Atrial Defibrillator Employing Transvenous and Subcutaneous Electrodes and Method of Use

Reference is made to commonly owned U.S. Pat. Application Ser. No. 07/922,587 for an "Atrial Defibrillator Employing Transvenous and Subcutaneous Electrodes and Method of Use", filed Jul. 30, 1992 by Bardy.

The present invention relates to medical electrical leads generally, and more particularly to implantable defibrillation electrodes and leads.

Early concepts of implantable defibrillators, such as disclosed in U.S. Pat. No. Re27,652 by Mirowski, et al., envision an electrode system employing a ventricular endocardial electrode and a plate electrode mounted to the heart directly, subcutaneously, or to the skin. U.S. Pat. No. 3,942,536 by Mirowski, et al., discloses a transvenous lead having electrodes intended for location in the right ventricular apex and in the superior vena cava. This electrode system is disclosed as useful for either ventricular or atrial defibrillation. Such systems were eventually tested in human beings, with some success. Currently available implantable defibrillators typically employ epicardial or subcutaneous patch electrodes, alone, or in conjunction with one or more transvenous electrodes.

It is generally believed that it would be desirable to produce an implantable defibrillation system which entirely avoids the necessity of a thoracotomy, and there has been substantial work directed towards development of multi-electrode systems to accomplish this result, as disclosed in U.S. Pat. No. 4,727,877 issued to Kallok, U.S. Pat. No. 4,708,145 issued to Tacker, et al., and as disclosed in U.S. Pat. No. 5,099,838, issued to Bardy. Other endocardial defibrillation electrodes are disclosed in U.S. Pat. No. 4,481,953 issued to Gold et al., U.S. Pat. No. 4,161,952 issued to Kinney, et al., U.S. Pat. No. 4,934,049 issued to Kiekhafer et al. and in U.S. Pat. No. 5,042,143 issued to Holleman, et al. The Kinney, Gold, Holleman and Kiekhafer patents all disclose endocardial defibrillation leads employing defibrillation electrodes fabricated from elongated coils of biocompatible metal, mounted exposed to the exterior of the defibrillation lead, for location in the right ventricle and other locations within the heart. The above-cited Smits patent and the Mehra application both disclose a variety of endocardial defibrillation electrodes intended for use in the atrium, ventricle and coronary sinus, all of which employ electrodes taking the form of elongated coils of conductive biocompatible metals.

A return to lead systems employing only two electrodes for ventricular defibrillation is suggested in U.S. Pat. No. 4,922,927, issued to Fine et al. This patent proposes the use of an electrode system as in the above-cited Mirowski reissue Patent, using only a right ventricular electrode and a subcutaneous electrode, which may correspond to prior art subcutaneous electrodes or may be the metal enclosure of the defibrillator. The right ventricular electrode carries an elongated coil electrode fabricated of a copper-zirconium alloy coated with iridium oxide. The use of biphasic pulses in such a two electrode system is also recommended.

Concurrent with the development of lead systems adapted to treat ventricular fibrillation, there has also been some work directed to the development of lead systems to treat atrial fibrillation. Synchronized cardioversion using two electrodes located on a lead located in the right atrium is disclosed in U.S. Pat. No. 3,738,370, issued to Charms. A later system is disclosed in U.S. Pat. No. 3,952,750, issued to Mirowski et al., employing one electrode in the atrium and presumably a second electrode at an unspecified location. Neither of these references discloses a specific embodiment for the electrodes located in the atrium.

