US3431912A - Standby cardiac pacer - Google Patents

Description

March 1969 J. w. KELLER, JR

STANDBY CARDIAC PACER Z of 4 Sheet Filed May '3,

INYENTOR. JOHN WALTER-KEL LER JR.

ATTORNEYS Sheet 3 of 4 March 1969 J. w. KELLER, JR

STANDBY CARDIAC PACER Filed May 6, 1966 m m 01 R W 97 m m E WQL R v E w 0 mm N K T we w 1 R M 55 9mm 8 6 mm E 3 we T @8 m mo N w mm 3 m A. a 8. NE H ME 5. 9% E M KP N I l m 9mm J 5 mm Q H 5 T 4 1: m0 Em w O\ 2oc m S u v we we 5 T 3 5 2m 8 6 N 6 mmwm 0% #78 vm 8 mm my 81% @m mix 2% Em E cmEownq March 11, 1969 J- W. KELLER, JR

STANDBY CARDIAC PACER JOHN WALTER KELLER,JR.

ATTORNEYS United 1 States Patent 4 Claims ABSTRACT OF THE DISCLOSURE A cardiac pacer providing electrical stimulation to a defective heart only in the absence of natural heart beats includes an electrode implanted in the heart which serves both to deliver artificially-generated pulses to the heart and to detect pulses of natural or artificial origin. The pacer includes inhibiting circuitry which responds to a signal originating from a natural heartbeat by preventing the producton of an artificial stimulus for a period of time at least equal to the normal interpulse interval of the pacer or, alternatively, by advancing the interpulse timing of the source so that an artificial pulse is immediately generated and then prevented from reaching the heart. The source then operates normally unless another natural heartbeat is detected by the electrode before the interpulse interval has been completed. The pacer also includes circuitry designed to simulate electronically a refractory delay period of about 0.4 second, during which time a detected pulse following a prior pulse is ineffective to activate the inhibiting circuitry described above, since such a subsequent signal normally will not cause the ventricle to contract.

This invention deals with cardiac pacers. With fixed rate pacing, essential when the patients heart is incapable of providing any sensible signals of physiologic origin as a point of departure, the principal object of the invention is continuously to monitor the heart for physiologic signals resulting from spontaneous heart action and, whenever one is sensed, to introduce a temporary pause into the sequence of contraction-promoting stimuli, thus to allow the heart, if it will and can, to respond mechanically and naturally to its own depolarization signal and, at the same time, to ensure that the pacer shall be ready at all times to deliver artificial stimuli when they are needed. Other objects are to simplify and improve the construction of a cardiac pacer and to conserve the power of its battery and so to prolong battery life.

In the normal heart the left auricle accepts oxygenated blood from the lungs and delivers it to the left ventricle which pumps it through the body tissues. At the same time the right auricle accepts deoxygenated blood from the veins and delivers it to the right ventricle which pumps it through the lungs Where the products of the body metabolism, notably carbon dioxide, are removed and oxygen is absorbed. Thus each side of the normal heart operates as a two-stage pump, the two auricles, which together constitute the atrium, contracting simultaneously and the two ventricles contracting simultaneously.

Contraction of the atrium is accompanied and slightly preceded by a transient electrochemical potential known in medical terminology as the P Wave. After an interval known as the AV (atrioventricular) delay of the order of second another electrochemical potential known as the QRS complex normally appears in the ventricle and, in the normal heart, this initiates the contraction of the ventricle. Following each of its contractions the ventricle remains inert and insensitive to electrical influences throughout a period known as the refractory period which endures for about second. Following immedi- Patented Mar. 11, 1969 ately after this refractory period is a vulnerable period of the heart of a few millisecond duration, in which, if an electrical stimulus is applied to the heart, though it does not respond by contraction, it may be damaged.

