricular tip terminal (V L  TIP)  44 , a left ventricular ring terminal (V L  RING)  45 , a left atrial ring terminal (A L  RING)  46 , and a left atrial shocking coil terminal (A L  COIL)  48 , which are adapted for connection to the left ventricular tip electrode  26 , the left ventricular ring electrode  25 , the left atrial ring electrode  27 , and the left atrial coil electrode  28 , respectively. 
   To support right ventricular sensing, pacing and shocking, the connector further includes a right ventricular tip terminal (V R  TIP)  52 , a right ventricular ring terminal (V R  RING)  54 , a right ventricular shocking coil terminal (RV COIL)  56 , and an SVC shocking coil terminal (SVC COIL)  58 , which are adapted for connection to the right ventricular tip electrode  32 , right ventricular ring electrode  34 , the RV coil electrode  36 , and the SVC coil electrode  38 , respectively. 
   At the core of the stimulation device  10  is a programmable microcontroller  60  that controls the various modes of stimulation therapy. 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. Any suitable microcontroller  60  may be used that carries out the functions described herein. 
     FIG. 2  illustrates an atrial pulse generator  70  and a ventricular pulse generator  72  that generate stimulation pulses for delivery by the right atrial lead  20 , the right ventricular lead  30 , and/or the coronary sinus lead  24  via a switch  74 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial pulse generator  70  and the ventricular pulse generator  72  may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The atrial pulse generator  70  and the ventricular pulse generator  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 interchamber (A—A) delay, or ventricular interchamber (V—V) delay, etc.), as well as to keep track of the timing of refractory periods, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc. 
   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, cross-chamber, etc.) by selectively closing the appropriate combination of switches 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  20 , coronary sinus lead  24 , and the right ventricular lead  30 , through the switch  74 , for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial and ventricular 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. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. 
   Each of the atrial sensing circuit  82  or the ventricular sensing circuit  84  preferably employs one or more low power, precision amplifiers with programmable gain and automatic gain or sensitivity control, bandpass filtering, and a threshold detection circuit, to selectively sense the cardiac signal of interest. The automatic sensitivity control enables the stimulation 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  for triggering or inhibiting the atrial and ventricular pulse generators  70  and  72 , respectively, in a demand fashion, in response to the absence or presence of cardiac activity, respectively, in the appropriate chambers of the heart. The atrial and ventricular sensing circuits  82  and  84 , in turn, receive control signals over signal lines  86  and  88  from the microcontroller  60 , for controlling the gain, threshold, polarization charge removal circuitry, and the timing of any blocking circuitry coupled to the inputs of the atrial and ventricular sensing circuits  82  and  84 . 
   For arrhythmia detection, the stimulation device  10  includes an arrhythmia detector  77  that utilizes the atrial and ventricular sensing circuits  82  and  84  to sense cardiac signals, for determining whether a rhythm is physiologic or pathologic. As used herein “sensing” refers to the process of noting an electrical signal. “Detection” refers to the step of confirming that the sensed electrical signal as the signal being sought by the detector. As an example, “detection” applies to the detection of both proper rhythms (i.e., “R wave” or “R wave”) as well as improper dysrhythmias including arrhythmia and bradycardia (e.g., detection of the absence of a proper rhythm.) 
   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  77  by comparing them to a predefined rate zone limit (e.g. bradycardia, normal, low rate ventricular tachycardia, high rate ventricular tachycardia, 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 stimulation, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”). 
   Cardiac signals are also applied to the inputs of a data acquisition system  90 , which is depicted as an analog-to-digital (A/D) converter for simplicity of illustration. The data acquisition system  90  is configured to acquire intracardiac electrogram (EGM) signals, convert the raw analog data into digital signals, 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  20 , the coronary sinus lead  24 , and the right ventricular lead  30  through the switch  74  to sample cardiac signals across any pair of desired electrodes. 
   Advantageously, the data acquisition system  90  may be coupled to the microcontroller  60  or another detection circuitry, for detecting an evoked response from the heart  12  in response to an applied stimulus, thereby aiding in the detection of “capture”. In the embodiment shown in  FIG. 2 , the microcontroller  60  includes an automatic capture detector  65  that searches for an evoked response signal following a stimulation pulse during a “detection window” set by timing control circuitry  79 . The microcontroller  60  enables the data acquisition system  90  via control signal  92  to sample the cardiac signal that falls in the capture detection window. The sampled signal is evaluated by automatic capture detector  65  to determine if it is an evoked response signal based on its amplitude, peak slope, morphology or another signal feature or combination of features. The detection of an evoked response during the detection window indicates that capture has occurred. 