An electrode lead system specifically designed for atrial defibrillation is disclosed in the article "Elective Countershock in Atrial Fibrillation With an Intracardiac Electrode--A Preliminary Report, by Jain, et al., published in the Journal of the Association of Physicians of India, Vol. 18, pp. 821-824, 1970. This lead was provided with a 10 mm silver electrode for location in the right atrium and was tested in conjunction with either a second electrode located in the right atrium or a second, cutaneous electrode located on the left side of the chest wall. A second electrode system specifically designed for use in atrial cardioversion is disclosed in the article "Safety and feasibility of transvenous cardioversion in atrial tachycardia", by Blanc et al., published in Cardiac Pacing, edited by Gomez, Futura Pub. Co., 1985, pp 1526-1529. This electrode system employed a single lead with electrodes located in the atrium and pulmonary artery. More recently, the use of electrodes located in the right atrium and coronary sinus for atrial defibrillation has been disclosed in U.S. Pat. No. 5,165,403 by Mehra, issued Nov. 24, 1992, incorporated herein by reference in its entirety.

The present invention is directed toward the provision of a defibrillator and defibrillation lead system particularly optimized for use in defibrillation or cardioversion of the atrium. The lead system includes a coronary sinus/great vein electrode and a subcutaneous plate electrode located in the left pectoral region which may optionally take the form of a surface of the defibrillator housing. In some embodiments, the electrode system may also include a superior vena cava electrode and/or a ventricular electrode. As discussed in U.S. Pat. No. 5,099,838, issued to Bardy on Mar. 31, 1992, incorporated herein by reference in its entirety, the coronary sinus/great vein electrode to subcutaneous electrode defibrillation pathway may be advantageously used in performing ventricular defibrillation. However, the inventor has determined that this electrode configuration may be advantageously employed to perform atrial defibrillation as well. Thus, the invention may usefully be practiced in a device which is intended to perform atrial cardioversion or defibrillation only, or in a device which also performs ventricular cardioversion or defibrillation.

The present invention is preferably practiced in a defibrillator/cardioverter which delivers an asymmetrical biphasic capacitive discharge pulse between the electrodes. Only the coronary sinus and subcutaneous electrodes may be employed during atrial defibrillation or alternatively an additional electrode (e.g. a superior vena cava electrode) may also be employed. If three electrodes are employed they may be arranged so that two of the three electrodes are connected together during the delivered pulse, as disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al. on Sep. 4, 1990, also incorporated herein by reference in its entirety or sequential pulses may be delivered between two electrode pairs.

If the device is configured to perform atrial and ventricular defibrillation or cardioversion, the coronary sinus/great vein electrode to subcutaneous electrode pathway may also be employed as part of the electrode system employed for ventricular cardioversion or defibrillation. For example, the coronary sinus/great vein electrode and subcutaneous electrode might be employed in conjunction with a right ventricular electrode, as disclosed in the above-cited Bardy patent. Alternatively, an entirely different pathway may be employed for ventricular defibrillation, for example from a right ventricular electrode to either said coronary sinus/great vein electrode or said subcutaneous electrode.

FIG. 1 illustrates a first embodiment of an implantable defibrillator and lead according to the present invention.

FIG. 2 illustrates a second embodiment of an implantable defibrillator and lead according to the present invention.

FIG. 3 illustrates a functional schematic diagram of an implantable pacemaker/cardioverter/defibrillator in which the invention may usefully be practiced in conjunction with the electrodes illustrated in FIGS. 1 and 2.

FIG. 1 illustrates a defibrillator and lead set according to the present invention. The ventricular lead takes the form of the lead disclosed in the above cited patent issued to Bardy, and includes an elongated insulative lead body 16, carrying three concentric coiled conductors, separated from one another by tubular insulative sheaths. Located adjacent the distal end of the lead are a ring electrode 24, an extendable helix electrode 26, mounted retractably within an insulative electrode head 28, and an elongated coil electrode 20. Each of the electrodes is coupled to one of the coiled conductors within the lead body 16. Electrodes 24 and 26 are employed for cardiac pacing and for sensing ventricular depolarizations.