The specification of a copending application of J. Walter Keller, Ser. No. 548,239, filed May 6, 1966 describes a cardiac pacer having a high degree of flexibility. In the total absence of sensible potentials from the heart of the patient a free-running multivibrator delivers pulses to an electrode imbedded in the ventricle of the heart at a pre-set rate, for example, 70 pulses per minute, which means a period of about 0.85 second. These pulses cause contractions of the ventricle which drive blood through the body. In the presence of a sensible signal originating in the heart, whether in the atrium or in the ventricle, this signal is picked up, amplified, delayed by a period of about second to correspond with the physiological AV delay, and applied to the multivibrator. This signal trips the multivibrator from its OFF state to its ON state, and so acts to advance the next pulse due from the multivibrator on the time scale and so to deliver an output pulse A second after the sensed heart potential. Selection, as between a potential sensed in the atrium of the heart (a P-Wa-ve) and a potential sensed in the ventricle of the heart (a QRS complex) is controlled by a switch which, like the remainder of the apparatus, may be implanted in the patients body but which may be operated from the outside by a magnet. The AV delay is necessary in the case of an atrial signal. In the case of a signal sensed in the ventricle, the simulated AV delay provides that the pacer stimulus shall occur at a time when it does the heart no harm. Additionally, the pacer includes apparatus which simulates the refractory period of the human heart and so prevents tripping of the multivibrator until after the lapse of the refractory delay following the most recent sensing of a heart potential. Thus it prevents stimulation of the heart muscle by two successive electrical signals which may occur too close togetheron the time scale for the patients good.

With that apparatus an electrical stimulus is delivered to the heart with every pulse generated by the multivibrator; regularly at the preset rate when no heart potentials are sensed and when a heart potential, if sensed, occurs within the simulated refractory period, and with some irregularity when a heart potential is sensed in the nonrefractory period and acts to advance the next stimulus on the time scale. Because, even in cases of failure of atrial signals the heart may manifest a spontaneous QRS complex, in response to which it contracts without the assistance of an external stimulus, some of these simuli are unnecessary. Though a stimulus thus delivered to the heart in its own refractory period is ineffective to cause contraction, a long sequence of ineffective or unnecessary stimuli can, at best, do the heart no good. Moreover, every unnecessary stimulus represents a waste of battery power.

The present invention is based on the realization that one of the commonest forms of heat failure is failure of the P-Wave originating in the atrium of the heart to be conducted to the ventricles while one of the last physiological functions to fail completely in the case of a cardiac patient is its own spontaneous QRS complex, in response to which the ventricle contracts naturally. On the other hand, it frequently happens that the natural QRS complexes and the natural contractions that follow them take place at a dangerously low rate: 40 beats per minute, or less.

In accordance with the invention, therefore, and taking, as a point of departure, a free-running oscillator adjusted to deliver pulses to the ventricle, in the absence of sensed signals from the heart, at a regular rate of about 70 pulses per minute, which corresponds to an interpulse interval of 0.85 second, the apparatus picks up any QRS complex from the ventricle which may exist, amplifies it and utilizes it not to deliver a new stimulus to the ventricle, but rather to initiate a new oscillator period while inhibiting delivery of the stimulus. Thus the ventricle is allowed to respond in the normal physiologic fashion to its own QRS complex. At the same time, the sensed QRS complex actuates a refractory delay simulator so that a new signal which might originate in the heart within the refractory delay period is ineffective to alter the period of the oscillator or the regularity of its pulse rate. If the heart should deliver no new QRS complex within the new oscillator period, the oscillator delivers an artificial stimulus to the heart upon its completion. If a second QRS complex should arise within the artificial refractory period, it is of no effect whatever and the next oscillator pulse takes place as described above. If, to the contrary, a new QRS complex arises, and a consequent natural contraction takes place, after the conclusion of the refractory period but before the termination of the new oscillator period it acts again to postpone the delivery of the next artificial stimulus, resetting the oscillator to its time zero, whereupon it proceeds, again, to measure a new period of about 0.85 second.

In one embodiment of the invention the QRS complex starts the retiming operation without causing the oscillator to generate a pulse. Especially when the oscillator is of the multivibrator variety and is constructed of two transistors of opposite conductivity types, in which the ON state is represented by conduction of both transistors for a very brief period such as two milliseconds or less while the OFF state is represented by nonconduction of both transistors, postponement or suppression of the shift serves to conserve battery power. Moreover, since no pulse is delivered from the multivibrator, no steps need be taken to block it from reaching the heart.

