US3908667A

US3908667A - Cardiac pacer

Info

Publication number: US3908667A
Application number: US324331A
Authority: US
Inventors: Robert I Bernstein
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-01-17
Filing date: 1973-01-17
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

An implantable demand cardiac pacer provides a stimulating pulse determined by a free-running multivibrator. The heartbeat is monitored to detect the presence of a naturally occurring R-wave in order to inhibit the generation of the pulse stimulus until one preset interpulse period after the last naturally occurring detected R-wave. A monostable multivibrator is responsive to the detection of the natural R-wave so as to cause the inhibition of the generation of the pulse stimulus provided by the free-running multivibrator to the heart, such as by preventing the passage of the pulse that closes a transistor switch in the output circuit of the pacer so as to prevent the passage of the generated stimulus to the heart or by injecting a charge in the firing circuit of the free-running multivibrator so as to duplicate the end-of-pulse condition of the free-running multivibrator and, thus, resynchronize the firing thereof. Such a stimulator may also utilize a tripolar arrangement of electrodes having a sensing anode, a stimulating anode and a common cathode so as to optimize the placement of the various electrodes with respect to the heart.

Description

United States Patent 1191 Bernstein 1 1 Sept. 30, 1975 1 1 CARDIAC PACER Primary Evuminer-William E. Kamm Inventor: Robert I. Bernstein, 100 Deed-dd Attorney, Agent, or FirmHubbe11, Cohen and Stiefel Dr., Tenafly, NJ. 07670 [22] Filed: Jan. 17, 1973 [57] ABSTRACT An implantable demand cardiac pacer provides a stim- [21 1 App! 324531 ulating pulse determined by a free-running multivibrator. The heartbeat is monitored to detect the presence 52 us CL 12 419 2 422; 307 273; of a naturally occurring R-wave in order to inhibit the 307 315; 32 191; 32 207; 323 214 generation of the pulse stimulus until one preset inter- [51] Int. Cl. A61N 1/36 Pulse Period after the 111st naturally Occurring detected 53 Field f Search 12 419 p 419 R, 42 R-wave. A monostable multivibrator is responsive to 12 422 423; 331/113 307/265 2 8 the detection of the natural R-wave so as to cause the 307/273, 291, 315; 328/58, 191, 193, inhibition of the generation of the pulse stimulus pro- 328/196 207 214 vided by the free-running multivibrator to the heart, such as by preventing the passage of the pulse that [56] References Cited closes a transistor switch in the output circuit of the UNITED STATES PATENTS pacer so as to prevent the passage of the generated 3 278 756 10/1966 Weber 328/207 .Stimu lus the heart or by T 1 in the 3:345:990 10/1967 Bcrkovits 128/419 P mg of the freHumm-g mumvlbrmor so as to 1431912 3/1969 Keller Jr. n [28/419 F duplicate the end-of-pulse condition of the freeg 11/1969 Grembutch 133/419 p running multivibrator and, thus, resynchronizc the fir- 3,528,428 9/1970 Berkovits 128/419 P ing thereof. Such a stimulator may also utilize a tripo- 3 669,120 6/1972 Nielsen 1. 128/419 P lar arrangement of electrodes having a sensing anode, 1 3 7/1972 Coe v 307/315 a stimulating anode and a common cathode so as to 3,673937 7/1972 C016 128/419 P optimize the placement of the various electrodes with 3.693.627 9/1972 Berkovits 128/419 P respect to the heart 3,757,791 9/1973 Bcrkovlts 128/419 P 3,795.247 3/1974 T112118! 128/419 P Claims, Drawing Figures 3. 25 32 1-FQ-M- 42 I8 34 VOLTAGE -o- I Patent Sept. 30,1975 Sheet 3 of 5 FIG. 5.

R-WAVE INHIBITED MODE I v & o \Lzlo 5A. I A A l l I '1 FIG. 5B. a

US. Patent Sept. 30,1975 Sheet4 of5 3,908,667

FIG. 6.

FIXED RATE FIG. 6A.

FIG. 68.

I (T T FIG. 6C.

TIME

U.S. Paten Sept. 30,1975 Sheet 5 of5 3,908,667

CARDIAC PACER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to implantable demand cardiac pacers.

2. Description of the Prior Art Implantable Demand Caridac Pacers are well known. The principle underlying this type of device is to inhibit the generation of a heart stimulating pulse which is supplied to the heart via electrodes when a naturally occurring QRS complex or R-wave is sensed and to pro vide such a stimulating pulse when no such R-wave is sensed. Such prior art devices attempt to provide electrical stimulation of an abnormal heart in a manner which satisfactorily integrates stimulated action of the heart with natural heart beat action. The size and power requirements of such implantable demand cardiac pacers are quite critical as, ideally, the greater the simplicity of the device, the less the chance for failure and the lower the power drain of the device, the longer the stimulator may operate without recharging battery replacement, which is most desirable from both thepatients and physicians point of view. Prior art implantable demand cardiac pacers have not proved satisfactory with respect to their required power consumption and attempts to solve this problem have focussed on longer-life power sources rather than making the circuit design more efficient. These disadvantages of the prior art are overcome by the present invention.

SUMMARY OF THE INVENTION An implantable cardiac pacer operated in the R-wave inhibited demand mode is provided. In the absence of naturally occurring R-waves the cardiac pacer freeruns with a desired preset interpulse period. The occurrence of a natural R-wave inhibits the next electronic stimulus that otherwise would occur and synchronizes the timing circuit of the stimulator so that the first permitted electronic stimulus occurs one preset period after the last detected naturally occurring R-wave. Stimulus inhibition is accomplished either by injection of charge on to a capacitor in the timing circuit which controls the free-running period, and the consequent duplication in the timing circuit of the initial condition which obtains immediately after an electronic stimulus so that the free-running multivibrator which provides the stimulus does not fire and recycles; or by blocking the passage of the free-running stimulating pulse output to the stimulating electrodes. In the event of the occurrence of electromagnetic interference or other phenomenon which simulates excessively rapid R-waves, the inhibition mechanism ceases to function and the cardiac pacer reverts to its free-running mode. In such an instance, such as where the monostable multivibrator which fires in response to the detection of an R- wave is caused to fire at an excessive rate, the freerunning multivibrator becomes isolated from the inhibition circuitry so as to convert the demand cardiac pacer into a fixed-rate pacer whose rate is determined by the timing parameters of the free-running multivibrator, as long as such interference occurs. The same pair of electrodes may be utilized to both sense the occurrence of R-waves from the heart as well as to provide the stimulating pulse to the heart, or the device may be a tripolar device in which a separate sensing anode and stimulating anode are provided together the output stage of the amplifier also serving as the first stage of the monostable multivibratorthereby reducing the burden on the other stages of the amplifier and decreasing the power drain of the stimulator.

