CA1120549A

CA1120549A - Implantable demand pacemaker

Info

Publication number: CA1120549A
Application number: CA000313250A
Authority: CA
Inventors: Robert A. Neumann (Sec 33(3)
Original assignee: Medtronic Inc
Current assignee: Medtronic Inc
Priority date: 1977-10-17
Filing date: 1978-10-12
Publication date: 1982-03-23
Also published as: US4166470A

Abstract

ABSTRACT OF THE DISCLOSURE
Cardiac stimulating apparatus comprising a first external unit for transmitting electromagnetic energy within the patient's body to be received by a seccond, surgically implanted unit within the patient's body and adapted to be solely powered by the transmitted electromagnetic energy. The first external unit transmits a power signal and a control signal, the power signal comprising a first pulse for energizing the second internal unit, and the control signal for controlling the internal unit to pace the patient's heart in a variety of modes, e.g., atrial, ventricular and atrial/ventricular sequential pacing.

Description

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This invention relates to electronlc devices implant~
able within the human body and in particuilar to cardiac monitoring and stimulating apparatus.
Heart pacemakers such as that described in U.S. Patent No. 3,057,356 issued in ~he name of Wilson Greatbatch and assigned to the assignee of this invention, are known ~or pro-viding electrical stimulus to the heart, whereby it is contracted at a desired rate in the order of 72 beats per minute~ Such a heart pacemaker is capable of being implanted in the human body and operative in such an environment for relatively long periods of time. Typically, such pacemakers are implanted within the chest beneath the patient's skin and above the pectoral muscles or in the abdominal region by a surgical procedure wherein an incision is made in the selected region and the pacemaker is implanted within the patient 1 5 body. Such a pacemaker provides cardiac stimulation at low power levels by utilizing a small, completely implanted transistorized, battery-operated pacemaker connected via flexible electrode wires directly to the myocardium or heart muscle. ~he electrical stimulation provi.ded by this pacemaker is provided at a fixed rate.
In an article by D.A. Nathan, S. Center, C.Y. Wu and W. Keller, "An Implantable Synchronous Pacemaker for the Long Term Correction of Complete Heart Block", American Journal of Cardiology, 11:362, there is described an implantable cardiac _ pacemaker whose rate is dependent upon the rate of the heart's natural pacemaker and which operates to detect the heart beat signal as derived from the auricular sensor electrode and, after a suitable delay and amplification, delivers a corresponding stimulus to the myocardium and in particular, the ventricle to initiate each heart contraction.
Such cardiac pacemakers, separately.or in combination, tend to al].eviate some examples of complete heart block. In a 311?~

heaxt block, the normal electrical inkerconnec-tion in the heart between its atrium and its ventricle is interrupted whereby the normal command signals directed by the atrium to the ventricle are interrupted with the ventricle contracting and expanding at its own intrinsic rate in the order of 30-40 beats per minute.
Since the ventricle serves to pump the greater portion of blood through the arterial system, such a low rate does not provide sufficient blood supply. In normal heart operation, there is a natural sequence between the artial contraction and the ventri-cular contraction, one following the other. In heart block,there is an obstruction to the electrical signal due, perhaps, to a deterioration of the heart muscle or to scar tissue as a result of surgery, whereby a block in the nature of a high electrical impedance is imp~sed in the electrical flow from the atrium to the ventrical.
When the heart block is not complete, the heart may periodiaally operate for a periocl of time thus competing for control with the stimulation provided by the artificial cardiac pacemaker. Potentially dangerous situations may arise when an electronic pacemaker stimulation falls into the "T" wave portion of each natural complete beat. As shown in Fig. 1, the "T" wave follows by about 0.2 seconds each major beat pulse (or !'R" wave causing contraction of the ventricles of the heart). Within the "T" wave is a critical interval known as the "vulnerable period"
and, in the case of a highly ahnormal heart, a pacemaker impulse falling into this period can conceivably elicit bursts of tachylcardia or fibrillation, which are undesirable and may even lead to a fatal sequence of arrhythmias.
Cardiac pacemakers of the demand type are known in the prior art such as that disclosed by United Kingdom Patent No.
826,766 which provides electrical pulses to stimulate the heart only in the absence of normal heartbeat. As disclosed, the heartbeat is sensed by an acoustical device disposed external of the patient's bod~, responding to the presence of a heartbeat to provide an inhibit signal defeating the generation of heart stimulating pulses by the pacemaker. In the absence of the patient's natural heartbeat, there is disclosed that the pace-maker generates pulses at a fixed frequency.
In U.S. Patent No. Re. 28,003, of David H. Gobeli, assigned to the assignee of this invention, there is disclosed an implantable demand cardiac pacemaker comprising an oscillator circuit for generating a series of periodic pulses to be applied via a stimulator electrode to the ventricle of the hPart. The stimulator electrode is also used to sense the "R" wave of the heart, as derived from its ventricle to be applied to a sensing portion of the cardiac pacemaker wherein, if the sensed signal is above a predetermined threshold lever, a corresponding output is applied to an oscillator circuit to inhibit the generation o~
the skimulator pulse and ~o reset the oscillator toinltiate timiny a new periQd. l'he Eol~owing patents, each assiyned to the assignee of this invention, provide further examples of demand type heart pacemakers: U.S. Patent No. 3,648,707 of Wilson Greatbatch; U.S.
Patent No. 3,911,929 of David H. Gobeli; U.S. Patent No. 3,927,-677 of David El. Gobeli et al; U.S. Patent 3,999,556 of Clifton Alferness; and U.S. Patent No. 3,999,557 of Paul Citron et al.
Demand type pacemakers are particularly adapted to be used in patients having known heart problems such as arrhythmias.
For example, if such a patient's heart develops an arrhyt~nia, failing to beat or to beat at a rate lower than a desired minumum, the demand type pacemaker is activated to pace the patient's heart at the desired rate. Of particular interest to the subject invention, are those patients that have recently undergone heart surgery; typically, these patients are apt to develop any and all known arrhythmias in the immediate post-3r3~ 5i9L~