Under some circumstances the natural physiologic QRS complexes and the natural contractions of the ventricle in response to them continue for long periods, i.e., hours, days or weeks. Furthermore, any patient who has had the benefit of an implanted cardiac pacer may improve in his condition. Hence, with apparatus of the type described above the oscillator may withhold its pulses for hundreds, indeed for thousands of consecutive periods. This may raise concern in the mind of the attending physician lest, when it is called upon to deliver a new pulse, it should fail to do so. This difficulty may be avoided by arranging, in accordance with a second embodiment, that the sensed cardiac activity, instead of postponing the occurrence of the next spontaneous oscillator pulse, shall advance it on v the time scale, thus to cause the oscillator to generate a pulse and, in so doing, to reset itself and initiate a new period. This pulse must now be blocked from reaching the heart. This is accomplished, in accordance with the second embodiment, by arranging that at the same instant that the oscillator pulse is forced by an external signal, a forward-acting clamp prevents the pulse from reaching the heart as a stimulus. If, to the contrary, a simulus required by the heart has just been delivered to it by the oscillator, then it must be arranged that a QRS complex which might originate too soon after the delivery of the artificial pulse shall not again force such a pulse. In one embodiment, this is accomplished by arranging that the outgoing pulse itself shall actuate a backward-acting clamp which disables the forward-acting clamp and so permits the required artificial stimulus to be delivered to the ventricle of the heart. In another embodiment, advantage is taken of the difference between the wave form of an artificial stimulus and that of a natural QRS complex to reach the same result. In either case, those pulses which the oscillator generates spontaneously, each taking place a full period after the last event, reach the heart; while those oscillator pulses that are forced by the occurrence of a sensed heart potential are blocked from the heart.

The invention will be fully comprehended from the following detailed description of illustrative embodiments thereof taken in connection with the appended drawings in which:

FIG. 1 is a schematic circuit diagram showing a first embodiment of the invention;

FIG. 2 is a group of wave form diagrams referred to in the exposition of FIG. 1;

FIG. 3 is a schematic circuit diagram showing a second embodiment of the invention; and

FIG. 4 is a schematic circuit diagram showing a third embodiment of the invention.

Referring now to the drawings, the central constituent around which the remainder of the apparatus is built is a free-running oscillator pulse source; illustratively, a multivibrator employing two transistors of opposite conductivity types designated Q and Q respectively. This multivibrator is proportional to deliver, in the absence of signals originating in the heart, a sequence of pulses each of about two milliseconds duration at a regular rate of about seventy pulses per minute, i.e., with a period or interpulse interval of about 0.85 second. In the ON state of the multivibrator both transistors are conducting. This state endures for about two milliseconds. In the OFF state both transistors are nonconducting, and this state endures for the remainder of the 0.85 second interpulse interval. The pulse duration is determined by the magnitudes of R and C While the interpulse interval is determined by the magnitudes of C R and R Accordingly, the pulse rate may be altered by changing any of these elements. Each pulse as it is generated by the multivibrator appears across the resistor R and is applied to the base electrode of a transistor Q, to drive the latter into conduction. When it conducts, a voltage drop takes place across a load resistor R and is delivered through a capacitor C to an electrode 1 embedded in the ventricle of the heart. The external electrical circuit is completed through a large-area electrode 2 which may be implanted in the abdominal or chest area of the patient. When the entire apparatus is implanted in the patients body, this electrode may advantageously be constituted of the external case or wall of the apparatus.

A Zener diode Z having a breakdown potential of about 8 volts, connected between the abdominal electrode 2 and the ventricular electrode 1, serves to protect the circuit against damage which might occur, for example, due to electrical stimulation of some part of the patients body either by a stray field or as a consequence of some unrelated medical procedure.

