US 3835865 A
The output circuit of an electric stimulator is connected to an organ such as the heart through a coupling capacitor. A semiconductor switch is triggered with a short pulse so that it partly discharges the capacitor and stimulates the organ. Immediately following the turn off of the first switch, a second switch is activated to recharge the capacitor to its initial state during which time reverse current is delivered momentarily to the heart. In one embodiment, after the first switch turns off to terminate the discharge of the capacitor which stimulates the heart, the current through the heart is reversed immediately by recharging the capacitor through a diode which is connected to a supply line and is in series with the capacitor and the heart. Turn-off of the first switch starts the diode conducting heavily. A high input impedance detector determines whether there is a natural electric signal on the organ and turns on a generator which provides the trigger pulse if there is no natural signal. The circuit is arranged so that the detector and output circuit present a high impedance to the organ.
Description (OCR text may contain errors)
Bowers I [4 1 Sept. 17, 1974 BODY ORGAN STIMULATOR  Inventor: David Lee Bowers, Wauwatosa, Wis.
 Assignee: General Electric Company,
 Filed: July 7, 1972  Appl. No.2 269,611
Related U.S. Application Data  Continuation-impart of Ser. No. 53,842, July 10,
 U.S. Cl 128/419 P, 128/421 [51 IntIClI AGln 173 6  Field of Search 128/2.l R, 419 CD, 419 E, 128/419 P, 419 R, 419 S, 421, 422, 423;
Primary Examiner-William E. Kamm Attorney, Agent, or Firm-Ralph G. I-Iohenfeldt; Fred Wiviott 5 7] ABSTRACT The output circuit of an electric stimulator is connected to an organ such as the heart through a coupling capacitor. A semiconductor switch is triggered with a short pulse so that it partly discharges the capacitor and stimulates the organ. Immediately following the turn off of the first switch, a second switch is activated to recharge the capacitor to its initial state during which time reverse current is delivered momentarily to the heart. In one embodiment, after the first switch turns off to terminate the discharge of the capacitor which stimulates the heart, the current through the heart is reversed immediately by recharging the capacitor through a diode which is connected to a supply line and is in series with the capacitor and the heart. Turn-off of the first switch starts-the diode conducting heavily. A high input impedance detector determines whether there is a natural electric signal on the organ and turns on a generator which provides the trigger pulse if there is no natural signal. The circuit is arranged so that the detector and output circuit present a high impedance to the organ.
8 Claims, 7 Drawing Figures  References Cited UNITED STATES PATENTS 3,547,127 12/1970 Anderson 128/419 P 3,662,758 5/1972 Glover 128/419 E 3,669,120 6/1972 Nielsen 128/419 P FOREIGN PATENTS OR APPLICATIONS 707,011 3/1965 Canada 128/419 R fix-amas- EAIENIEDSEPI (I974 sum 1 or 3 I INVENTOR,
DAVID L. BOWERS Waugh-m Attorney J md 9/9 s u 55E Qz L 5 kf as; Em
EAIENIEU SEPI 71914 SHEET 2 OF 3 INVENTOR, DAVID L.BOWERS By Wwfldm Attorney BODY ORGAN STIMULATOR CROSS REFERENCE TO RELATED APPLICATION This is a continuation-in-part of copending application Ser. No. 53,842, filed July 10, 1970, now abandoned.
BACKGROUND OF THE INVENTION Electric pulse generators have been used internally and externally of the body for stimulating a body organ to function in the absence of natural electric or natural nerve impulses. Stimulation of the bladder, ureter and the atrium and ventricles of the heart are common examples. The first generation of organ stimulators were essentially fixed rate pulse generators which were connected to the organ to furnish it with artificial pulses whether or not its natural or intrinsic electric or nerve impulses had reoccurred.
