CA1067960A - Synchronous pacemaker with upper rate stabilization - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is a synchronous artificial cardiac pacemaker having a pulse generator which includes a timing circuit that, in the absence of any heart activity for a predetermined maximum time interval, provides stimulating pulses at a lower base pacing rate to a ventricular electrode to effect contraction of the heart to maintain a minimal cardiac output level when the heart is asystolic. A first sensing circuit is coupled to the ventricular electrode and a reset circuit is coupled to the timing circuit. Natural ectopic heart beats indicative of ventricular activity are picked up by the ventricular electrode, sensed by the sensing means and applied to the reset circuit which resets the operation of the timing circuit, so that if the ventricles are contracting at a rate exceeding the base pacing rate, the operation of the pulse generator is inhibited. A second sensing circuit is coupled to an atrial electrode and, through a memory circuit and a time delay circuit, to the timing circuit. When atrial heart activity is picked up, the sensing circuit produces a signal that sets the memory circuit and after a time delay, triggers the timing circuit into producing a stimulatig pulse for application to the ventricle in synchronism with the natural atrial contraction of the heart. Therefore, when the heart's natural conduction system frm the sino-atrial mode of the atrium to the ventricles is blocked while atrial activity is normal, the pacemaker synchronously paces the heart. If the atrial rate becomes excessive, the pulse generator circuit operates through an upper rate limit circuit coupled to the timing circuit and the operation of the memory circuit to synchronously pace the ventricles at an average rate not exceeding the upper rate of the timing circuit.

Description

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BACKGROUND OF THE INVENTION
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ield of the Invent on This invention relates to artificial cardiac pacemaker and more particularly, to a cardiac pacemaker which may or may not be implantable within the human body and which will respond to the changing needs of the body but will not compete with the na~ural cardiac electrical activity of any kind.
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S~a~e of the Prior Art .
The implantable cardiac pacemaker~ shown ln U S. Pat.
No. 3,057~356 permits innocuous, painless, long-term cardiac st~mulation at low power levels by utilizing a small, completely implanted transistorized and battery-operated pacemaker connected via flexible electrode wires directly to the cardiac muscle.
Such an asynchronous pacemaker or pacer, while providing fixed-rate s~imulation not automa~ically ch~nged in accordance with the body's needs, has proven efective in alleviating the symptoms o~ complete heart block. An asynchronous pacer, however, has the possible disadvantage of competing with the natural, physiological cardiac pacemaker during episodes of nonmal sinus conduction.
~ A synchronous or P-wave pacer, shown in U.S. Pat. No.
- 3,2533596, has been proposed for producing a stlmulus following --each P-wave or atrial beat. Whe~ the body signals a need for in-reased heart rate, as indicated by an increasing atrial beat, the ;synchronous pacer responds with an increased ventricular s~imu-~lation rate. However, the function-o the known synchronous pacer .
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is not responsive to irregular.ventricular ectopic act~vity and may compete against such beats. Thus, while the synchronous pacer will not compete agains~ normally conducted beats, it can compete against ectopic or abnormally conducted beats. Any competition between the natural and the artificial pacer may be undesirable because such competition may possibly lead to incidents of tachy-.. . .
cardia or even fibrillation.
There has also been developed a ventricular inhibited ordemand-type pacer, shown in U.S. Pat. No. 3,478,746, in which .
case the artîicial stimuli are initiated only when required ~nd subsequently can be eliminated when the heart re~urns ~o : -sinus rhythm above a predetermined base rate. The demand pacer - solves the problem arising in asynchronous pacer by inhibiting itself in the presence of ventricular activity b~t coming "on li~e" and filling in missed heart beats in the absence of ven-tricular activity after a base time in~erval. When the dem~nd -pacer comes "on l;ne", it operates as an asynchronous pacer~ the rate of which is not responsive to atrial activity.
-To-cope with the problems of the synchronous pacer, a --further pacer has been developed, shown in U.S. Pat. No. 3~648,707~
which stimulates the heart asynchronously~in the absence of cardiac elec~rical activity of any kind~ inhibits itsel and becomes complete~ dorman~ ~or a suitable i~terval in the presence of a .
~-single ventricular beat, ectopie or conducted from the atrium~
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~-and st~mulates the heart synchronously in the presence o a~rial -~-~ctivity not acc~mpanied by arry~mic ventricular activity. -;~

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To the best of my knowledge, all versions of the atrial synchronous pacer that have been publicly disclosed and/or marketed have stlmulated the heark synchronously in the presence of sensed atrial activity in a certain range extending from the asynchronous lower rate of the pacer circuit up to a maximum upper rate. Any natural P-wave rate exceeding the upper rate results in a sudden change of patient's heart beat and a concurrent sudden loss of cardiac output. Presumably, the patient's atrial rate had risen to the upper limit due to exertion or stress re-quiring increased cardiac output, and the sudden change in cardiac output could cause that patient to suddenly become faint and possibly be endangered in view of the circumstances causing the atrial rate to rise. ~-This particular trait of the synchronous pacer was de-liberately designed in because a physiological condition of the cardiac muscle left it ~ulnerable to irrltation by external elec-trical stimulae during a predetermined time period following a complete depolarization of the heart muscle. For example, if the depolarization of the heart is caused by a pacer pulse stimulus ~7 synchronously following a sensed atrial depolarization;, the heart muscle must repolarize over a predetermined time interval, named the T-wave interval. If a second atrial depolarization is sensed :i .
, ~ ~too soon ollowing the first depolarization, the pacer stimulus, if applied to the heart during the vulnerable period might con-ceivably elicit bursts of tachycardia or fibrillation which are undesirable and may even lead to a fatal sequence Oe arrhythmias.
; With this fear in mind, advantage was taken of the fact ..

that sense amplifier circuits in synchronous and demand pacer ~ 3 ,: : :

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circuits have a buil~-in re~ractory period in which they are in-sensitive to any incoming signal following too closely a previously sensed signal in order to prevent their sensing the pacer!:s own stimulus. This refrac~ory period o the sense ampliiers in synchronous pacers was allowed to establish the maximum allowable rate at which the pacer would synchronously ~ollow the atrial de-polarizations. As described in U.S. Pat. No. 3,648,707 the re-fractory period of the a~rial sense amplifier might be set at 500 milliseconds, (conforming to a maximum synchronous rate of D 120 beats per minut~), so that if the atrial depolarizations followed each other by less than 500 milliseconds, and a second P-wave signa~ would arrive at the P-wave sense amplifier while it - was still refractory, only every other atrial depolarization would .. ..
be sensed and the pacer rate would be o~e-half the atrial rate.
~` Through the same mechanism, i~ the atrial rate increases even more, and the atrial depolarizations o cur within 500 milliseconds, only one would be sensed9 and the pacer would operate at one-third the atrial rate. -; ~ : - .
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~lternatively, pacer circ-uits have been designed to revert D - to an asynchronous base rate mode once ~he atrial depolarizations.. ~ - ' ~ . .' ', ' .
~- occur at a rate exceeding the upper limit. The asynchronous mode rate is the lower limit at w~ich the pacer circui~ functions in the absence of any heart activity. In th~s case clrcuit designers have again ta~en advantage o a pre-exîsting pacer circuit to j : : ' i ~e~ec~ this operation.

s mentioned earlierj the sudden ~ransition from a high ~pacing~rate to a lower rate, while physiologically beneficial in ~ - :
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the sense that cardiac stimulation du~ing the vulnerable period is prevented, causes sudden symptomatic interruption in cardiac out-put and that may indirectly cause physical harm to the patient.