operative period. Current therapy for such patients involves the implanting at the time of surgery of cardiac leads wi-th their electrodes connected to the patient's heart and the other ends of the leads being connected to an external pacemaker to provide pacing for arrhythmia management.
` In addition, the same pacemaker leads that interconnect the internally planted electrodes and its external pacemaker, are also connected to an external monitoring unit for providing signals indicative of the patient's heart activity to the exter-nal monitoring unit. A significant advantage of such pacemaker leads is that they may be used for recording of direct epicardial electrograms, which provide high quality precision data as to the patient's heart activity. The study of such wave shapes, i.e., morphology, is an invaluable aid in a diagnasis of arrhythmias. In this regard, it is unders-tood that a normal EKG
having its electrodes atkached to various portions of the patient's skin does not provide the high qualit~ output sigrlal ~or dia~nosls o arrhythmias as is obtained by cardiac electrodes attached directly to a patient's heart. For example, the output signal as obtained from such directly attached electrodes has a bandwidth in the order of 500 Hz and a signal to noise ratio in the order of 40 to 1, with no more than 30 db frequency loss.
Such a high quality EKG signal cannot be obtained from a standard EKG monitor as is attached only to the outer skin of the patient.
However, the use of pacemaker leads directed through the patient's skin presents certain probl~ms. Typically, if the external leads are left in the patient for any length of time, e.g~, 5 to 7 days, an infection may develop at the exit side of the leads, and the leads may be accidentally pulled with subsequent damage to the patient's heart. Further, such leads present microand macro shock hazards to the patient. for example, there are small residual char~es on many objects wi-thin a surgical environment and if the leads are accidentally exposed ; to such a charge, it will be applied Via the leads to the patient's heart possibly inducing an arrhythmia therein.
Further, relatively high voltage such as carried by an AC power-line are typically found in the operating room; the electrogram recording apparatus is so powered and the contemplated accidéntal contact of the external leads with such an AC powered line would have serious consequences or the patient. In addition, it is necessary to remove the cardiac leads approximabely 5 to 7 days after their surgical implantation. Further, there is consider-.
able electrical envir~mental noise within an intensive care unit where a post-operative cardiac patient would be placed.
: Illustratively, such noise results from fluorescent lights or other electrical equipment typically found in an intensive care unit and is capable of inducing millivolt signals into such cardiac leads of similar arnplitude to those si~nals derived from the patient's heart. Thus, such environmental noise-induced signals may serve to lnhibit the external pacemaker from pacing, even though the patient's heart may not be beating. Further, it is contemplated that after the surgical implantation of such demand pacemakers, that the connections of the atrial and ven-;~ trlcal leads to the external pacemaker may be reversed, with resulting pacer-induced arrhythmias.
The prior art has suggested artificial pacemakers having a transmitter or unit disposed externally of the patient's body and a receiver surgically implanted within the patient, having leads directly connected to the patient's heart. For example, in the West German Auslegeschrift 25 20 387, enti-tled Testing Arrangement for Arti~ic.ial Pacemakers, there is descri-bed a pacemaker having an external transmitter for transmittingexternal energy by radio fre~uency ~RF) waves to an internally planted unit for supplying elec-trical stimulation to the heart.

Further, it is c~isclosed that the internally planted unit is capable of transmitting informatiorl to a monitoriny device disposed externally of the patient's body, for indicating various characteristics of the pacemaker.
The present invention will be described further by way of example only, with reference to the accompanying drawings, in which:
Figure 1 illustrates the ~oltage wave produced by a human heart during one complete heartbeat;
Figure 2 i5 a schematic drawing above described of a demand heart pacemaker circuit of the prior art;
Figure 3 is a pictorialshowing of the manner in which an artificial heart pacemaker in accordance to the teachings of this invention;
Figure 4 is a block diagram of a transmitter or ex-ternal unit and a receiver or internal unit in accordance with the teachings of this invention;
Figure 5 is a detailed circuit diagram of the receiver as shown in Figures 3 and 4; and Figures 6 and 7 comprise a detailed circuit diagram of the transmitter as shown in Figures 3 and 4.
Further, in a pair of articles entitled "A Demand Radio Frequency Cardiac Pacemaker", by W.G. Holcomb et al, appearing in Med. & Biol. En~., Vol. VII, pp. 493-499, Pergamon Press, 1969, and "An ~ndocardio Demand (P&R) Radio Frequency Pacemaker", by W.G. Holcomb et al, appearing in the 21st ACEMB, page 22Al, November 18-21, 1968, there is described a demand-pacemaker including an external transmitter 10' as shown in Yigure 2, labeled PRIOR ART, for generating an RF signal from its primary coil or antenna 1~' to be received by a receiver 12' internally implanted within the patient's skin 14'. In addition, the receiver 12' in turn transmits heart activity in terms of the currents of the heart's "R" wave to synchroni.ze the activity of a pulse generator 26 within the external transmi~ter 10'~ Ag shown in Figure 2, the receiver 12' includes two separate electronic circuits each sharing common leads connected to the pacemaker electrodes, which are surgically connected to the patient 15 heart~ The first circuit, i.e., the EKG transmitter section, consists of a rectifying circuit of diodes D20-D23 for providing power to a transistor amplifier Q10, to which is applied the EKG signal; the amplified EKG signal is applied in turn to a coil 34'a for transmittion to the transmitter 10'.
The primary coil or antenna 16' receives and applies the EKG
signal via a detector 30, to be amplified by an amplifier 30, which provides the indicated EKG signal to be analyzed upon a display not shown. The second electronic circuit of the receiver 12' is the stimulus receiver, which furnishes the stim-; ulating pulse to the cardiac electrodes. In particular, the output of the pulse generator 26 of the transmitter 10' is :