In due course a stand-by type of organ stimulator was developed. The stand-by type of heart pacer, commonly called a Pacemaker, includes a pulse generator which is under the control of a heart signal detector. If an intrinsic heart signal is detected, the detector turns off the pulse generator or otherwise inhibits delivery of an artificial stimulating pulse to the heart. If no intrinsic signal is detected on the heart within a period that corresponds with the desired beating rate of the heart, the pulse generator becomes inhibited and delivers one or more artificial stimulating pulses to the heart as required.
A rather sophisticated detector is required for intrinsic heart signals. These signals are composed of various frequency components and there is often a broad spectrum of noise frequencies present. The amplitude of the signals is low and must be detected down as low as 2 millivolts ordinarily. There are some unpredictable phenomena of electrical and chemical nature occurring at the interface of the stimulating electrodes and the heart tissue that result in signals being produced which confuse detection of the natural signals that must necessarily be detected for proper control of the pulse generator. When the heart coupling capacitor is slowly recharged through the heart as it is in prior art devices following the short duration pulse which stimulates the heart artificially, the noise modulates the recharging current thus accentuating the noise which the detector must distinguish from other frequency components of the intrinsic heart signals.
Another problem encountered with heart pacers is the migration of metal ions from the heart attachment electrodes into the tissue of the heart. Besides the deleterious effect this may have on heart tissue it is also suspected of being a contributor to electrical instability of the heart immediately after stimulation and during the detection time. It has been proposed in the prior art to minimize metal ion migration by stimulating the heart with alternately positive and negative pulses which are spaced from each other by the desired beating period of the heart. This was expected to attract the ions back to the electrodes on every alternate pulse but experience has shown that it is not fully effective, probably because there is too much time between pulses. More over, it did not contribute to the electrical stability of the heart during the critical detection time.
An object of the present invention is to mitigate the above disadvantages by providing a stimulator with a new type of fast recovery output circuit.
Another object is to provide an output circuit which puts the heart in a substantially stable electric state during the detection period, or in other words, to stabilize the electrode surfaces, the interface between the electrodes and tissue and to minimize the effect of polarization barriers.
A further object is to provide a fast recovery output circuit which minimizes low frequency noise components and eliminates them from the detector input so that improper detection due to capacitor charging or electrode polarization is vitiated.
A still further object is to provide means for charging the heart coupling output capacitor during the refractory period of the detector so that erroneous detection is prevented.
Another object is to provide an output circuit and a stimulator which results in a high impedance being presented to the heart to facilitate detecting the low energy signals which are produced by a depolarization of heart cells.
Another object is to provide for fast recovery of the electric stability of the heart regardless of variations in the heart load impedance or in the intensity or energy of the stimulating pulse.
Briefly, the stimulator output circuit comprises two conductive devices which may be transistor switches each of which has collector and emitter terminals serving as load circuit tenninals or one may be a diode and the other may be a switch. One of the devices may be triggered by a pulse which occurs at the instant when it is desired to stimulate the organ. Conduction by this device discharges a coupling capacitor or charges the same, depending on the embodiment, through the organ to provide the stimulus. The other device is prevented from conducting or is substantially nonconducting when the first one is conducting. When the first one terminates conduction, the second'one immediately becomes conductive and recharges or discharges, respectively, the capacitor to substantially the same voltage that it had initially. The recharging circuit also includes the organ so the recharge current is substantially equal to the discharge current plus losses regardless of the fact that the organ impedance is variable. When the second device has stopped conducting, the remainder of the charge deficiency is made up on the capacitor through a high resistance which is supplied from the d-c power source. Thus, the second device is inactive when the capacitor recharge is substantially completed and the organ signal detector and the organ will only see the high valued charging resistance which is in parallel with the second device.
As illustrated, the output circuit is incorporated in a stand-by known type of heart pacer or stimulator. The pacer includes a detector of any electric signals appearing on the organ. If the desired intrinsic signal does not appear when it should, the detector turns on a pulse generator which controls the output circuit and causes an artificial stimulus to be applied. The output circuit is, however, usable in and functions in the same way in other organ stimulators.
How the above mentioned objects and other more specific objects are achieved will appear from time to time through the course of the ensuing more detailed description of a preferred embodiment of the invention taken in conjunction with the drawing.
DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a heart stimulator which incorporates the new output circuit;
FIGS. 2-4 are schematic diagrams of alternate forms of the new output circuit;
FIGS. 56 depict wave forms which are applicable to the invention; and
FIG. 7 is a schematic diagram of an alternative embodiment of the output circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, an organ which is to be stimulated is symbolized by a resistive load marked R The load is connected to a stimulator output terminal 10 by means of a lead 11. In the case of a heart pacer, all the other circuit elements shown in FIG. 1 are encapsulated in a resin and covered with a body compatible coating so that the whole device may be implanted in the body and connected to the organ with a conductive catheter or other leads. Alternatively, the device may be located outside the body and leads 11 may constitute a conductive catheter which runs through a blood vessel and is attached to or in contact with the organ which is to be stimulated.
It is apparent that stimulating signals from the stimulator will appear on output terminal 10 and that signals which are intrinsic to the organ will also appear thereon.
A conductor 12 is connected to output terminal 10 for picking up any signals that appear on the organ. This conductor leads to a detector at the far left of the figure and has a reference numeral 13. The detector 13 is basically a preamplifier and a filter which produces a ringing signal at its output when it is shocked, the output signal being exemplified by the oscillatory wave form 14. A similar signal is produced regardless of whether the incoming signal to detector 13 is positive or negative. The detector 13 and its associated circuitry are so designed that the system is refractory for a short period of time following receipt of an incoming signal. In other words, there is a lapse of time before an output signal can be produced following a preceding signal.
Detector output signals 14 are furnished to a gated threshold trigger 15 through a capacitor 16. The structural details of the trigger 15 may be accomplished by a skilled electronic designer so they will not be discussed here. For present purposes it is sufficient to observe that input to trigger 15 of a positively swinging portion of the oscillatory wave 14 results in the output of a pulse 17 which appears on the top of a resistor 18.
Also associated with gated threshold trigger 15 is an inhibiting transistor 19. The collector resistor of this transistor is marked 20. Connected across the collector and emitter of the transistor is a resistor 21 across which the oscillatory signal voltage 14 is developed and applied to trigger 15 ordinarily. However, under certain circumstances it is necessary to inhibit the output of pulses 17 by trigger 15 in which case an inhibiting signal is applied to the base of transistor 19 by means of a conductor 22. How this inhibiting signal is developed will be discussed later. At the present, it is sufficient to note that when transistor 19 is rendered conductive the oscillatory signal 14 will be shunted to ground and will not operate trigger 15.
Assume for the sake of explanation that an intrinsic organ signal has been detected on output terminal 10 and that a corresponding pulse 17 has been produced by trigger 15. The pulse, or consecutive pulses, are furnished to an interference rejector 23. Rejector 23 is essentially a selective filter which produces output signals such as 24 only if signals 17 are coming at a predetermined rate. The details of rejector 23 are not pertinent to the present invention.
Each pulse signal 24 is applied to the base of a transistor 25 which has its collector resistor 26 connected to positive line. Transistor 25 controls a timing pulse generator 27 which is enclosed in broken lines and will now be described.
Timing pulse generator 27 has a timing network including series connected resistors 28 and 29 and a timing capacitor 30 which are connected between positive line and ground. The capacitor 30 charges and will cause the pulse generator to turn on when it reaches a certain voltage level unless the capacitor is prematurely discharged in whole or in part. There is a voltage divider including resistors 31 and 32 for controlling the bias on the timing pulse generator. Point A in the divider has a specific positive voltage value with respect to ground. A capacitor 33 parallels resistor 32 and stabilizes the d-c voltage at point A. When timing capacitor 30 charges to above a certain value, it causes a transistor 34 to become forward biased through its emitter- Base circuit and a resistor 35 leading to point A. As the timing capacitor 30 charges, its voltage is increasing toward the point where transistor 34 will be forward biased for producing a pulse 36 on its collector. This pulse will be produced unless capacitor 30 is prematurely discharged. When capacitor 30 is charged sufficiently, it will supply the necessary current to forward bias the emitter to base circuit of transistor 34 through a path which includes resistor 35 at which time it also renders the emitter to collector circuit of transistor 34 conductive and causes the pulse 36 to appear on its collector. The pulse duration is about two milliseconds which is desirable for stimulating the heart. Pulses of other duration may be chosen for stimulating other organs.