At the proximal end of the lead is a bifurcated connector 14 which carries three electrical connectors, each coupled to one of the coiled conductors. The defibrillation electrode 20 may be fabricated from platinum, platinum alloy or other materials known to be usable in implantable defibrillation electrodes. The atrial lead includes an elongated insulative lead body 15, carrying two concentric coiled conductors, separated from one another by tubular insulative sheaths. Located adjacent the J-shaped distal end of the lead are a ring electrode 21 and an extendable helix electrode 17, mounted retractably within an insulative electrode head 19. Each of the electrodes is coupled to one of the coiled conductors within the lead body 15. Electrodes 17 and 21 are employed for atrial pacing and for sensing atrial depolarizations. At the proximal end of the lead is an in-line connector 13 which carries two electrical connectors, each coupled to one of the coiled conductors.

The coronary sinus lead takes the form of the coronary sinus lead disclosed in the above cited patent issued to Bardy, and includes an elongated insulative lead body 6, carrying one coiled conductor, coupled to an elongated coiled defibrillation electrode 8. Electrode 8, illustrated in broken outline, is located within the coronary sinus and great vein of the heart. At the proximal end of the lead is a connector 4 plug which carries an electrical connector, coupled to the coiled conductor.

An implantable pacemaker/cardioverter/defibrillator 10 is shown in combination with the leads, with the lead connector assemblies 4, 13 and 14 inserted into the connector block 12. Optionally, insulation of the outward facing portion of the housing 11 of the pacemaker/cardioverter/defibrillator 10 may be provided using a plastic coating, for example parylene or silicone rubber, as is currently employed in some unipolar cardiac pacemakers. However, the outward facing portion may instead be left uninsulated, or some other division between insulated and uninsulated portions may be employed. The uninsulated portion of the housing 11 serves as a subcutaneous defibrillation electrode, used in conjunction with electrode 20 to defibrillate either the atria or ventricles.

The electrode system and method of the present invention has the important advantages of simplicity of construction and of use. Unlike systems involving subcutaneous electrodes located at approximately the level of the heart, the pectoral location of the electrode of the present invention allows for implant using only a single incision. This particular electrode configuration has advantages in regard to optimizing current distribution with respect to the atrium of the heart. The coronary sinus/great vein electrode, located adjacent left atrial tissue and the pectorally located defibrillator housing 11 provide a defibrillation pulse vector which includes a substantial portion of atrial tissue, and provides an alternative electrode location for those patients in which a the right ventricle to left pectoral defibrillation pulse pathway described in the parent application to the present case is not optimally effective. In addition, by adding a right ventricular electrode, a combined atrial and ventricular defibrillator may be provided.

FIG. 2 illustrates an alternative defibrillator and lead set according to the present invention. In this embodiment, the pacemaker/cardioverter/defibrillator 110 is intended only to treat atrial arrhythmias. The atrial lead includes an elongated insulative lead body 115, carrying three concentric coiled conductors, separated from one another by tubular insulative sheaths. Located adjacent the J-shaped distal end of the lead are a ring electrode 121 and an extendable helix electrode 117, mounted retractably within an insulative electrode head 119. Mounted proximal to the J-shaped bend is a superior vena cava electrode 120. Each of the electrodes is coupled to one of the coiled conductors within the lead body 115. Electrodes 117 and 121 are employed for atrial pacing and for sensing atrial depolarizations. At the proximal end of the lead is a bifurcated connector assembly 113 which carries three electrical connectors, each coupled to one of the coiled conductors.

The coronary sinus lead takes the form of the coronary sinus lead disclosed in the above cited patent issued to Bardy, and includes an elongated insulative lead body 116, carrying one coiled conductor, coupled to an elongated coiled defibrillation electrode 118. Electrode 118, illustrated in broken outline, is located within the coronary sinus and great vein of the heart. At the proximal end of the lead is a connector 114 plug which carries an electrical connector, coupled to the coiled conductor.

An implantable pacemaker/cardioverter/defibrillator 110 is shown in combination with the leads, with the lead connector assemblies 114 and 113 inserted into the connector block 112. The uninsulated portion of the housing 111 serves as a subcutaneous defibrillation electrode, used in conjunction with electrodes 118 and 120 to cardiovert or defibrillate the atria.

