IL37845A - Cardiac pacer for providing artificial demand cardiac electrical impulses - Google Patents

Description

37845/2 »Va»n o»pm ηρυοκ^ a? asip Cardiac pacer for providing artificial deoaad cardiac electrical impulses MEDCOR, INC. 0/ 35946 FIELD OF THE INVENTION The resent invention relates to cardiac pacers, and in particula electronic, long life implantable demand cardiac pacers .

BACKGROUND OF THE INVENTION Today the lives of thousands of cardiac patients are being maintained and extended by the cardiac pacer which produces arti ficial heart stimulation pulses to the heart's ventricle upon failure of the heart to produce normal rate ventricle contrac¬ tions .

Cardiac pacers can be classified as continuous or demand pacers. The first provides a- continuous sequence of artificial a heart stimulation pulses at a normal rate of 60 to 85 pulses per minute. Demand pacers produce an artificial pulse only in the absence of normal or in the presence of irregular ventricular depolarization causing ventricular contraction* Early examples of demand cardiac pacing systems are shown in patents to Davies, No. 826,766 (British) and Berkovits, No. 3,345,990. These early systems, however, were not basically implantable, lacking light weight, small size, and long life without servicing. The transistor and modern batteries have made the implantable cardiac pacer possible. Examples of necessary to provide a simple and accurate means for detecting energy depletion in the portable energy source or battery.

Additionally, it is desirable in cardiac pacers to regulate the interval between artificially produced pulses to be independent of varying heart load* Prior art designs have been unable to develop circuitry which provides for all of these requirements and desirable features in a single low voltage, redundant battery pacer while maintaining the unit size, complexity and weight of the cardiac pacer at a level compatible with body implantation.

In past designs a simulated electronic refractory period providing a long refractory period to precluded resetting of the timing generator during electrical stimulation and a short period to accommodate premature ventricular contraction has not been available. With only a short simulated refractory period it is possible to alter the timing of the artificial heart stimulating pulse. With only a long simulated refractory period, on the otherjjhand, continuous artificial stimulation can occur in the presence of tachycardia heart beats.

Accordingly, it is a general object of the present invention to provide a demand cardiac pacer of very long implant life and reliability in combination with many sophisticated cardiac pacing capabilities.

It : .s a specific object of the present invention to provide a cardiac pacer having a low voltage redundant portable power supply fo long life and reliability.

It ; s a further specific object of the present invention to proviie a cardiac pacer having a low current consumption.

It is a further specific object of the present invention to provide a demand cardiac pacer having a variable refractory period wHich provides for continuous artificial cardiac pulsing in the presence of high frequency noise but inhibits artificial pulsing at normal heart rates up to and including tachycardia rates.

It is a further specific object of the present invention to provide a cardiac pacer having a low current drain pulse generating means which maintains the pulse interval and duration substantially constant with variations in the pulse load. 4 BRIEF DESCRIPTION OF THE DRAWINGS The Structure and the operation of the present invention will be irbre fully understood by referring to the following description of a preferred embodiment presented for purposes of illustrat on and to the accompanying drawings, of which: Figure 1 is an electrocardiograph waveform of a normal he'art beat; Figure 2 is a schematic diagram with block function indications of a complete demand cardiac pacer; Figure 3 is a schematic diagram of a modified pulse generating portion for a cardiac pacer; and Figure 4 is a schematic diagram of a further modified pulse generating portion of the cardiac pacer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 indicates the electrocardiograph waveform of a normal heart beat of 0.86 second period comprising a P wave produced immediately preceding the contraction of the atrium, a QRS -complex signal causing the ventricle to contract followed by an S-T wave. A refractory period extends after the QRS complex for approximately 0.3 second during which the heart will not respond to artificial stimulus* A vulnerable period of very short duration follows the refractory period and indicates a time when artificial stimulation could cause heart damage.

Referring to Figur 2, there is shown a schematic diagram with broken lines indicating functional blocks of a complete demand cardiac pacer exemplary of the present invention.

In reference to the functional blocks of the Figure 1 circuitry, a low voltage redundant energy source 12 provides electrical operating power to the pacer over high and low (or ground) lines 14 and 16 respectively* Pulse lines 18 and 20 conduct artificial heart stimulation electrical pulses from a pulse generator 22 to the ventricle of a heart load 24 and natural QRS signals from the heart 24 through series capacitances within the generator 22 to a high gain, low current, feedback amplifier 26 ♦ Normally the pulse line 20 is attached to a plate embedded within the body or on an external surface of an implanted pacer while the line 18 actually contacts the vent ricle muscle of a heart 24 via a catheter electrode not shown.