SUMMAR~ OF THE INVE:NTION
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Accordingly in recognition of the above-stated dis-- .advantages of the prior art synchronous pacemaker, the present invention provides an artificial cardiac pacemaker which in response ; to atrial electr;cal activity, not accompanied by natuxal ven-`: tricular activity, provides ventricular stimulating impulses O suitably delayed and in synchronism with the atrial activity, - and when the atrial activity exceeds an upper limit for artificial stimulation, the pacer continues to provide ventricular stimulating : ~mpulses at a rate approaching the upper limit. The pacer provides - the ventricular stimulating impulses in partial synchronism with the atrial activity.
-- -In a preerred embodiment of my invention, the artificial .cardiac pacer provides stimulating electrical pulses to the heart - at a pred~termined rate in the absence of natural cardiac electrical -activity of any kind. In response to a ventricular beat~ either O ~ ectopic or conducted and from any source, the pacer inhibits it--self and becomes completely dormant for a suitable interval. In - - response to:a~r;al electrical activity, not accompa~ied by natural . ., '''5' -~e~tricular ac~ivity, the pacer provides stimula~ing pulses suit-:~ab-ly delayed and in synchronism with ~he atrial beats.
-~Advantageously ~o the pat~ent, the:artificial cardiac pacer of my invention does not~suddenly rever~ ~o a lowe~ stimu--lating rate when atrial activity exceeds an upper limit, thus .. ; .
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lowering cardiac output when the patient may need it the most.
According to a broad aspect of the invention there is provided a synchronous heart pacemaker, comprisîng: means for selectively detecting natural electrical heart signals; means for generating an artificial pacing stimulus in timed synchronism with the detected heart signal; means for applying the generated pacing stimulus to the heart to elicit a responsive heart beat; and means for stabilizing the synchronous pacing rate at an -:
average rate approaching but not exceeding the upper pacing rate such means further comprising: means for inhibiting the generation of a subsequent pacing stimulus for a predetermined time interval corresponding to an upper pacing rate; and means for varying the time elapsed from the detection of electrical heart signals recurring at a rate exceeding the upper pacing rate and the synchronous generation of artificial pacing stimulus to include the predetermined time interval.
The foregoing aind additional advantages and characterizing eatures of the present invention will become clearly apparent upon reading . ..
of the ensuing detailed description of an illustrative embodiment thereof : :
together with the included drawings depicting the same. .. ~.
BRIEF DESCRIPTION OF THE DRAiWINGS
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Figure 1 illustrates the voltage wave produced by an animal heart ; :
during one complete heart beat;
Figure 2 is a block diagram illustrating a preferred embodiment of an artificial pacer according to the present invention;
Figure 3, on the first sheet of drawings, is a block diagram of the relationship of Figures 3a and 3b;
Figures 3a and 3b are schematic diagrams of the circuitry included in the block diagram of Figure 2; and .
Figure 4, on the first sheet of drawings, is a block diagram of the relationship of Figures 4a and 4b;
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:; 30 . ~ Figures 4a and~4b illustrate the signals sensed by or developed at various points in the schematic and block diagrams during several modes o aperation of the prefer~ed embodiment of my invention.
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DETAILED DESCRIPTION ~F T~IE ILLUSTRATED EMBODIMENT
The human heart beat is represented electrically as a complex wave consisting of wha-t are designated "P, " "Q," "R," "S" and "T" waves all as shown in Figure 1. The P-wave electrically represents an atrial beat associated with atrial depolarization ~hich beat commands heart rate as a function of signals from the rest of the body depicting the required cardiac output. The major and most pronounced electrical pulse in the heart is the :
R-wave and is normally of a magnitude between 2 to 20 millivolts in the .
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ventricle. The R-wave, which s.~imulates and represents ventricular ~contraction, ~ypically has an amplitude relation to the P-wave of at least three to one. The R-wave normally is generated by de-polarization of the ventr}cles, but when not so produced due to some cardiac malfunction, i~ is the function of the artificial pacer to provide periodic electronic pulses to the heart to supply -a missing R-wave. If both the natural and artifi~al pacer supply an R-wave~ however, competition fox control of the heart results and a possibly dangerous situation arises when the pacer elec-tronic pulse occurs in a T-wave region. The T-wave poxtion of each complete heart beat follows the R-wave or major beat pulse thereo~ by about 0.3 seconds. Within the T~wave i5 a critical , interval known as the "vulnerable period" and, in the case of a highly abnormal heart, a pacer impulse falling into this period can conceivably elicit bursts of ~achycardia or ~ibrillation which -', are undesirable and may even lead to fatal sequence of arrythmias.
,.~ The waves depicted,in Fig. 1 may be picked up and re corded or dirplayed by electrocardiogram apparatus. When an ,e~ectrocardiogram records or displays an artificial cardiac pacer _ 'O impulse, that impulse may be referred to as a pacer artifact or ' ~p~ke. Thus in the drawings, the artifact is designated, "A''.
, ~: Fig. 2 shows in block diagram fonm an artificîal cardiac -~ - -*acex constructed in accordance with the present invention. The t ~ pacer includes pulse generating means 10 for providing stimulating ' '~

,ff ~ -pulses ~o the hear~ under certain condi~ions and at a preset rate ~`f~ ~c~ntrcflled by oscillator tim;ng means 12 i~cluded thereinO A
first electrode, 14 is coupled to pulse generating means 10 and is ~dapted to be operatively connected to a patient's heart on or in he ven~ricle thereo~. An indifferent electrode 16, which functions f~ : ' ~ 367g6~
as a reference or indifferent electrode, can be placed in con~act with another portion of the patient's body or even with a selected portion of the patient's heart. A disc shape ls shown for the indifferent electrode 16, since indifferent electrodes for pacers nften comprise conductive discs or shields on the surface of the implanted pulse generator. The pacer includes a second electrode 18 adapted to be operatively connected to a patient's heart on or in the atrium thereof. The atrial electrode 18 is coupled to another component of the pacer as will ~e d scribed LO presently.

The electrode arrangement shown is prefer~ed, although others can be substltuted,without departing from the spirit and scope of the present inventionO Only one ve~tricular electrode 14 is needed and is placed surgically on or in ~he ventricle of ~he heart. The in~ifferent electrode 1~ can be subcutaneously - ' implanted. At least electrode 14 is of the type which provides .~ .
a sensing function as well as a stimulating func~ion, although . .
, -a separate electrode structure for sensing ventricular electrical , ~ signals can be provided. Elec~rode 18 is placed surgically on or '~O in the atrium of a patient's heart. , ~ Indifferen~ electrode 16 is coupléd through a lead 20 , ~nd a ground bus 22 connected thereto to pulse generating means 10.
lectrode 16 serves an electrical ground or pulse generating means . .
O and also as a ground ~or other circuit components of the p~cer ~Y-`Will be described presently.
hie~pacer provided by the present invention also com-prises a first signal responsive means 24 which is responsive to 8- ' , :
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ventricular electrical signals in the heart and thus also may be called an R-wave detector. First signal responsive means 24 is coupled to ventricular electrode 14 through leads 26 and 28.
Ventricular electrode 14 is also coupled to the output of pulse generator 10 through leads 26 and 30. Signal responsive means or R-wave detector 24 is operatively connected through a lead 39 to a RESET terminal of pulse generator 10, and a ventricular signal, such as an R-wave, produced in the heart is sensed by means 24, the signal is amplified therein by sense ampli~ier 25, and pulse generator 10 is reset or recycled so that no stimu-lating pulse will be sent to the heart by the pacemaker. The sense amplifier 25 of R-wave detector 24 is rendered insensitive to any input signal on lead 28 for a period of time, characterized as the refractory period, established b~ a refractory circuit 34.
In this case, the refractory period is chosen to be 300 milliseconds (ms). The refractory circuit 34 is rendered operative to inhibit the sense amplifier 25 by an amplified R-wave received through diode 37 and leads 3~ and 38 or by an oscillator output signal received from oscillator 12 on lead 40 and through diode 42. A detailed description of the circuitry and operation of the R-wave detector ; 24 will be presented further on in the specification.
Pulse generator means 10 comprises an oscillator 12 ;, which, in the absence of an amplified R--wave signal at its RESET
input or an output of the upper rate memory circuit 44 at its TRIGGæR input (the operation of which will be described in due course) free b 03cillates at a preset rate of, for example, 60 beats per minute (a pulse interval of 1000 ms). The 60 beats per~minute established by the oscillator 12 constitutes the lower pacing rate of the~pacer. The oscillator output signal developed .
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by the oscillator 12 is conducted by a lead. 46 to a pulse ampli~ier 48 which amplifies the output signal to a voltage and current su~ficient to s~imula~e the patient's ventricles, and that pacemaker stimulus is applied to the ventricular electrode 14 ~hrough leads 30 and 26. The pacemaker stimulus or artiact is desig~ated "A" in the drawings. The oscillator 12 has an upper limi~ above which it is inhibited from either freely oscillating in case of a circuit or battery malfunction or being driven by the second signal responsive means 48 (in a manner to be described).
That upper limit is established by a rate limit circui~ 50 which is rendered operative to inhibi~ the oscillator 12 ~or a pre determined period following the receipt of an oscillator output signal ~hrough leads 46, 40 and 52. The upper limi~, or example, may be 120 beats per minute ~hich conforms to a time interval be~ween oscillator output signals of 500 ms.
The second signal xesponsive means 48 is responsive to ~:
natural atrial beats of the heart picked up by the atrial electrode .
18 and conducted to a sense amplifier 54 by lead 56 and may be - designated a P-wave detector. The sense amplifier 54 is also ~, !0' rendered refrac~ory or insensi~ive ~or a refractory pPriod (lS0 ms, .:. . .
.: ~for example) established by a refractory circuit 58. Re~ractory , :~

;: circui~ 58, through diodes 37 and 60 and leads 36,39, 40, 62 and ~4 is rendered operati~e whenever the R-wave detector 24 amplifies ..~ . --.an~R-wave to prevent the P-wave detector 48 from also respondi~g to ~,~ --~he same ~-wave i.it is subsequently picked up by the atrial ., --Plectrode 18 a~ter ~raveling from the ven~icle to the atrium.
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~ Refractory circuit 58 through diodes 42 and 60 and leads 40, ~:`
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61, 62 and 64 also renders sense amplifier 54 refractory to oscillator output signals developed by th~ pulse gene~ator 10 and associated cardiac signals for 150 ms. Finally, refractory circuit 58, through leads 62 and 64, also renders sense amplifier 54 re-fractory for 150 ms following the sense amplifier's own detection and amplification of a P-wave signal.

