US 3253596 A
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May 31, 1966 .1. w. KELLER, JR
CARDIAC FACER 3 Sheets-Sheet 1 Filed May 27, 1963 Z mw n VZ *A me 1M r M a W uw ,a
May 31, 1966 .1. w. KELLER, JR 3,253,596
CARDIAC FACER Filed May 27, 1963 2. Sheets-Sheet 2 United States Patent O 3,253,596 CARDIAC PACER John Walter Keller, Jr., Miami, Fla., assignor to Cordis Corporation, Miami, Fla., a corporation of Florida Filed May 27, 1963, Ser. No. 283,271 7 Claims. (Cl. 12S-421) This invention relates to cardiac pacers and more particularly to an implantable cardiac pacer having fail-safe features,
In the animal heart, blood is pumped from the atrium to the ventricles and then from the ventricles into either the lungs or the aorta and from there on into the body. At the instant that the atrium begins to contract, a depolarization of the muscle cells in the atrium has taken place. This action manifests itself as an electrical signal commonly referred to in electrocardiograph practice as the P- wave. The depolarization of the ventricular musculature cells, which proceed-s the ventricular contract-ion and ventricular pumping action when observed in usual electrocardiograph practice, is referred to as the QRS complex, or the R-wave. In normal healthy animals, including man, the R-wave always follows synchronously the P-wave, since the ventricular pumping act-ion of the heart always follows the atrial pumping of the heart. The delay between the atrial signal and the ventricular signal is of the order of W10 of 'a second in man, and less in smaller animals. n
This synchronous pumping of the two major sections of the heart is provided for by a conducting system from the atrium to the ventricles, commonly referred to as the AV conduction system. Frequently in older human beings there is a blocking of the AV conduction system so that the atrial pumping and the ventricular pumping are notsynchronous and the ventricular pumping is usually at a very much lower rate than the atrial pumping. It is the purpose in this invention to replace the physiological stimulation system provided by the AV conduction system and the following Purlcinje ores with an artificial pacemaker which provides synchronous ventricular stimulation through electrodes implanted on the heart and which is small enough to be itself implanted within the patients body.
On the other hand, if the P-wave should, for physiological reasons, weaken so as to be insufficient for synchronization pu-rposes or if the detecting portion of the pacer should fail, it is highly desirable that stimulation of ventricular action should continue even though asynchronously.
Accordingly, objects of the invention are to provide an implantable cardiac pacer which incorporates predetermined maximum -and minimum rates of ventricular stimulation in the event that synchronous stimulation between these limits is not possible.
Further objects are to provide these modes of operation in a very simple and reliable construction which can be made very small -so as to facilitate implanting the device in a patient. Other related objects are to provide such a pacer which draws very little power so that long operation without service can be expected and which incorporates means for recharging batteries while the pacer is implanted.
According to a primary aspect of the invention, a cardiac pacer includes sensing means responsive to a physiological heart pacing signal for producing a trigger signal, means for delaying said trigger signal for -a period substantially equal to a normal atrial-ventricular delay, a two-slate free running oscillator one -state of which can be terminated by the arrival of a delayed trigger signal and the other state of which is unalected by the arrival of a signal, means responsive to the return of said oscillator to said one state for producing ventricular stimulation, whereby the minimum rate at which the pacer operates Vis determined bythe natural period of the oscillator and the maxi- Cil c ICC mum rate at which -said pacer can operate is determined by the natural duration of said other state, .the natural durations ot each of lsaid states being independently predeterminable, and the arrival of delayed trigger signals at frequencies between said minimum and maximum synchronously controls said oscillator.
For the purpose of illustration a preferred embodiment of the invention is shown in the accompanying drawings in which FIG. 1 is a schematic diagram of the electronic circuitry for an implantable cardiac pacer;
FIG. 2 is an illustration of the waveforms to be Ifound within the circuit of FIG. l during normal operation; and
FIG. 3 is a schematic diagram of a battery charging apparatus for the device of FIG. l.
Referring now to FIG. l the cardiac pacer circuitry may conveniently be divided into six component circuits: amplifying circuits 10 and 20; the AV delay circuit 30; the pacing and .blocking oscillator 40; the stimulating pulse generator 5U; and the output amplifier 60. The amplifying circuits and 20 and the delay circuit 30 comprise input means for a cardiac signal; the pulse generator 50E and output amplifier 6) comprise output means delivering a cardiac stimulation pulse; and the oscillator 40 comprises a channel interconnecting the input and output means.