If, as and when a natural QRS complex should be sensed by the electrode 1 embedded in the ventricle, this is applied by way of resistor R and capacitors C C to the input point of a two-stage amplifier of conventional construction comprising transistors Q and Q to which power and needed biases are supplied by batteries 13 -13 The output of Q appears across a resistor R and is applied through a capacitor C to the second stage transistor Q The output of the transistor Q appears across a resistor R and is applied by way of a capacitor C and.

a diode D to the base of a transistor Q The several resistive and capactive elements mentioned above, or some of them, are advantageously proportioned to provide, for a amplifier Q Q a low frequency cutoff of about ten cycles per second and a high frequency cutoff of about two hundred cycles per second. The range between these two cutolf frequencies include all components of interest that are found in a sensed cardiac potential, and excludes spurious signals outside of the range of interest. The collector electrode of a transistor Q is connected by way of a resistor R alone to the base electrode of Q while the collector electrode of Q, is connected by way of a resistor R and a capacitor C to the base electrode of Q Thus the transistor pair Q Q constitutes a monostable or one-shot multivibrator. It remains in its stable state, in which Q is conducting, until driven by the amplified incoming signal to its metastable state in which Q conducts and Q is nonconducting. It remains in its metastable state, throughout which it is insensitive to incoming signals, for a time determined by the magnitudes of the circuit elements, whereupon it returns by itself to its stable state.

The monostable multivibrator Q Q is included to simulate the refractory period of the heart. Accordingly, the magnitudes of the circuit elements are selected to provide for a duration of about second for the metastable state.

Whatever may bethe wave form of the QRS complex as initially sensed, the series-connected capacitors C C C pass only its transient components, and these are of alternately opposite polarities. Hence either the first of these transient components or the second passes through the diode D increasing the potential of the base of Q and causes it to conduct. Regeneration takes place practically instantaneously and cuts off conduction of Q The monostable multivibrator is now in its metastable state, in which it is insensitive, during the refractory delay period which it provides, to any other incoming signal which might occur too soon for the good of the patient.

When the incoming signal from the ventricle trips the multivibrator Q Q from its stable state to its metastable state, it delivers a negative outgoing pulse a which endures until the monostable multivibrator returns, after the simulated refractory delay period, to its stable state. This is converted by the capacitor C which passes only transients, into a negative-going spike b coinciding with the leading edge of the pulse and a positive-going spike c coinciding with its trailing edge. A diode rectifier D properly poled, blocks the second spike and passes the first spike to the control terminals of two switches S S which, since they may be of any desired construction, are shown in highly abstracted manner: two conduction terminals, normally out of contact with each other and a control terminal which, when actuated, establishes electric contact between the conduction terminals. The conduction terminals of the first switch S are connected to the left hand terminal of the capacitor C and to the -7 volt bus, respectively. The conduction terminals of the second switch are connected to the right hand terminal of the capacitor C and to the negative terminal of an auxiliary 7-volt battery B whose positive terminal is connected to the 7 volt bus. With these connections, each momentary actuation of the switches abruptly brings the potential of the left hand terminal of the capacitor C to 7 volts and its right hand terminal to l4 volts. The connection endures only for "a few microseconds, during the existence of the negative voltage spike c through the capacitor C and is then immediately released.

The operation of the system will be best understood from consideration of FIG. 2 in which the graph A shows, in solid lines, two spontaneous output pulses of the multivibrator Q Q spaced apart on the time scale by a period P, illustratively about 0.85 second. It also shows in broken lines three additional pulses of the same kind which, were it not for the invention, would also be delivered by the multivibrator, each spaced from its predecessor by the same period P, but which are, in fact, suppressed. Finally, it shows a pulse that is not suppressed.

Each time the transistors Q and Q enter their conducting state, thus to generate an output pulse, the capacitor C is rapidly charged to a potential nearly equal to the full potential of the batteries B B of the power supply, its left hand terminal being connected through Q, to the zero-volt bus and its right hand terminal, through Q to the 7 volt bus. This charge brings the base electrode of the transistor Q to a potential close to -7 volts. Thereupon the transistors enter their nonconducting or OFF state, and the interpulse interval commences. Simultaneously with the entry of the transistors Q and Q into their OFF states, the potential of the left hand terminal of the capacitor C falls to 7 volts and that of its right hand terminal, following it, falls to l4 volts. As the interpulse interval, thus started, progresses, the charge of the capacitor C leaks away through the resistors R R R R and R Eventually, as the charge leaks off the capacitor, the potential of the base electrode of Q rises to a point at which it commences to conduct, whereupon regeneration takes place virtually instantaneously, both transistors conduct and a new pulse occurs. The first such cycle of the capacitor potential, commencing with the first spontaneous pacer pulse and ending with the second, is shown in curve D.