The pulse generator 22 generates pulses of 1 millisecond duration with a preselected time interval (approximately 860 millisecond) elapsing between pulses unless the generator is reset to recommence the time interval. The resetting function is achieved by sensing, amplifying, and processing the signal which re suits on lines 18 and 20 whenever a natural QRS signal or artificial stimulating pulse is produced. i The amplifier 26 receives signals representing the natural QRS signals or artificial heart stimulating electrical pulses, at an input 28 and presents them highly amplified at an output 30. Separate paths of DC negative feedback and AC negative feedback within the amplifier 26 provide stabilization and allow a very high gain in amplifier 26 despite changes in characteristics of the active elements and of the source 12.

The high gain characteristics of the amplifier 26 force it into heavy saturation when the input 28 receives large signals which occur during artificial stimulation. The amplifier 26 is held in saturation for a period substantially longer than the duration of the artificial heart stimulating electrical pulse inputted to it, thereby delaying the last output from the amplifier 26 beyond the termination of each artificial pulse. Because the signals received at input 28 from natural QRS signals are of a lower level than the signals from artificial pulses, saturation of amplifier 26 is minimal and signal delay negligible during natural heart stimulation. This variable saturation delay of amplifier 26 forms a substantial portion of a simulated refractory period and gives this simulated refractory period a variable characteristic* The output 30 of the amplifier 26 is fed to a comparator 32 which produces at an output 34 a unipolar signal representative of the amount by which the electrical magnitude of the output 30 4 of the amplifier 26 exceeds a threshhold surrounding a quiescent level at the output 30.

The unipolar pulse output 34 of the comparator 32 begins the running of a fixed portion of the simulated refractory period. This fixed period is triggered by an inhibiter circuit 36 a short interval after it receives as input the unipolar pulses out of comparator 32, The fixed portion of the simulated refractory period is defined by an inhibit condition of inhibiter 36 during which an output 38 of the inhibiter 36 is disabled. In the short interval between input of a unipolar pulse to inhibiter 36 and the establishment of the inhibit condition, the unipolar pulse appears at the output of inhibiter 36 where it is effective to reset generator 22 and recommence the time interval of generator 22 via a reset circuit 40 between generator 22 and inhibiter 36* The pulse generator 22 has an internal capacitor charge timing circuit 42, the time constant of which determines the time interval between pulses. A capacitive discharge circuit 43 discharges the capacitor resulting in the generation of a pulse and the recommencement of t^ time interval. The reset circuit 40 on receiving a unipolar signal also effects a discharge of the capacitor. The discharge path 43 used in generating a pulse is included' ithin the feedback loop of a monos able pulse width regulator 44 which establishes and regulates the duration of the artificial heart stimulating pulse from the generator 22 and which further isolates the time interval between pulses from . , loading effects due to variations in the load 24 from patient to patient or in the same patient across lines 18 and 20. Finally a push-pull voltage doubler circuit 46 receives the regulated pulse from the monostable circuit 44 and doubles the magnitude of the voltage output so that the pulse present across the lines IS and 20 is nearly twice the magnitude of the voltage supplied by the source 12.

Thus, the pulse generator 22, in the absence of any natural heart beat signals, generates continuously pulses of a preset interval |and width. When a natural beat signal is received, however, the pulse generator is immediately reset to the begin-ning of a1 pulse time interval without the generation of an arti-ficial piiIse, unless that natural pulse occurs within a simulated variable efractory period after the last natural or artificial stimulateng pulse, in which case the pulse generator is not reset The fixe< plus saturation portions of the refractory period totallin approximately 350 milliseconds insure that no signals detected by the amplifier are e fective to reset the pulse gen-erator if they occurred during a long refractory period after an artifici 1 stimulation pulse. The pulse time interval thus runs from the last artificial heart stimulation pulse until at least after this long refractory period.