A latching voltage keeping this multivibrator circuit in conduction appears across the resistor 35 and is additive to the bias voltage across resistor 31. Another transistor 37 in the timing pulse generator also conducts and the potential appearing on its emitter is applied to a resistor 38 and conductor 22 to the base of the inhibiting transistor 19 at the far left of the drawing. A filter network including capacitor 39 and a parallel resistor 40 prevents the emitter of transistor 37 from floating above ground potential and provides emitter impedance during the conductive state of transistor 37. The purpose of applying a signal to transistor 29 is to make it conductive and thereby inhibit threshold trigger 25 so that it does not sense or respond to artificial heart pacing signals. As explained earlier, when transistor 19 conducts, the top of resistor 21 is placed at ground potential and bias is removed from threshold trigger 15 so it produces no output pulse 17.
Whenever detector 13 senses the presence of an intrinsic or natural signal on the heart, timing capacitor 30 is discharged prematurely in this example so that it will not forward bias transistor 34 nor produce a timing pulse 36. Capacitor 30 is prematurely discharged through a circuit which includes a diode 41, a resistor 42 and a collector to emitter path of transistor 25. Recall that transistor is rendered conductive whenever a natural signal is detected so it causes capacitor to be discharged to restart the timing cycle.
When one or more pulses 36 are produced by timing pulse generator 27 they produce a signal voltage across a resistor 43. This voltage is delivered through a resistor 44 to the base of a transistor 45. Pulses 36 turn on transistor 45 and cause it to produce output pulses such as 46 on its collector. There is a collector resistor 47 for transistor 45 and connected to it is a filter circuit including resistors 48 and 49 and a capacitor 50. The filter shapes pulses 36 in square form for being applied to the base of a transistor 51.
Transistor 51 amplifies and with its associated circuitry permits setting the current level of the new output circuit and, accordingly, the current which is furnished to the organ for stimulating it. Connected between the emitter of transistor 51 and positive line is a resistor 52 in series with a parallel combination comprising a fixed resistor 53 and a variable resistor 54. These resistors determine the bias current of transistor 51 and its load current which flows through an adjustable resistor 55 and a fixed resistor 56. The voltage appearing on the wiper 57 of variable resistor 55 is preferably a square wave of two millisecond duration in heart stimulators. Wiper 57 can be set for zero output voltage when variable resistor 54 and its calibrated current indicating dial, not shown, is set for zero stimulating current. These pulses 62 are delivered through a resistor 58 to one of the terminals 59 of the new fast recovery output circuit.
The FIG. 1 embodiment of the new stimulator output circuit comprises a first controllable conductive device which may be any suitable switch but is here shown as transistor 60 whose load terminals comprise a collector and emitter of the transistor. The transistor 60 has a re sistor 61 in its emitter circuit and functions as an emitter follower. The value of resistor 61 in one embodiment is 47 ohms and transistor 60 is a silicon type 2N930. Current through resistor 61 and, as it will appear, the current delivered to the organ, is equal to the voltage on the base of transistor 60 minus its base-toemitter drop divided by the value of resistor 61. Thus, the amplitude of the input pulse to the base of transistor 60 determines the current through resistor 61 and the voltage on the latter follows the base.