BRIEF DESCRIPTION OF DRAWING FIG. 1 is a block diagram of the preferred embodiment of the present invention;

FIG. 2 is a schematic diagram of the embodiment shown in FIG. 1;

FIG. 3 is a block diagram of an alternative embodiment of the stimulator shown in FIG. 1;

FIG. 4 is a schematic diagram of the embodiment shown in FIG. 3;

FIGS. 5 and 6 are graphic illustrations of timing diagrams helpful in explaining the operation of the embodiments shown in FIGS. 1 through 4;

FIG. 7 is a schematic diagram of another alternative embodiment of the stimulator shown in FIGS. 1 and 2 for providing a tripolar stimulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings in detail, and initially to FIGS. 1 and 2, the presently preferred embodiment of the R-inhibited demand cardiac pacer, generally referred to by the reference numeral 10, of the present invention is shown. The pacer 10 is preferably of the type which is implantable beneath the skin of the patient and includes electrodes for preferably connecting the pacer 10 to the heart of the patient in order to provide a stimulus thereto as well as to sense the occurrence of a naturally occurring heartbeat which, in the present instance, is preferably indicated by the occurrence of a QRS complex or R-wave. Preferably, in the presently preferred embodiment one pair of

electrodes

12 and 14 are utilized both for providing a stimulus to the heart and for sensing the occurrence of a naturally occurring R-wave, although, if desired, separate pairs of electrodes can be utilized for each function or, as will be described in great detail hereinafter with reference to an alternative embodiment of the present invention, separate sensing and stimulating anodes may be utilized with a common cathode electrode. In the presently preferred embodiment illustrated in FIGS. 1 and 2, one electrode 12 is preferably placed in contact with the endocardium, such as by means of a pervenous catheter, or if desired, electrode 12 may be connected to the myocardium, such as surgically. The other electrode 14 preferably serves as the indifferent electrode and is preferably located one centimeter or more from electrode 12 such as by the use of a bipolar pervenous catheter or a separate pervenous catheter and is preferably located beneath the skin of the abdomen or chest or at some appropriate point on the myocardium. The location of the

electrodes

12 and 14, should be noted, is preferably conventional.

The pacer 10 of the present invention preferably includes an amplifier means 16 which receives as an input signal thereto the R-wave sensed by

electrodes

12 and 14. The amplifier 16 preferably includes a plurality of transistor amplification or

gain stages

18, 20 and 22, each transistor stage having an emitter, a base, and a collector, 24, 26, 28, respectively, for transistor 18; 30, 32 and 34 respectively, for transistor 20; and 36, 38 and 40 respectively, for transistor 22. As shown and presently preferred, transistors 20 and 22 are PNP transisters and transistor 18 is an NPN transistor. Preferably,

transistor stages

18, 20 and 22 are each connected in a conventional common emitter configuration with respective associated circuit elements utilized in such a configuration and these configurations will not be described in greater detail hereinafter. As shown and preferred in FIG. 2, the energizing source for the pacer of the present invention is D.C. supplied by a conventional battery 42, such as a rechargeable battery of a value capable of providing an output potential sufficient to provide a stimulating pulse to the heart in the absence of the occurrence of a natural R-wave. As shown and preferred in FIG. 2, the

common emitter stages

18, 20, 22 of amplifier 16 are base biased through the connection of their

respective bases

26, 32, 38 to a tap on battery 42 which provides the bias potential thereto. Thus, the

transistor stages

18, and 22 are base biased, these transistors preferably providing three stages of gain for amplifier 16 with transistor stage 22 being the output stage of the amplifier 16.

The output of amplifier 16 is coupled to a conventional monostable multivibrator 44 which is preferably a two stage multivibrator comprising a pair of transistor stages, transistor 46 and transistor stage 22 which stage 22 performs two functions, one function being, as previously discussed, as the output stage of the amplifier l6 and the other function being as the first stage of the monostable multivibrator 44. The second transistor stage 46 includes an emitter 48 a base 50 and a collector 52, with the base 50 of transistor stage 46 being coupled to the collector 40 of transistor stage 22 through a capacitance 54 and a load resistor 56.

The pacer 10 of the present invention also preferably includes a free-running astable multivibrator 58 having a pair of

transistor stages

60 and 62, transistor stage 60 preferably being an NPN transistor and transistor stage 62 preferably being a PNP transistor. Transistor 60 preferably includes an emitter 64, a base 66 and a collector 68 and transistor 62 preferably includes an emitter 70, a base 72 and a collector 74. The configuration of the astable multivibrator 58 is preferably operated with an RC time constant provided by a resistor 76 connected between the base 66 and the collector 68 of transistor 60, and a capacitor 78 connected between the base 66 of transistor 60 and the collector 74 of transistor 62. As will be explained in greater detail herein after, astable multivibrator 58 preferably free-runs with a repetition period determined principally by the RC time constant 7678 the voltage across the battery 42, the voltage appearing across the capacitor 78 immediately after an astable multivibrator firing and the values of base-to-emitter cut-in voltages of

transistors

60 and 62. The duration of the astable multivibrator 58 pulse is determined principally by capacitor 78, resistance 76, the impedance seen looking into the collector 74 of transistor 62 and the voltage amplitude of the pulse at the collector 74 of transistor 62. Preferably, the astable multivibrator 58 interpulse period and pulse duration are one second and one millisecond respectively, although a range of variation of both values may be preferably obtained through appropriate choice of the circuit values, such as preferably selecting these values to provide an interpulse period in the range of 0.5 seconds to 1.2 seconds and a pulse with the range of 0.5 milliseconds to 5 milliseconds. Transistor stages 60 and 62 preferably normally conduct during the occurrence of the pulse, such as the one millisecond pulse, and are substantially cut off otherwise.

The pacer 10 of the present invention also preferably includes an output circuit 80 which preferably comprises a two stage arrangement including a pair of transistor stages 82 and 84, each having an emitter, a base and a collector, 86, 88, 90, respectively for

transistor

82 and 92, 94, 96 respectively for transistor stage 84. As shown and preferred, transistor stages 82 and 84 are preferably connected in a Darlington type current amplifier configuration which preferably amplifies the astable multivibrator pulse and provides a low output impedance for the pacer 10.