The pectoral location of the subcutaneous electrode of the system illustrated in FIG. 2 also allows for implant using only a single incision. This particular electrode configuration also has advantages in regard to optimizing current distribution with respect to the atrium of the heart. The coronary sinus/great vein electrode, located adjacent left atrial tissue provides a defibrillation pulse vector which includes a substantial portion of atrial tissue. By adding the superior vena cava electrode 120, improved current distribution in the right atrium is accomplished. The electrodes are preferably used to deliver pulses simultaneously or sequentially between a first electrode pair comprising coronary sinus/great vein electrode 118 and superior vena cava electrode 120 and between a second electrode pair comprising coronary sinus/great vein electrode 118 and subcutaneous electrode 111.

FIG. 3 is a functional schematic diagram of an implantable pacemaker/cardioverter/defibrillator in which the present invention may usefully be practiced. This diagram should be taken as exemplary of the type of device in which the invention may be embodied, and not as limiting, as it is believed that the invention may usefully be practiced in a wide variety of device implementations, including cardioverter and defibrillators which do not provide anti-tachycardia pacing therapies.

The device is provided with an electrode system including electrodes as illustrated in FIG. 1 or FIG. 2. If the electrode configuration of FIG. 1 is employed, the correspondence to the illustrated electrodes is as follows. Electrode 311 corresponds to electrode 11, and is the uninsulated portion of the housing of the implantable pacemaker/cardioverter/defibrillator. Electrode 320 corresponds to electrode 20 and is a defibrillation electrode located in the right ventricle. Electrode 318 corresponds to electrode 8 and is a defibrillation electrode located in the coronary sinus and great vein. Electrodes 324 and 326 correspond to electrodes 24 and 26, and are used for sensing and pacing in the ventricle. Electrodes 317 and 321 correspond to electrodes 17 and 19 and are used for pacing and sensing in the atrium.

If the electrode configuration of FIG. 2 is employed, the correspondence to the illustrated electrodes is as follows. Electrode 311 corresponds to electrode 111, and is the uninsulated portion of the housing of the implantable pacemaker/cardioverter/defibrillator. Electrode 320 corresponds to electrode 120 and is a defibrillation electrode located in the superior vena cava. Electrode 318 corresponds to electrode 118 and is a defibrillation electrode located in the coronary sinus and great vein. Electrodes 317 and 321 correspond to electrodes 117 and 121 and are used for pacing and sensing in the atrium.

If the electrode system illustrated in FIG. 2 is employed, electrodes 324 and 326 in FIG. 3 are not employed, and the ventricular sensing and pacing functions described below are not activated. Similarly, the ventricular anti-tachycardia and defibrillation therapies discussed below are not available. In such case, only the atrial sensing, pacing and anti-arrhythmia functions discussed below would be activated. However, it is also believed within the scope of the invention to combine the electrode systems of FIGS. 1 and 2 together or to combine them with other additional electrodes, in which case the all of the atrial and ventricular pacing, sensing and tachyarrhythmia treatment functions discussed below may be activated.

Electrodes 311, 318 and 320 are coupled to high voltage output circuit 234. Electrodes 424 and 326 (if present) are located on or in the ventricle and are coupled to the R-wave amplifier 200, which preferably takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on R-out line 202 whenever the signal sensed between electrodes 612 and 614 exceeds the present sensing threshold.

Electrodes 317 and 321 are located on or in the atrium and are coupled to the P-wave amplifier 204, which preferably also takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on P-out line 206 whenever the signal sensed between electrodes 617 and 621 exceeds the present sensing threshold. The general operation of the R-wave and P-wave amplifiers 200 and 204 may correspond to that disclosed in U.S. Pat. No. 5,117,824, by Keimel, et al., issued Jun. 2, 1992, for an Apparatus for Monitoring Electrical Physiologic Signals, incorporated herein by reference in its entirety.

