Abstract:
Apparatus and method for either manually or automatically initiating the cardioversion of a malfunctioning heart. The apparatus includes a single intravascular catheter electrode system which allows for a much more compact cardioverting system capable of being completely implanted within the patient. The heart function is continuously monitored, and when the function becomes abnormal, the malfunctioning heart is shocked by a voltage of sufficient amplitude to restore the heart to normalcy. If the heart does not return to its normal functions after a given interval, then it is again shocked. Normal heart activity ensures that the shocking mechanism remains inert.

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of U.S. Pat. Application Ser. No. 124,326 filed Mar. 15, 1971, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     During the past several decades, coronary heart disease has come to occupy the first position among the causes of death in the developed areas of the world. In the United States, for example, this disease is responsible for over one-half million deaths yearly. And of this number, more than half occur suddenly, outside the hospital, and therefore before the patient is able to obtain the necessary medical assistance. Although the precise cause of sudden death in coronary heart disease has not yet been entirely clarified, the available evidence permits the medical field to ascribe death in the majority of these cases to grave disturbances in cardiac electrical activity culminating in ventricular fibrillation. 
     The frustrating but related problem is the present inability to deal effectively with lethal and non-lethal arrhythmias outside of a hospital setting. Within the hospital environment, recent experience has clearly demonstrated that ventricular fibrillation and its frequent precursor, ventricular tachycardia, are reversible phenomena when prompt defibrillation of the heart is instituted. Under such circumstances, cardiac function can frequently be restored to normal without the patient suffering from residual disability. Unfortunately, however, the state of the art makes defibrillation very much dependent upon a highly specialized medical environment, thus limiting such treatment to elaborately equipped modern hospitals. 
     There is no question that a great need exists for a defibrillator which could be carried by those who are prone to having one of the many life threatening arrhythmias generally discussed above. Thus, in some patients having coronary heart disease, a fatal outcome from ventricular tachycardia or ventricular fibrillation could be avoided, even in the absence of immediate medical assistance. The first step, of course, is the detection of those prone to suffering from cardiac malfunctions leading to ventricular tachycardia or ventricular fibrillation. 
     While it is not possible to predict with unerring certainty which patients suffering from coronary heart disease will be the victims of sudden death, several high risk groups of patients can be recognized. For example, patients who have experienced myocardial infarction, even though they may be surviving in good health, run a substantial risk of dying suddenly, a risk several times greater than that associated with the general population. Further, if patients with myocardial infarction have a history of serious ventricular arrhythmias and/or of cardiac arrest, or if evidence of persistent myocardial irritability is present, it may logically be assumed that the risk of sudden death is increased substantially. Patients like those described above would greatly benefit from an automatic, standby or demand defibrillator. 
     Also, such an automatic defibrillator would be an asset to those hospitalized patients who have suffered myocardial infarction both in the coronary care unit and those who have been discharged from the well-equipped coronary care unit. Under such circumstances, the defibrillator could be utilized temporarily for the remainder of the expected hospital stay; or the automatic defibrillator could be permanently implanted for use both in the hospital and after discharge. 
     Another recognizable class of patients particularly in need of an automatic defibrillator is the class composed of those who have not shown prior histories of myocardial infarction but who show severe symptoms of coronary heart disease, such as ventricular arrhythmias resistant to medical treatment or angina pectoris. 
     From the brief discussion above, there should be little doubt that the possible applications for an automatic defibrillator are numerous. Such automatic standby defibrillators have been developed and are described in U.S. Pat. No. Re. 27,757 entitled STANDBY DEFIBRILLATOR AND METHOD OF OPERATION and U.S. Pat. No. Re. 27,652 entitled ELECTRONIC STANDBY DEFIBRILLATOR. 