The monostable multivibrator 44 is preferably coupled to the astable multivibrator 58 through a rate sensitive triggering circuit 98. The rate sensitive triggering circuit 98 preferably includes the output stage 46 of the monostable multivibrator 44 whose collector 52 is coupled through a diode 100-resistance 102 arrangement through a capacitor 104 and therefrom through another diode 106-resistance 108 arrangement to the emitter 110 of a transistor stage 1 14 also having a base 1 16 and a collector 118. The collector 118 of transistor stage 114 is coupled to the base-collector circuits of transistor stages 60 and 62 of the astable multivibrator 58 as well as to the base 88 of output transistor 82. The capacitor 104 is connected across the battery 42 through

resistors

108 and 120 so that the voltage across capacitor 104 is substantially equal to the voltage across the battery 42 before the monostable multivibrator 44 pulse occurs.

As shown and preferred in FIGS. 1 and 2, the pacer 10 preferably also includes an inhibitor circuit 124 which comprises an impedance, such as resistance means 126 connected between the collector 74 of transistor stage 62 and the base 88 of output transistor 82, resistor 126 also being connected in parallel to the emitter 110 of the transistor stage 114. Preferably, the value of resistance 126 is chosed large enough to assure that any positive pulse produced at the collector 74 of transistor stage 62 is insufficient to bring the base 88 of transistor stage 82 into conduction during the presence of the negative voltage applied to the base 88 of transistor stage 82 through diode 106 and when a natural R- wave causes the astable multivibrator 58 to be triggered into firing so as to prevent the pulse produced thereby from actuating the output circuit 80 which comprises transistors 82 and 84. As will be described in greater detail hereinafter, this is accomplished by the negative voltage supplied to the base 88 of transistor 82 through capacitor 104 and diodes 100 and 106 due to the firing of the monostable multivibrator 44. The feedback path from the collector 96 of transistor stage 84 of the output circuit 80 to the emitter 36 of transistor stage 22 through a diode 122 prevents the output stage pulse from actuating the inhibitor circuit 124 prior to the intended completion of the output pulse.

As shown and preferred in FIG. 2, a capacitor 130 is preferably connected across the battery 42 for minimizing the peak pulsatile current drawn from the battery 42. This capacitor 130 may be omitted when such minimization is not desired or required.

OPERATION Referring now to FIGS. 5 and 6 and in addition to FIGS. 1 and 2 and describing the operation of the pres ently preferred embodiment of the pacer of the present invention.

Referring initially to waveform A of FIG. 5, a typical R-wave which is detectable at

electrodes

12 and 14 is graphically illustrated.

If the R-wave has a slope, by way of example, of at least 1/15 volt per second and an excursion, by way of example, of at least one millivolt, it will cause triggering of the monostable multivibrator 44. This criterion, which requires the input waveform to exceed both the minimum slope and a minimum voltage swing, permits the recognition of R-waves while substantially rejecting other physioelectric waveforms. In the absence of naturally occurring R-waves the astable multivibrator 58 comprising transistor stages 60 and 62 free-runs with a repetition period determined principally by the RC time constant 76-78, the voltage across the battery 42, the voltage appearing across capacitor 78 immediately after an astable multivibrator 58 firing, and the values of base-to-emitter voltages of transistor stage 60 and transistor stage 62 at which the astable multivibrators internal loop gain becomes large enough to cause it to fire. The duration of the astable multivibrators 58 pulse is determined principally by capacitor 78, resistance 79, the impedance seen looking into the collector 74 of transistor stage 62, and the voltage amplitude of the pulse at the collector 74 of transistor stage 62. As previously discussed, typical preferred values of interpulse period and pulse duration for the astable multivibrator 58 are one second and one millisecond respectively, although a range of variation of both values may be utilized. Transistor stages 60 and 62 conduct during the duration of the pulse and are substantially cut off otherwise, as can be observed in waveform D of FIG. 5 which shows the signal present at the base of transistor 82 in the R-wave inhibited mode and waveforms A and C of FIG. 6 which show the voltages present at the collector of output transistor 84 and the base of output transistor 82, respectively, in the fixed rate mode which occurs when no R-wave appears prior to the termina' tion of the astable multivibrators free-running period or when interference causes the pacer 10 to revert to free-running operation.

The astable multivibrator 58 preferably produces a positive pulse on the collector 74 of transistor stage 62. This pulse causes a base current to flow in transistor 82 through resistance 126. This current is amplified by transistor 82 and causes saturation of transistor 84. The collector 96 potential of transistor 84 thereupon falls to a low voltage for the duration of the pulse, which is one millisecond in the example given. This voltage decrease is conveyed to electrode 14 through a capacitor 132 so that the electrode 14 is preferably made negative with respect to electrode 12 for the duration of the pulse. Electrode 14 serves as a negative terminal for stimulating the heart and electrode 12 preferably serves as the positive terminal thereof. If a natural R-wave occurs, the waveform it produces being illustrated by waveform A of FIG. 5, it is sensed between the

electrodes

12 and 14 and is conveyed to the input of the amplifier 16 through another capacitor 134 which is connected.

connected to the collector 40 of transistor output stage 22 to which the capacitor 54 is connected in parallel. This waveform is coupled to the base 50 of transistor 46 through capacitor 54. Transistor 46 is normally cut off, but when the waveform is applied to the base 50 thereof, it brings transistor 46 into conduction, which transistor 46 forms the monostable multivibrator 44 with transistor stage 22 and produces a single-shot negative pulse at the collector 52 of transistor stage 46. This waveform is illustrated by waveform B of FIG. 5 which shows the collector voltage present at collector 52. The duration of the monostable multivibrator 44 pulse is determined principally by the value of capacitor 54, the saturated output impedance of transistor stage 22, the descent in collector voltage of transistor stage 22 at the beginning of the pulse, and the values of base-to-emitter voltage of transistor stages 46 and 22 which cause the loop gain of the monostable multivibrator 44 to rise to a sufficient value to initiate the regenerative return to the stable state.