Switch matrix 208 is used to select which of the available electrodes are coupled to wide band (0.5-200 Hz) amplifier 210 for use in digital signal analysis. Selection of electrodes is controlled by the microprocessor 224 via data/address bus 218, which selections may be varied as desired. Signals from the electrodes selected for coupling to bandpass amplifier 210 are provided to multiplexer 220, and thereafter converted to multi-bit digital signals by A/D converter 222, for storage in random access memory 226 under control of direct memory access circuit 228. Microprocessor 224 may employ digital signal analysis techniques to characterize the digitized signals stored in random access memory 226 to recognize and classify the patient's heart rhythm employing any of the numerous signal processing methodologies known to the art.

The remainder of the circuitry is dedicated to the provision of cardiac pacing, cardioversion and defibrillation therapies, and, for purposes of the present invention may correspond to circuitry known in the prior art. An exemplary apparatus is disclosed of accomplishing pacing, cardioversion and defibrillation functions follows. The pacer timing/control circuitry 212 includes programmable digital counters which control the basic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI and other modes of single and dual chamber pacing well known to the art. Circuitry 212 also controls escape intervals associated with anti-tachyarrhythmia pacing in both the atrium and the ventricle, employing any anti-tachyarrhythmia pacing therapies known to the art.

Intervals defined by pacing circuitry 212 include atrial and ventricular pacing escape intervals, the refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals and the pulse widths of the pacing pulses. The durations of these intervals are determined by microprocessor 226, in response to stored data in memory 226 and are communicated to the pacing circuitry 212 via address/data bus 218. Pacer circuitry 212 also determines the amplitude of the cardiac pacing pulses under control of microprocessor 224.

During pacing, the escape interval counters within pacer timing/control circuitry 212 are reset upon sensing of R-waves and P-waves as indicated by a signals on lines 202 and 206, and in accordance with the selected mode of pacing on timeout trigger generation of pacing pulses by pacer output circuitry 214 and 216, which are coupled to electrodes 317, 321, 324 and 326. The escape interval counters are also reset on generation of pacing pulses, and thereby control the basic timing of cardiac pacing functions, including anti-tachyarrhythmia pacing. The durations of the intervals defined by the escape interval timers are determined by microprocessor 224, via data/address bus 218. The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals, which measurements are stored in memory 226 and used to detect the presence of tachyarrhythmias.

Microprocessor 224 operates as an interrupt driven device, and is responsive to interrupts from pacer timing/control circuitry 212 corresponding to the occurrence sensed P-waves and R-waves and corresponding to the generation of cardiac pacing pulses. These interrupts are provided via data/address bus 218. Any necessary mathematical calculations to be performed by microprocessor 224 and any updating of the values or intervals controlled by pacer timing/control circuitry 212 take place following such interrupts.

For example, in response to a sensed or paced ventricular depolarization or R-wave, the intervals separating that R-wave from the immediately preceding R-wave, paced or sensed (R-R interval) and the interval separating the paced or sensed R-wave from the preceding atrial depolarization, paced or sensed (P-R interval) may be stored. Similarly, in response to the occurrence of a sensed or paced atrial depolarization (P-wave), the intervals separating the sensed P-wave from the immediately preceding paced of sensed atrial contraction (P-P Interval) and the interval separating the sensed P-wave from the immediately preceding sensed or paced ventricular depolarization (R-P interval) may be stored. Preferably, a portion of the memory 226 (FIG. 4) is configured as a plurality of recirculating buffers, capable of holding a preceding series of measured intervals, which may be analyzed in response to the occurrence of a pace or sense interrupt to determine whether the patient's heart is presently exhibiting atrial or ventricular tachyarrhythmia.