3o B - 6a applied via closed switch 24 to superimpose a high voltage pulse upon the output of the 2 MHz oscillator, which is subsequently ampl.ified by amplifier 20 and applied via detector 18 to the antenna 16'. The high vol~age pacemaker pulse as superimposed upon the RF carriex, is received by the coil 34'b and rectified by the diode D25 and the capacitor C25 to actuate an electronic switch primarily comprised of transistors Q12 and Q13, which are closed thereby to apply the high voltage pulse via FET Qll to the pacemaker electrodes, the FET Qll serving to regulate the current passing to the pacemaker electrodes. The transistors Q12 and Q13 are voltage-responsive and disconnect the coil 34'b from the pacemaker electrodes in the absence of the high voltage pace-; maker pulse.
It is understood that the RF carrier as derived fromthe oscillator 22 is continuously applied to the coil 16'. The secondary coil 34'a receives a continuous RF wave ~rom the prim ary coil 16'. An EKG signal is derived Erom the paaemaker electrodes and ls applied to the base of the ampli~ier transistor Q.10, which in turn provides a correspondingly varyinc~ load to the ~oil 34'a, whereby a correspondi.ng voltage fluctuation of induced across the coil 16'. In other words, the voltage appear ing across the coil 16' is amplitude modulated in accordance with the patient's heart activity or EKG signal. Though the circuitry shown in Figure 2 provides a relatively simple circuit of ener-gizin~ the receiver 12' implanted within the patient, the EKG
signal as derived from the patient does not contain sufficient precision to provide a diagnostic quality display of the patient's EKG. ~ypically, to provide a diognostic quality display o~ the patient's EKG it is necessary to transmit the EKG signal with a bandwidth of 100 Hz with a signal to noise ratio in the order of 40 tol and with no more than a 3 db frequency loss; the circuitry shown in Figure 2 does not provide such quality primarily due to ~J.~
the ampli~ude modulationtype of signaItransJnission, which i8 sensi-Live to the xelative positions in terms of distance and angle of orientation between the coils 16' and 34'a. In this regard, if the distance or the angle between the axes of the coils 16' and 34'a vary due to the patient's movement, the amplitude o~ the signal as seen by the detector 18 also will vary. Thus, in an amplitude modulation system, this body movement will provide a distortion in the EKG signal detected. In addition, the extxaneous noise to which such a pacemaker would be exposed such - 10 as radiation from fluorescent lights or AC power lines, as well as other extraneous artifacts, may appear as amplitude modulation to introduce further errors in the signal received from the transmitter 10'~
According to the present invention thcrc is provided cardiac stimulating apparatus for;ap~lying stimulating signals to the atria and vertricles of a patient's heart, said cardiac stimulating apparatus comprising:
a. a first unit disposed externally of the patîent's body and including generator means for controlling the generation and transmission of a first power signal and a second control signalinthe form of electromagnetic energy in~o the patient's body and disposed in a timed relation to the first power signal;
and b. a second uni~ adapted to be surgically implanted within the patient's body, said second unit including first means responsive to the ~ransmitted irst power signal for pro-viding thesole energization of said second unit, and second means for respectively applying the first and second stimulating signals to the atria and ventricles of the patient's heart, said second means includes decodîng means responsive to the transmitted second control signal for initiating the operation of and energi~ing o said second means and controlling the ~z~
selective application of one of the ~irst and second stimulatiny signals in accordance with the timed relation o~ the sec~nd control signal.

Referring once more to the drawings and in parkicular to Figure 3, there is shown an artificial pacemaker and monitoring system in accordance with the teachin~s of this invention, including a transmitter or ex~ernal unit 10 that generates stimulating pulses to be applied via a pair 15 o conductors to an incapsulated coil 16 whereby electromagnetic energy in the for~ of RF radiation is transmitted through the skin 14 of the patient to be sensed by an internal unit 12 and in particular, as`shown in Figure 4, a coil 34a. The internal unit or receiver 12 is solely powered by the RF radiation transmitted to it for stimulating in various ~odes of operation the atrium 40 and ventrical 42 of the patient's heart, as by leaas 17 and 19, respectively.
Further, the external unit 10 is connected via con-ductor 59 to a monitorin~ unit 63, illustratively taking the form of a 78000 series unit of Hewlett-Packard for providing a :

f?, display of the atrial and ventrical activity of the patient's heart.
With reference now to Figure 4, there is shown the transmitter 10 including a control 50 for controlliny a variety of selected paciny functions and capable of operatillg in a demand or asynchronous mode for atrial, ventrical, or atrial-:~ ventrical sequential pacing. In an illustrative embodiment of this invention, the control 50 may be appropriately adjusted to effect asynchronous atrial pacing from 50 to 8C0 BPM and demand atrial pacing from 50 to ~80 BPM. It is contemplated that for use with postsurgical patients, arrhythmias may readily develop and by the application of heart stimulatin~ pulses in the range of 180 to 800 BPM that the patient's heart may be forced out of its arrhythmic beating pattern. Further, the control 50 is adapted for asynchronous or demand ventrical pacing in the range of 50 to 180 BPM. In an atrial-ventrical sequenti~l pacin~ mode, the control 50 may be adjusted to provide stimulation froln 50 to 180 BPM with a delay between the atrial and ventrical pulses adjustable from 0 to 300 ms. The control 50 selectively applies the output of an R~F transmitter 52 by an antenna switch 56 via the set 15 of leads to the primary coil or antenna 16 for transmission of a corresponding electromaynetic wave to be received and detected by the receiving coil or antenna 34a.
Significantly, the transmitter or external unit 10 operates in first mode for effecting pacing and in a second mode for processing of EKG or ~lectrograph information derived from the implanted receiver 12. In the second mode of operation, pulse width ~odulated data indicative of the amplitude of the heart's activity is transmitted from the coil or antenna 34b of the internal unit 12 to the coil 16 of -the external unit 10 to be applied via the antenna switch 56 to a pulse widthdemodulation 54 and a dernultiplexer 55. As will be explained in detail later, ~2~