Before the monostable multivibrator 44 pulse, the voltage across capacitor 104 is preferably equal to the voltage across battery 42. At the beginning of the monostable multivibrator 44 pulse, the collector voltage of transistor stage 46 drops to its saturation value, causing the voltage across capacitor 104 to be applied to a series cirucit consisting of diode 100, diode 106, and the impedance seen looking into the emitter of transistor 114. This brings transistor 114 into conduction and reduces the base voltage of transistor 62 so that transistor 62, which had been cut off, comes into conduction. As a result, the astable multivibrator 58 is triggered into firing although the charge on capacitor 78 has not changed sufficiently for it to fire spontaneously. The voltage present at base 66 of transistor stage is illustrated in waveform C of FIG. 5 for the R-wave inhibited mode. The astable multivibrator 58 produces its usual pulse and causes the same charge to appear on capacitor 78 as in a spontaneous firing. The timing of the astable multivibrators 58 free-running period is thereby synchronized with the occurrence of the natural R-wave. If another natural R-wave does not occur prior to the expiration of the astable multivibrators 58 free-running period, the multivibrator 58 will fire spontaneously, and the output circuit 80 will cause a stimulus to appear between

electrodes

12 and 14.

When a natural R-wave causes the astable multivibrator 58 to be triggered into firing the pulse it produces is prevented from actuating the output circuit 80 comprising transistors 82 and 84 by means of the negative voltage supplied to the base 88 (See waveform D of FIG. 5) of transistor 82 from capacitor 104 through diode 106 of the monostable multivibrator 44. As was previously discussed, the value of resistance 126 is preferably chosen large enough to assure that the positive pulse produced at the collector 74 of transistor 62 is insufficient to bring the base 88 of transistor 82 into conduction. The duration of the negative waveform from the monostable multivibrator 44 preferably exceeds the duration of the positive pulse from the astable multivibrator 58 by a sufficient amount to assure that transistor 82 is held cut off and that the output circuit 80 doe not deliver any stimulus to the heart, the energy that would have been delivered by the inhibited stimulus being conserved entirely within the battery 42. The voltage present at the base 66 of transistor stage 60 throughout this interval is illustrated in waveform C of FIG. for the R-wave inhibited mode.

Each time the monostable multivibrator 44 fires, a low resistance path is preferably furnished for the discharge of capacitor 104 into the collector 52 of transistor 46 on one side and on the other side through resistance 108, diode 106 and the circuit elements in series with diode 106. When the monostable multivibrator 44 returns to the cut off condition, capacitor 104 recharges. This recharge path includes resistor 120 which is chosen to have a very large resistance value so that the recharge time constant is preferably much longer than the discharge time constant. If successive firings of the monostable multivibrator 44 occur too rapidly, there will only be time for the voltage across capacitor 104 to reach a small value prior to each firing. This diminished voltage can be less than the combined cut-in voltages of diode 100 and 106, and the base 116 of transistor 114, so that the rapid firing of the monostable multivibrator 44 will not trigger the astable multivibrator 58. The astable multivibrator 58 then is isolated from the circuits on the cathode side of diode 106 and free-running operation occurs. In this manner, the demand pacer is converted into a fixed rate cardiac pacer, as illustrated in waveforms A, B and C of FIG. 6, in the presence of interference that causes the monostable multivibrator 44 to fire too rapidly. The interference rate, such as above 5 pulses/second by way of example, at which this conversion occurs is preferably established by the selection of the circuit values that determine the discharge and recharge rates of capacitor 104. Waveform A of FIG. 6 illustrates the voltage present at the collector 96 of transistor 84 in this fixed rate mode, waveform B of FIG. 6 represents the voltage present at the base 66 of transistor 60 in the fixed rate mode, and waveform C of FIG. 6 represents the voltage present at the base 88 of transistor 82 in the fixed rate mode.

After each firing of the monostable multivibrator 44, a charge is left on capacitor 54 which causes the base 50 of transistor 46 to be negative with respect to its emitter 48, tending to prevent a signal from amplifier stage 22 from bringing transistor stage 46 into conduction, and thereby producing a refractory period in the responsiveness of the pacer 10 to signals received be

tween electrodes

12 and 14. This charge leaks off capacitor 54 with a time constant determined by capacitor 54, resistance 56 and the output impedance of stage 22. By varying these parameters, the duration of the refractory period can be established as desired.

As was previously mentioned, it is not necessary that the same pair of

electrodes

12 and 14 be utilized both for stimulation and the reception of natural R-waves. If the same electrodes are employed for both purposes, as in the embodiment described above, or if different electrodes are employed but a substantial portion of the stimulus appears at the receiving electrodes, it is preferred that degeneration of the stimulus be prevented. Unless precautions are taken, such degeneration will occur through the output stimulus appearing at the input of the amplifier 16 and causing the monostable multivibrator 44 to fire. This would cause a negative voltage to be applied to the base 88 of transistor 82 and would cut off the conduction of the output circuit 80 comprising transistors 82 and 84 prior to the intended termination of the stimulus. This undesired result is preferably prevented by applying a gate signal to the amplifier 16 to obstruct its operation until the intended termination of the stimulus. This gating signal is obtained from the collector 96 of the transistor 84 and applied through diode 122 to the emitter 36 of the amplifier output stage 22.

ALTERNATIVE EMBODIMENT Referring now to FIGS. 3 and 4 as well as FIGS. 5 and 6, an alternative embodiment 10a of the cardiac pacer 10 of the present invention will now be described. For purposes of explanation, like functioning components will be indicated by identical reference numerals with those utilized in the description of FIGS. 1 and 2 followed by the letter a. The operation of the alternative embodiment shown in FIGS. 3 and 4 is substantially identical with that previously described with reference to the embodiment showed in FIGS. 1 and 2 with the exception that in the preferred embodiment of the pacer 10, the astable multivibrator 58 is caused to fire when a natural R-wave occurs and the inhibitor circuit 124 prevents this firing from actuating the output circuit although not interfering with the firing of the astable multivibrator 58, this causes the charge placed on the capacitor when determined the free-running period of the astable multivibrator 58 to be fixed solely by the parameters of the astable multivibrator 58. In this alternative embodiment of the pacer 10a, however, as will be described in greater detail hereinafter, charge injection is utilized to prevent the astable multivibrator from firing when an R-wave occurs.

Except for the differences to be described in greater detail hereinafter, the amplifier stage 16a and the monostable multivibrator stage 44a are preferably substantially identical. As described with reference to the preferred embodiment 10 illustrated in FIGS. 1 and 2, the amplifier 16a and monostable multivibrator 44a having a common transistor stage 22a which serves both as the third or output stage of the amplifier 16a and the first stage of the monostable multivibrator 44a. As shown and preferred, the transistor stages of the alternative embodiment of the cardiac pacer 10a are all preferably base biased by means of battery 42a as in the embodiment 10 described with reference to FIGS. 1 and 2.