The output of P-wave detector 48 is coupled to an upper "
rate memory circuit 44 which in the normal course of operation of the pacer (i.e., below the upper limit in a P-wave synchronous -mode) introduces a P-R delay of about 100 ms between the detection ;~ of a P-wave and the production of a pacer impulse A by the pulse generator 10. The upper rate memory circuit 44 comprises a memory ` 66 that can be "Set" by the amplified P-wave conducted on lead 62 to its "Set" input and can be "Cleared" by either an amplified R~

,.!'~ wave signal developed by R-wave detector 24 ~diode 37 and leads 36, 39 and 40) or by an oscillator output signal developed by oscillator ~- 12 ~through leads 40 and 61 and diodes 42 and 60) that are each ~ ~ simultaneously applied to both the S~et and Clear inputs of memory - -~ 66.
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Whenever the memory 66 is Set by an amplified P-wave !~

signa~l at lts Set input, it generates a memory output signal ; ~ on lead 68. IVhenever the memory 66 is Cleared by either an ; amplifled R-wave signal or the oscillator output signal, memory 66 is~cleared and the memory O-ltpUt signal is removed from lead 68. The~memory 66 may be a bistable multivibrator of flip-flop of any conventional deslgn,~ havlng bistable states of operation.
The memory output slgnal is conducted by lead 68 to a ;`
P-R interval time delay circuit 70.~ The circuit 70 may be of any ;7~6~1 conventional time delay circuit design having the capability of responding to a memory output signal and that has a preset in-terval of from 80 to 150 ms, which interval is chosen to cor-respond to the average interval between the naturally occuring P-wave and the subsequent R-wave of a normal heart beating within a normal range. Preferably, the P-R interval is set to 100 ms.
The circuit 70 will not respond to a memory output signal that does not last at least as long as the preset interval, for reasons to be explained.
- 10 The P-R time delay circuit 70 responds to the memory signal to produce a TRIGGER signal that is conducted by lead 72 - to the TRIGGER input of the oscillator 12. The oscillator 12 responds by immediately producing the oscillator output signal on , .
lead 46, which is amplified and applied to the patient's ventricle.
The oscillator output signal also, as mentioned earlier, resets the oscillator 12, clears memory 66 and renders the R-wave and P-wave detectors refractory. Thus, the total time interval from the detection of the P-wave until the stimulation of the ventricle is approximately 80 to 150 ms, and the pacer circuit is non-responsive to any subsequent R-wave signal for at least 300 ms.
Referring now to Figures 3a and 3b considered side by side ~ -in the manner depicted in Figure 3 there is shown a schematic of a complete diagram of an illustrative embodiment of an artificial -1 .
cardiac pacemaker pulse generator circuit operable as ; ~ described above with respect to Figure 20 The circuits 10, 24, 44 and 48 and the interconnecting leads may be enclosed or her-metically sealed within an electromagnetic interference shield, which, ln this~illustrative embodiment, may form the circuit's ndifferent electrode 160 Each of the circuits 10, 24, 44 and .. . .

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1~:)67~60 48 are energized ~y a common power source comprising a plurality of batteries B~ (not shown) serially coupled in parallel across a capacitor (not shown) and each of the circuits.
The R-wave and P-wave detector circuits, 24 and 48, are identical e~cept for time constants of the refractory portions of the circuit, that will be further explained, Consequently, only the particular circuitry of the R-wave detector 24 is de-picted and described.
In Figure 3a, the R-wave is applied through a filter circuit comprised of capacitors Cl and C2 and resistors Rl and R14 to the gate electrode of an FET Ql. The amplified output of FET Ql is in turn applied to the base of transistor Q2 for a second stage of amplification. Capacitor C6, connected in series with resistor R5 and diode CRl across positive and ground buses 72 and 74, is normally charged. When transistor Q2 i9 rendered more or less conductive by the greater conduction or lower con-duction of FET Ql, the voltage on capacitor C6 changes accordingly.

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If the sensed amplitude of the R-wave is above a predetermined level, e.g., ~ 3mV, the charge on capacltor C6 is altered to a yoltage sufficient to turn normal b non-conducting transistor Q3 on. The sensing and amplif~ing circuit of Figure 3a is responsive to ,, ~ . .
positive and negative R-wave signals. In particular, if a heart signal of a polarity is present to produce a negative potential upon the collector oi~transistor Q2, a current will be drawn through the emitter-to-base path of transistor Q3 and capacitor C6, whereby transistor Q3 is turned on. On the other hand, if a . ~ , .
~- ~ heart signal o~ an opposite polarity is placed upon the input ~;

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o the sensing and amplifying circuit, a, positive potential appears upon the collector of transistor Q2~ thereby tending to turn transistor Q3 o~. However, when the posi~ive potential is removed from the collec~or of transistor Q2, capacitor C6 (dis-charged through diode CRl) charges through the base-emitter junc-tion of transistor Q3 whereby transistor Q3 is turned on at that time.
A magnetically-actuated switch Sl is connected through diode CR5 between the collector of transistor Q3 and the ground bus 74. Similarly, ~he switch Sl is connected to the same point in the P-wave detector circuit 48, and it serves to disable both o~ ~he sense amplifier circuits when it is magnetically closed.
The switch Sl is inserted into the circuit to permit ~he doctor to disable the sensing and amplifying circuit by actuating the switch Sl with a suitable magnetic field and thereby clamping the collector of transistor Q3 to the forward voltage of diode CR5. With the sense ampli~ier ci~cuits defeated, the oscillator circuit 12 (Fig. 2) will freely oscill.ate and produce stimulating signals at a rate dependent on the battery voltage. Thus, the doctor may determine the operability o~ the pulse generator and ~the state of its batteries by monitoring the free-running rate and comparing the presently measured rate to the rate it exhibited at -the time of implant.

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Further, the collector o~ tr~nsistor Q3 is coupled in ~urn by a capacitor C7 and diode C~2 to the base of transistor , -~4. I the amplitude of the heart signal is above the predetermined 3 -level, the transistor Q3 is rendered conductive, thereby raising the voltage applied through the capacitor C7 to the base of transistor ~ 7 ~ ~

Q4 toward the B~ potential, thereby turning transistor Q4 on.
In this manner, the heart signal is amplified and sensed, and i ~t is above the predetermined level, transistor Q4 is rendered eapable of conducting, When normally non-conductive transistor Q4 is rendered capable of conducting, transistor Q5, which has its emit~er-collertor current conduction path in series with the ~mi~ter-collector current conduction path o~ transistor Q4 and resistors Rll and R12 across ~he ~t source, is also rende~ed conductive since on~ard bias as applied to its base by resistor R9. l~hen.
transistors Q4 and ~5 conduct, the voltage at the junction of resistors Rll and R12 abruptly decreases tending to make capacitor C8 charge to the voltage drop across re~istor Rll. Charging current for capacitor C8 is drawn from B~ through the emitter-base junction o~ transistor Q6 which tends to render it con-ductive~ Capacitor C8 can charge t~ the voltage drop across .. ~
.: resistor Rll less.the: emitter-base forward voltage drop of ~ransistor Q6.

~lthough the amplified R-wave signal at the base of ~9 tr~nsis~or Q4 is of relatively short duration, while ~ransis~ors -. -.Q4, Q5 and Q~ are rendered conductive, ~oltage through ~ransistor Q6,~resistor R13 and diode ~ tends to maintain ~ransis~or .