In tachycardia conditions the heart beats up to 220 times per minute as contrasted to a normal rate of between 60 and 85· It is desirable to preclude any artificial stimulation during this condition and yet to produce continuous stimulation with noise, interference, or beat rate signals above tachycardia present at the input of amplifier 26. The shorter fixed refractory period, approximately 250 milliseconds, allows resetting of, the pulse generator 22 up through 220 beats per minute of natural tachycardia contractions. High frequencies at the input to ampli fier 26 produced by noise, interference, or faster beat rates such as from fibrillation keep the inhibiter 36 in an inhibit condition continuously, prevent reset circuit 40 from receiving any unipolar pulse signals and, allow pulse generator 22 to generate artificial pulses continuously Further features of this invention can be understood by describing in detail the operation of each block of the preferred embodiment of Figure 2.

Turning to this more detailed description, the source 12 further comprises batteries 50 and 52 (typicall two series connected mercury cells each) ♦ Negative terminals of batters 50 and 52 are connected to line 16 and positive terminals are connected respectively to emitters of transistors 54 and 56. The bases of these transistors are circuit grounded to line 16 via a resistance 58, and their collectors are joined at line 14» The low saturation voltage drop between their so that little voltage or energy is lost in them between the batteries 50 and 52 and the lines 16 and 14. If eithe:: battery short circuits or open circuits, however, the transistors 54, 56 operate as an OR gate so that the lines 14 and 16 cbntinue to be fed by the one remaining good battery. A capacitor 59 across lines 14 and 16 provides transient isolation.

The amplifier 26 receives at input 28 both the QRS complex signals from line 18 as well as the signals resulting from artificial heart stimulating electrical pulses produced by the pulse generator 22. Line 28 feeds a two stage DC coupled amplifier composed of first stage NPN transistor 60 and collector coupled second stage PNP transistor 62. The collector and emitter of transistor 60 are connected to the lines 14 and 16 through high resistances 64 and 66 respectively. The collector and emitter of transistor 62 are connected to lines 16 and 14 respectivel through high resistance 68 and high resistance 70 in series with high resistance 72. The base of transistor 60 is biased by a resistance 74 from the emitter of transistor 62 which also DC stabilizes the amplifier in addition to biasing transistor 60. The emitter of transistor 60 is connected via a capacitance 76 to the junction between resistances 70 and 72 providing AC feedback for the amplifier 26. The emitter of transistor 62 is circuit grounded through a capacitor 71 and normally closed 37845/2 ^ switch 73· In its normally open position, switch 73 adds a series resistor 75 between ground and capaeitor 71.

Uo receipt of a signal at amplifier 26 resulting from an artificial electrical pulse, the amplifier 26 is heavily driven into one saturation state and remains there for a substantial time after the disappearance of the pulse signal at its input. Upon cc-ing out of this saturation state, the amplifier overshoots to saturation in the opposite state and remains there for a substantial period* It produces a pulse at output 3G when coming out of saturation, altogether approximately 130 milliseconds after the input to amplifier 26, Thus the relatively slow recovery from saturation of amplifier 26 provides for a final pulse output substantially delayed by a delay period beyond the last input variation. This- saturation characteristic is imparted to amplifier 26 by the AC and 3X1 feedback in conjunction with a phasing change produced by capacitors 71 and 76 as is understood by those skilled in the art to produce step response overshoot.

Because of the amplifier's limited low frequency response only the QBS complex of the natural heart signal is recognized, and it will not saturate on receiving natural QBS signals because their magnitude is substantially less than the magnitude of signals from artificial stimulation pulses. No amplifier delay is thus produced.

The pulses at output 30, through a differentiating capacitance 78 from the collector of transistor 62, are of Accordingly, the output 30 is fed to the base of an NPN . k transistor 80 and to the emitter of an NPN transistor 82 and has a path to ground through resistance 84. The collectors of., transistc s 80 and 82 are joined and feed through a resistance a PNP transistor 88 , and the high voltage line 14 through a re-. sistance The emitter of transistor 88 contacts the high line 14 throu¾fi a normally closed switch 85 while the collector is connecte to output 34, and, via serially connected capacitance 92 and r¾sistance 94, to the base of transistor 82 and to a re-sistance 96 to ground. Parallel capacitor 8 > resistor 87 and zener di' de 91 connects line 14 to the emitter of transistor 88 in the normally open position of switch 85.

Wheiji a pulse of either polarity appears on output 30 one of the transistors 80 and 82 is turned on, thereby turning on transistor 88 and regeneratively holding transistors 82 and 80 on for a peitiod specified by the time constant of capacitance 92 and resistanpes 94 and 96.