Transistor stages

18a, 20a and 22a are all preferably common emitter stages as in the presently preferred embodiment 10. The pacer 10a also preferably includes an astable multivibrator 58a comprising transistor stages and 152 each having an emitter, a base and a collector, 154-156-158, respectively for transistor 150, and -162-164 respectively, for transistor 152. The astable multivibrator 58a configuration is essentially conventional with the collector 158 of transistor 150 being coupled to the base 162 of transistor 152. the output of the astable multivibrator 58a, which is provided at the collector 164 of transistor 162, is cou- -pledto the output circuit 80a which preferably consists of an output transistor 66 having an emitter 168, a base and a collector 172. Collector 164 of transistor 152 is coupled to the base 170 of output transistor 166. The monostable multivibrator 44a is coupled to the astable multivibrator 58a through an inhibiting charge injection circuit 174 comprising a parallel RC arrangement of a resistor 176 and a capacitor 178 connected to the collector 52a of the second stage 46a of the monostable multivibrator 44a, and a parallel diode 180-resistor 182 arrangement, with the diode 180 connected to the base 156 of transistor 150 and in parallel to the collector 164 of transistor 152. The inhibition charge is injected onto a timing capacitor 184 which is connected between base 156 of transistor 150 and a resistance 190 which is connected to collector 164 of transistor 152. v

In the absence of natural R-waves, the astable multivibrator 58a formed by

transistor stages

150 and 152 free-runs with a repetition .period determined principally by an RC time constant provided by capacitor 184 and a resistance 186 connected between the battery 42a and the base 156 of transistor 150, the voltage at the point in battery 42a to which resistor 186 is connected, the voltage appearing across capacitor 184 immediately after a multivibrator 58a pulse, and the values of base-to-emitter voltages of

transistors

150 and 152 atwhich the multivibrators 58a internal loop gain becomes large enough to cause a new pulse. The duration of the pulse is determined principally by capacitor 184,

resistors

190 and 192 which are connected in parallel to capacitor 184, the output impedance at the collector 164 of stage 152, and the voltage amplitude of the pulse at the collector 164 of transistor stage 152. As was previously mentioned with reference to the embodiment shown in FIGS. 1 and 2, by way of example, typical preferred values of interpulse period and pulse duration are one second and 1.5 milliseconds, respectively, although a range of variation of both these values may be achieved through appropriate choice of the circuit components to provide any desired range, such as a pulse duration in the range of 0.5 to 5 milliseconds.

Transistors

150 and 152 conduct during the duration of this pulse and are preferably cut off otherwise.

The astable multivibrator 58a comprising transistors 1 50 and 152 produces a positive pulse at the collector 164 of transistor stage 152, assuming the polarities illustrated in FIG. 4. This pulse, which is supplied to the base 170 of output transistor 166, causes the output transistor 166 to switch from the cut off to the conducting condition, the voltage at the collector 172 of output transistor 166 dropping from the value of the voltage across the battery 42a to a much lower value for the duration of the astable multivibrator 58a pulse. This condition is illustrated in waveform A of FIG. 6 which shows the collector 172 potential and waveform C of FIG. 6 which shows the base 170 potential throughout this interval. This voltage drop at the collector 172 of output transistor 176 is conveyed to electrode 14a through a capacitor 191 connected between the collector 172 and electode 14a, causing electrode 14a to go negative with respect to electrode 12a for the duration of the astable multivibrator 58a pulse. The location of electrodes 12a and 14a is preferably as previously de scribed with reference to

electrodes

12 and 14 for the embodiment shown in FIGS. 1 and 2. The above conditions exist in the absence of natural R-waves. When a natural R-wave occurs, the waveform produced is sensed between electrodes 12a and 14a and is conveyed to the input stage 18a of amplifier 16a through a capacitor 134a which is coupled to the base 26a of transistor stage 18a of amplifier 16a. As was previously mentioned, amplifier 16a preferably comprises three transistor stages of

gain

18a, 20a and 22a, preferably being connected in a common emitter configuration.

The input waveform, which is an R-wave such as graphically illustrated in waveform A of FIG. 5, is amplified in amplifier 16a and this amplified waveform present at the collector 40a of the third or output transistor stage 22a of amplifier 16a appears across resistor 136a connected thereto and is coupled to the base 50a of transistor 46a through capacitor 54a. Transistor 46a is preferably normally cut off, but when the waveform applied to its base 50a brings it into conduction, it forms the single-shot multivibrator 44a together with transistor 22a and produces a single negative pulse, assuming the polarities as illustrated, at the collector 52a of transistor 46a. The duration of this single-shot. pulse is determined principally by the value of capacitor 54a, the saturated output impedance of stage 22a, the swing in collector voltage of stage 22a at the beginning of the single-shot pulse, and the value of base-to-emitter voltage of stage 46a which causes the collector current of transistor 46a to decrease from its saturation value.

Before the occurrence of the single-shot multivibrator 44a pulse, the voltage across the capacitor 178 is established by a voltage divider network comprising resistor 200 connected to the collector 52a of transistor 46a, resistor 176 connected parallel with capacitor 178 and resistor 182. At the beginning of the single-shot pulse, the collector voltage of transistor stage 46a drops to its saturation value, causing the voltage across capacitor 178 to be applied to the series circuit consisting of diode 180, capacitor 184, and resistor 192. This brings diode 180 into conduction and injects a charge onto capacitor 184 such that the side therof closest to diode 180 is negative with respect to the opposite side thereof and the base-to-emitter voltage of transistor is brought to the same value as immediately after the termination of a pulse produced by free-running operation of the astable multivibrator

58a comprising transistors

150 and 152. In this manner, a charge is injected onto capacitor 184 which duplicates the endof-pulse or initial condition obtaining upon the completion of one free-running pulse. A stimulus pulse will then not occur until the subsequent expiration of one free-running interpulse period; however, if another R- wave occurs prior thereto, the charge on capacitor 184 will again be restored to the post-pulse initial condition without the occurrence of a stimulus. The ,voltage present at the base 156 of transistor 150 during this interval is graphically illustrated in waveform E of FIG. 5. The sensed R-wave thereby causes the inhibiting of the stimulating pulse and synchronizes the timing of the following stimulating pulse. In addition, the energy that would have normally been delivered to the heart by the stimulating pulse, if it were not inhibited, is conserved within the battery.