     The automatic standby defibrillators described in the above-identified patents employ a sensing probe carrying one electrode which is positioned in the right ventricle of the heart. A second electrode, separate from and unconnected to the sensing probe, is generally a flat plate either placed on the outer surface of the chest, sutured under the skin of the anterior chest wall or applied directly to the ventricular myocardium. As a result, these systems require much more surgery and, depending on the position of the second electrode, may ultimately be only partially implanted. In addition, such systems require a capacitor of sufficient size and capacity to store approximately several hundreds of watt-seconds, which is the necessary energy required in those systems to produce defibrillation. Therefore, it is evident that an automatic standby or even a manually initiated defibrillating system which requires a much lower energy level, thereby being much smaller and compact in size and which may be totally and completely implanted within the patient, would be highly desirable and would represent a great improvement. 
     Another drawback of the prior art relates to the possibility of developing ventricular fibrillation in the course of correcting, for example, atrial fibrillation. In the prior art, the heart is often brought out of artial fibrillation by delivering an electrical shock across the chest by means of precordial electrodes, or paddles. With such an arrangement, it is necessary to develop a large electrical potential across the paddles, but impossible to centralize the potential across the atria. Accordingly, it is essential to very carefully time the pulses delivered to bring a malfunctioning heart out of atrial fibrillation, or the large shock across the ventricles may induce ventricular fibrillation. This is yet another indication of the need for a low-energy intravascular electrode system capable of accurately delivering electrical pulses to localized areas of the heart. 
     Two other disadvantages of the prior art high-energy cardioverting systems, and the need for improvement, should be noted. First, because of the extreme pain which could be experienced by a patient undergoing high-energy cardioversion, it is the practice to administer a general anesthetic before an attempted cardioversion. With a low-energy intravascular electrode system, the use of a general anesthetic can be avoided. 
     Secondly, the application of high-energy electrical shocks across the chest of a patient carries with it substantial risk of cardiac damage. Again, by way of a low-energy intravascular electrode system capable of localizing defibrillating electrical shocks, the potential risk of injuring the myocardium is considerably decreased. 
     In this regard, efforts have been made to experiment with defibrillators other than that described above. For one, see Hopps et al., &#34;Electrical Treatment of Cardiac Arrest: A Cardiac Stimulator-Defibrillator,&#34; Surgery, Vol. 36, No. 4 (Oct. 1954), at pages 833-849. There, the researchers attempted to bring dogs out of ventricular fibrillation by using an intracardiac electrode carrying two very closely spaced electrodes. The authors concluded that shocks applied through an intracardiac catheter were not effective in cardiac defibrillation. A second example can be seen in recently issued U.S. Pat. No. 3,738,370. There, a single catheter carries a pair of electrodes spaced together so closely as to allow placement of both electrodes in the atrium. Though the patentee alleges effective cardioversion, the energy levels are higher than are actually necessary. The electrode spacing and location are seen to be the reasons. Therefore, to date, the medical field is without a simple, low-energy cardioverting device using a single intravascular electrode catheter capable of treating a wide range of atrial and ventricular arrythmias. 
     SUMMARY OF THE INVENTION 
     The present invention relates to just such a cardioverting device. As used herein, a cardioverter is an electronic system which, after detecting one of the above-noted lethal or non-lethal arrhythmias, reestablishes normal heart function in the heart of the user. &#34;Cardioverting&#34; or &#34;cardioversion&#34; as used herein is intended to encompass the correction of a number of arrhythmic heart conditions, both lethal and non-lethal. Those arrhythmic heart conditions include atrial tachycardia, atrial flutter, atrial fibrillation, junctional rhythms, ventricular tachycardia, ventricular flutter, and ventricular fibrillation, and any other non-pacemaking related arrhythmic condition which may be corrected by applying electrical shocks to the heart. Obviously then, &#34;defibrillation&#34; is included in the term cardioversion as a method of applying electrical shocks to the heart to defibrillate fibrillating atria or fibrillating ventricles. 
     The system of the present invention may be installed in patients particularly prone to develop ventricular tachycardia and/or ventricular fibrillation, or other types of tachyarrhythmias which may be corrected by cardioverting, either on a temporary or a permanent basis. And, because of extremely small and compact size, the system including both electrodes may be totally and completely implanted under the skin of the patient, or alternatively, may be carried externally, save for the sensing probe carrying the two electrodes. 