The fast recovery output circuit includes another controllable conductive device or switch which is in the form of a transistor 63 which may be of the same type as transistor 60. Transistor 63 has a collector and emitter as load terminals and is adapted in the embodiment to charge an organ coupling capacitor 67 rapidly and to restore the capacitor to its initial state of charge if it has been partially discharged for the purpose of delivering a stimulating pulse to the organ. From a terminal 70 which is connected to positive line, transistor 63 is supplied with base-emitter bias current through a resistor 65. The collector resistor of the transistor is marked 64 which, in this example, has a value of 1,000 ohms to limit current although its value may be reduced to zero under certain circumstances. Connected between the emitter of transistor 63 and positive line terminal 70 is a resistor 66 of preferably very high value such as one megohm. The anode of a diode 68 is connected to the emitter of transistor 63 and the cathode of the diode is connected to the collector of transistor 60. The base of transistor 63 is connected to the cathode of the diode by means of a conductor 71.
The output circuit operates in the following manner. Assume that organ coupling capacitor 67 is initially uncharged when battery 72 is connected to power input terminal 70. Coupling capacitor 67 is then charged through a series circuit starting at positive terminal and including bias resistor 65, the base-emitter of transistor 63, capacitor 67, output terminal 10, lead 11 and then the organ load R which is returned to the negative side of battery 72. With the flow of bias current, of course, there is an amplified current flow through the collector circuit of transistor 63 which causes capacitor 67 to be charged very rapidly. When capacitor 67 is near full charge, transistor 63 turns off because the difference between the voltage on its base and the capacitor voltage is too small to forward bias the transistor. Full charge of capacitor 67 to line voltage results from conduction through high resistor 66. When capacitor 67 is fully charged it is ready for delivering a stimulating pulse to the organ load R on demand.
A stimulating pulse will be delivered when first transistor 60 is pulsed on. When this occurs, capacitor 67 discharges through a series circuit which includes diode 68, the collector-emitter path of transistor 60, resistor 61, and the organ load R from which there is a ground return to the capacitor 67. Note that when transistor 60 conducts, the base of transistor 63 is pulled down to substantially ground potential in which case the latter transistor is not forward biased and is non-conducting. Upon termination of the trigger pulse which turned on transistor 60 the latter turns off and transistor 63 then becomes forward biased again to recharge capacitor 67 very rapidly. The recharge is initiated simultaneously with termination of discharge of the capacitor. When the coupling capacitor 67 is almost fully recharged, the difference between the voltage on the capacitor and on the base of transistor 63 will be inadequate to sustain conduction of the transistor and it turns off. Then, due to continued conduction through high valued resistor 66, the voltage on capacitor 67 will approach line voltage.
The amount of current delivered to the organ during the discharge interval of capacitor 67 depends upon how hard transistor 60 is driven by its trigger pulse. The voltage amplitude of trigger pulse 62 and, hence, stimulating current amplitude depends on where the wiper of variable resistor 54 is set as is explained earlier.
The wave form for a discharge and recharge cycle of capacitor 67 for one current intensity setting of the output circuit shown in FIG. 1 is shown in FIG. 5. Before transistor 60- conducts the voltage on terminal 10 and the load R is negative by an amplitude indicated by the numeral 73. When the two millisecond pulse terminates, discharge of the capacitor 67 also terminates and the potential applied to the organ is less negative and has an amplitude indicated by the numeral 74. The shaded area under the curve is indicative of the stimulus energy delivered to the organ. Immediately after transistor 60 is off, transistor 63 turns on'to recharge capacitor 67 by the amount of charge that it has lost. The recharging curve is marked 75 and the shaded area under it corresponds with the energy dissipated in the organ plus losses. Net current flow through the organ is substantially zero and the fact that flow is quickly reversed tends to minimize ion migration from the metal electrodes which connect to the organ.
It is immaterial that recharge current will tend to stimulate an organ, such as the heart, in the same manner as the discharge current from capacitor 67 because the heart is already undergoing contraction and is refractory to further stimulation at that time.
FIG. 6 shows the capacitor 67 charging and discharging curves which occur when the impedance of the organ is much lower than in the previous case. In this case most of the energy which is stored in capacitor 67 is dissipated in the organ during discharge of the capacitor. Nevertheless, a balancing current is quickly driven through the heart over a short interval immediately following the stimulation of the heart as indicated by the charging current curve 76.