Diode D4 merely protects the emitter-base junction of Q, from back voltage stress, thus enhancing Q s reliability.

Suppose, now, that a QRS complex X is sensed by the ventricular electrode 1 after the second pacer pulse and the simulated refractory period which follows it. This signal finds the monostable multivibrator Q Q in its stable state, switches it to its metastable state causing the delivery of an output spike b to the control electrodes of the clamping switches S S Acting through these clamping switches, this signal abruptly restores the potentials of the terminals of the capacitor C now about half discharged, to 7 volts and l4 volts, respectively whereupon, as shown in graph D of FIG. 2, the capacitor C commences to discharge for a third time without switching the multivibrator Q -Q The period of this third discharge embraces the instant at which, had it been otherwise, the third pulse of graph A would have been generated.

Suppose, again, that another QRS complex X is sensed by the ventricular electrode and again after the lapse of the refractory period. As before, the monostable multivibrator Q Q in its OFF state because the refractory period has elapsed, delivers an output pulse when it is switched to its metastable state and, acting through the clamping switches S S abruptly restores the terminals of the capacitor C to their charged potentials of 7 volts and 14 volts. The same events can take place over and over again, two further QRS complexes X and X being shown in curve B.

After the last of these events, suppose that no further QRS complex is sensed until after the lapse of the preassigned interpulse period P. Then, as the discharge of the capacitor C progresses, it reaches the firing threshold of the transistor Q the free-running multivibrator Q Q shifts by itself from its OF-F state to its ON state and a new pulse, shown solid in graph A, is generated and delivered to the ventricle.

Graph C illustratively shows the waves of the hydrostatic pressure which result in the ventricles from these events. The first two of these waves and the last one, denoted 5,, are stimulated by the pacer pulses. The others, denoted N are the natural physiologic consequences of the QRS complexes X X X X The ventricular electrode 1, of course, senses any potential that appears in the heart, whether it be a natural QRS complex or a stimulus delivered by the pacer. But the pacer stimulus, as applied to the blocking capacitor C is of negative polarity, and this endures for the 2 millisecond output pulse of the multivibrator Qe-Qq. The series-connected capacitors C C C partially convert this into two spikes of current which coincide, in time, with the start and the end of the pacer output pulse. Of these, the first is of negative polarity and the second of positive polarity. Consequently the diode D blocks the first spike from tripping the refractory delay simulator Q Q passing only the second one. Thus, operation of the clamping switches S S by a pacer pulse passing around the feedback path is prevented until after the termination of the output pulse, transistors Q and Q being now in their OFF states, in which case actuation of the clamping switches S S is harmless.

In the circuit path through which the capacitor C is discharged, R R and R are connected in series with the resistors R and R The magnitudes of these elements are such that their influences on the interpulse interval are negligible. Rather, R is included to limit the strength of the recharging current, R limits the magnitude of the current entering the base of Q In addition to providing a path for the recharging current, R provides base stabilization of Q While straightforward, the apparatus of FIG. 1 is not without complexity, and, in the illustration, the auxiliary battery B or its circuit equivalent is required. This may be eliminated, without addition of complexity of apparatus, by going to an indirect approach which may be instrumented with the apparatus of FIG. 3. Here the freerunning multivibrator Qg-Qq or other oscillatory pulse source, the refractory delay simulator, i.e., the monostable multivibrator Q Q are as described above. In contrast with the apparatus of FIG. 1, however, each pulse delivered by the monostable multivibrator Qg-Q4 and originating in a sensed QRS complex is applied to the freerunning multivibrator Q Q not in a fashion to postpone its next spontaneous pulse on the time scale, but rather to initiate the retiming operation by advancing the instant of occurrence of the next pulse due. Thus the negative-going output pulse a from the monostable multivibrator Q -Q is applied through the capacitor C directly to the base electrode of the transistor Q being prevented from reaching the zero volt bus by a diode D It acts to drive the free-running multivibrator Q Q into its ON state, thus to deliver a pulse across the resistor R This, of course, takes place only provided the QRS complex actually switches the monostable multivibrator Q Q from its stable state to its metastable state. Since the metastable state endures for about to second, the second one of two consecutive QRS complexes that may occur within less than this period is blocked from the free-running multivibrator Q Q Also, when Q -Q are on as the result of a spontaneous QRS complex, clamp Q turns ON and prevents signals from Q Q from reaching the base of Q Therefore, output stimuli are blocked by spontaneous QRS complexes but the timing cycle of Q -Q is reinitiated.