Amplifying stages 10 and 20 are arranged for high gain with low current consumption. The P-wave pick-up lead is coupled into the rst amplifying stage 10 through capacitor C1. Transistor T1 is biased to a conduting stage by R1 and the P-wave causes a positive-going impulse to be coupled into the second amplifying stage through the capacitor C2. NPN transistor T2 is biased at a low-current operating point by resistors R3 and R4 so as to conserve power and a positive-going impulse applied to the base circuit generates a still further amplied negativegoing impulse which is passed on through the capacitor C3 to the AV delay circuit 3). l
The AV delay circuit is an essentially conventional one-shot or one-cycle multivibrator arranged so that the transistor T4 is normally in its on or conducting state, being so biased by resistor R5. A negative-going impulse applied to the base circuit of transistor T4 by the amplifying circuit 20, xcauses that transistor to be cut oli and the multivibrator to reverse states, the transistor T3 being switched to its on state. The delay circuit remains in this condition until the condenser C5 is discharged through the resistor R5, the delay being determined by the respective values of these two components. It should be noted that only negative-going impulses are applied to the base circuit of transistor T4, posit-ive impulses being blocked by the diode D1 and that, once triggered, the circuit 30 is insensitive to further signals from the :amplilier 20 until the delay period has passed.
The output signal from the collector circuit of transistor T4 is coupled through the condenser C4 into the pacing and blocking Oscillator 40. When the delay circuit 30 is rst triggered, a positive output pulse is generated as the transistor T4 is cut off. This positive-going impulse, however, is blocked by the diode D2 and does not affect the oscillator 40. The return of the delay circuit 30 to its normal state, howevera generates a negative-going output pulse which is passed by the diode D2.
The pacing and blocking oscillator is an essentially conventional free running multivibrator in which the transistors T5 and T6 alternate being in the on or conducting state at a rate determined by their respective coupling networks R7-C7 and Ril-C8. In the absence of any signal being applied from the delay circuit 30 the oscillator will run at a frequency having a period equal to the sum of the normal on times for the two transistors. If, however, a negative-going pulse is applied to the base circuit of the transistor TS while it is lin its on state, it will 'be cut off prematurely and the total period of the entire circuit will be `f orrespo'ndingly shortened, It should be noted, however, that the duration of the on state for the transistor T6 is not also affected. The circuit 40 will not be aiiected at all if a negative-going pulse is applied While the transistor T6 is in its on state, the transistor T5 -being already cut off. The ability of negative-going pulses applied to cut short one of the on states then provides a means for synchronoizing the oscillator 40 to a frequency higher than its free-running frequency.
The emitter of the transistor T6 is not connected directly to ground as is that of the transistor T but rather is connected through the base-emitter circuit of a transistor T10. The emitter-collector output circuit of the transistor T shunts transistor T4 of the delay circuit 30. When the transistor T6 is in its on state, the transistor T10 is also placed in a conductive state and, 'by shunting the output of the transistor T4, reduces the loop gain of the one-shot multivibrator delay circuit so that no new delay period can be initiated. It should be understood that this operation is not merely a shunting of the output pulses from the circuit 30 but rather it completely nullifies the effect of any signal applied to the input of the amplifying circuit 10, that is no simulated AV delay can be initiated.
The output signal from the collector circuit of the transistor T6 is in turn coupled, through the capacitor 10, to a pulse generating circuit 50. The pulse generating circuit utilizes complementary symmetry to minimize current drain. By using transistors of complementary conduction types, the signal polarity inversion which takes place at each stage of amplification can be offset by the -complementary conduction arrangement of the following transistor so that increased current consumption in one transistor also increases the current consumption in the next. The two transistors T7 and T8 are arranged in a positive feedback loop and are biased so that each is normally'cut off. A negative-going pulse from the pacing oscillator 40, drives the PNP transistor T7 into conduction and its positive-going output signal in turn causes the NPN transistor T8 to also conduct. The negative-going output signal from the transistor T8 is coupled back to the base circuit of the transistor T7 through the capacitor C12 so that regeneration takes place until both transistors T7 and T8 are saturated. As the feedback loop is completed through the condenser C12 however, the period for which the transistors can remain saturated is limited and, once the capacitor C12 is charged, both transistors fall back into their cut-off condition. The duration of the conduction period is determined by the time constant of the resistor R12- capacitor C12 pair. Diodes D3 and D4 prevent the more powerful feedback signals frominterfering with the normal operation of the pacer oscillator 40.
A portion of the output signal from the transistor T8, taken from an emitter-lead resistor R15, drives a conventional common-emitter output amplifier 60 and the output from collectorfs circuit of this amplifier is coupled to the ventricle lead through blocking capacitor C15. Both the input and output signals are taken with reference to a local ground connection 61 which may, as explained and claimed in copending application Serial N-o. 300,547, filed August 7, 1963, 'be a conductive casing on the pacer itself which provides a substantial area of connection to the bulk of the pat-ients body.