Transistors 80 , 82 and 88 are in the off condition except during a pulse output from amplifier 26 and no bias current is drawn by the comparator 32 except during an output from arapli- 26 fier -3£¾ thereby minimizing the current drain of comparator 32 * Forward conduction voltages of transistors 80 and 82 establish a partial noise discriminating threshhold.

The output 34 of comparator 32 is fed to the inhibiter, 36 . consisting of a conduction path from the output 34 through.a ser- ies diode 98 and resistance 100 to both a capacitance 102 and a resistance 104 which in turn leads into a grounded emitter NPN . transistor 106. A variable resistor 107 from ground to the base to transistor 106 is provided to discharge capacitor 102 through the cutoff point of transistor 106 and to adjust the refractory period. Similarly, from the output 34 a high value resistance 108 conducts to the collector of transistor 106 from which the output 38 of the inhibiter 36 is taken.

As thus designed, the inhibiter 36 operates to pass pulses from the output 34 of the comparator 32 to the reset circuit 40 via the output 38 until the transistor 106 has been turned on, at which time the output 38 becomes grounded. Thus the first pulse from the comparator 32 both passes to the reset circuit 40 and charges the capacitor 102 to the point where the transistor 106 turns on and inhibits further pulses from the comparator 32 passing to the reset circuit 40 until the capacitance 102 has dis- resistor 107 charged through resistance 104/and transistor 106, thereby defining the fined portion of the refractory period. Each pulse from comparator 32, in the meantime, though not passing to the reset circuit 40, recharges the capacitor 102 and effectively recommences the timing of the fixed portion.

In perspective then, the amplifier 26 receives signals from both natural QRS ventricular stimulation and artificially generated pulse stimulations. The former are amplified without delay as acting via reset circuit 40 to reset the pulse generator if the inhibiter 36 was not in an inhibit condition when the QRS.. signal occurred. When an artificial stimulating pulse is generated, a pulse signal passes through amplifier 26 with the same results as for the QRS signal except that saturation behavior of the amplifier 26 produces a series of pulses at its output occurring substantially after the passage of the artificial stimulating pulse and retriggering the fixed portion of the simulated refractory period to cause a longer overall duration to the inhibit condition .

A variation in the characteristics of the simulated refractory period is achieved by placing both the switches 73 and 85 in the normally open condition which places resistance 75 between ground and capacitor 71 and places the parallel combination of resistance 87, capacitance 89 and zener 91 between line 14 and the emitter of transistor 88. The addition of resistor 75 in the circuit of amplifier 26 alters the phasing and damping of the closed loop amplifier to prevent pulse step response overshoot which causes saturation with signals from artificially stimulated beats. he capacitance 89 charges each time that the capacitor 102 is charged and discharges through resistor 87 at a slower rate thaj: the discharge of capacitor 102. With each pulse out of amplii'ier 26, the capacitor 102 will be charged to a variable level defending inversely upon the charge remaining in capacitor 89. Witi more rapidly occurring signals at the input to amplifier 26, capacitor 89 discharges less between pulses to,., comparator 32 from amplifier 26 resulting in a lower charge and* voltage on capacitor 102. In turn, the period of inhibition , produced by inhibiter 36 is shorter with more frequently received heart signals.

The simulated refractory period thus varies with the rate at which amplifier 26 receives signals from either natural or artificially stimulated beats. This allows a relatively long simulated refractory period for normal beating rates, either natural or artificially induced, but allows a short simulated refractory period to accompany tachycardia heart beats to prevent a continuous inhibit condition. The zener diode 91 limits the charge on capacitor 89 so that a minimum simulated refractory period is established and so that higher frequency signal or noise received by amplifier 26 will cause a continuous inhibit condition and continuous artificial pulse generation as before.

Of course in any pacer according to this invention, the switches 73 and 85 may be deleted and the pacer constructed with or without the resistors 75 and 87, capacitor 89 and zener 91 to operate with whichever simulated refractory period is desired* It is also clear that capacitor 89 can be connected to line 16 instead of line 14, or that the entire three elements in parallel may be inserted at any point in the charge path of capacitor 102 other than between it and line 16.

A re 3d relay 110 forms a normally open contact between. the output 3i and ground, but may be activated by an external magnet to close the contact grounding the output 38, and temporaril grounding the input of the reset circuit 40.

The reset circuit 40 is composed of PN transistor 112 fed at its b£ se by the output 38. The emitter and collector of the transistc|r 112 are connected respectively to the collector and base of 4 PNP transistor 114 which has its collector grounded and emitier connected to its base through a resistance 116.