Q4 in conduc~ion. Therefore, as long as capacitor C8 charges ,~

~ ~nd transistor Q6 xemains conductive, transistors Q4 and Q5 are . ~ . .
:-also:~maintained:conduc~i~e. -The conduction of transistor Q6 o-te~ds to charge capacitor C9 which is coupled to the base of transistor QS~to battery voltage. While capacitor C9 char~es, *ransistor Q5 remains conductive. ~However, when capacitor C8 7~6(~

is fully charged, transistor Q6 turns off and capacitor C9 begins to discharge. The charge time of capacitor C8 establishes the relatively short pulse width of the amplified R-wave signal on lead 39. ~hile capacitor C9 discharges the base voltage on transistor Q5 is lowered to a level insufficient to allow it to conductJ thus turning it off. The discharge time of capacitor C9 is selected to be about 300 ms, and during that time the circuit ` is incapable of responding to an incoming R-wave signal. When transistor Q5 is turned off, the capacitor C8 becomes reverse biased and transistor Q6 is further turned off and capacitor C8 discharges.
Thus although transistor Q6 is conductive only a short period . . ~ . , of time, to produce the signal b) on conductor 39, the circuit is ~ `

refractory for a period of time dependent on the values of capacitors C9, C10 and R9.

`:t It will be noted in Figure 3a that the refractory circuit 34 IS integral with the sense amplifier circuit 25, the two - circuits sharing lead 40 for the transmission of outgoing .~ . , amplified R-wave signals and the receipt of incoming oscillator reset signals. An incoming oscillator reset signal is conduc~ed by resistor R13 and diode CR3 to the base of transistor Q4, rendering transistor Q4 conductive in the manner hereinbefore described. Again in this case, transistors Q5 and Q6 are rendered conductive. Capacitor C9 is charged and re-establishes the re-fractory period as it discharges. Capacitor C10 is a relatively small capacltor and it acts as a filter to prevent high frequency noise from triggerlng transistor Q4 into conduction.

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Turning now to the pulse generator 10, (Figure 3b) it includes oscillator 12 comprising the circuitry associated with transistors Q7, Q8, Q9 and Q10, the rate limit circuit 50 compris-: ing the circuitry associated with transistor Q12, and the pulse amplifier circuit 48 comprising the circuitry associated with trans-: istors Qll, Q13 and Q14.
: Referring first to the oscillator portion, in the absence of the normal operation of the patient's heart as indicated ., - by the amplified R-wave or P-wave signal, a timing capacitor Cll is charged by current through the timing resistor R14 at a rate determined by the R-C time constant of the series time constant means; the rate of charging capacitor Cll to a reference voltage lev01 de~ermines the base rate at which stimulating pulses are to be applied by the artificial cardiac pacemaker to the patient's ventricle, The voltage charge developed on capacitor Cll is ap-plied through resistors R15 and R16 eo the emitter of transistor -Q10, and upon reaching a predetermined level in excess of the reference voltage established upon its base at the junction of . . .
- resistors R17 and R18, transistor Q10 is rendered conductive. In 5l ~, 20 turn, trans1stor Q10, upon being rendered conductive3 raises the i~ voltage applied to the base of transistor Q9 whereby it is also , ... .
turned on. As seen in Figure 3b, the collector of transistor Q10 is coupled directly to the base of transistor Q9 and upon being rendered conductive, raises the voltage applied thereto, turning transistor Q9 on. When transistor Q9 is rendered conductive, potential is applied through resistors R20 and R21 to the base of the first output transistor Q11, thereby turning it on for a pulse width period determined in a manner to be explained. : :

.~ 7'9~0 The conduction o transistors ~9 and Q10 of the oscillator circuit and the subsequent conduction o~ the f~rst output transistor Qll is c~nditioned on the fact that the rate limit transistor Q12 is .also conduct-ive, for the emi~ter-collector path of transistor Q12 is in series with the reference voltage divider circuit of resistors R17 and ~18. For the time being, the "on" or conduc~ing state of transistor Q12 is presumed.
The emitter-collector path o~ the ;rst output tran-sistor Qll is also connec~ed in series with the emitter~collector .
path of transistor Q12 and with resistors R22 and R23. With positive voltage applied to its base on the conduction of tran-sistors Q9 and Q10, transistor Qll is turned on to lower the vo~tage at the junction of resistors R22 and R23, which junction -is coupled to the base of second output transistor Q13. The emitter-collector path of second output transistor Q13 is connected in series with resistor 24, diode CR9 and resistor R25 and transistor Q13 is rendered conductive by the lower voltage on its base. . .
. ~As the voltage at the collector of ~ransistor Q13 is _.
raised, a correspondingly more positive voltage is applied through resis~or R28, diode CR6 ~42 o Fig. 2) and the resistors R~7 and R38 to the base Qf reset ~ra~sistor Qi, whereby it is ~e~dered conductive, thereby discharging timing capacitor Cll . :. ~ . .
hrough its collector-emitter pa~h in preparation for the next . cycle of operatio~ of the R-C timing circuit comprising Rl4 and ., .. .
C~ -The reset siV~nal is also conducted through diode CR6 and ~ leads 40 and 39 ~o ~he refractory circuit of the R-wave detec~or '~ .; ~; . ~ :
.~:",. ~ .

., .: .

, ,. ' ! , ~ . . ~ . ~ : , , . . . ' ~7961D

24 and the Clear input of the memory 66, and through additional diode 60 to the refractory circuit of the P-wave detector 48 and the Set input of the memory 66.
Departing for a moment from the further explanation of the pulse generator 10, the response of detector circuits 24 and 48 to the reset signal from the pulse generator will be explained.
As explained earlier the mechanism by which the sense amplifiers are rendered re~ractory centers on the sustained low voltage on the base of transistor Q5. Without repeating the full operation of transistors Q4, Q5 and Q6, let it suffice to say that for the time period while capacitor C10 discharges from battery voltage to a voltage level insufficient to reverse bias transistor Q5, any sensed signal on the base of transistor Q4 will fail to render it conductive and produce an amplified R-wave signal on lead 39. The same explanation would apply to the circuitry of the P~wave detector.
Referring back to the pulse generator circuit 10, the conduction of transistor Q13 allows battery current to flow through resistor R24, diode CR9 and reslstor R25 and the voltage ., ~- 20 across resistor R25 is conducted to the base of the power output transistor Q14 that is rendered conductive thereby. The con-:
~` duction of transistor 14 precipitates the rapid discharge of voltage on output capacitor C13 through ventricular electrode 14, ~ ~`
. : :
the patient~s heart and back through the indifferent electrode 16 .i The condenser C13 recharges in the interval between the pacemaker output pulses from the battery source B+ through resistor R29 and the aforementioned electrodes. Capacitor C13 discharges ;~ ~ rapidl~ and the amplitude and pulse width of the pacemaker !
.!
!
,.~ :

~ ~7 9 ~
output signal is selected to exceed the heart's stimulation threshold. However, capacitor C13 recharges slowly due to the high resistance of resistor R29,and the recharge current is held below the heart's stimulation threshold.
The condenser C14 and the zener diode CR10 protect the pacemaker circuitry from any large amplitude electrical signals picked up by electrode 14 from external sources.
Referring to the pulse width of the pacer output pulses or artifacts, pulse width controlling circuitry transistor Q8 and capacitor C12 are provided. When transistors Q9 and Q10 are non-conductive, capacitor C12 tends to charge with capacitor Cll through the series circuit including resistors R14, R15, R16, d;od~ c R~
capacitor C12, ~ de ~R~and resistor R25. The volta~e on cap-acitor C12 will follow the voltage on capacitor Cll.
When transistor Q13 conducts, however, a voltage is .
; developed across the voitage divider circuit in series therewith ~ comprising resistors R26 and R27. The junction of resistors .. ; . .
R26 and R27 is connected to the base of normally non-conductive, ~pulse width extending transistor Q8, the emitter-collector path _ of which is connected between the junction of resistors R15 and ~-R16. Transistor Q8 is rendered conductive by the voltage on resistor R27 developed when transistor Q13 is conductiva so long -a~s the voltage on power source battery B~ is normal. The voltaga on capacitor Cl2 discharges through resistor R16, ~15 and reset ;~ ~ . .:
~; , . . .
. ~.~ . . .

., ;~ : : .

~ 20 ~ ~
.