FIG. 2 illustrates the various wave forms found in the circuitry during its operation under normal physiological conditions, the Roman numeral designation of each wave form being in correspondence with indicated portions of the circuit of FIG. 1. The sensed P-wave is indicated at I and its :amplifiedV version is shown at II. The output signal from the AV delayed circuit 30 is indicated at III, the start yof the square topped Wave being shown as occurring simultaneously with the arrival of the P-wave. The output of the pacer and blocking oscillator 40 is shown at IV, this signal Ibeing a square topped wave which begins simultaneously with the negative-going portion of the wave from lmal physiological signals is as follows.
the AV delayed circuit 30. The stimulation pulse output is shown at V and, as may be seen, the durati-on of the pulse from the blocking oscillator 40 is adjusted so as to be longer than the stimulating pulse and is preferably also long enough to extend beyond any physiological signals which may be caused by the application of the stimulating pulse. In this Way oscillation of the circuit is prevented and a mixmum rate of stimulation is established.
The nature `and electrical connections of the elements of a presently preferred embodiment of the invention are clearly shown in the drawing, whereas the exact characteristics, ratings, or commercial designations, so far as material for the proper operation of the device, are identified in the following table which refers to the corresponding designations in the drawing. It should fbeunderstood that adjustments and mutual correlations may have to be applied upon initial testing yfor proper performance, according to routine practice in the manufacture of devices of thi-s type.
TABLE OF COMPONENTS Resistors R1, R3 megohms-- 2.2 R4 do 1.2 R5 kilo-ohms 680 R7 megohms 3.9 R8 do 6.8 R12 kiloohms 47 R15 d0 l0 R20 do 470 R21 do 680 R22, R24 do 470 R23 megohms 2.2 R25 do 6.8 R26, R27 kiloohms 820 R28 megohms 6.8 R29 kiloohms 470 R30, R31 do 100 R32 do l0 R33 do 47 Capacitors Microfarads C4 0047 C5 01 C7, C8 0l C10 .0047 C12 .01 C15 3 3 Transistors T1, T2 T.I. 475 T3, T4 T.I. 474 T5, T6 T I. 474 T7 2N 863 T8, T9 2N 706 B T10 2N 706 B Diodes D1, D2 1N 625 D3, D4 1N 625 D10, D11 1N 482 A The operation of this circuit in conjunction with nor- The P-wave or atrial signal is amplified in the stages 10 and 20 and is used to trigger the circuit 30 so as to produce a delayed trigger signal. The amount of the delay is adjustable -by altering t-he value of the resistor R5 and is set so that the delay introduced corresponds approximately to the normal AV delay. If the delayed trigger signal is produced at a time when transistor T5 isl in its conducting state, as will be the case when P-wave's are being produced at a normal rate, the circuit -40 will be caused to.
invert states and to deliver a negative-going pulse to the stimulation pulse generator 50. The stimulating impulse lful stimulating impulse normally has a significant effect on the input signal to the pacer. This effect is indicated in broken lines at I in FIG. 2. If this signal were allowed to initiate a successive operation of the AV delayed circuit 30, the pacer would oscillate at a fast rate determined only by the delay introduced by the circuit 30. As explained previously, however, the blocking oscillator 40, operating through the transistor T10, renders the sensing and delay circuits insensitive for a predetermined period, for example 0.5 sec., following each stimulating impulse thereby establishing a maximum rate of stimulation which is lower than that which would correspond to the stimulated AV delay alone. Oscillation is accordingly prevented.
Similarly, if the patients system should be producing P-waves or similar signals at a rate which is dangerously high for ventricular stimulation, the successive amplified impulses will arrive at the AV delay circuit 30 while the transistor T6 of the blocking oscillator is still in its conducting state. The simulated AV delay will not be then initiated as the transistor T will be shunting the one-shot multivibrator feedback loop. This blocking feature provided by the oscillator circuit 40 sets a maximum rate at which the stimulating signals may be applied to the ventricles so that neither atrial brillation nor external electrical interference can cause dangerous rates of ventricular stimulation.
If, on the other hand, there should be a failure in the 4physiological production of the P-waves or a failure to properly sense the P-waves or a failure in any one of the circuits 10, 20 and 30, the oscillator circuit 40 will continue to provide stimulations Iat its free-running rate without synchronization. This free-running feature then provides a minimum rate of ventricular stimulation. A minimum rate of 60 pulses per minute is suitable for a human patient. This rate calls for a normal on period for the transistor T5 of approximately 0.4 second with the on period for T6 being 0.5 second as determined previously.