     More particularly, the present invention relates to a device for reliably sensing the differences between a properly functioning heart and one which has suddenly developed an arrhythmia correctable by cardioverting, and for then delivering a cardioverting shock to the arrhythmic heart. The device is adapted to continue delivering intermittent shocks to the heart in the event that the heart fails to return to its normal behavior pattern, and has the ability of automatically regaining sensing control over a functional heart thereby ensuring that further shocks are inhibited after successful cardioversion has taken place. 
     The standby defibrillator of the present invention has as its basic element, a capacitor capable of storing electrical energy in an amount sufficient to cardiovert a malfunctioning human heart. Upon discharge of this capacitor, a shock is delivered to the heart through the two stimulating electrodes. A sensing probe carries both electrodes, the whole unit forming a single intravascular catheter electrode system. One electrode forms the distal tip of the intracardiac catheter and is positioned within the right ventricle (when dealing with a ventricular arrhythmia) while the second stimulating electrode is spaced from the first electrode and is positioned either within the heart or in the vascular system outside the heart. 
     The capacitor is associated with a sensing circuit connected to the proximal end of the intracardiac catheter and is adapted to respond to a signal recorded at the distal end of the catheter. The signal sensed by the catheter must, of course, be inherently related to some distinctive characteristic of ventricular tachycardia or ventricular fibrillation; and in a specific embodiment of the present invention, the pressure in the right ventricle is sensed. When this pressure falls below a given value, on the order of 5 to 15 mm hg, the heart is malfunctioning and, therefore, the capacitor is discharged into the heart. 
     Between the sensing circuit and the capacitor, means are provided for delaying the repetition of depolarizing discharges for a preset period of time (on the order of 10 to 30 seconds). This delay is essential to give the heart the opportunity to be converter spontaneously to a normal cardiac rhythm, and also to ensure that the abnormal heart conditions are, in face, critical. Only in the absence of a successful conversion is a subsequent shock delivered to the heart. In a particular embodiment of the present invention, the time delay is brought about with the aid of a sawtooth generator, a relay and the charge time of the storage capacitor. 
     The above discussion, for the most part, has been directed to ventricular arrhythmias. It should be remembered, however, that the present invention is also well suited to treat arrhythmias of the atria. When being used in such a mode of operation, one electrode of the catheter is positioned in the right atrium while the other electrode resets outside the heart. 
     Accordingly, an aspect of the present invention is to provide either a manual or an automatic standby cardioverting device having a single intravascular catheter electrode system. 
     Another feature of the present invention is the provision of either a manual or an automatic standby cardioverting device capable of defibrillating a malfunctioning heart extremely rapidly and at relatively low energy levels. This device is extremely compact and includes an electrode system which is totally and completely implantable in the body of a patient. The unit lies dormant during normal heart activity but applies a shock to the heart when the heart functions become abnormal. If the unit is automatic, it reliably senses the difference between a normally functioning heat and one which has suddenly developed abnormal function, and then automatically delivers a cardioverting shock to the heart. Further, the unit is capable of delivering multiple shocks in the event that the heart is not successfully cardioverted by the initial shock; and is designed to automatically regain sensing control over a functioning heart, thereby inhibiting further shocks after successful cardioversion. Still another feature of the present invention is that it may sense conditions requiring either cardioversion or pacing. 
     The particular design of the intravenous catheter is an important aspect of the present invention. First, it is important that the catheter be flexible. Otherwise, the catheter could not safely be inserted and positioned in the heart. Second, it is important that the electrodes on the catheter have a sufficient contact surface so as to effectively discharge the stored energy through the heart. Thirdly, the electrode placement in the heart and, hence, the spacing between the distal and proximal electrodes on the catheter, are important to the effectiveness of the device. In the preferred embodiment of the invention, the intravascular catheter includes at least two spaced electrodes for discharging energy into the heart, with the electrode spacing being such that when the distal electrode is located in the heart cavity undergoing the arrythmia to be cardioverted, the proximal electrode lies outside the heart. 