the internal unit or receiver 12 is capable of sensin~ and monitoring the atrial and ventrical activity of the patient's heart and for transmitting pulse width modulated signals lndi-cative thereof in timed sequence with a timing signal, to the transmitter or external unit lO. In this regard, the demodul-ator 54 and the demultiplexer 55 separates the atrial and ven-trical signals transmitted from the internal unit 12, as well as demodulates the pulse width modulated signals to provide corresponding output signals indicative of the amplitude of the atrial and ventrical signals, via conductor 59 to the external monitoring device 63, as shown in Figure 3, whereby a graphical display thereof may be provided with a diagnostic quality, so that the attending physician may accurately analyze the patient's heart activity. With such information, the physician is able to predict pending heart failure or arrh~thmias. In this regard, the subject invention is capable of achieving diagnostic quali-ty displays, i.e. i9 able ko transmit to the receiver unit 10, heart signals with a barldwiclth frequency of 100 Hz, a signal noise ratio in the order o~ 40 to l with no more than 3 db frequency loss. Further, a sensing amplifier 58 provides an inhibit signal to the control 50 whereby the control 50 is inhibited from operation during spontaneous cardiac activity.
As shown in Figure 4, the receiver or internal unit 12 - comprises an RF detector 60 for receiving the RF signal as derived fron~ the input coil or antenna 34a, which separates the detected RF signal into power and control components, the power component energizing the power storage circuit 66. As will be explained in detail later with respect to Figure 5, the power storage circuit 56 provides power for energizing the elements of the receiver 12. The RF detector 60 also provides a data signal to a paciny pulse and command decoder 62, which decodes the control signal transmitted from the transmitter lO to detect the 3~

mode of pacing in which the receiVer 12 is to be operated in and for applying energizing pulses to a pacing output circuit 64.
As shown in Figures 4 and 3, the output of the pacing output circuit 64 is connected via leads 17 and 19 to the atrial and ventrical portions 40 and 42 of the patient's heart. In addition, the atrial and ventrical leads 17 and 19 are also connected to a monitoring or second portion of the internal unit 12, which comprises two operational amplifiers 68 and 70 for amplifying and applying respectively atrial and ventrical signals to a 1~0 multiplexer-modulator circuit 74. As will be explained in detail with respect to Figure 5, the circuit 74 operates to energize sequentially the coil or antenna 34b and thereby trans-mit via the coil 16 to the receiver 10 signals indicative of the atrial and ventrical activity of the patient's heart, accompanied by at least one timiny signal In addition, the circuit 74 .
modulates the ventrical and atrial signals in a manner tha~ i~
not adversely effected by environmental noise, as occurs to amplitude modulated sicJnals. In an illustrative embodiment of this invention, circult 74 pulse width modulates each of the signals before transmitting same to the receiver 10. It is contemplated that the circuit 74 ma~ also frequency modulate the atrial and ventrical signals to transmit them accurately to the external unit 10.
In Figure 5, there is shown a detailed schematic diagram of the receiver 12 wherein the functional blocks as shown in Figure 4, are shown and identified with similar numbers.
The transmitter 10 transmits from its primary coil 16 to the secondary coil 34a an RF signal comprised of a train of ampli-tude modulated pulses. Each signal of such train comprises a first or power pulse that i5 stored in the power storage circuit 66 to provide energization ~or the elements of the receiver 12.
The initial power pulse has a width in the order o~ at least 20 fL?4~