In the absence of sensed spontaneous QRS complexes, the multivibrator Q Q runs free, and each of its output pulses appears as a voltage drop across the resistor R and is from there passed to the base electrode of Q rendering it conductive through the base-emitter path of Q Current then flows through Q and the load resistor R to develop a voltage drop across the load resistor, and this is passed through a blocking condenser C to the ventricular electrode 1. This voltage drop is, incidentally, applied by way of the feedback path to the two-stage amplifier Q Q and from there via diode, D1 of FIG. 3, to the monostable multivibrator Q Q which, provided it is in its stable state, is immediately switched to its metastable state, thus to deliver a new pulse through the capacitor C This occurs in a matter of a few microseconds so that the pulse reaches the free-running multivibrator Q Q long before its own two millisecond output pulse has terminated and hence, through the action of clamp Q would shut off the output too early. Thus, if a stimulus precedes a QRS complex, Q dominates. If the QRS complex is first in time, clamp Q dominates, preventing output stimuli. In the embodiment of FIG. 3 diodes D and D provide the pacer with equal sensitivities to either plus or minus polarity of the QRS complexes. With this arrangement, a negative signal drives Q OFF and a positive signal drives Q ON. The results of the two actions are alike; i.e., the monostable multivibrator is driven to its metastable state. The consequence of the present arrangement, with its two diodes is that the state of the monostable multivibrator is shifted by an incoming pulse of either polarity, and therefore at the earliest possible moment after the delivery of a stimulus to the heart.

At the same time that the transistor Q is driven into its conductive condition as described above its current,

passing through the base-emitter path of the transistor Q drives the latter, too, into its conductive condition, in which case it holds the base electrode of transistor Q close to the potential of 7 volts. Thus the transistor Q serves as a backward-acting or retrograde clamp to disable the transistor Q which, when not so disabled, operates as a forward-acting or antegrade clamp. Disabling of the antegrade clamp by the retrograde clamp ensures that the spontaneous pulse output from the free-running multivibrator Q5QI1 shall appear as a voltage drop across the resistor R and so permit delivery of a stimulus to the ventricle without impediment.

On the other hand, if a ventricular QRS complex should be sensed within less than A to second of the last preceding QRS complex it will find the monostable vibrator Q Q in its metastable state in which the transistor Q; is in its conducting condition, its emitter circuit being returned to the -7 volt bus through the base-emitter path of the transistor Q The resulting voltage drop between base and emitter of Q turns the transistor Q ON, in which case its emitter-collector path acts as a virtual shortcircuit across the load resistor R This, in turn, holds the base electrode of the transistor Q close to the potential of -7 volts in which case it cannot be driven into conduc tion by an output pulse from the free-running multivibrator.

Thus, each spontaneous pulse generated by the multivibrator Q Q is delivered as a stimulus to the heart while each pulse which is not generated spontaneously but, rather, forced by a sensed heart potential is blocked, thus to allow the heart to carry out its normal physiologic assignment and undergo a natural contraction in response to its own QRS complex. As in the case of the direct approach first described, the result is to stimulate the heart when stimulation is needed, and to withhold stimulation when it is not needed.