In conjunction with this means for providing a mini- Vrnurn rate of stimulation, note should be made of the particular arrangement of the battery cells providing current to the circuitry. The circuits 40, 50 and 60, which are essential for providing a minimum rate of ventricular stimulation, are connected across all five of the cells BI-BS. The circuits 10, 20 and 30, however, which are concerned with the gener-ation of a delayed synchronizing signal are each supplied by only a portion of the battery. By this arrangement if there is a failing in any one cell there will lbe a loss of synchronization of the stimuli wit-h the P-waves which will provide a valuable indication to a physician of the `battery failure but there will be no complete loss of stimulation as would endanger the patient.
While the cardiac pacer illustrated in FIG. 1 and the associated table of components draws very little power and can thus be expected to provide long -battery life, another aspect of the invention provides means for recharging the batteries `while the pacer is implanted within the patient. As illustrated in FIG. 3 the recharging apparatus associated with the pacer includes an induction coil L1 and a full-wave rectifier bridge, including diodes D-D9, the output of which is connected -across all five battery cells Bl-BS. With the incorporation of this charging circuit into the pacer, the batteries may be recharged simply anduwithout discomfort to the patient by applying an alternating magnetic field to the portion of the patients body carrying the pacer. By using a relatively low frequency, no physiological effects will be introduced and the recharging can take place while the patient is asleep.
While a particular embodiment has been shown by way of illustration it should be understood that the invention includes all modifications and equivalents falling within the scope of the appended claims.
1. An electrical cardiac pacer circuit comprising: input means responsive to a cardiac signal to produce a trigger signal, output means for producing a cardiac stimulation signal, and a channel interconnecting the input means and output means, said'channel including an oscillator having means responsive to said trigger signal to cause said output means to produce a stimulation signal synchronously with the trigger signal, and said oscillator being capable of self oscillation in the absence of a trigger signal to cause said output means to produce a stimulation signal synchronous with said self oscillation, said oscillator being permanently connected to said output means so as to automatically cause production of a stimulation signal in the presence or absence of a cardiac signal.
2. The circuit according to claim 1 further characterized in that said oscillator comprises a multivibrator which includes time constant means rendering said multivibrator insensitive to cardiac or other signals during a predetermined delay period following said trigger signal. 3. The circuit according to claim 1 characterized in that said oscillator comprises a multivibrator connected to said input means and including time constant 'means allowing the multivibrator to be free running at a given rate and including an input causing the multivibrator to perform a cycle of oscillation in response to said trigger signal recurring at a rate higher than said given free running rate.
4. The circuit according to claim 3 further characterized in that said multivibrator includes time constant means rendering said multivibrator insensitive to further sign-als during a substantial part of its cycle.
5. The circuit according to claim 3 further characterized in that said multivibrator produces a feedback signal to said input means during a substantial part of the multivibrator cycle `and said multivibrator and input means are coupled by a feedback circuit including means to render the input means ineffective during said part of the cycle.
6. The circuit according to claim 1 characterized in that said oscillator generates yand applies a control signal to said output means in response to said trigger signal thereby to cause said output means tol produce a stimulation signal, said oscillator independently generating said control signal at a predetermined minimum frequency in the absence of a trigger signal occurring at or above said minimum frequency.
7. The circuit according to'claim 1 characterized in that said circuit is implantable in a human or animal body together with -a battery comprising a plurality of cells, said oscillator and -output means being connected across the entire battery and said input means comprising an amplifier and delay means each connected across exclusively different cells such that a failing of one of said exclusively different cells will dis-able said input means while the battery as a whole continues to supply said oscillator and output means, whereby said failing of said one cell will -be indicated by a loss of synchronization of the cardiac signal with the stimulation signal before a dangerous loss of said stimulation signal occurs.
(References on following page) 7 8 References Cited by the Examiner Y Hickman et al., IRE Transactions on Bo-Medical UNITED STATES PATENTS ezotromcs, October 19611, vol. BME-8, No. 4, pp. 258- 3,061,742 10/1962 Harrison 331-113 X Kanfo'witlsetf-,OSuggeZGynecology and Ob- 3110 l 5 ster1cs,pp. cto er 3 llg tem et all 13S 4 Stephenson et a1., Journal of 'Thoracic and Cardiovasa asz et a' "v 1 cular Surgery, vol. 38, No. 5, pp. 604-609, November OTHER REFERENCES 1959' Chardack et al., Surgery, v01. 48, No. 4, pp. 643- 10 RCHARD A' GAUDET Prlmmy Exammer' 654, October 1960, WILLIAM E. KAMM, Assistant Examiner.