Detection of atrial or ventricular tachyarrhythmias, as employed in the present invention, may correspond to tachyarrhythmia detection algorithms known to the art. For example, presence of atrial or ventricular tachyarrhythmia may be confirmed by means of detection of a sustained series of short R-R or P-P intervals of an average rate indicative of tachyarrhythmia or an unbroken series of short R-R or P-P intervals. The suddenness of onset of the detected high rates, the stability of the high rates, or a number of other factors known to the art may also be measured at this time. Appropriate detection methodologies measuring such factors are described in U.S. Pat. No. 4,726,380, issued to Vollmann, U.S. Pat. No. 4,880,005, issued to Pless et al and U.S. Pat. No. 4,830,006, issued to Haluska et al. An additional set of tachycardia recognition methodologies is disclosed in the article "Onset and Stability for Ventricular Tachyarrhythmia Detection in an Implantable Pacer-Cardioverter-Defibrillator" by Olson et al., published in Computers in Cardiology, Oct. 7-10, 1986, IEEE Computer Society Press, pages 167-170. However, one of the advantages of the present invention is that it is believed practicable in conjunction with most prior art tachycardia detection algorithms.

In the event that an atrial or ventricular tachyarrhythmia is detected, and an anti-tachyarrhythmia pacing regimen is desired, appropriate timing intervals for controlling generation of anti-tachyarrhythmia pacing therapies are loaded from microprocessor 224 into the pacer timing and control circuitry 212, to control the operation of the escape interval counters therein and to define refractory periods during which detection of R-waves and P-waves is ineffective to restart the escape interval counters.

Alternatively, circuitry for controlling the timing and generation of anti-tachycardia pacing pulses as described in U.S. Pat. No. 4,577,633, issued to Berkovits et al on Mar. 25, 1986, U.S. Pat. No. 4,880,005, issued to Pless et al on Nov. 14, 1989, U.S. Pat. No. 7,726,380, issued to Vollmann et al on Feb. 23, 1988 and U.S. Pat. No. 4,587,970, issued to Holley et al on May 13, 1986, all of which are incorporated herein by reference in their entireties may also be used.

In the event that generation of a cardioversion or defibrillation pulse is required, microprocessor 224 employs the an escape interval counter to control timing of such cardioversion and defibrillation pulses, as well as associated refractory periods. In response to the detection of atrial or ventricular fibrillation or tachyarrhythmia requiring a cardioversion pulse, microprocessor 224 activates cardioversion/defibrillation control circuitry 230, which initiates charging of the high voltage capacitors 246, 248, 250 and 252 via charging circuit 236, under control of high voltage charging control lines 240 and 242. The voltage on the high voltage capacitors is monitored via VCAP line 244, which is passed through multiplexer 220 and in response to reaching a predetermined value set by microprocessor 224, results in generation of a logic signal on Cap Full (CF) line 254, terminating charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse is controlled by pacer timing/control circuitry 212. Following delivery of the fibrillation or tachycardia therapy the microprocessor then returns the device to cardiac pacing and awaits the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization.

One embodiment of an appropriate system for delivery and synchronization of ventricular cardioversion and defibrillation pulses and for controlling the timing functions related to them is disclosed in more detail in commonly assigned U.S. Pat. No. 5,188,105 by Keimel, issued Feb. 23, 1993, and incorporated herein by reference in its entirety. Embodiments of appropriate systems for delivery and synchronization of atrial cardioversion and defibrillation pulses and for controlling the timing functions related to them are disclosed in more detail in PCT Patent Application No. WO92/18198 by Adams et al., published Oct. 29, 1992, and in U.S. Pat. No. 4,316,472 by Mirowski et al., issued Feb. 23, 1982, both incorporated herein by reference in their entireties.

However, any known cardioversion or defibrillation pulse control circuitry is believed usable in conjunction with the present invention. For example, circuitry controlling the timing and generation of cardioversion and defibrillation pulses as disclosed in U.S. Pat. No. 4,384,585, issued to Zipes on May 24, 1983, in U.S. Pat. No. 4,949,719 issued to Pless et al, cited above, and in U.S. Pat. No. 4,375,817, issued to Engle et al, all incorporated herein by reference in their entireties may also be employed.