The forward-acting and backward acting clamps of FIG. 3 require no more apparatus than do the two recharging clamps S S of FIG. 1. While, in the apparatus of FIG. 3, the free-running multivibrator Q -Q may deliver many more pulses than does the same multivibrator in the apparatus of FIG. 1, these pulses represent but a small power drain on the battery as compared with the power required for the actual stimulus which, in FIG. 3 as in FIG. 1, is conserved when it is not needed. Additionally, advancement on the time scale of the generated pulse permits dispensing with the auxiliary battery of FIG. 1.

The purpose served by the backward-acting clamp Q, is to disable the forward-acting clamp Q and so to guard against the possibility of a stimulus delivered by the pacer to the heart from coming around the feedback path, through the amplifier and the monostable multivibrator Q Q to trip the multivibrator Q6 Q7 and so to block the output pulse before it is ON an effective length of time. The same result can be secured by taking advantage of the Waveform of the outgoing pacer pulse, and by arranging that only its trailing edge, in contrast to its leading edge, shall trip the monostable multivibrator Q Q Thus, by removing the signal path to the base of Q and the diode D which it includes, as shown in FIG. 4, the monostable multivibrator Q Q can be tripped only by the positive going spike which corresponds to the trailing edge of the stimulus pulse. Thus this spike reaches the free-running multivibrator slightly after its transistors Q and Q have been turned OFF, at which instant the base of Q; is at, or very nearly at, zero potentiala higher potential than any that can be derived from the switching of Q so that the fed back pulse is ineffective to alter the state of the multivibrator Qg-Qq- A QRS complex, however, consists of a sequence of pulses of which the polarities alternate. Hence restriction of the switching of the monostable multivibrator Q -Q to positive going pulses is, in the case of a QRS complex, inconsequential.

The invention having now been described, what is claimed is:

1. In cardiac pacer apparatus, a source proportioned to deliver a sequence of electric pulses with an interpulse interval of preassigned duration conformable t the rate of the normal heart beat of a patient, means including an electrode for applying said pulses as stimuli to the ventricle of said patients heart, thereby to initiate contractions thereof, sensing means including said electrode for detecting a physiologic signal originating in said ventricle, inhibiting means whereby said sensed physiologic signal inhibits the delivery of stimuli to said ventricle throughout a period at least equal to said interval and commencing with the sensing of said signal, and blocking means interposed between said sensing means and said inhibiting means, said blocking means being actuated both by a pulse delivered as a stimulus to said ventricle and by a natural physiologic signal originating in said ventricle for preventing a sensed physiologic signal from alfecting the operation of said source until after the termination of a simulated refractory period following the delivery of said stimulus.

2. Apparatus as defined in claim 1 wherein said inhibiting means comprises means, actuated by said physiologic signal, to cause the generation of a new pulse and to initiate a new interpulse interval and auxiliary means, also actuated by said physiologic signal, for blocking said new pulse from said ventricle.

3. Apparatus as defined in claim 1 wherein said blocking means comprises a refractory delay simulator device having a receptive state and an insensitive state, said simulator device being proportioned to be driven from its receptive state to its insensitive state by a physiologic signal, to remain in its insensitive state for a preassigned refractory period and thereupon to return to its receptive state 'whereby, when two consecutive physiologic signals, separated by less than said refractory period, reach said device, it responds by passing the first one to said pulse source and by blocking the second one from said pulse source.

4. Apparatus as defined in claim 3 wherein said simulator device comprises a monostable multivibrator proportioned to rest, normally, in its stable state, means under control of each physiologic ventricular signal for driving said monostable multivibrator to its metastable state, said multivibrator being proportioned to remain in said metastable state for a period less than the interbeat period of a normal heart and thereupon to return to its stable state, and means, operative only throughout the duration of said metastable state, for blocking transmission of an ensuing physiologic signal from said pulse source.

References Cited UNITED STATES PATENTS 3,311,111 3/1967 Bowers 128422 3,345,990 10/1967 Berkovits 128419 FOREIGN PATENTS 826,766 1/ 1960 Great Britain.

OTHER REFERENCES Nathan et al., Progress in Cardiovascular Diseases, vol. 6, No. 6, May, 1964, pp. 538-565 (only p. 542 relied on) 128-419P.

WILLIAM E. KAMM, Primary Examiner.

US. Cl. X.R.

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