ms that is detected by the detector 60 in the form o~ a tuning capacitor Cl connected in parallel with the antenna or secondary coil 34a. A positive voltage derived from capacitor Cl is applied through a diode Dl to charge a capacitor C3 to a value determined by a zener diode D3a, illustratively a plus 10 volts.
Further, a negative voltage is established through diode D3 to i charge a capacitor C4 to a value limited by the zener diode D4a r illustratively a negative lO volts. As a review of the schematic ¦ of Figure 5 indicates, these negative and positive voltages energlze the elements of the receiver 12 and are applied to various points throughout the receiver 12~ Illustratively, the first power pulse has a pulse width in excess of 20 ms and an amplitude in excess of 30 volts (peak to pea~), whereby the capacitors C3 and C4 are charged with a voltage that will not be discharged for a period in the order of 800 ms, which is long enough to permit the monitoring portion o the receiver 12 to ~ pick up a P wave and an R wave, as shown in Figure 1, of I approximately 75 BP~. Further, the ~ir~t power pulse is respect-ively derived from the control 50 of the external unit lO once each 800 ms to continue the energization of the internal unit 12 to monitor the patient's heart. When the apparatus is oper-ating in the telemetry mode only, the pulse width of the initial power pulse is increased to about 100 ms in a manner to be described. Similarly, duriny the transmition period between the pacing and telemetry mode and during the refractory period of the patient's heart, a second power pulse, again of 100 ms length, is transmitted to power the receiver 12.
Further, as indicated in Figure 5, the train of pulses also include a series of between 0 and 3 command pulses.
If no command pulses are transmitted, the receiver 12 is comman--ded to operate in its monitoring mode and no pacing will be provided. If a single command pulse occurs within 2 ms after the initial power pulse, the pacing pulse and command decoder 62 decodes such lnstruction to cause the receiver 12 to pace the atrium at a pulse width equal to the width of the command pulse.
If two pulses follow within 4 ms of the initial power pulse, the decoder 62 causes the receiver 12 to pace the atrium on the occurrence of the first command pulse, and to arm the ventricular output circuit 64b on the occurrence of the second command pulse.
If there is a third command pulse occurring within a period of up to 300 ms of the trailing edge o~ the initial power pulse, the decoder 62 will effect a corresponding delayed actuation of the ventricular output circuit 64b to apply a pacing pulse to the patient's ventricle. By providing a variable delay before the occurrence of the ventricular pacing pulse, a sequential atrial-ventricular pacing mode can be effected.
~he command pulses are derived from the capacitor Cl and applied to a detector circuit comprised of resistor Rl and capacitor C2 which responcls only to the envelope of the power and co~nand pulses, t~picall~ haviny a width in the order of 20 ms and 0.5 ms, respectively, to turn on transistor Ql, when a ~0 power or command pulse has been so detected. The voltage established upon capacitor C4 is coupled across resistor R3 and transistor Ql, whereby its output as derived from its collector is limited to a value less than that to which capacitor C4 is charged. As explained, the collector of transistor Ql is turned on in response to power or command pulses, whereby positive going pulses of a duration correspondent to the power or command pulses are applied via a data line 81 to actuate the atrial and ventrical output circuits 64a and 64b in a manner to be explained.
Upon the occurrence of the ini-tial power pulse, a corresponding negative going signal is developed ~t the collec-tor of the nonconductive transistor Ql and is applied via a a diode D5 and resis~or R4 connected in parallel ~nd NAND yate 80 to set a flipflop 82 upon the trailing edge of the initial power pulse. The resistor R4, a capacitor C5 and NAND gate 80 act as a discriminator to prevent the passage of any pulses shorter than approximately 20 ms, i.e., any command pulses. As a .result, the flipflop 82 only responds to the power pulse and I derives at its output terminal 13 a timing pulse of approximately

2 ms commencing at the trailing edge of the initial power pulse and being applied to input terminal 2 of the NAND gate 102, thereby enabling the NAND gate 102 for a period of 2 ms. Thus, if a command pulse appears upon the data line 81 within this timing window of 2 ms after the trailing edge of the initial power pulse, an output is derived from the NAND gate 102 and inverted by digital inverter 104 to actuate the atrial output circuit 64a and in particular to render conductive FET Q4, whereby a pacing pulse is applied via the lead 17 to stimulate the atrium ~0 of the patient.
In addition, the 2 ms pulse derived from pin 13 of the ~lipflop 82 is inverted by inverter lO0 and is applied to input t.erminal 3 of flipflop 84, which responds to its trailing edge, that is, at the end of the window in which the atrial trailing pulse can be activated. The flipflop 82 also generates at its output terminal a 4 ms negative pu~se that is applied to the reset input terminal R of the flipflop 84, which in turn provides a posi~ive ; output signal at its output Q following the termination of the . output pulse derived from terminal 13 of flipflop 82. This out-put pulse derived from flipflop 84 provides an enabling signal to the NAND gate ~6, whereby the second, ventricular arming command pulse may be applied via the enabled NAND gate 86 to the input terminal 11 of a flipflop 88, which in turn applies a positive enabling signal from its output terminal 13 to a NAND
yate 90. In this manner, a second window is defined illustrat--15~

d ively in a period bet~een 2 ms and 4 ms ~ollowing the trailing edge of the first power pulse, in which window the second ven-tricular arming command pulse may appear to arm the ventricular output circuit 64b in preparation to be actuated by the third command pulse.
At this time, the NAND ~ate 90 is enabled or armed for a period illustratively up to 333 ms to wait the third, ventri-cular pacing command pulse on the data line 81 and applied to pin 3 of the NAND gate 90. If the third, ventricular pace command pulse occurs during the 333 ms window, it is passed via the enabled NAND gate 90, inverted by an inverter 92 to render . conductive an FET Q2 of the ventrical output circuit 64b, where-by a negative ventricular stimulating pulse is applied via the conductive FET Q2 and FET Q3, a capacitor C9 and lead l9 to energize the ventrical 42 o the patientls heart.
. As a further feature of this invention, the ~ate ak whlch the patient's ventrical can be paced is llmited to 180 ventricu.lar BPM by the provision of a .~lipflop 106. As seen in Pigure 5, the ventricular stimulating pulse as derived from the output of the inverter 92 i5 also applied to reset the flipflop 106, which responds thereto by providing a one shot output pulse of a period of 333 ms from its output terminal lO to be applied to the reset terminal R of the flipflop 8~ for a corres-ponding period, whereby flip10p 88 may not be set to enable the above described ventrical pulse path (including NAND gate 90 and inverter 92) for a like period. In this manner, it is assured that the patient's ventricle 42 may not be paced at a rate above 180 BPM or more often than once each 333ms. as may occur in the event of a failure of a circuit component.
Thus, depending on the coded signal transmitted to the internal unit 12, the artificial pacemaker is capable of operating to apply pacing pulses to either of the pati~ent's -lb-2~