In a commercial embodiment, the capacitor 67 has a value of 22 microfarads. In general terms, it charging time constant is the sum of the saturation resistance of transistor 63, resistor 64 and R times the capacitance in farads of capacitor 67. This is a non-linear impedance dependent on the base drive through resistor 65 and the voltage differential between the source voltage and the voltage on capacitor 67 at the time when the discharge pulse is completed.
In the current mode of operation the discharge time constant is a function of the constant source current through cathode resistor 61 assuming that transistor 60 is in a nonsaturating mode.
If resistor 61 is zero, the circuit will be operating in the voltage mode and the time constant will be the sum of the load resistance R, the saturation resistance of transistor 60 and the forward resistance of diode 68 multiplied by the capacitance of capacitor 67.
Note that in this output circuit the charge on capacitor 67 is restored to its initial state in a short interval immediately after the organ is artificially stimulated. This balanced recharge current, in the case of the heart restores its electrical stability very quickly so that later the detector 13 does not receive the kind of noise that it does when the capacitor is recharged slowly over most of the interval between artificially stimulated or natural heart beats as is the case in prior art devices. Noise due to capacitor charging or electrode polarization at the tissue interface is especially harmful in prior art devices because the noise is rich in low frequency components which interfere with proper operation of detector 13 since it is tuned to respond to frequencies in range of to 40 Hz which prevail in the natural R- wave on the heart. This frequency range is the predominant one in the intrinsic electrocardiograph signal. Thus, with the new output circuit, the effect of noise is removed and the heart is electrically stable when detector 13 is called upon to sense whether the next natural or intrinsic heart signal will be produced or whether it will be called upon to dictate production of stimuli if This is a low impedance for the heart to encounter and makes signal detection on the heart unstable and uncertain.
In the present circuit, coupling capacitor 67 is recharged in about to milliseconds. The detector 13 is designed, as indicated earlier, so that it is refractory for a little longer time and does not sense the signal coincident with the recharging of capacitor 67.
FIG. 2 shows an alternative form of the new stimulator output circuit. In this figure parts which are the same as in FIG. 1 are given the same reference numerals. In a sense, this embodiment is the converse of the preceding one because the coupling capacitor 67 is now charged to stimulate the heart upon receipt of a trigger pulse on terminal 59. When there is a trigger pulse, transistor 60 turns on for about two milliseconds and capacitor 67 charges from positive line terminal 70, through load R diode 68, transistor 60 and resistor 61. The right side of the capacitor 67 is then positive but it is not fully charged because of the load resistance R When transistor 60 turns off with expiration of the trigger pulse, it effectively disconnects the base of transistor 63 from ground and the base-emitter of transistor 63 becomes foward biased because the voltage on the left side of the capacitor 67 is more negative than the line. This causes initial heavy conduction through the collector-emitter path of transistor 63 into capacitor 67 and finally lesser conduction through high resistor 66. Current for this charge equalization flows from positive line, through paralleled transistor 63 and resistor 66 and back to positive line through heart load impedance R, Thus, the stimulating and recharging waveform is the inverse of that shown in FIG. 5 but there are the same balanced currents through the heart.
FIG. 3 is another alternative form of the output circuit which uses PNP transistors. As in FIG. 1, this version stimulates by discharging capacitor 67 when a trigger pulse is applied to the base of PNP transistor 80 and resistor 81. The trigger pulse in this case is negative. Capacitor 67 is first charged positive on its right side. A trigger pulse on the base of transistor 80 then permits discharge beginning at the positive right side of the capacitor, through R the emitter-base of transistor 80, the diode 68 and back to the left side of the capacitor. Immediate recharge occurs through a circuit starting with positive line and including R capacitor 67, the emitter-base of transistor 82 which is then turned on. Final charge is again applied through high resistor 66.