~67~60 transistor Q8 in that case. At the same time, current conducted through transistor Q13 and resistor R24 is applied t~ ~he opposite - plate of the capacitor C12 tending to charge capacitor C12 in the opposite direction.
When transistors Q7 and Q8 are rendered conductive, voltage on the emitter of transistor Q10 drops towards ground potential, and tends to render transistor Q10 non-conductive.
However, ~he charging voltage from transistor Q13 through resistor R24 and capacitor C12 tends to maintain transistor Q10 in conduction for an interval dependent on its rate of charge of capacitor C12.
By careful selection of the values of the resistors mentioned above and resistor R16 and capacitor C12, this interval which controls the interval during which transistor Q14 is conductive, and .~, .
consequently the pulse width of the pacer output pulse, can be preset. By the addition of a remotely variable resistance in the ~-,, i .
circuit, the pulse width may be changed as found physiologically beneficial to the patient. A typical pulse width of the stimu-,.. .
; lator pulse, ranges from .5 to 1.2 ms, whereupon the transistor - Q10 is turned off. As a result, transistors Q9, Qll~ Q13 and Q14 are turned off to terminate the pulse output of the pacemaker .-"
~ circuit 10 as derived from the transistor Q14. As transistor Q13 . ......................................................................... .
is rendered nonconductive the reset pulse on the base of transistor Q7 terminates, thus permitting capacitor CIl to recharge to initiate .
the next cycle of operation m a manner as explained above.
~; 3 ~ Further, capacitor C12 begins to recharge when transistors Q7 and Q8 are rendered non-conductlve. The pulse width of the pacemaker artifact is automatically widened, when the battery source voltage B~
~ :3 :~

-~-3 7g6~

drops due to its depletion, to insure that the pacemaker stimulus has sufficient energy to retain capture, i.e., depolarize, the heart muscle. The pulse width extending transistor Q8 accomplishes this by failing to conduct, due to a lower voltage at its base and across resistor R27, whereupon the capacitor Cl2 must discharge through the additional resistor Rl5 and reset transistor Q7. The additional resistor in the circuit extends the discharge time and the "on" time of transistor QlO to increase the pacemaker stimulus pulse width.
Thus, as the pulse generator circuit 10 has been described heretofore, it operates in an inhibited mode designated Mode I and a demand mode designated Mode II upon the timely . .
~ receipt of a reset signal that renders transistor Q7 conductive to ,i . . .
, discharge capacitor Cll and reset the timing interval, and in an asynchronous mode designated Mode III when the R-C timing -` circuit is allowed to fully time out whereupon the oscillator -circuit resets itself. The operation of the circuit of Figures ;

2 and 3 in the remaining synchronous modes designated Modes IV -and V requires an explanation of the rate limit circuit of the pwlse generator 10 and the upper rate memory circuit ~4 to follow. ; ;
~, In ~igure 3bj the pulse generator circuit includes the aforementloned normally conductive rate limit transistor Q12 whose collector is connected to the base of transistor Q10 through resistor Rl~ and whose emitter is connected to the ground bus 74.
The base of rate limit transistor Q12 is connected through resistor R30 to the battery voltage and also through capacitor C15 and re-, j sistor R31 to the collector of normally non-conductive second output .i : : :: , :, ,,,, ~
., : ..
~ - 22 -.~ .

.~: ' , ~

~679~(~

transistor Q13. The transistor Q12 serves in prior art oscillator circuits to prevent the oscillator circuit from stimulating the patient's heart at too fast a rate in the event one of the oscillator timing elements should become defective in any manner.
For example, if the resistance R17 becomes an open circuit, the transistor Q10 would turn on prematurely with the result that a very rapid, possibly dangerous series of stimulating pulses would be applied to the patient's heart. In operation, transistor Q12 is normally biased to a conducting state by the battery voltage through resistor R30. To terminate the pulse width of the artificial stimulus, transistor Q10 is rendered non-conductive in a manner as explained above, whereby transistors Q9, Qll and Q13 are also rendered non-conductive. However, while transistor Q13 has been conductive, capacitor C15 connected to the base of :::
. transistor Q12 charges. When transistor Q13 is turned off, the negative charge established upon capacitor C15 serves to bias ., , off transistor Q12, thereby preventing transistor Q12 from being turned on again for a period dependent upon the discharge time of capacitor C15. As shown in Figure 3b, capacitor C15 dis-. ~ , -~ 20 charges primarily through resistors R30, R31, R26, R27, R24, diode : i , ,.
CR9 and R25, the discharge time being in the order of 500 ms.
While transistor Q12 is rendered non-conductive, transistor Q10 and therefore transistors Q9, Qll, Q13 and Q14 may not be turned C on. Thus, if one of the elements within the oscillator circuit becomes defective, thereby tendlng to turn transistor Q10 on pre-maturely, transistor Q12 serves a protective function, preven~ing J the premature conduction of the noted transistors and therefore liDits the rate at which stimulating pulses may be applied to the ~; patient's~heart, to a rate in the order of 120 beats per minute.
:: i : ~ .: .

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, ,-~ , .
~ 23 -6~ 9 ~
The rate limit circuit, therefore, inhibits pacemaker-runaway in Modes I through III. In this preerred embodiment of my invention, advantage is also taken of its action in the P-wave synchronous mode of Modes IV and V. ~n short~ in the synchronous mode, amplified P-wave signals that Set the memory circuit of the upper rate memory circuit 44 produce a trigger signal that is applied directly across the timing capacitor Cll and very rapidly increases the voltage thexeon to that necessary to render tran-sistor Q10 conduc~ive and produce a ventricular stimulation pulse in the manner explained abov~. However, if the rate limit has not yet timed out, the pulse signal will be ineffect;ve until the rate limit circuit does t~me out. Consequently, the pacemaker cannot be driven in synchronism wî~h the heart's P-waves at a rate exceeding 120 beats per minute. ;
Turning now to the upper rate memory circuit 44 in Fig 3a, it comprises a memory circuit (~6 in Fig. 2) including the i~ver~er 82, NOR gates 84, 86 and 88, and a time delay circuit ':
(70 in Fig. 2) including resistor R32, diode CR7, capacitor Cl7 .
and transistor Q15. Of course, the inverter 82 and NOR gates 84, . . ~ .
0 86 and 88 may also comprise solid state circuits that are conventional .
~ in na~ure and, for co~venience sake) are not reproduced herein.
: ., ~- As is well known, an :inverter responds ~o an input signal at its ~ ~nput terminal~f either positive or negative polarity, inverts

3 ~~he polarity o~ that signal and produces the inverted signal at its output terminal. A NOR gate such as gatP 84 functions to invert the polarity of and pass or block æignals at its two input ~',.'i, ~ :
-*erminals (gO and 92, for e~ample) as follows:

.. . .
i, . .
., i : . ~ .
. . .
.;, .

~79~

_90 _ 92_ _ _ _ OUTPUT ~941 _ LOW LOW HIGH
HIGH LOW LOW
LOW HIGH LOW
HIGH HIGH LOW
In Figure 3a, the input terminal 92 is normally HIGH
in the absence of a Set signal clue to the fact that inverter inverts its normally LOW input voltage-. If the presence of a reset signal from the pulse generator lO or an amplified R-wave from R-wave detector are characterized as HIGH signals on the Clear input ~terminal 90) and the presence of any of the afore-mentioned inpu~ signals on the Set input generates a LOW signal from inverter 82 at input terminal 92, the three NOR gates interconnected as shown in Figure 4 respond as follows: ~
92 96 _ --LOW (normal) HIGH ~normal) LOW
HIGH ~input) HIGH (normal) LOW
lHIGH ~input) LOW ~input) LOW --`LOW (normal) LOW ~input) HIGH
': 20 From the logic table it can be appreciated that the ~ .; .
~" ~ ~ memory output is normally LOW or is rendered LOW in all possible situations except when only an mput signal appears on the Set .$
input. The memory is bistable; that is, it remains in a stable HIGH or LOW state at~terminal 96 until a combination of }IIGH and LON~slgnals capable of changing its state next appears at the Set~or~Clear inputs. ;
Connected between the Set input and the inverter 82 -is~a differentiating circuit comprising resistors R33 and R34 and capacitor~CI8~deslgned~to assure that a HIGH Clear signal lasts ~ ~longe- ~h n~lhe transl~nt~LOW Set signal on input 92.

"?': ~ , ~ 25 - ~

` 1~67~613 . .