atrium 40 or ventrical 42 or to operate in an atrial-ventrical sequential pacing mode, wherein the atrium 40 is first paced and after a selected time delay, the ventrical 42 is paced. In addition, if no pacing is desired, only the initial power pulse is applied, which provides a power energization for a subsequent period in which the remaining elements of the internal unit 12 remain energized, and a second monitoring portion of this circuit, as will now be described, is energized for transmitting to the external unit 10, atrial P-type and ventricular R-type waves as sensed by electrodes applied directly to the patient's heart. In the following, the monitoring portion of the receiver 12 is explained whereby the atrial and ventricular signals are time multiplexed and pulse width modulated, to provide a train of pulses ener~izing the coil 34b, to induce similar signals ;nto the primary coil 16 of the external unit 10, whereby correspond-ing atrial and ventricular signals may be separa-ted and app:Lied to the external monitor 63, as shown in Figure 3. During periods of RF transmission when the antenna switch 56 as shown in Fi~ure 5 is in a position to transmit only the RF transmission, the monitoring portion of the receiver 12 is not operative to trans-mit the atrial and ventricular signals, because the magnetic field created by the RF transmission from the coil 16 is of significantly greater magnitude than that of the atrial and ventricular signals emanating from the coil 34b.
As shown generally in Figure 4 and in detail in Figure 5, the electrodes connected to the atrium 40 and ventrical 42 of the patient's heart are connected by the leads 17 and 19 to operational amplifiers 68 and 70, whereby the atrial and ventricular signals are amplified and applied to the multiplexer modulator circuit 74. In particular, the atrialand ventricular/
signals are applied to a time multiplexiny portion of the circuit 74 for placing these signals in a desired time sequence, along q)~

with a timing signal. In particular, the circuit 74 comprises an operational amplifier 94 having internal feed back and oper-ated as a free~running oscillator to provide a square wave output that is applied to a counter 110, from whose three output terminals are derived in sequence three timing signals, which axe applied in turn to four corresponding FET's Q6, Q7, and Q8 seguentially enabling the three FETIs i.n a timed sequence. In particular, the output of the atrial operational amplifier 68 is applied to the FET Q6l while the output of the ventricular operational amplifier 70 is applied to the FET Q7. A negative voltage, as derived from the capacitor C3 is applied to the FET
Q8 to provide the desired timing signal. Thus, the multiplexer modifies the square wave output of the oscillator 94 to provide therefrom in sequence a first signal indicative of the atrial activity of the patient's heart, a second signal indicative of the ventricular activity of the patient's heart and a third positive going timing pulse to be applied to a pulse width modulating circuit essentially comprised of the operational amplifier 98.
In an illustrative embodiment of this invention, the three timing outputs of the counter 110 are 1.0 ms in width, occurring every 3 ms to thereby sample the P-wave signal appear-ing at the patient's atrium 40 or the R-wave signal appearing at the patient's ventrical 42. In the illustrative embodiment, the sampling frequency is 333 Hz. Thus, with the average P-wave signal lasting longer than 20 ms, the P-wave is sampled about six times as it rises and falls to provide about six sample amplitudes of signals to be applied to the amplifier 98. In similar fashion, a number of sampled amplitudes of the R-wave are derived and applied in sequence with the pulses indicative of instan~aneous P-wave amplitudes to the amplifier 98. The instantaneous sampled amplitudes are transformed into pulse l~Z~ 9 width modulated signals by amplifier 98~
In particular, the output of the square waVe generator formed by the oscillator 94 is applied to a second free running oscillator 96, which in turn generates a triangle wave output to be applied via resistor R16 to a first inpu-t of the opera-tional amplifier 98 acting as a comparator and pulse width modu-lator. ~ second input to be summed with the first, is derived via a resistor R15 from the commonly connected output electrodes of the FETIs Q6, Q7 and Q8. Thus, when no signal is derived from either the atrial or ventricular amplifier 68 or 70, only the triangular output of the oscillator 96 is applied to the amplifier 98, which in turn produces a square wave output of a first fixed period indicating a zero amplitude signal. The out-put o~ the amplifier 98 is limited to fixed voltages set by the zener diodes Dll and D12, 3.3 volts illustratively. In addition, resistors ~17 and R18 along with diodes D9 and D10 provide a source of power for the signals applied to the coil 34b, whereby the operational amplifier 98 is essentially used as a comparator or pulse width modulator and not a power amplifier.
~lhe sampled amplitudes of the atrial and ventricular signals are summed with the triangu3ar wave output derived from : the oscillator 96. Summing these two voltages allows the rising triangular wave derived from the oscillator 96, to exceed the reference potential as applied to pin 3 of the amplifier 98, whereby the amplifier 98 is turned "on" earlier and "off" later in time, thus widening the output pulse of the amplifier 98. In this manner, each of the atrial and ventricular signals whose amplitude indicate the intensity of the corresponding atrial and ventrical heart activity, are sampled pulse-width modulated and applied as energizing signals to the coil 34b.
Thus, there is provided a three-phase timing or multi-plexing function, whereby a first pulse-width modulated signal 19- , ~12~r~9~9 indicative of a signal appearing at the atrial electrode is provided, followed by a second, pulse-width modulated signal indicative of the signal appearing a-t the ventrical electrode, followed by a ]00% positive indicating signal. The third phase signal is used as a timing reference, whereby the demodulator 54 within the transmitter 10 determines the presence of and demodu-lates the atrial and ventricular signals.
Referring now to Fig. 6, there is shown a detailed circuit diagram of the timing and mode control circuit 50 and the RF oscillator 52 of the transmitter 10 as generally shown in functional block dlagrams in Figs.~ 4. The timing function of the control circuit 50 is implemented by a multivibrator 128 which provides a square-wave clock signal whose rate is deter-mined by the variable capacitor 122. The square-wave clock signal is applied therefrom to a divider circuit 130, whose out-put is applied in turn via a standby switch 124b, which is a portion of the mode sequence switch 124 as generally shown in Fig. 4, to a series of one-shot multivibrators rom which the various control signals for effec~ing the actuation of the RF
oscillator 52 are derivecl to provide RF signal via the antenna switch 56 to the antenna or pximary coil 16, whereby similar RF
signals are applied to control and energize the receiver 12, as explained above. As seen in Fig. 6, the square-wave clock signal is applied via the standby switch 124b to a first, 10 ms one-shot multivibrator 163 and then to a second 20 ms one~shot multi-vitrator 165~ whose output i5 applied via a NORgate 167,a NOP~gate 182 and an inverter 184to excite the RF oscillator 52 to provide the 20ms first power pulse to the receiver 12. Further, the output of the 20 ms one-shot multivibrator 165 is also applied to a 1 ms one-shot multivibrator 132 and then to a one-shot multivibrator 134, from whose Q output terminal a 1 ms pulse is applied via a closed, atrial control switch 126g, the NOR gate 182 and inverter 184 to energize the RF oscillator 52 to provide the flrst cor~mon pulse. In similar fashion, the Q output of the one~shot multi-vibrator 134 is applied to a further one-shot multiplier 136, whose Q output in turn is applied to a one-shot multiplier 138, whose Q output provides after the 1 ms delay pro~ided by the one-shot multiplier 136, an arming control pulse. The arming control pulse is applied via the NOR gates 174, 176 and 182l and the inverter 184 to the ~F oscillator 52 to generate the second command pulse.
In similar fashion, the output of the one-shot multi-plier 138 is applied to the inputs of a pair of one-shot multipliers 140 and 142. If the gang connected AV (atrial-ventrical) sequential switch 126e-1 and e-2 are disposed in their uppermost position as seen in Fig. 6, the output of the one-shot multiplier 140 is applied to a ventrical one-shot multiplier 144, whose Q ou~put is applied via NOR yate~ 174 and 176, a closed ventrical switch 126f, and NOR gate 182 to excite the RF o~cillator 52 to transmit the third control or ventrical pulse to 32 the receiver 12. If, on the other hand, the switches 126e-1 and e-2 are in their lowermost position and switch 126f is in its open position, the ou~put of the variable, one-shot multiplier 142 actuates the ventrical one-shot multiplier 144 to apply via the NOR gates 174, 176 and 178, the inverter 180 and the closed AV sequential switch 126e-2, whereby the first control or actual pulse and a second control or actual pulse are applied, with a selectable period therebetween as determined by the setting of the potentiometer 120, to excite the RF oscillator 52 for corresponding first and second pulses of RF energy.
In order to operate the transmitter 10 and the receiver 12 in a demand mode, the demand mode control switch 126d is disposed to its lower position (the other switches rernaining in the positions shown in Fig. 6), whereby a NOR ~ate ~.~Z6~5~