     Based upon the foregoing preferred criteria, and in light of the well-known average heart sizes, the electrode spacing can be determined. For example, in the average adult, the heart measures about 121/2 cm. from the base to the apex. On the other hand, the length of the right ventricular inflow tract, that is, the internal spacing between the tricuspid annulus and the apex, is approximately 71/2 cm. Thus, taking into consideration the average thickness of the ventricles, the spacing between the base of the right atrium and the superior vena cava is approximately 31/2 cm. See, for example, S. E. Gould, Pathology of the Heart, 2d. Ed., Charles C. Thomas, publisher, p. 95; and S. A. O. Eckner, et al., &#34;Dimensions of Normal Human Hearts,&#34; Archives of Pathology, vol. 88 (1969), pp. 497-507. 
     While it is preferable that an electrode be tailored to the user, based upon his precise heart measurements, useful ranges of electrode spacings can be set out for commercial production. 
     For ventricular cardioversion, the most adjacent distal and proximal electrodes are spaced at least two and one-half to three inches apart. As a practical range, the electrodes would be spaced apart somewhere between two and one-half and six inches apart. More preferably, the spacing would be between three and five inches, and for commercial exploitation, would be between 4 and 41/2 inches. For atrial cardioversion, on the other hand, the electrodes would be spaced at least 21/2 to 3 inches apart, with the maximum electrode spacing again being somewhere between 21/2 and 6 inches, and in the commercial embodiment, would be spaced apart somewhere from between 21/2 and 4 inches. 
     These and other aspects of the invention, as well as many of the attendant advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of the combination sensing probe and electrode structures defining the single intravascular catheter electrode system which forms a part of the present invention; 
     FIG. 2 indicates the catheter system of the present invention positioned in the heart; 
     FIG. 3 illustrates a typical pressure curve for the right ventricle of a normally functioning heart; 
     FIG. 4 is a circuit schematic of one embodiment of an electronic circuit which may be employed in the standby defibrillator of the present invention; and 
     FIG. 5 is a graph of voltage versus time illustrating the operation of the sawtooth generator forming a part of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For purposes of description, the present invention will be explained for use in accomplishing ventricular defibrillation. However, it should be understood that the present invention could be employed for performing cardioversions with respect to other arrhythmic conditions as described above. 
     FIG. 1 shows a single intravascular catheter electrode system 10 which comprises a pacing tip shown generally at 11, a distal electrode shown generally at 12, and a proximal electrode indicated generally at 13. The term &#34;catheter&#34; as used herein is intended to mean an intravascular or transvenous lead either with or without a lumen. The electrode system 10 is designed so that it is protected from the corrosive environment inside the human vascular system, and is sealed so as to be inert to body fluids. Electrical connection is made via a coil spring wire 14, which is completely molded in a silicone rubber, substantially cylindrical casing 15, and which associates with a hollow conductive cap 16 affixed to the extreme distal end of catheter system 10. Cap 16 serves as the electrodes or pacing tip if it is desired to use the catheter system in a pacemaking mode. A coil spring is used so that a stylet may be passed therethrough to facilitate placement of the catheter system into proper position. Cap 16 may, for example, be made of solid platinum irridium. 
     Distal electrode 12 comprises a plurality of spaced, conductive metal rings 17, three being shown in the illustrated embodiment. Metal rings 17 may, for example, be made of solid platinum irridium and fit snugly to prevent lateral movement around casing 15. A spacing 18 is left between adjacent rings 17 in order to provide the electrode with the necessary flexibility for positioning in the vascular system without undue stress and strain. A plurality of rings define electrode 12 to provide sufficient surface area and hence good electrical contact with the heart tissues when the catheter is implanted. All of the rings 17 are electrically connected together in series by a low impedance wire 19. Wire 19 may, for example, be platinum tinsel wire or three serve copper tinsel wire. The wire should be sufficiently conductive to adequately deliver the voltage levels used in defibrillating. 