In the P-R time delay circuit, transistor Q15 is connected in emitter ~ollower relation to battery source bus at its collector, timing to capacitor Cll via lead 72 at its emitter and capacitor C17 at its base. When voltage on its base exceeds that on its emitter, transistor Q15 is rendered conductive to a degree sufficient to raise the voltage on its emitter to that on its base less the base-emitter foward voltage drop of the transistor.
The delay circuit connected between output 96 and lead 72 in its normal quiescent state is non-conductive and does ~:
not provide a TRIGGER signal to timing capacitor Cll. This is because ~th a LO~ output at 96, capacitor C17 is discharged through diode CR7 and cannot bias transistor Q15 into conduction The LOW
state ma~ correspond to ground potential or a negative voltage that is reflected on the base of transistor Q15. However, when the memory is Set, a HIGH positive voltage at 96 tends to charge capacitor . 1 C17 through resistor R32 at a rate dependent on the R-C tim0 con-stant of resistor R32 and capacitor C17. For example~ the values of the HIGH positive voltage, resistor R32 and capacitor C17 may `
be chosen so that Gapacitor C17 charges over an interval of lOO
ZO ms to the ref~rence voltage n the base of transistor QlO (Figure 'A; ~ 36 to forward bias transistor Q15 into conduction (unless the voltage on capacitor Cll has already rea~hed the reference voltage leve])~as soon as the voltage on capacitor al7 exceeds the voltage then~on capacitor Cll plus the base-emitt0r ~orward voltage drop Ragardless of the voltage on capacitor Cll at the~time durm g the~lOO ms interval that transistor Q15 conducts, capacitor Cll then charges at the R~C time co~stant of resistor R32 and capacitor G17 so that at the end of ~c ~ 26 ~ 13167~6~3 100 ms it has..reached the reference voltage (less the emi~er~
collector vol~age drop of transistor Q15).
I~ the rate limit circuit of the pulse generator 10 has not yet timed out following the 100 ms time delay, capacitor Cll continues to charge to battery voltage and will render trans-istor Q9 and Q10 conductive when the rate limit interval elapses and rate limit transis~or Q12 again conducts. As described here-i~before, when the rate limit circuit does time out, and trans-istors Q13 and Q14 are then also rendered conducti~e, a pacing .;:10 pulse or stimulus is applied to the ventricular electrode.14, a -~. reset signal is conducted from pulse generator 10 on lead 40 ~o -. . .
.~ ~he Clear input and through diode CR8 to the Set input terminal.

.. The reset signal is simultaneously HIGH at input 90 and LOW

(through the action of inverter 82) at input 92 which switches t-he state o the memory to the LOW state at output 96. The volt-: age on capacitor C17 rapidly discharges through diode CR7 and . . . .
` .output 96 and renders transistor Q15 incapable of conducting.
.. . .
:.. Capacitor Cll is simultaneous~y discharged.

It should be noted with respect to the above-explained .
.. ..
.~Q operation o~ the.time delay circuit that i the memory outpu~ is ~reset to the LOW state within 100 ms since it was set to the HIGH ;
state, the capacitor C17 may charge to a leve~ suficient to bias :~
transistor Q15 into conduction and consequently charge capacitor . ~:
-~Gll to a corresponding level.~ This operation is reflected in the :- operation of the:circuit in accordance with Mode I o~ Fig. 4a.

.However, if the memory circuit is Cleared, it means that a reset : ` ~signal~was produced in the circuit and that the reset signal :`~

;

27 ~:
: .1 ~ 67 ~ 6 ~
simultaneously is applied to reset transistor ~7, to discharge capacitor Cll. In that way, the pulse genera~or 10 is inhibited rom producing a pacemaker stimulus.
Re~erring now to Figs. 4a and 4b, several modes of operation of the artificial cardiac pacer of my invention will be explained. In these figures, the P-wave and R-wave developed by the heart and picked up P-wave and R-wave electrodes 18 and 14, respectively, the pacemaker artifact A~ the sawtooth voltage wave form of the adjustable timing means of the oscillator, and wave forms developed at points a) through h) in the circuit o Fig. 2 are shown in consecutive points in time The artifact A
is shown in broken lines superimposed on the P-wave and R-wave .
wave forms to indicate that it is not picked up by the respective detectors. In the P-wave wave form, the R-wave is shown in broken lines, and conversely, in the R-wave wave orm, the P-wave is shown in broken lines to indicate tha~ they are not picked up by the respective detectors. T~e R-wave developed ! ~ -thereby, sho~n in broken lines in the P-wave wave form, is deiayed in ~ime to show that these signals would reach the ) atrial electrode after a time delayed transmission through the ~--heart. The sawtooth wave form of the R-C oscillator voltage is , . ~
developed a~ the junction of the R-C timing means of resistor R14 and capacitor Cl7 of Fig. 3 included within the oscillator 12.
`All~o the wave forms depicted in Figs. 4a and ~b are stylized for ease of illustration of~the principle of operation o the invention and may not re~lect actual ~mplitudes or polarities or shapes o~
~ the~;signals.

-3~

~ ~7 9 60 Five modes of operation of the circuits of Figs. 2 and 3, as indica~ed at ~he tops of tl~e wave form diagrams of Figs. 4a and 4b, will be discussed. Mode I of the operation of the circuit represents the inhibited action thereof in the presence of normal heart activity. In Mode I, and with reference to Fig. 2, the P-wave signal is picked,up by the atriial electrode 18 and is ampli-fied by the P-wave detector 48 and appears as signal a) on lead 62.
The amplified P-wave signal a) is applied to the Set input of me~oxy 66, which responds thereto to produce the memory output signal on lead 68 depicted as c) in Fig. 4. The memory output signal c~ is applied to the P-R time delay circuit 70 in Fig. 2 which, a~ter a period of 100 ms, will TRIGGER, unless memory 66 is CLeared before the 100 ms elapses, the oscillator 12 into oper-ation. The amplified P-wave signal a) also is applied to the r(efrac~ory circuit 58 which produces the refractory signal g) tha~

. .
iæ applied to the.sense amplifier 54. The refra tory signal g) :~

has an invariable pulse width of 150 ms; however, the refractory .j period may b~ restarted during the 150 ms interval~

In the Mode I si~uation, the ventricle of the heart - :
, i :
..~ contracts in response to the contraction o the~atrium, the 100 ms ~ of~ the time delay 70 elapses, and the natural R~wave signal is ~ - ! ` : ' .picked up by the ventricular le~d 14 and amplified by the R-wave etector.24 to produce the amplified R-wave signal o wave foxm ~;~
b).~ ~The ampli~ied R-wave signal b~ is applied to the Clear input .Yterminal of memory 6~ to:clear tha~ memory thus termina~ing the . .~ .: j .
~ memory ou~put ~ignal c). If we assume that the tLme interval ~: ~
. ;l :
between the production of wa~e~forms a) and b~ is 80 ms, the P-~

ime delay:circui~ 70 ails ~o TRIGGER the oscillator 12 as would.
,~ ~ , . . .

~ 29 _ :
:
.

~ ~7 ~ ~
be indicated in wave form d). With re~erence to the R-C oscilla-tor curve of Fig. 4, it should be noted that the amplified R-wave signal b) is applied to the RESET input of oscillator 12 to reset the voltage of the wave form to or near ground potential, therefore restarting the timing of the oscillator 12. Consequently, it may ` be seen that in Mode I, the artificial cardiac pacer circuit of - Figs. 2 and 3 is inhibited from producing a stimulating impulse due to the timely natural response of the ventricle to the atrial depolarization.
L0 Mode II of operation of the pacemaker circuit o~ Figs. 2 and 3 takes place when the patient's ventricle spontaneously depolarizes~ wi~hou~ a prior depolarization of the atrium or the ` applicat;on of an electrical stimulus. Such an isolated, or ectopic R-wave, if it occurs before the timing circuit of the oscillator 12 ^ fully times out, will reset the timing circuit of the oscillator to prevent the application of an artificial s timulus to the heart.
- This mode of opera~ion of the pacer cîrcuit of Fig. 2 is referred to as the ventricular inhibited mode o operation. When an ectopic R-wave is picked up by the ventricular electrode 14 and 20 amplified in the R-wave detector 24, it is applied through conduct- :
or~39 to the RESET input of the oscillator 12, to the refractory circuit 34 of the R-wave sense amplifier 25, to the refractory ~-circuit 58 of the P-wave sense amplifier 54, and to the Clear input o~ the memory 66. As shown in ~ig. 4, at the ins~ant the amplified .. ,: , ~.
~R-wave signal b) is applied to the RESET input of oscillator 12, ~, .
~ the voltage on capacitor Cll drops back again to 0 or groundO
i .. . .
~ - hlso, although the P-wave detector 48 is tuned to rèject all in--: .
~ ~oming wave forn~ except the P-wave, extra insurance is added by ,., - ~ .

~ 30 ~

.
.. .

10 ~79 6~
triggering the refractory circ~i~ 58, so that the sense ampLifier 54 is incapable of detecting any incoming signal ~or a period of 150 ms. Since the ventxicular electrode 14 is closer to the ectopic source of depolarization in the ventricle than is the atrial 18, it follows that the R-wave depicted in dotted lines in the P-wave wave ~rom in Fig. ~a will reach the atrial electrode after a certain time delay. Consequently, if due to the particu-lar wave of the ectopic R-wave, the sense amplifier 54 could have possibly amplified it, this occurrence is prevented by triggering the refractory circuit 58 during that time period.
The third mode of operation o~ the artificial cardiac pacer of Figs. 2 and 3 involves the situation where neither the atrium nor the ventricle of the heart depolarizes wi~hin a cer-tain prese~ interval of the pulse generator timing me~ns in the ... . ..
-oscillator 12. This interval is preset, and it may conform to an interval of 1000 ms which corresponds to a heart beat rate of > ~ -60 beats per minute. Thus, if the heart beat ra~e of the patient's ~-~ atrium and v~ntricle falls below 60 beats per minute, the artifi--~- ~cial cardiac pacemaker is designed to respond to asynchronously -stimulate the heart. This mode of operation may be called the demand mode, and it is illustrated in Fig. 4a in the time rame 'l : : ' - ' -ollowing th~ ectopic R-wave of Mode II. With reference to the oscillator voltage wave form, it will be observed ~hat at Mode II, the voltage WQS reduced to 0~ and it steadily increases ~ - in~the sawtooth pattern depicted until a pred~ermined reerence -J voltage (+ Ref.) is reached, whereupon the oscillator circuit ? I ~ .