153 is permitted to respond to a sensed R-wave signal as received at a first input of NOR gate 153, In addition, NOR gate 153 may be also enabled by the end of the timing sequence signal AS
derived from the Q output of the ventrical one-shot multiplier 144. As seen in Fig. 6, the output of the NOR gate 153 energizes a sensing onP shot multivibrator 154 to apply an output via NOR
gate 155 to successively energize a 100 ms one-shot multivibrator 150 and a 10 ms one-shot multivibrator 152. The output of the 100 ms one-shot multivibrator 150 is applied through NOR gate 167 and serves to disable the NOR gate 182 for a corresponding period of time and thereby turn on the RF oscillator 52, for a 100 ms period following a delay after a normal patient's heart activity is sensed or when a timing period has been completed.
During the 100 ms period, which falls within the refractory period o the patient's heart, the oscillator 52 is turned on and a 100 ms power pulse is transmitted to provide additional power to the receiver 12. The outpuk of the l0 ms multivibrator 152 i5 applied to a multivibrator 148 whose 1 ms output is applied to the antenna switch 56 to permit the signals as derived from the receiver 12 indicative of the a-trial and vent-ricular signals of the patientls heart and a timing signal, to be applied to the pulse-width modulator 54 and the multiplexer 55, as will be explained with regard to Fig. 7. In similar fashlon, a telemetry switch 124a may be closed whereby a one-shot multivibrator 156 is energized to provide from its output Q a 4 ms inhibit signal via the NOR gate 158, the inverter 160, the NOR gate 162 and the inverter 164 to the divider circuit 130, whereby the energization of the RF oscillator 52 is terminated for a corresponding period to permit the atrial and ventricular signals as derived from the receiver 12 to be monitored. In a further feature of the timing and mode control circuit 50, a high rate control switch 126c may be depressed to apply a first si~nal via the inverter 168 and a second siynal via NOR gate 170, NOR gate 162 and inverter 164 to the divider circuit 130, where-by the divider circuit 130 operates to divide the output of the multivibrator 128 by a smaller factor to effect the actuation o the RF oscillator 152 at a higher rate, e.g., by a factor of 4, whereby the patient's heart may be stimulated at the higher rate.
In addition, there is provided a standby switch 124b (as a part of the mode sequence switch 124) which, when disposed in its standby, lower position, applies the ~utput of the divider circuit 130 via an inverter 166 and the NOR gate 155 to excite a 100 ms multivibrator 150, whereby its output is applied via a NOR gate 167, and the NOR gate 182 to excite the RF
oscillator 52 for a corresponding period of 100 ms so that only a first power pulse of 100 ms duration is applied to energize the receiver 12.
In a further eature of this invention, a one-shot multivibrator 146 is provided that is responsive to the output of the ventrical one sh~t multivibrator 144, to provid~ at its Q
output a slgnal for a period illustratively of 333 ms to dlsable the NOR gate 176 thus preventing the application of a ventrical pulse to the patient's heart for a corresponding period. By the proVision of the one-shot multivibrator 146, the repeated application of pacing pulses to the patient's ventricular is prevented ~ithin the aforementioned period, e.g., 333;ms; thus in the event of a circuit element failure, a higher rate of pacing pulses may not be applied to the patient's ventrical.
Referring now to Fig. 7, there is shown a detailed schematic circuit diagram of the elements of the pulse-width ~:
modulator 54, the amp 53, the antenna switch 56 and the demulti-plexer 55. In particular, the antenna switch 56 is comprised of a relay 200 that is responsive to the presence of an RF signal as derived rom the RF osciIlator 52, to apply the RF signal to the antenna or coil 16 as shown in Fig. 4. In the absence of the RF signal, the relay 200 is thrown to its uppermost position whereby the signals in the form of positive and negative going spikes correspondlng to the leading and trailing edges o the pulses transmitted from the transmitter 12 via the antenna 16, are applied to the amp 53 comprised of a plurality of serially connected operational amplifiers 202, 204 and 206. The output of the operational amplifier 206 is applied via a 60 Hz filter -208 to a positive buffer 210 and therefrom to a negative buffer 212. As shown in Fig. 7, the output of the positive buffer 210 is applied to a comparator 214b to be compared with a signal developed by a 50 K potentiometer. In similar fashion, the output of the negative buffer 212 is applied to a comparator 214a to be compared with a signal developed by the 50 K potentiometer.
The outputs of the comparators 214a and 214b are applied respect-ively to the clock tcl) and reset (R) inputs of a 1ip-flop 216 and have a width correspondiny to the aforementioned negative and positive spikes as de~ived ~rom the coil 16, It is understood that a pulse as cJenerated by the receiver 12 indicative of either an atrial, ventrical or timing signal will appear at the termin-als of the coil 16 as the positive and going spikes. In parti-cular, a positive going spike (corresponding to the leading edge of the pulse) above a predetermined lever provides an output from the comparator 21~b to reset the flip-flop 216, whereas a negative going spike (corresponding to the trailing edge of the pulse) above the predetermined lever provides a signal to clock the flip-flop 216.
The pulse as derived from the flip flop 216 is applied as a clock signal to a second flip-flop 218 for a purpose to be described and also to the demodulator 54, which comprises an FET
221 which turns on and off in response to its input pulse signal to thereby control the tirning period of an integrator essentially 5~9 comprised of an operational amplifier 222 I'he output of the operational amplifier 222 is a signal of an amplitude correspon-ding to the width of the pulse applied to the FET 220. In turn, the output of the operational amplifier 222 is applied to a DC
level remover circuit comprised essentially of the operational amplifier 224, whose output has an amplitude that is independent of any DC level and is further applied to the multiplexer 55 to be described.
A further comparator 214c is provided that is respon-sive to the output of the FET 221 to compare that signal with a reference level whereby an output is developed therefrom in the presence of a timing pulse as derived from the receiver 12 via the antenna 16. It is understood that the timing pulse has a width that is greater than the width of the atrial and ventrical pulses to provide at the output of the FET 221 a signal of greater amplitude, which the comparator 214c recognizes as the timing signal to reset a flip-~lop 218~ As a result, an output is developed from the Q terminal of the ~lip-flop 21a to trigger one-shot multivihrators 220 and 222, whose Q and Q outputs develop four timing signals to be applied to actuate ln timed sequence two distinct sample and hold circuits 236 and 238 corresponding, respectively, to the atrial and ventrical electro-grams of the patient. In particular, the Q and Q outputs of the flip-flop 220 are applied respectively to FET's 228 and 230 to apply at a predetermined time interval an atrial indicating pulse as amplified by amplifier 224 to the sample and hold 236 to provide therefrom a signal whose amplitude is indicative of the patient's atrial electrogram. In similar fashion, the Q and Q outputs of the one-shot multivibrator 222 are applied to FET's 232 and 234 to apply the ventrical indicating signal as derived from the DC level remover aircuit and as amplified by an amplifier 226 to the sample and hold amplifier 238, to provide ~Z~ 9 from its output and a signal indicative of the patient's ventrical electrogram.
Numerous changes may be made in the above-described apparatus and the dif~erent embodiments of the invention may be made without departing from the spirit thereof; therefore, it is intended that all matter contained in the foregoing description and in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