   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  10  to suit the needs of a particular patient. Such operating parameters define, for example, stimulation pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each stimulation pulse to be delivered to the patient&#39;s heart  12  within each respective tier of therapy. 
   Advantageously, the operating parameters of the stimulation device  10  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  60  by a control signal  106 . The telemetry circuit  100  advantageously allows intracardiac electrograms and status information relating to the operation of the stimulation device  10  (as contained in the microcontroller  60  or memory  94 ) to be sent to the external device  102  through the established communication link  104 . 
   The stimulation device  10  may further include a physiologic sensor  108 , commonly referred to as a “rate-responsive” sensor because it is typically used to adjust 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 stimulation 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  10  additionally includes a power source shown as a battery  110  that provides operating power to all the circuits shown in  FIG. 2 . For the stimulation device  10 , which employs shocking therapy, the battery  110  must be capable of operating at low current drains for long periods of time, preferably less than 10 μA, and also be capable of providing high-current pulses 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  110  preferably has a predictable discharge characteristic so that elective replacement time can be detected. 
   As further illustrated in  FIG. 2 , the stimulation device  10  is shown to include an impedance measuring circuit  112  which is enabled by the microcontroller  60  by control signal  114 . The known uses for an impedance measuring circuit  112  include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper lead positioning or dislodgment; detecting operable electrodes and automatically switching to an operable pair if dislodgment 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  112  is advantageously coupled to the switch  74  so that any desired electrode may be used. 
   Since stimulation device  10  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 stimulation or shock therapy to the heart aimed at terminating the detected arrhythmia. 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 (11 to 40 joules) energy, as controlled by the microcontroller  60 . Such shocking pulses are applied to the patient&#39;s heart through at least two shocking electrodes, and as shown in this embodiment, selected from the left atrial coil electrode  28 , the RV coil electrode  36 , and/or the SVC coil electrode  38  ( FIG. 1 ). As noted above, the housing  40  may act as a counter electrode (that provides a return electrical path) in combination with the RV electrode  36 , or as part of a split electrical vector using the SVC coil electrode  38  or the left atrial coil electrode  28  creating a dual defibrillation pathway. 
   As illustrated in  FIG. 1 , one possible defibrillation pathway, represented by vector A, exists between the RV electrode  36  and the left atrial coil electrode  28 . Another possible defibrillation pathway, represented by vector B, may exist between the RV electrode  36  and the housing  40 . A dual defibrillation pathway represented by both vectors A and B exists when the left atrial coil electrode  28  and the housing  40  are selected, concurrently, to function as parallel counter electrodes. In accordance with the present invention, successive defibrillation waveforms may be delivered along these defibrillation pathways in an alternating fashion. 
   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. 
   A number of defibrillation waveforms may be delivered during a given defibrillation regimen in response to fibrillation detection. Preferably, the device  10  verifies that fibrillation is still present following the delivery of one waveform prior to delivering the next waveform. In accordance with the present invention, if one waveform delivered along one pathway fails to defibrillate the heart, the next waveform is delivered along an alternative pathway by selecting, through switch  74 , an alternative counter electrode. 
     FIG. 3  illustrates a method  400  implemented by the device  10 , for automatically alternating the defibrillation pathway of successive waveforms during a defibrillation regimen. In this flow chart, 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 chart presented herein provides 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 chart of  FIG. 3  and other descriptions presented herein. 
   Method  400  is initiated at step  405  when ventricular fibrillation is detected by arrhythmia detector  77 . A defibrillation waveform is delivered at step  410  between the right ventricular coil electrode  36  to the left atrial coil electrode  28 , such that the defibrillation waveform follows a pathway represented by vector A shown in  FIG. 1 . At step  415 , arrhythmia detector  77  determines if the ventricular fibrillation has been terminated. If fibrillation has been successfully terminated, method  400  is concluded at step  450 . 
   If fibrillation is still detected at decision step  415 , a second defibrillation waveform is delivered from the right ventricular coil electrode  56  to the housing  40  at step  420 . The delivery of this waveform will follow the pathway illustrated by vector B shown in  FIG. 1 . At step  425 , arrhythmia detector  77  determines if fibrillation has now been successfully terminated. If so, method  400  is concluded at step  450 . 
   In one embodiment, method  400  may continue alternating between the left atrial coil electrode  28  as the counter electrode and the housing  40  as the counter electrode between successive waveforms, until defibrillation is confirmed or the maximum allowed number of waveforms to be delivered during the regimen has been reached. 
   In one embodiment, the first and second waveforms are the same type of waveform (e.g., a biphasic waveform). In an alternate embodiment, the first and second waveforms are different (e.g., a monophasic waveform and a biphasic waveform), where one type of waveform can be used for a certain electrode configuration, and the other waveform can be used for a different electrode configuration. 