~ produces the oscillator output signal e). The oscillator outpu~ ~
~; ~ . i .
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,,; ~ : :
~ w3~
,, . . ~ : ' ' . :

~i ~

~ 79 ~ ~
signal e) is applied back to reset the osci~lator 12 to reduce the voltage from the positive reference voltage level back to the 0 voltage level as depicted in Fig. 4a. Simultaneously, the s~gnal e~ is applied to the refractvry circuits 34 and 58 and to Clear the memory 66. As depicted in the R-wave ~orm of Fig. 4a, the oscillator output signal e) is amplified by the pulse ampli~ier 48 to produce the pacemaker stimulus or artifac~ A that is applied by the ventricular electrode 14 to depolarize the ventricle and pro-~ duce thP R-wave wave form, Since the oscillator output signal e) ;10 is applied to the refractory circuit 34 and 58, neither the R-wave detector 24 nor the P wave detector 48 is capable oE picking up the pacer artifact or the induced.R-wave.
- In Mode III it should be noted that following the gener-ati~n of the oscillatox output signal e) the rate limi~ circuit SV
r~sponds to INHIBIT the oscillator 12 for 500 ms as indicated ~y the inhibit si~nal h).
.. . .
-~ Mode n of operation of the artificial cardiac pacer ., .
~- circui~ of Figs. ~ and 3 involves t~e operation of the circuit in .~.,' . `: . .
he P-wave synchronous mode; that is, a mode wherein the atrial ~`20 depolari~ations or P-waves occur at a rate exceeding the base :
~ra e o~ the oscillator 12 but are not effective to trigger the . 1 ~ epolari~ation of the patientls ventricle due to a disorder in the ~ ~ -hèart's natural conduction system within the normal P-R interval;

which~in this instance is chosen to be 100 ms. I~ is also assumed . . ~ , in~:the~explanation o this; mode that the atrial rate is below the rate~limit:o 120 beats per ~inute, i.e., having an average ~s~ inter~al exceeding 500~ms, es~ablished by the rate limit circuit S0.

. ~ ~

67 ~ ~ ~
Re~erring now ~o Fig. 2, at the start of Mode IV, the P~wave signal is detected and ampliied in the P-wave de~ector 48, and the amplified P-wave signal a) Sets the memory 6~ o the upper;ra~e memory circuit 44. The Set state of the memory 66 1s indicated by ~he wave ~orm cj in Fig. 2. I~ should be noted that the P-wave has occurred after ~he termination of the refractory period of the sense amplifier S4 that was caused by the immediately preceding R-wave signal of Mode III. The memory output signal c) is applied to the P-P~ timQ delay circuit 70, and aft~r a period of lOO ms, the P-R time delay c~rcuit 70 triggers (wave form d)) the production o~ an oscilla~or outpu~
~ignal e) by oscillator 12. With re~erence to the R~C oscillator voltage wave fon~ in Fig. 4a at ~he instant the signal d) is depicted, full voltage is applied to the timing circuit means and the shape of the wave form ~mediately changes. W~th full voli~age (~ applied to ~he t;ming ci~cuit means, the reference voltage level is rapidly reached and exceeded causing the osc~lator 12 to produce the oscillator output signal e). The oscilla~or output signa~ e)g in the manner described hereinbefore, resets the oscillator 12 and Clears ~he memory 66, thus termina~ing the ~ ~ . . . . .
'. oscillato~ ou~put signal e) and th~ memory output signal c). The r .
- ~ oscillator output si~al e) is amplified by the pulse amplifier 48 ~ and applied to the ~entricul~r electrode l~ as artifact A and ;l --:elicits the R-~ave ~hat follows the arti~act ~ In ~he same -ma ~ er as describe~ hereinbefora, the sense a~plifier 54 is rendered ~ refractory for its refractor~ period by the amplified P-wave ,-t~ ~ signal a);~and then lOO ms into the l50 ms refr~ctory period, the oscilla~or output si~nal e3 is~appl;ed to the refractory clrcui~

-` 1~ 67~ ~ ~

58 to restart the reractory period of the sense amplifier 54 as shown in wave form g). Upon the production o the oscillator outpu~ signal e) the rate limit circuit 50 again inhibits the ~scillator 12 for 500 ms and renders sense amplifier 25 refractory for 300 ms.
All of the above described modes of operation are known from the prior art. In accordanc~ with my invention the pace~

.
maker circuît operates in a fifth mode that advantageously realizes a ~table upper rate Limit for the operation o a synchronous pacemaker.
Turning now to Mode V, depicted in Fig. 4b, i~ will be assumed that the atrial, P-wave rate increases above 120 beats per minute. As depic~ed in Fig. 4b, this means that the interval . ~ .
between P-waves decreases below 500 ms. For convenience o illustration, the P-wave interval is assumed to go from just above 500 ms at the left in the diagram to about 450 ms (133 beats per minute) through~the remaining depicted intervals. It will also be~assumed that the Mode IV rate limit signal f) is just timing -out as the first P-wave (counted from the left) is detected. 5 O Again, as in the Mode IV operation, an artifact A is produced . .~ , .. ~, ~ -ater lO0 ms from the detection and amplification~of the P-wave.
" :
-The~ra~e limit circuit is restarted to inhibit lthe oscillator ~or -` i 500 ms and the oscill2tor timing circuit voltage res~arts.

~The ~econd P-wave arrives about 450 ms after the irst :
~- ~-P-wave, and P-wave detector 48 responds ~o produce the amplified P-wave ~signal a), because the refractory circuit 58 has already : . :
t~med out. Signal a) Sets the memory circuit 66 to produce the memory output slgnal c). After lO0 ms, the osci11ator 12 is ~L~6796(1 triggered. However, the oscillator is incapable of responding ~ for 50 ms, and, as shown in the timing circuit voltage wave form, - the timing circuit voltage remains near B~ until the full 500 ms operation of the rate limit circuit 50 times out. As soon as the ~ oscillator 12 is no longer inhibited, it is triggered to produce : the oscillator output signal e) and the pacemaker artifact A.
As shown in Figure 4b, at the time that the oscillator output signal e) is produced, the P-wave refractory circuit 58 is fully timed out; consequently, it is restarted at that instant.
.
- 10 The third and fourth P-waves elicit a similar response from the pacemaker circuit of Figures 2 and 3. It should be -noted though, that the P-R intervals are widening until the ~;
fourth R-wave almost corresponds in time with the fifth P-wave.
Because of the chance that the natural P-wave and the driven R-wave could occur simultaneously, and because the P-wave de-tector 48 might not be able to distinguish the two unless perhaps, the detector filter circuits could be custom tuned for each patient, the P-wave detector 48 is rendered refractory both by the .
amplified P-wave a) and by the oscillator output signal e).
Consequently, when the fifth P-wave occurs, the P-wave , ; detector 48 is refractory due to the previous oscillator output ` ~ ~ signal e). Therefore, the upper rate memory circuit 44 is not Set by the fifth P-wave, and ~he pacemaker is inhibited until the sixth . `
P-wave occurs. As shown in Figure 4b, the pacemaker has delivered st1mulating artifacts A at a constant interval of 500 ms through ~ `
the fourth P-wave. But the interval between the fourth artifact A and the next artifact A is wider--approaching 650 ms--representing the skipped~P-wave. At the sixth P-wave, however, all refractory 1 ~ ~
~ ~ .