     Proximal electrode 13 comprises a plurality of spaced-apart conductive metal rings 20, with four being illustrated in the FIG. 1 embodiment. Metal rings 20 may, for example, be made of solid platinum irridium and fit snugly about the molded silicone rubber casing 15. A spacing 21 is left between adjacent rings 20 in order to provide the electrode with the necessary flexibility for positioning in the vascular system without undue stress and strain. A plurality of rings define electrode 13 to provide sufficient surface and good electrical contact with the heart tissues when the catheter is implanted. All of the rings 20 are elecrically connected together is series by a low impedance wire 22. Wire 22 may, for example, be platinum tinsel wire or three serve copper tinsel wire. The wire should be sufficiently conductive to adequately deliver the voltage levels used in defibrillating. Casing 15 seals wires 14, 19 and 22 so as to make them inert from body fluids as well as electrically insulates them from one another. 
     Although the embodiment shown depicts the distal electrode 12 as having three conductive rings 17 and the proximal electrode 13 as having four conductive rings 20, the number of rings of each electrode may be varied. Also the length of each of the rings 17 and 20, as well as the spacings 18 and 21 respectively between them, may vary. The important relationship is that catheter system 10 be sufficiently flexible to be easily positioned within the vascular system, and at the same time have sufficient surface area on each of the electrodes to provide good electrical contact with the heart when catheter system 10 is properly positioned to deliver energy sufficient to cardiovert the heart. In the specific embodiment shown, rings 17 and 20 are each 1/4 inch in length and spacings 18 and 21 are each 3/8 inch. the overall length of catheter system 10 may typically be approximately 60 to 70 cm. 
     Another dimension of importance is the interelectrode distance, d, shown in FIG. 1 between the distal electrode ring 17 farthest from cap 16 and the closest proximal electrode ring 20 thereto. Of course, the required distance will be slightly different from one patient to the next. However, a good average for ventricular cardioversion is between four and four and one-half inches for the interelectrode distance, d. For atrial cardioversion, the distance, d, would be somewhere between 21/2 and 4 inches for the preferred embodiment. 
     A sensing probe 23 may be carried by catheter system 10 if it is used as an element of an automatic cardioverter. The sensing probe comprises a main body portion 24 and, for example, a pressure sensitive bulb 26. Electrical connections to bulb 26 are made at junction box 28. The main body 24 of the sensing probe is in the shape of a flat ribbon, and the body of bulb 26 is spherical. 
     FIG. 2 shows one possible position of catheter electrode system 10 in a heart for effecting ventricular defibrillation. Catheter system 10 is introduced through a peripheral vein, such as, for example, the right jugular vein, by means of surgery very similar to that involved in the implantation of a pacemaking catheter. One very effective position has been found to be where the distal electrode 12 is wedged in the apex of the right ventricle and proximal electrode 13 is immediately superior to the right atrium or just outside the cardiac silhouette in the vascular system. In this position distal electrode ring 17 farthest from cap 16 should not be too close to the tricuspid valve and the proper intro-electrode distance, d, should be provided so that proximal electrode 13 lies just outside the cardiac silhouette. 
     When the catheter system 10 is being inserted in proper position as shown in FIG. 2, cap 16 of pacing tip 11 and electrode 12 are electrically isolated from one another. At this time, therefore, conventional pacemaking signals may be applied between cap 16 and electrode 12 by delivering the appropriate pacemaking signals to coil spring 14 and wire 19, respectively. Since the heart responds favorably to the pacemaking signals only if cap 16 of pacing tip 11 is properly positioned, this method is suitable for checking the position of catheter system 10. The proper location may, of course, be recognized by other methods such as, for example, fluoroscopy or pressure recordings. Once it is determined that catheter system 10 is properly located, it is secured in place and the pacemaking electronics, if unnecessary for the particular patient, are disconnected. The electronic circuit associated with the standby defibrillator of the present invention is then connected to electrodes 12 and 12 via wires 19 and 22, respectively. 