Each time the single-shot multivibrator 44a fires, some charge is removed from capacitor 178. Under normal conditions, this charge is replenished during the interval between R-waves. However, if external electromagnetic interference or other phenomena cause the single-shot multivibrator 44a to fire with sufficiently excessive rapidity, the charge on capacitor 178 will not be adequately replenished and diode 180 will not be caused to conduct by the firing of the monostable multivibrator 44a. Therefore, the balance of the circuitry connected to the output side of diode 180, that is astable multivibrator 58a and the output circuit 80a, will not be affected by the circuitry connected to the input side of diode 180 and will be effectively isolated therefrom, the pacer 10a thereby reverting to its freerunning mode of operation, such as illustrated in waveforms A, B andC of FIG. 6 which graphically illustrate the voltage at the collector 172 of output transistor 166, the voltage at the base 156 of transistor 150 of astable multivibrator 58a, and the voltage at the base 170 of output transistor 166, resepectively, both in the condition when no R-waves are present as well as the condition when excessive interference is present which causes the pacer a to revert to its free-running mode of operation. If it is desired that the first permitted stimulus follow the last R-wave by an interval that is longer or shorter than the free-running period, the circuit values may be chosen so that the charge injected onto capacitor 184 will be accordingly greater or by a predetermined value lesser, than the initial value obtaining upon the completion of a free-running pulse.

After each firing of the single-shot multivibrator 44a, a charge is left on the capacitor 54a which causes the base 50a of transistor 46a to be negative assuming the polarities shown, with respect to its emitter 48a, tending to prevent a signal from the amplifier 16a from bringing transistor 46a into conduction, and thereby producing a refractory period in the responsiveness of the pacer 10a to received signals. This charge leaks off capacitor 54a with a time constant determined by capacitor 54a, resistor 56a connected in parallel therewith, and the output impedance of transistor stage 22a. By varying these parameters, the duration of the refractory period can be determined accordingly.

As was previously mentioned, it is not necessary that the same pair of electrodes be utilized both for stimulation and the reception of R-waves. However, if the same electrodes are employed for both purposes, as in the illustrated embodiment, or if different electrodes are employed but a substantial signal is produced at the receiving electrodes by the occurrence of the stimulus, it is preferred that degeneration of the stimulus be prevented. Unless precautions are taken, degeneration will occur by the stimulus appearing at the input of the amplifier 16a causing a negative voltage swing at the base 1 156 of transistor 150. This negative voltage will truncate the duration of the pulse produced by the astable multivibrator

58a comprising transistors

150 and 152 and interfere with its operation in other ways. In order to prevent such degeneration, a gating signal is preferably applied to the amplifier 16a to obstruct its operation until the termination of the stimulus. This gating signal is preferably obtained from the collector 164 of transistor stage 152, where a positive pulse is produced simultaneously with the negative stimulus at electrode 14a. This positive pulse is applied through a diode 202 connected to the collector 164 of transistor 152 and a capacitor 204 connected thereto to the collector 34a of transistor 20a, and prevents monostable multivibrator

44a comprising transistors

22a and 46a from operating until the conclusion of the stimulus. In this manner, degeneration of the generated stimulus is prevented.

TRIPOLAR ALTERNATIVE EMBODIMENT Referring now to FIG. 7, an embodiment of a tripolar R-inhibited demand cardiac pacer 10b is shown. Such a tripolar arrangement is preferably utilized when it is physiologically desirable to have a common cathode electrode and two separate stimulating and sensing anode electrodes. Physiologically, the presently preferred best stimulating and the best sensing both occur when the cathode is located at the apex of the right ventricle of the heart. Therefore, in such an instance it is preferred to have two different anodes because, for the stimulation, it may be preferred that the anode be located outside the heart with the cathode inside the heart, while for sensing it may be preferred that both the anode and the cathode be within the heart, or, if desired, the reverse may be true. These considerations are normally determined by the geometry of the patients unique naturally occurring electric field which is due to the patients construction, the characteristics of the particular disease if present, or interference conditions if present. The embodiment illustrated in FIG. 7 preferably includes three

electrodes

210, 212 and 214 which comprise the common cathode 210, the stimulating anode 212 and the sensing anode 214, the R-wave being sensed between

electrodes

214 and 210 and the stimulating pulse being provided between electrodes 212 and 210.

The operation of the circuitry illustrated in FIG. 7, as well as the arrangement thereof is preferably substantially identical to that previously described with reference to FIG. 2 with the exception that in place of the use of a common pair of

electrodes

12 and 14 for both sensing and stimulating, three

electrodes

210, 212 and 214 are utilized having a common cathode 210, with one pair 210-214 being utilized for sensing and the other pair 210-212 being utilized for stimulating. In order to insure that electrode 210 is a common cathode, or negative for both sensing and stimulating; as opposed to the polarities illustrated in FIG. 2, each of the transistor stages shown in the embodiment in FIG. 2 is replaced by an equivalent one at opposite polarity, an NPN stage being substituted for a PNP stage, or vice versa, the diodes present in the embodiment of FIG. 2 are reversed in direction, and the polarity of the battery 42b is reversed. Other than that, the operation of the embodiment in FIG. 7 is substantially identical with that previously described with reference to the embodiment shown in FIG. 2, waveforms A through D of FIG. 5 preferably illustrating various waveforms present throughout the pacer 10b in theR-wave inhibited mode except that the waveforms of B, C and D and are replaced by their mirror images so that the voltage rises instead of falls and vice versa. Similarly, waveforms A through C of FIG. 6 graphically illustrate the fixed rate mode of the embodiment illustrated in FIG. 7, such as when no R-waves are detected or when excessive interferrence occurs as previously discussed with reference to the embodiment illustrated in FIG. 2; however, the actual waveforms of the pacer 10b are the mirror images of those illustrated in waveforms A through C of FIG. 6 for the embodiment of FIG. 2 since the operation and construction of the embodiment illustrated in FIG. 7 is substantially identical with that illustrated in FIG. 2 with the exceptions noted above, no further description thereof will be provided, sirnilarily functioning elements in the embodiment of FIG. 7 with those described with reference to the embodiment of FIG. 2 being indicated by identical reference numerals with those utilized in FIG. 2 followed by the letter b. Suffice it to say that the amplifier 16b comprises three transistor gain stages 18b, 20b and 22b, with transistor stage 22b being common to both the amplifier 16b and the monostable multivibrator 44b which also includes a second transistor stage 46b, the transistor stage 22b functioning as both the output stage of the amplifier 16b and the first stage of the monostable multivibrator 44b. The astable multivibrator 58b comprises transistor stages 60b and 62b and the output circuit b comprises a Darlington-type current amplifier connection of

transistor stages

82b and 84b.