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Claims

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

1. Cardiac stimulating apparatus for applying stimu-lating signals to the atria and ventricles of a patient's heart, said cardiac stimulating apparatus comprising:
a. a first unit disposed externally of the patient's body and including generator means for controlling the generation and transmission of a first power signal and a second control signal in the form of electromagnetic energy into the patient's body and disposed in a timed relation to the first power signal;;
and b. a second unit adapted to be surgically implanted within the patient's body, said second unit including first means responsive to the transmitted first power signal for pro-viding the sole energization of said second unit, and second means for respectively applying the first and second stimulating signals to the atria and ventricles of the patient's heart, said second means includes decoding means responsive to the transmitted second control signal for initiating the operation of an energizing of said second means and controlling the selective application of one of the first and second stimulating signals in accordance with the timed relation of the second control signal.

2. Cardiac stimulating apparatus as claimed in claim 1, wherein said generator means generates the first power signal in the form of a first pulse-like signal having a first relatively long pulse width and the second control signal in the form of first and second pulse-like command signals in first and second sequential time slots following the first pulse-like signal in the timed relation.

3. Cardiac stimulating apparatus as claimed in claim 2 wherein said decoding means comprises means responsive to the trailing edge of the first pulse-like signals for setting first and second timing windows corresponding to the predetermined transmission of the first and second pulse-like signals, and means responsive to receipt of the first and second pulse-like command signals within the corresponding first and second timing windows for applying selectively the first and second stimulating signals to the atria and ventricles of the patient's heart, respectively.