     With reference now to FIG. 3, there is illustrated a right ventricular pressure curve for a normally functioning heart. Only pulses 30 and 32 are illustrated for ease of description, though such pulses repeat at the rate of approximately 60 to 70 per minute in a normally functioning heart. FIG. 3 clearly shows that each pulse has a peak and that these peaks rise above a preset pressure indicated by the dotted line 38. This dotted line corresponds to the threshold between a healthy heart and one which is in need of cardioversion. When the height of the peaks 34 and 36 falls below the pressure indicated by line 38, the malfunction is sensed by probe 23 which, as will be described immediately below, initiates the cardioversion of the heart. 
     With reference then to FIG. 4, one embodiment of an electronic circuit which may be used with the standby defibrillator of the present invention will be described. The electronic circuitry of FIG. 4 may conveniently be broken down into several component parts. The first part is a pressure transducer shown at 40, this pressure transducer being directly associated with the pressure sensing probe 23. The next stage of the electronics is a combination of an amplifier and a sawtooth generator shown at block 42. The amplifier is adapted to amplify the signals received from pressure transducer 40. The sawtooth generator comprises a transistor and a capacitor connected between the collector electrode of the transistor and ground. The signal from the sawtooth generator is then passed to an output amplifier shown at 44, which amplifier in turn feeds its output signal to the base of a transistor associated with the relay stage shown at 46. The relay 46 is normally in its open state condition, but when it is closed, a DC signal is impressed upon a DC/DC converter stage 48. The DC/DC converter 48 boosts the input voltage from approximately 15 volts to the necessary defibrillating voltage. The DC voltage signal from the converter 48 is then fed to a storage capacitor 70 which is associated with a firing circuit, the entire combination shown at 50. When the firing circuit 50 triggers the discharge of the capacitor 70, the necessary defibrillating voltage signal is applied to the electrodes 12 and 13 illustrated in FIG. 1. Therefore, when the sensing probe 23 senses a malfunction in the heart, the capacitor, after a predetermined time delay, shocks the heart with the appropriate defibrillating voltage. 
     Still referring to FIG. 4, but in greater detail, the circuitry associated with the present invention functions as follows. The pressure transducer 40 takes the form of a resistive bridge, one resistor of which is defined by the pressure sensor 26 on the tip of the probe 23. The remaining legs in the bridge are defined by resistors housed in the junction box 28 shown in FIG. 1. The pressure transducer 40 is arranged so that the pressure sensed by element 26 is converted to an electrical signal, the amplitude of which is directly proportional to the sensed pressure. The output from the pressure transducer 40 is fed to a conventional multistage amplifier shown in block 42 which amplifies the received pulses and which then feeds these amplified pulses to the sawtooth generator also in block 42. The trimming potentiometer 52 is provided to balance the inputs to the associated amplifier. 
     With reference now to FIGS. 3 through 5, the operation of the sawtooth generator will be described. the sawtooth generator, if unaffected by the external environment, will have an output curve such as that shown at 54 in FIG. 5. However, if a signal is fed to the sawtooth generator, and if the signal is at least of a predetermined amplitude, then the output voltage of the generator on lead 56 will immediately drop to zero and then again begin to climb. Therefore, if the sawtooth generator receives repetitious pulses of at least the predetermined voltage, then its output will be similar to that shown in FIG. 5 by waveform 58. 
     If the heart functions sensed by the pressure transducer 40 are normal, following the curve shown in FIG. 3, then the amplified signal corresponding to a pulse in ventricular pressure will cause the output of the sawtooth generator to drop to zero. The threshold signal reaching the sawtooth generator can be adjusted by adjusting the amplification factor of the signal amplifier shown in block 42. Amplifier 44 has a threshold which is adjusted by potentiometer 62 so that the amplifier 44 actuates the relay 64 only after approximately 6 seconds of heart malfunction. If, then, the ventricular pressure falls lower than that value indicated by the dotted line 38, and so remains for the preset time interval, the amplified voltage reaching the generator will be insufficient to cause the generator output to drop to zero. Rather, the generator output will follow the curve shown at 54 in FIG. 5, and the threshold of amplifier 44 will be reached. Trimming potentiometer 60 is provided to balance the inputs to the associated output amplifier 44 from lead 56 and potentiometer 62. 