Resuitin. ly, when the input of the reset circuit 40 is grounded or lacks a pulse signal, both transistors 112 and 114 are held off and have no effect upon a voltage at the emitter of transistor 114.1 When a pulse comes through the inhibiter 36, both transistors 112 and 114 are turned on effectively grounding the emitter of transistor 114.

Transistors 112 and 114 are off except when resetting the pulse generator and thus contribute to maintaining a low current drain for the circuit. , The emitter of transistor 114 is connected to the ungrounded side of a grounded low value timing capacitance 118. That same side of capacitance 118 is fed from the high line 14 through a variable high value resistance 120 and a fixed high value resistance 122. A combination of resistance 120 and 122 and capacitance 118 forms a low current drain RC charging circuit which can be r. pidly discharged by the reset circuit 40 whenever a pulsej is prreserit at its input to ground the emitter of transistor.114.

The junction between the capacitance 118 and resistance 122 is conne< ted to the emitter of a PNP transistor 124. The base.. and colli!ctor of transistor 124 are respectively connected to the collecto: and base of a grounded emitter NPN transistor 126. A low currlnt drain reference voltage is established by a high resistan e 128 from the high line 14 to the collector of trans-istor 12 and by a high resistance 130 between the collector and emitter f transistor 126. When the voltage across the capaci-tance is increased to equal the voltage at the collector of transistor 126 plus the forward conduction voltage of the emitter-base diode of transistor 124, the capacitance 118 is discharged rapidly through transistors 126 and 124 and a collector rasistance 132 to ground until the capacitance has been thoroughly discharged. During this discharge interval the collector of transistor 126 is grounded producing a negative pulse at the collector of transistor 126 during the discharge.

This pulse is conducted from the collector of transistor 126 through a capacitance 134 and through a resistance 136 to the base of a PNP transistor 138 having its emitter tied to the hig'h line 14. A biasing resistance 140 connects the high line 14 to the junction between capacitance 134 and resistance 136. The collector of the transistor 138 is connected through a feedback The Rpulse which is generated at the collector of transistor 138 and fjasses to the base of transistor 126 is regulated in duration to about 1 millisecond because of timing circuit 134, 136 and jf. 0 and the feedback path from the collector of transistor 138 ija the base of transistor 126 utilizing circuitry already a part o. the pulse generator. This particular feedback config* uration reinforces the on condition of transistors 124 and 126 and contributes to maintaining the width of the pulse at the collector of transistor 138 substantially constant in the face of varying loads upon the collector of transistor 138 * Regulation of the pulse interval is achieved by isolation of the timing capacitance 118 from the output. Pulse duration decreases with source voltage, a feature which provides an additional indication of source condition and distinguishes it from other pacer problems .

Collector of transistor 138 feeds a push-pull voltage doub-ler circuit 46 through a voltage divider composed of resistors 148 and 150 to ground. The connection between these resistances feeds the base of a grounded emitter transistor 152 with its collector connected to one side of a voltage doubling capacitor 154 which has its other side connected to pulse line 18. The collector of transistor 152 is also connected through serially connected capacitance 156 and resistance 158 to the input 28 of amplifier 26 , thereby providing a path between line 18 and the input of amplifier 26. Finally the collector of transistor 152 is connected through resistances 160 and 162 to a high line 164 fed and c†coupled for short circuit protection from line 14 by . serial resistance 166 and shunt capacitance 168. The junction of resistances 160 and 162 is connected through a capacitance 170 . to the base of a PNP transistor 172 having its emitter connected to the line 164 and its collector connected to the pulse output line 20 and further having its base joined to the line 164 through resistance 174. The pulse lines 18 and 20 are connected to ground through respective resistances 176 and 178, and are connected to each other by zener diode 180 to prevent excessive* voltage across them.

During the time that no pulses are generated or received, capacitance 154 charges up to the full voltage between the lines 164 and 16. Then during the pulse interval when capacitance 118 is being discharged, the transistors 152 and 172 connect this capacitance 154 in series with the voltage between lines 164 and 16 across the pulse lines 18 and 20 producing there a pulse of voltage twice the voltage across the source 12.