   In the embodiment shown in  FIG. 3 , if fibrillation has still not been terminated at decision step  425 , the switch  74  selects the housing  40  and the left atrial coil electrode  28 , concurrently, to form a parallel counter electrode combination. A third defibrillation waveform is delivered at step  435  from the right ventricular coil  56  to the parallel counter electrodes forming a dual pathway illustrated by both vectors A and B shown in  FIG. 1 . 
   At step  440 , arrhythmia detector  77  determines if the fibrillation is now terminated. If so, method  400  is concluded at step  450 . If fibrillation continues to be detected, method  400  may return to any of steps  410 ,  420 , or  435 , as desired or programmed, to deliver another shock, as described above. Method  400  may continue to cycle between the following three defibrillation pathways, either in a linear, predetermined regimen, or alternatively, in another predetermined or programmed regimen: 1) the single pathway to the left atrial coil electrode  28 , 2) the single pathway to the housing  40 , and 3) the dual pathway to the concurrently selected left atrial coil electrode  28  and housing  40 . The energy, shape, and maximum number of waveforms in each regimen may be predetermined according to device specifications or programmed as desired by a clinician. 
   The method  400  shown in  FIG. 3  describes one algorithm for cycling between two single pathways and one dual pathway. The order by which the counter electrodes are selected for determining these pathways is preferably programmable. The primary counter electrode, that is the counter electrode used on the first defibrillation attempt, is preferably selected by a clinician based on clinical testing. 
   Depending on the implanted system, a number of single counter electrodes or parallel counter electrodes could be selected. The concept of the present invention for alternating waveform delivery between different single, dual or even multiple pathways may be used successfully in any implantable defibrillator system equipped with at least three or more electrodes suitable for delivering high voltage therapy. Both atrial and ventricular high-voltage therapy could be delivered using a method of alternating or cycling counter electrodes. 
     FIG. 4  illustrates another method  500  implemented by the device  10 , for automatically alternating the defibrillation pathway of successive waveforms during a defibrillation regimen, by learning and following the best pathways (i.e., counter-electrodes). Method  500  is initiated at step  505  when ventricular fibrillation is detected by arrhythmia detector  77 . A defibrillation waveform is delivered at step  510  between the right ventricular coil electrode  36  to the left atrial coil electrode  28 , such that the defibrillation waveform follows a pathway represented by vector A shown in  FIG. 1 . Otherwise, if this pathway has proven not to be ineffective, then the defibrillation waveform is delivered using a previously stored preferred counter electrode. 
   At step  515 , arrhythmia detector  77  determines if the ventricular fibrillation has been terminated. If fibrillation has been successfully terminated, method  500  is concluded at step  550 . If fibrillation is still detected at decision step  515 , a second defibrillation waveform is delivered from the right ventricular coil electrode  56  to the housing (or case electrode)  40  at step  520 . The delivery of this waveform will follow the pathway illustrated by vector B shown in  FIG. 1 . 
   At step  525 , arrhythmia detector  77  determines if fibrillation has now been successfully terminated. If so, method  500  proceeds to step  527  where it logs the housing  40  as the preferred counter electrode for future use at step  515 , and is then concluded at step  550 . 
   If fibrillation has still not been terminated at decision step  525 , the switch  74  selects the housing  40  and the left atrial coil electrode  28 , concurrently, to form a parallel counter electrode combination. A third defibrillation waveform is delivered at step  535  from the right ventricular coil  56  to the parallel counter electrodes forming a dual pathway illustrated by both vectors A and B shown in  FIG. 1 . 
   At step  540 , arrhythmia detector  77  determines if the fibrillation is now terminated. If so, method  500  proceeds to step  542  where it records the dual counter electrode configuration as the preferred counter electrode for future use at step  515 , and is then concluded at step  550 . 
   As an optional addition to method  500 , whenever a maximum energy shock fails with a given counter electrode, then that counter electrode is excluded from the next therapy regimen delivered. However, if all 3 counter electrodes fail the first time when used at maximum energy, then they are presumed to be functioning properly. All these 3 counter electrodes are relisted in an “approved” category, and process  400  of  FIG. 3  is initiated. It is noteworthy to mention that the impedances will be different between the different counter electrodes. Thus, the device  10  will be able to deliver the optimal waveform timing for each counter electrode which may vary. 
   Thus a system and method have been described for cycling between alternative single and combined parallel counter electrodes when delivering successive defibrillation waveforms during a defibrillation regimen. While detailed descriptions of specific embodiments of the present invention have been provided, it would be apparent to those reasonably skilled in the art that numerous variations of the methods described herein are possible in which the concepts of the present invention may readily be applied. The descriptions provided herein are for the sake of illustration and are not intended to be exclusive.