     The output from the amplifier 44 is fed to the relay circuit 46. The relay contacts shown generally at 64 are initially set in the open-circuit condition, thereby isolating the 15 volt source from the DC/DC converter 48. Further, the relay 46 is set to close only after the current passing though coil 66 reaches a predetermined value. With reference to FIG. 5, the voltage output of the sawtooth generator must be at the level 68, the threshold of amplifier 44, before the current in the coil 66 is sufficient to switch the relay 64 into its closed-circuit state. 
     When the relay 64 closes, then the 15 volt source is connected directly to the DC/DC converter 48. From FIGS. 3 through 5, it can be seen that approximately six seconds must elapse, with the heart continuously malfunctioning, before the relay 64 switches from its open-circuit mode to its closed-circuit mode. This will be apparent when one realizes that each &#34;tooth&#34; of the curve 58 corresponds to one peak of the right ventricular pressure response and, as noted above, the peaks of the pressure curve repeat at approximately 60 to 70 per minute. Therefore, absence of a coordinated ECG-ventricular pressure peak must exist for approximately six seconds before input voltage is fed to the DC/DC converter 48. If the heart returns to its normal function at any time during that 6 seconds, then the sawtooth generator output response would drop to zero and the six second cycle would begin again. 
     With the relay 64 closed and a 15 volt or other DC signal appropriate from implantable battery sources being impressed upon the converter 48, an output of sufficient voltage to produce defibrillation appears at the output terminals of the converter 48 when capacitor 70 is fully charged. Simultaneously, this voltage signal is fed to a resistive chain and finally to the base of transistor 72 via a neon tube 74. A silicon controlled rectifier (SCR) 76 is triggered on when transistor 72 becomes conductive. 
     The operation of the firing circuit 50 is as follows: the voltage signal from the converter 48 is fed to the capacitor 70. When the capacitor 70 is fully charged, the transistor 72 becomes conductive, due to the now-conducting neon tube 74. The resistor chains and the tube 74 are interconnected in such a manner that when the voltage across the capacitor 70 reaches the full voltage level from converter 48, then the tube 74 becomes conductive. When the tube 74 conducts, so too does transistor 72 and, therefore, SCR 76. Then, the full defibrillating voltage passes through electrodes 12 and 13 thus shocking the heart with a voltage sufficient to cause defibrillation. 
     As noted above, it is important that a time period elapse between the detection of a heart malfunction and the delivery of the defibrillating shock to the heart. As also noted above, an approximately 6 second delay occurs between the first detection of a malfunction and the closing of relay 64. Then, there is an additional delay, on the order of a few more seconds, which is brought about by the charge time of capacitor 70. That is, when the relay 64 closes, 6 seconds after the initial malfunction, the capacitor first begins to charge. Preferably, less than 20 seconds elapse between the initial sensing of heart fibrillation and the discharge of the capacitor into the heart. Naturally by varying the rise time of the sawtooth generator and the charge time of the capacitor, the twenty seconds may be enlarged or contracted as desired. And, as mentioned above, if at any time during the delay period the heart returns to normal, then the delay period automatically begins again. 
     In recent animal experiments using dogs in which fibrillation was induced by an alternating current electrical signal, defibrillation was achieved successfully in less than 3 watt-seconds at voltages as low as a few hundred volts. Even more recently, success in atrial cardioversion was achieved at an energy level of a mere 0.1 watt-seconds. However, it is possible that effective results at lower energies may be achieved by appropriate selection of the system&#39;s parameters. 
     Above, a specific embodiment of the present invention has been described. It should be understood, however, that this description is given for illustrative purposes only and that many alterations and modifications may be practiced without departing from the spirit and scope of the invention. Just as a few examples, it should be understood that while in the specific embodiment of the present invention, the pressure in the right ventricle is sensed as an indication of heart malfunction, other sensing arrangements may be practiced. Further, a single SCR is used as a triggering device. It is possible to substitute this device for a plurality of SCR units or, alternatively, with a vacuum relay. Still further, while the above description shows a single storage capacitor, a series of capacitors could be employed. It is, therefore, the intent that the present invention not be limited to the above but be limited only as defined in the appended claims.