In addition to the current required to produce a heart stimulation pulse, the pulse generator 22 requires very little ambient current due to the high impedances of reference resistances 128 and 130 and charge resistances 120 and 122 along with capacitance 118, and due to the off condition of transistors 124, 126, 138, 152 and 172 except during a pulse when they are saturated and dissipate little energy. emitter connected to the line 164. The collector of transistor 192 is connected to the pulse line 20 and via resistance 19 to ground. A resistance 196 connects the other output pulse terminal 18, to ground and a zener diode 198 connected between the lines 18 and 20 provides over voltage protection. The collector of transifstor 192 is also connected to the feedback network composed of resistance 144 and diode 146 preferably without capacitor 142 and also is connected through resistance 200 to the base of transistor 184.

The operation of the circuitry of Figure 3 is similar to that of Figure 2 but the same function is achieved more reliably at lower weight and power consumption by having one fewer transistor. During the standby state when no pulses are generated or received the capacitance 186 charges up to the full voltage of source 12. Then during a pulse the transistors 184 and 192 switch this capacitance in series with the source 12 voltage across the output pulse terminals 18 and 20 providing there a voltage pulse of twice the voltage from source 12* Figure 4 shows a further modified embodiment of Figure 2. The charge and discharge circuits for the capacitance 118 are basically the same as shown in Figure 1 with the exception that the collector of transistor 126 is connected to the base of transistor 124 through a parallel combination of a low forward voltage diode 202 and capacitance 204 with the diode 202 oriented for conduction from base to collector. A resistance 206 connects the high line 14 to the collector of transistor 126 and a pulse conducting capacitance 208 conducts from the collector of transistor 126 through a resistance 210 to the base of a PNP transistor 212 having its emitter connected to the line 14, and its collector connected through a resistance 214 to ground. The pulse line 20 is connected to the collector of transistor 212 and the pulse line 18 is connected through a voltage doubling capacitance 216 to the collector of transistor 126, The feedback network of components 144 and 146 is connected from the collector of transistor 212 to the base of transistor 126. A capacitance 218 may be provided for connection from the collector of transistor 126 to the input 28 of the amplifier 26. A resistor 220 connects line 14 to the junction between resistor 210 and capacitor 208.

During standby conditions of no pulse generation or reception the capacitance 216 charges up to the full voltage between lines 14 and 16 through the load across terminals 18 and 20.

Then during the pulse state, when capacitance 118 is being discharged, the transistors 126 and 212 connect capacitance 216 in series with the source 12 across the load 24. The diode 202 prevents the resistance 206 from effecting the very high impedance reference impedances 128 and 130 during standby condition. The diode can be replaced by an FET, or be any low resistance . device conducting only during discharge of capacitor 118. Hedundanj 37845/2

Claims (26)

1. A cardiac pacer which is implantable for providing artifi- v and having cial demand electrical impulses/ resettable generating means for generating an artificial heart stimulating pulse a predeter- ,and mined interval after the last reset thereof/ detecting means for detecting a signal representative of artificial and natural heart stimulation pulses: said device further comprisi resetting means for resetting said generating means in response to a detected natural heart stimulation signal; said generating means being adapted to reset itself upon the generation of an artificial heart stimulation pulse; inhibiting means for inhibiting said resetting means from resetting said generating means for a period after the detection of each natural and artificial heart stimulation signal ; a portable supply source of redundant electrical energy having a voltage output substantially below the necessary voltage magnitude for artificial heart stimulation; and amplifying means for augmenting the voltage magnitude of the stimula ing pulse above the source voltage.

2. Apparatus according to claim 1 wherein said generating means is further characterized by having a low current noninductive push-pull voltage doubler output circuit whereby the magnitude of the artificial heart stimulation pulse from said generating means is approximately twice the maximum magnitude of voltage from said supply source. 37845/2

3. .Apparatus according to claim 1 wherein said pulse generating means includes means operative to maintain the width of each stimulating pulse and the interval between such pulses substantially constant despite variation in the load on said generating means.

4. Apparatus according to claim 1 further characterized by having a normally off state for the majority of the active elements of said apparatus.

5. Apparatus according to claim 1 wherein said detecting means produces a plurality of output pulses beginning with the detection of each said heart stimulation signal and continuing for a delay period which increases with the level of said detected heart stimulation signal whereby natural heart stimulation signals have a negligible delay period while artificial stimulation signals have a substantial delay period; and said inhibiting means are adapted to provide a fixed period of inhibition after the last output of said detecting means.