,~
q`~ ` _ 35 _ 3 ~

1~ ~'79 ~0 and ra~e limit circuits are dorman~, and ~he R wave ~ollows in the 100 ms interval--in the identical ~ashion with the response o the pacemaker circuit to the first depicted P-wave.
Consequently, as can be seen from Fig. 4b, in its fifth mode of operation~ the pacemaker circuit of my invention responds to a P-wave rate exceeding a prede~ermined upper limit--120 beats per minute- to stabilize the rate at approximately the upper limit~
Periodic~lly the rate drops for one beat ~o a lower rate that still substantially exceeds the base rate o~ the oscil-lator circuit or half the actual P-wave rate. The average pace-maker rate equals the upper rate limit when it in turn is equaled by the P-wave rate and then gradual~y drops slightly as the P-wave , rate continues to climb.
` Physiologically speaking, the upper rate stabilization `~ of synchronous pacemaker operation achieved by m~ lnvention has -`
the advantage that the patient's cardiac output is not abruptly halved. -Although the average pacemaker xate stabilizes near the . , , ~ upper ra~e as the natural P-wave rate increases 9 the concurxen~
.
stabiliza~ionl in cardiac ou~put will itself tend to bring down ., ~
; the P-wave rate relatively slowly as the patient then modera~es ! his activlty. The u~per rate limit chosen for a particular patient can be selected to fi, his expected level o~ physical ~-~ activity. With my înventlon~ all patients will benefit by the i . , ~, added comfort and safety from physical harm during periods of trenuous~activity or mental s~ress that raise their cardiac ~output requlrements.

, ~ 36 ~ I

7~
From a reliability and physiological viewpoint, the periodic re-establishment of constant timed synchronism (the 100 ms delay) of the pulse generator stimulus with the selectively detected P-waves, when the heart's atrial rate exceeds the upper or maximum pacing rate, allows the physician to verify, from an ECG strip, that the implanted pacemaker is functioning correctly ; and is not merely running asynchronously at the upper pacing rate. ~lso, from an engineering viewpoint, the operation of the - pac.omaker circuit as hereinbefore describeda in rendering the .
I P-wave detector circuit refractory following both a detected .~ P-wave, a detected R-wave and a pulse generator output or reset ~; signal recogniæes that the P-wave detector sensing circuit would have difficulty distinguishing between a natural P wave and a pacemaker driven R wave that ¢losely coincides in time with the : na~ural P-wave, as i~ bound to occur (in Mode V) when the natural atrial rate exceeds the upper pacing rate.

. . ~ . .
~ . Having thus described my invention with particularity - "! ~ . , , ~ , `.~ with referenc~ to the preferred forma it wilI be obvious to those skilled in ~he ar~, after understanding my invention, that other ~
I changes and modi~ications may be made therein without departing .j ~from~the spirit and scope of the invention, and I aim in the appended claims to cover such changes and modifications as are -wit~in the scope of the inventio~.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A synchronous heart pacemaker, comprising: means for selectively detecting natural electrical heart signals; means for generating an artificial pacing stimulus in timed synchronism with the detected heart signal; means for applying the generated pacing stimulus to the heart to elicit a responsive heart beat; and means for stabilizing the synchronous pacing rate an an average rate approaching but not exceeding the upper pacing rate such means further comprising: means for inhibiting the gener-ation of a subsequent pacing stimulus for a predetermined time interval corresponding to an upper pacing rate; and means for varying the time elapsed from the detection of electrical heart signals recurring at a rate exceeding the upper pacing rate and the synchronous generation of artifical pacing stimulus to include the predetermined time interval.

2. The synchronous heart pacemaker of claim 1 further comprising:
means for periodically interrupting the selective detection of natural electrical heart signals recurring at a rate exceeding the upper pacing rate so that the generation of a subsequent pacing stimulus is in timed synchronism with the detected heart signal.

3. The synchronous heart pacemaker of claim 1 wherein said generating means further comprises pulse generator means operable to generate electrical pacing pulses applicable to the patient's heart at a synchronous rate extend-ing between a lower, base pacing rate and an upper maximum pacing rate in-cluding timing circuit means controlling the generation of the pacing pulses at the lower, base pacing rate upon the failure of the pacemaker to detect such natural heart signals within a maximum time interval, and means for periodically interrupting the selective detection of natural electrical heart signals recurring at a rate exceeding the upper pacing rate so that the generation of a subsequent pacing stimulus is in timed synchronism with the detected heart signal.

4. The synchronous heart pacemaker of claim 3 further comprising: a first electrode coupled to said pulse generator means, said first electrode being adapted to be operatively connected to a patient's heart on or in the ventricle thereof to transmit the artificial pacing pulses to the patient's ventricle and to pick up natural ventricular electrical signals; and reset circuit means coupled to said timing circuit means and said first electrode and responsive to a ventricular electrical signal to restart the maximum time interval and prevent the pulse generator from stimulating the heart for the maximum time interval.

5. The synchronous heart pacemaker of claim 3 further comprising: a second electrode coupled to said pulse generator, said second electrode being adapted to be operatively connected to a patient's heart on or in the atrium thereof to pick up natural atrial electrical signals; and trigger circuit means coupled to said pulse generator means and said second elec-trode and responsive to the atrial electrical signals to trigger said pulse generator means into generating a pacing pulse synchronously with a detected atrial electrical signal.

6. The synchronous heart pacemaker of claim 5, wherein said periodic-ally interrupting means further comprises: signal responsive means coupled to said second electrode and said trigger circuit means and responsive to atrial electrical signals picked up by said second electrode for rendering said trigger circuit means operable; and refractory circuit means coupled to said signal responsive means and said pulse generator means and responsive to a generated pacing pulse to inhibit said signal responsive means from responding to atrial electrical signals for a predetermined refractory time period so that as the synchronous pacing time interval increases as the atrial heart rate increases, an atrial signal will fall within the refractory time period and will not be sensed to produce a pacing pulse.

7. The synchronous heart pacemaker of claim 5 further comprising:
time delay circuit means coupled to said second electrode and said trigger circuit means and adapted to respond to each detected atrial electrical signal for causing said trigger circuit to trigger said pulse generator means into generating a pacing pulse in timed synchronism, after an atrial-ventricular time delay, with the detection of the atrial electrical signals;
and said varying means further comprises: memory circuit means coupled to said second electrode and said time delay circuit means and responsive to the atrial electrical signal for maintaining the response of said time delay circuit for a time period exceeding the elapse of the predetermined time interval of said upper rate limit circuit means, so that said trigger circuit means will trigger said pulse generator into generating a pacing pulse in synchronism with the atrial electrical activity of the heart.

8. The synchronous heart pacemaker of claim 8 further comprising:
memory circuit means having input means coupled to said first and second signal responsive means and output means coupled to said time delay circuit means, said memory circuit means responsive to the second signal for main-taining the second signal at said time delay circuit for a time period exceeding the elapse of the predetermined time interval of said upper rate limit circuit means, whereby the pulse generator operates to produce a pacing pulse in synchronism with atrial electrical signals detected from the atrium but at an average rate not exceeding the maximum pacing rate.

9. The synchronous heart pacemaker of claim 7 further comprising:
refractory circuit means coupled to said second signal responsive means and said pulse generator means and responsive to a generated pacing pulse to inhibit said second signal responsive means to periodically interrupt the production of the second signal when the atrial electrical signal recurs at a rate exceeding the maximum pacing rate, so that the predetermined minimum time interval of the upper rate limit circuit means will elapse before a succeeding atrial electrical signal is picked up by said second electrode.

10. The synchronous heart pacemaker of claim 11 wherein: said input means of said memory circuit means is coupled to said circuit means; and said memory circuit means is responsive to the first signal for removing the second signal at said time delay circuit on the occurrence of the first signal.

11. The synchronous heart pacemaker of claim 1 wherein said generating means further comprises an oscillator responsive to a trigger signal for generating an electrical pulse signal adapted to be applied over a lead to the ventricle portion of the heart to elicit a responsive heart beat, said oscillator further being operable to generate said pulse signal upon the absence of any trigger signal for a designated time period, said oscillator including rate limit means for preventing said pulse signals from being generated at a rate exceeding an upper rate limit; said detecting means further comprises: sensing means responsive to the occurrence of a cardiac signal caused by the contraction of the atrium of the heart for providing a signal each time said atrium contracts, said sensing means further including means for inhibiting the response by said sensing means to an atrium contrac-tion caused signal which occurs during a fixed time, less than said desig-nated time, after the provision of each pulse signal; and said varying means further comprises: memory means, which in response to said sensing means signal, provides said trigger signal to said oscillator until said pulse signal is generated, said memory means being reset each time a pulse signal is generated and set each time a sensing means signal is provided.

12. The synchronous heart pacemaker of claim 11 further comprising:
second sensing means responsive to a cardiac signal caused by the contrac-tion of the ventricle of the heart for providing a second sensing means signal each time said ventricle naturally contracts; and means for providing said second sensing means signal to reset said memory, to inhibit the response by said first sensing means for said fixed time and to reset said oscillator to commence said designated time period.

13. The synchronous heart pacemaker of claim 11 wherein said memory means include means for delaying the provision of said trigger signal to said oscillator for a selected time after the provision of said sensing means signal.