As was described with reference to the embodiment illustrated in FIG. 2, the rate sensitive triggering circuit 98b comprises transistor stage 114b, transistor stage 46b,

diodes

122b and 100b, capacitor 104b and resistors l02b and 108b. Thus, when a natural R-wave causes the astable multivibrator 58b to be triggered into firing, the pulse it produces is prevented from actuating the output circuit 82b-84b by the positive voltage, assuming the polarities shown, supplied to the base 88b of transistor 82b from capacitor 104b by the firing of the monostable multivibrator 44b. Resistor 126b is preferably chosen large enough to assure that the nega' tive pulse produced at the collector 70b of transistor stage 62b is insufficient to bring the base 88b of transistor 82b into conduction during the presence of the positive voltage applied to the base 88b of transistor 82b through diode l06b. The duration of the positive waveform from the monostable multivibrator 44b exceeds the duration of the negative pulse from the astable multivibrator 58b by a sufficient amount to assure that transistor 82b is held out off and the output circuit 80b does not deliver any stimulus to the heart. In the absence of natural R-waves, the astable multivibrator 60b-62b free-runs with a repetition period determined principally by the RC time constant 76b-78b, the voltage across the battery 42b, the voltage appearring across capacitor 76b immediately after an astable multivibrator firing, and the values of base-to-emitter voltages of transistors 60b and 62b at which the multivibrators 58b internal loop gain becomes large enough to cause it to fire. The duration of the astable multivibrator 58b pulse is determined principally by capacitor 78b, resistor 79b, the impedance seen looking into the collector 74b of transistor 62b, and the voltage amplitude of the pulse at the collector 74b of transistor 62b. Transistors 60b and 62b conduct during the duration of this pulse and are substantially cut off otherwise. During stimulation, electrode 210 is made negative with respect to electrode 214 during sensing. The balance of the operation and circuitry associated with the ernbodiment shown in FIG. 7 will not be described in any greater detail except to say that where the description of the embodiment shown in FIG. 2 calls for a positive pulse or polarity this term would be replaced by a negative pulse or polarity or vice-versa due to the above described differences in the circuitry of the embodiment shown in FIG. 7. As in the embodiment of FIG. 2, the demand pacer 10b is converted to a fixed rate pacer in the presence of interference that causes the monostable multivibrator 44b to fire too rapidly, such as in excess of 5 pulses/second.

It is to be understood that the above described embodiments of the invention are merely illustrative of the principles thereof and that numerous modifications and embodiments of the invention may be derived within the spirit and scope thereof, such as by utilizing a rechargeable battery of a lower value than required together with a current intensifier, such as a voltage or current doubler or tripler which provides as the output thereof the desired potential value for providing the requisite stimulating pulse as well as for biasing the circuitry.

What is claimed is:

1. A heart pacing apparatus comprising a freerunning means for generating a heart simulating electrical pulse stimulus having a predetermined interpulse period and pulse duration; electrode means comprising means for monitoring the beating action of the heart for detecting the presence of a naturally occuring R- wave and adapted to be positioned relative to the heart for directing said free-running pulse stimulus into the heart for stimulating heartbeat action; coupling means for interconnecting said electrode means and said pulse stimulus generating means; amplifier means operatively connected to said monitoring means for initially amplifying said detected R-wave to provide an initially amplified output signal having a predetermined level therefrom when said R-wave is received; monostable multivibrator means operatively connected to said amplifier means for receiving said initially amplified output signal and generating a single-shot pulse therefrom having a predetermined pulse duration; and means responsive to said single-shot pulse stimulus for inhibiting the generation of said free-running pulse stimulus to said heart until said one present interpulse period after the last naturally occurring detected R- wave, said monostable multivibrator means comprising at least a first transistor means and a second transistor means operatively connected together for generating said single-shot pulse, said first transistor means providing both additional amplification of said detected R- wave in said amplifier means and cooperating with said multivibrator means second transistor means for raising said level of said initially amplified output signal to a predetermined value to cause said multivibrator means to provide said single-shot pulse; whereby said naturally occurring R-wave inhibits the generation of said stimulating pulse and synchronizes the timing of the next succeeding stimulating period.

2. An apparatus in accordance with claim 1 wherein said free-running means comprises astable multivibrator means comprising resistance means and capacitance means, said multi-vibrator means having an associated RC time constant determining said interpulse period, said apparatus further comprising a DC source, said stimulus being dependent on said DC source value.

3. An apparatus in accordance with claim 1 wherein said interpulse period is substantially within the range of 0.5 to 1.2 seconds.

4. An apparatus in accordance with claim 1 wherein said free-running means comprises astable multivibrator means comprising resistance means and capacitance means for providing an associated RC time constant and at least one transistor means having an associated base-to-emitter voltage being present in said transistor means immediately after the termination of said free-running pulse stimulus, said RC time constant providing said interpulse period, said responsive inhibition means comprising means for injecting a charge onto said capacitance means in response to the generation of said single-shot pulse, said charge being equivalent to the charge present on said capacitance means immediately upon the completion of the generation of said free-running pulse stimulus; whereby said free-running pulse stimulus generation is inhibited at least until the subsequent expiration of one free-running interpulse period.

5. An apparatus in accordance with claim 4 wherein said injection means injects said charge onto said capacitance means in response to the generation of said single-shot pulse each time a naturally occurring R- wave is detected, whereby the generation of said freerunning pulse stimulus is inhibited until said one preset interpulse period after the last naturally occurring R- wave.

6. An apparatus in accordance with claim 4 wherein said charge injection means comprises another capacitance means, said capacitance means being operatively connected to the output of said monostable multivibrator second transistor means, said other capacitance means being operatively connected to said associated astable multivibrator capacitance means for injecting said charge onto said capacitance means, said charge having been stored on said other capacitance means during said interpulse period, said stored charged being diminished each time said monostable multivibrator generates said single-shot pulse, said stored charge being replenished during a predetermined minimal charging interval to a predetermined value to enable said charge to be injected onto said capacitance means, said other capacitance means being coupled to said capacitance means through a gating means having a minimum bias potential, said charge not being injected when said stored charge is not replenished to at least said bias potential value, whereby said free-running pulse generation will not be inhibited.

7. An apparatus in accordance with claim 1 wherein said apparatus further includes refractory means interposed between said amplifier means and said monostable multivibrator means for preventing said amplifier output signal from causing the generation of said single-shot pulse for a predetermined refractory period, said refractory period being dependent on an RC time constant associated with said refractory means.