6. Apparatus according to claim 5 wherein said detecting means comprises a high gain amplifier having a saturation characteristic whereby the amplifier alternates between two states of saturation in response to each artificial heart stimulation signal, each change in said amplifier into and out of saturation producing an output pulse from said detecting means, the time passing before the final recovery from saturation resulting in the delay of said delay period. 37845/2

7. Apparatus according to claim 6 wherein said detecting means further comprises means for producing a unipolar signal representative of the deviation of the output of said amplifier from a noise threshold surrounding a quiescent state of said amplifier with the output of said means for producing a unipolar signal adapted to recommence the inhibiting period of said inhibiting means.

8. Apparatus according to claim 7 wherein said generating means is further characterized by having a low current push-pull voltage doubler output circuit adapted to produce said artificial heart stimulation pulse at a voltage magnitude substantially higher than the magnitude provided by said supply source, and means for maintaining the width of each artificial heart stimulation pulse and the interval between such pulses at substantially constant values with variation in the load for said artificial heart stimulation pulse.

9. Apparatus according to claim 7 wherein said inhibiting means further comprises a capacitor; means for charging said capacitor in response to a stimulation signal upon the occurrence of each stimulation signal; and means for discharging said capacitor over an interval which defines said inhibiting period, said resetting means being inhibited whenever the charge of said capacitor is above a selected magnitude.

10. Apparatus according to claim 1 wherein the period of inhibition produced by said inhibiting means is variable, decreasing with increasing rates of detected signals to pro 37845/2

11. Apparatus according to claim 10 wherein said inhibiting means in producing the variable period of inhibition is characterized by having a first RC timing circuit means for imparting a stable charge state to said timing circuit, means for applying to said timing circuit a variable change in charge from said stable charge state upon the detection of each heart stimulation pulse, the gradual return to said stable charge state defining said variable period and a second RC timing circuit with a longer time constant cnau saiu I SL circuit, means for imparting a stable charge state to said second timing circuit, means for applying to said second timing circuit, a change in charge from said stable charge state upon the detection of each heart stimulation pulse, and means for decreasing the change in charge applied to said first RC timing circuit with each detected signal in correspondence with the charge deviation of said second RC circuit from its stable charge state.

12. Apparatus according to claim 1 wherein said source has an output voltage magnitude which decreases near exhaustion of energy from said source; said generating means includes means responsive to the voltage magnitude of said source for controlling said predetermined interval to partially represent said output voltage magnitude; and means responsive to activation are provided to cause continuous pulse generating by said generating means .

13. Apparatus according to claim 12 further including means responsive to a magnetic field for causing continuous generations of artificial pulses for source energy depletion deter 57845/2

14. Apparatus according to claim 1 wherein said supply source is further characterized as having redundant batteries of matched characteristics in parallel and a diode device connected from each said battery in the direction of battery conduction to a point forming a terminal of said source.

15. Apparatus according to claim 1 characterized by said generating means including: an astable multivibrator operating at said predetermined interval; a one-shot mul ivibrator producing a pulse of a preselected pulse width which defines the width of said artificial heart stimulating pulse upon a change of state of said astable multivibrator; said one-shot multivibrator having a feedback path including said astable multivibrator with said feedback path adapted to maintain said pulse width substantially constant with variations in the load for said pulse; and said one-shot multivibrator including means operative to isolate the predetermined interval of said astable multivibrator from loading effect.

16. Apparatus according to claim 1 comprising a source of electrical energy providing an electrical potential between first and second terminals thereof; a first capacitor having an associated means for supplying energy from said source to said first capacitor; means for producing a reference signal of potential intermediate the potential of said source terminals; means for discharging said first capacitor in response to the signal on said first capacitor achieving a set relationship to said reference signal; said discharging means including a 378Λ5/2 rectifying device and a capacitor in parallel therewith; said 'rectifying device conducting only during discharge of said first capacitor; a first transistor of one conductivity type; a second transistor of the opposite conductivity type; with the emitter, base, and collector of said first transistor connected respectively to said first capacitor at a first terminal thereof, to a first terminal of said rectifying device for conduction from the emitter of said first transistor through said controlled conduction device and to the base of said second transistor, and with the emitter and collector of said second transistor respectively connected to a first terminal of said source including a connection to a second terminal of said first capacitor and to a second terminal of said rectifying device including a connection to a first junction; said discharging means having said first junction connected to said first terminal of said source during discharging of said first capacitor; a second capacitor connected from said first junction to a first terminal of a load; means for charging said second capacitor to the potential of said source during charging of said first capacitor with said first junction at the potential of a second terminal of said source during discharging of said first capacitor to said load at a second terminal thereof.