In the illustrated device, delivery of the cardioversion or defibrillation pulses is accomplished by output circuit 234, under control of control circuitry 230 via control bus 238. Output circuit 234 determines whether a monophasic or biphasic pulse is delivered, whether the housing 311 serves as cathode or anode and which electrodes are involved in delivery of the pulse. An example of output circuitry for delivery of biphasic pulse regimens may be found in the above cited patent issued to Mehra and in U.S. Pat. No. 4,727,877, incorporated by reference in its entirety.

An example of circuitry which may be used to control delivery of monophasic pulses is set forth in commonly assigned U.S. Pat. No. 5,163,427, by Keimel, issued Nov. 17, 1992, also incorporated herein by reference in its entirety. However, output control circuitry as disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al on Sep. 4, 1990 or U.S. Pat. No. 4,800,883, issued to Winstrom on Jan. 31, 1989 both incorporated herein by reference in their entireties, may also be used in conjunction with a device embodying the present invention for delivery of biphasic pulses.

In the event that, as in FIG. 1, both atrial and ventricular defibrillation are available, atrial defibrillation may be accomplished using lower pulse energy levels than required for ventricular defibrillation and may employ a different electrode set. For example, electrodes 311 and 318 may be employed for both atrial and ventricular defibrillation, with electrode 320 added or substituted for either of electrodes 311 or 318 for treating ventricular fibrillation. Alternatively, the same set of electrodes may be used for both atrial and ventricular defibrillation.

In modern implantable cardioverter/defibrillators, the particular therapies are programmed into the device ahead of time by the physician, and a menu of therapies is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber in which the tachycardia is diagnosed or to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher level cardioversion pulse may be selected thereafter. Therapies for tachycardia termination may also vary with the rate of the detected tachycardia increases. For example, fewer attempts at anti-tachycardia pacing may be undertaken prior to delivery of cardioversion pulses if the rate of the detected tachycardia is above a preset threshold. The references cited above in conjunction with descriptions of prior art tachycardia detection and treatment therapies are applicable here as well.

In the event that atrial or ventricular fibrillation is identified, the typical therapy will be delivery of a high amplitude defibrillation pulse, typically in excess of 10 joules in the case of ventricular fibrillation and in excess of 5 joules in the case of atrial defibrillation. Lower energy levels will be employed for cardioversion. As in the case of currently available implantable pacemakers/cardioverter/defibrillators, and as discussed in the above-cited references, it is envisioned that the amplitude of the defibrillation pulse may be incremented in response to failure of an initial pulse or pulses to terminate fibrillation. Prior art patents illustrating such pre-set therapy menus of anti-tachyarrhythmia therapies include the above-cited U.S. Pat. No. 4,830,006, issued to Haluska, et al., U.S. Pat. No. 4,727,380, issued to Vollmann et al. and U.S. Pat. No. 4,587,970, issued to Holley et al.

While the invention is disclosed above embodied in a dual chamber pacemaker/cardioverter/defibrillator, the invention may also be usefully practiced in substantially simpler devices. For example, the electrode system illustrated in FIG. 2, including electrodes 117, 119 and 118, with or without electrode 120, may simply be coupled to an implantable atrial cardioverter as disclosed in U.S. Pat. No. 3,738,370, issued to Charms, incorporated herein by reference in its entirety. A simple device of this type is believed workable in some patients. However, inclusion of the ability to detect and terminate ventricular tachycardias and fibrillation is believed of extreme importance in patients in whom delivery of atrial cardioversion or defibrillation pulses unintentionally in initiates ventricular arrhythmias.

There are major efforts presently underway to reduce the size of current implantable defibrillators to further simplify implant and enhance patient comfort. As the devices become smaller, it is anticipated that the surface areas of the defibrillator housings may become small enough to interfere ability of the housing to function efficiently as a subcutaneous defibrillation electrode. In such cases, it is envisioned that the surface area of the subcutaneous electrode may be increased by means of a supplemental plate electrode electrically coupled to the defibrillator housing or employed as an electrode in place of the defibrillator housing. This supplemental electrode may be simply placed in the pectoral implant site adjacent the defibrillator or may in some cases be clipped or otherwise attached to the inward facing surface of the defibrillator housing.