8. An apparatus in accordance with claim 1 wherein said apparatus further comprises means operatively connected to said amplifier means input for applying a gating signal thereto to obstruct said amplifier operation until the termination of a free-running generated pulse stimulus, whereby said monostable multivibrator single-shot pulse generation is inhibited until the conclusion of said free-running pulse stimulus.

9. An apparatus in accordance with claim 1 wherein said free-running means comprises astable multivibrator means comprising resistance means and capacitance means for providing an associated RC time constant, said time constant determining said interpulse period; said capacitance means charging to a predetermined firing value for actuating said astable multivibrator means; said apparatus further comprising rate sensitive triggering means interposed between said monostable multivibrator means and said astable multivibrator means for triggering said astable multivibrator means substantially at the beginning of said generated singleshot pulse irrespective of said astable multivibrator means time constant, said astable multivibrator means generating said stimulating pulse to charge said capacitance means to a value equivalent to said spontaneous firing value when triggered by said rate sensitive triggerin g means, whereby the timing of said astable multivibrator free-running interpulse period is synchronized with the detection of said naturally occurring R-wave.

10. An apparatus in accordance with claim 9 wherein said responsive inhibition means comprises means operatively connected between said astable multivibrator means and said coupling means to inhibit the passage of said free-running stimulating pulse to said electrode means in response to the generation of said singleshot pulse, said single-shot pulse being opposite in polarity to and greater in duration than said free-running stimulating pulse for inhibiting said coupling means from passing said generated free-running stimulating pulse to said electrode means.

11. An apparatus in accordance with claim 10 wherein said coupling means comprises a transistor switch and said passage inhibition means comprises means for biasing said switch in an open condition wherein said electrode means and said free-running multivibrator means are uncoupled.

12. An apparatus in accordance with claim 11 wherein said biasing means comprises means having an impedance of a value to assure that the potential of the output pulse of said free-running multivibrator is less than the level required to being said transistor switch into conduction, said switch coupling said electrode means to said free-running multivibrator only when in said conduction condition.

13. An apparatus in accordance with claim 11 wherein said coupling means further comprises addi tional transistor amplifier means.

14. An apparatus in accordance with claim 13 wherein said amplifier means is a Darlington type current amplifier means.

15. An apparatus in accordance with claim 9 wherein said rate sensitive triggering means includes capacitance means chargeable to a predetermined value to trigger said astable multivibrator means during said single-shot pulse interpulse period, the recharging time constant of said capacitance means being longer than the discharge time constant thereof, said charge being diminished each time said monostable multivibrator means generates said single-shot pulse and being replenished during said interpulse period to a predetermined value to trigger said astable multivibrator means, said triggering value only being obtained when the charging interval is substantially equivalent to at least said interpulse period, said astable multivibrator means being free-running at a predetermined rate having the desired interpulse period.

16. An apparatus in accordance with claim 1 wherein said electrode means comprises a sensing anode operatively connected to said amplifier means, for providing said detected R-wave to said amplifier means a stimulating anode operatively connected to said coupling means for providing said stimulus pulse to said heart and a common cathode connectable to said heart, said stimulating pulse being provided between said stimulating anode and said cathode and said detected R-wave being sensed between said sensing anode and said common cathode, whereby a tripolar R-inhibited demand pacer is provided.

17. An apparatus in accordance with claim 1 wherein said amplifier means comprises a plurality of transistor means having gain.

18. An apparatus in accordance with claim 17 wherein said amplifier means and said multivibrator means first transistor means comprises at least three stages of gain.

19. An apparatus in accordance with claim 17 wherein said apparatus further comprises means for biasing said plurality of transistor means.

20. An apparatus in accordance with claim 17 wherein each of said plurality of transistor means comprises a common emitter configuration.

Claims

1. A heart pacing apparatus comprising a free-running means for generating a heart simulating electrical pulse stimulus having a predetermined interpulse period and pulse duration; electrode means comprising means for monitoring the beating action of the heart for detecting the presence of a naturally occuring R-wave and adapted to be positioned relative to the heart for directing said free-running pulse stimulus into the heart for stimulating heartbeat action; coupling means for interconnecting said electrode means and said pulse stimulus generating means; amplifier means operatively connected to said monitoring means for initially amplifying said detected R-wave to provide an initially amplified output signal having a predetermined level therefrom when said R-wave is received; monostable multivibrator means operatively commected to said amplifier means for receiving said initially amplified output signal and generating a singleshot pulse therefrom having a predetermined pulse duration; and means responsive to said single-shot pulse stimulus for inhibiting the generation of said free-running pulse stimulus to said heart until said one present interpulse period after the last naturally occurring detected R-wave, said monostable multivibrator means comprising at least a first transistor means and a second transistor means operatively connected together for generating said single-shot pulse, said first transistor means providing both additional amplification of said detected R-wave in said amplifier means and cooperating with said multivibrator means second transistor means for raising said level of said initially amplified output signal to a predetermined value to cause said multivibrator means to provide said single-shot pulse; whereby said naturally occurring R-wave inhibits the generation of said stimulating pulse and synchronizes the timing of the next succeeding stimulating period.

5. An apparatus in accordance with claim 4 wherein said injection means injects said charge onto said capacitance means in response to the generation of said single-shot pulse each time a naturally occurring R-wave is detected, whereby the generation of said free-running pulse stimulus is inhibited until said one preset interpulse period after the last naturally occurring R-wave.

9. An apparatus in accordance with claim 1 wherein said free-running means comprises astable multivibrator means comprising resistance means and capacitance means for providing an associated RC time constant, said time constant determining said interpulse period; said capacitance means charging to a predetermined firing value for actuating said astable multivibrator means; said apparatus further comprising rate sensitive triggering means interposed between said monostable multivibrator means and said astable multivibrator means for triggering said astable multivibrator means substantially at the beginning of said generated single-shot pulse irrespective of said astable multivibrator means time constant, said astable multivibrator means generating said stimulating pulse to charge said capacitance means to a value equivalent to said spontaneous firing value when triggered by said rate sensitive triggering means, whereby the timing of said astable multivibrator free-running interpulse period is synchronized with the detection of said naturally occurring R-wave.

10. An apparatus in accordance with claim 9 wherein said responsive inhibition means comprises means operatively connected between said astable multivibrator means and said coupling means to inhibit the passage of said free-running stimulating pulse to said electrode means in response to the generation of said single-shot pulse, said single-shot pulse being opposite in polarity to and greater in duration than said free-running stimulating pulse for inhibiting said coupling means from passing said generated free-running stimulating pulse to said electrode means.

13. An apparatus in accordance with claim 11 wherein said coupling means further comprises additional transistor amplifier means.