17. Apparatus according to claim 1; wherein said supply source has a finite electrical energy and a low voltage to supply power to said generating means and has an output voltage between terminals which decreases near exhaustion of energy from said source; and wherein said generating means includes means for applying a reference voltage intermediate the voltages at said source terminals, a capacitor and associated charging path be 37845/2 capacitor through a semiconduc or junction of an active semiconductor device with the level of the forward voltage of said junction relative to said reference voltage defining a point of capacitor charge at which a discharge cycle begins and at which a heart stimulation electrical pulse is generated, a decrease in the output potential of said source increasing the interval between said heart stimulation electrical pulses.

18. Apparatus according to claim 17 wherein said generating means is resettable in response to a signal to recommence the predetermined interval, means are provided for generating the resetting signal and means are provided for inhibiting resetting of said generating means under the influence of a magnetic field whereby said artificial pulses are generated continuously with an inter-pulse interval indicative of source energy depletion.

19. Apparatus according to claim 1, wherein said detection means includes means operative to reset said generating means upon the detection of stimulation signals, said detecting means further include means operative to inhibit the resetting of said generating means for a variable refractory interval after the occurrence of each detected heart signal, and means for producing said variable interval with a duration varying in response to predetermined characteristics of said detected natural and artificial heart signals to prevent continuous artificial pulse generation during normal and tachycardia heart rates . 37845/2

20. Apparatus according to claim 19, wherein said means for producing said variable refractory interval comprises means for developing a plurality of signals from the detection of each said heart stimulation signal, said plurality of signals spanning a period which increases with the level of the detected heart stimulation signals, means for producing a further portion of said interval upon the occurrence of each of said plurality of signals.

21. Apparatus according to claim 20 wherein: said means for developing a plurality of signals includes an amplifier which saturates in response to detected artifical stimulation signals and has substantially no saturation in response to detected natural stimulation signals and said means for producing a further portion includes an RC timing circuit excited by each of said plurality of signals to leave a stable level of charge to which said timing circuit tends to gradually return; said inhibiting means including means responsive to a range of levels of charge during the gradual return to said stable level for producing a reset inhibition, and to said stable level of charge for producing no reset inhibition.

22. Apparatus according to claim 19, wherein said detecting means in being adapted to inhibit the resetting of said generating means is further adapted to inhibit said resetting for an interval which varies inversely with the rate of occurrence of each detected heart stimulation signals, said interval being longer for normal heart beat rates and shorter for tachycardia whereby artificial stimulation is prevented during tachycardia.

23. Apparatus according to claim 22 wherein: said variable interval producing means comprises a first RC timing circuit means for imparting a stable charge state to said first timing circuit; means for applying to said timing circuit a variable change in charge from said stable charge state upon the detection of each heart stimulation pulse; the gradual return to said stable charge state defining said variable interval; a second RC timing circuit with a longer time constant than said first RC timing circuit and means for imparting a stable charge state to said second timing circuit; means for applying to said second timing circuit, a change in charge from said stable charge state upon the detection of each heart stimulation pulse and means for decreasing the change in charge applied to said first RC circuit with each detected signal in correspondence with the charge deviation of said second RC circuit from its stable charge state.

24. Apparatus according to claim 1 wherein said supply source of electrical energy is portable and has a voltage output substantially below the necessary voltage for artificial heart stimulation and said pulse generating means has a push-pull voltage doubler output which non-inductively generates said artificial heart stimulating pulses at a voltage level approximately twice the voltage output of said source.

25. Apparatus according to claim 24 wherein said generating means is resettable, with each said artificial stimulating pulse being timed to occur said predetermined interval after the last reset of said eneratin means and in the absence of fined by said predetermined rate; and means are provided for sensing natural and artificially generated heart stimulating signals and for resetting said generating means on the occurrence of said stimulating signals; said sensing means being adapted to inhibit resetting of said generating means for a variable simulated refractory period after the last sensed signal .

26. An electronic apparatus for cardiac pacing substantially as hereinbefore described with reference to and as illustrated in Fig. 2 or in Fig 2 as modified by either of Figs. 3 or 4 of the accompanying drawings. For the Applicants