The FIG. 4 alternative uses PNP transistors. I-Iere stimulation occurs when the capacitor 67 is being charged. To charge, a negative pulse is applied to the base of transistor 80 turning it on and charging capacitor 67 through a circuit including transistor 80, diode 68, capacitor 67 and R This keeps the other transistor turned off. The left side of capacitor 67 is positive but below line potential because of the current limiting effect of R When transistor 80 turns off, transistor 82 turns on and the charge on capacitor 67 is equalized by conduction of transistor 82 through R Thus, a balanced reverse current again flows through the heart immediately after it is stimulated.
FIG. 7 is another embodiment of the fast recovery output circuit concept. This embodiment is ideal for replacing the slow recovery output circuits in existing implantable stimulator designs since the new circuit employs fewer components than the previously described embodiments or the piror art slow recovery circuits. In FIG. 7 components which have the same characteristics as in the previous embodiments are given the same reference numerals. Components which have the same function are given the same numerals with a prime mark. Different components are differently marked. In FIG. 7 the coupling capacitor 67 may be considered charged initially. When organ stimulation is demanded, a pulse 62 is applied to terminal 59 of a selectively conductive device which in this case is the base of a transistor 60. A resistor 61 may be optionally connected between battery return and the emitter of transistor 60. When transistor 60 conducts, due to application of a trigger pulse 62 from the stimulator, capacitor 67' which may be considered to be charged positively on its left side in this example, discharges through a series circuit including the load terminals of transistor 60, resistor 61, the ground path, and the organ load R In a heart stimulator, the duration of conduction and stimulation is about 1 millisecond. When pulse 62 terminates transistor 60 turns off and discharge of capacitor 67' terminates.
Simultaneously with termination of the capacitor discharge, restoration of the capacitor to substantially its initial charge state begins. The capacitor 67 is recharged rapidly from source terminal 70 through a resistor 91 and a diode 90 in series with it. The series recharge circuit begins at source terminal 70 and includes resistor 91, diode 90, capacitor 67 the organ load R and returns through what is shown as ground circuit to battery 72. Diode 90 conducts until the difference between the source voltage at terminal 70 and the voltage on the capacitor is less than the drop across the diode 90. Recharging or restoration of the capacitor to its initial charge state is then completed by conduction through high resistor 66. Generally, capacitor 67 will be restored completely to its initial charge state within 100 to 200 milliseconds of the time at which organ stimulation is terminated. Thus, the circuit has high input impedance in about 200 milliseconds or less after the stimulus pulse occurs and the input impedance at the terminals of the stimulator approaches the value of resistor 66 which is in parallel with the impedance of the sensing amplifier 13.
Some additional current is drawn by this output circuit as a result of diode 90 conducting when transistor 60 is on. It is possible to select a resistor 91 which is high enough in value to limit diode current to about 2 or 3 percent of the organ load current which resistor will still be low enough .in value to provide sufficient recharging current during the fast recovery phase.
The FIG. 7 circuit used in an implantable stimulator embodiment employs a resistor 91 having a value of 10 kilohms, resistor 66 of 100 kilohms, and the capacitor 67' of 3.3 microfarads. These values may vary, of course, depending on other parameters of the circuit. Other values may be used as long as the high input impedance is maintained during the period when intrinsic organ signals are being sensed. Those skilled in the art will appreciate that comparable results can be obtained with a circuit that reverses the polarity of the d-c source and the polarities of diode 90 and transistor 60.
The fast recovery output circuit of FIG. 7, like those previously described, impresses reverse current on the organ shortly after it is stimulated so that there are stable potentials at the interface of the organ and its electrodes during the sensing period. The reflected input impedance of the stimulator is maintained high so that the generated signals from the organ are not attenuated significantly. The effect of noise is removed and the organ is electrically stable when detector 13 is called upon to sense if an intrinsic organ signal has been produced or if it will be called upon to dictate production of an artificial stimulus if the intrinsic signal does not occur within the desired time interval.
In summary, several versions of the new output circuit for organ stimulators have been described. Each provides a balanced current through the heart or other organ which is opposite in sense to the stimulating pulse current. In all cases the reverse current is applied immediately after the stimulating current in a short period of time which is much shorter than the period between natural or artificially induced stimuli. Detection of the electrical state of the heart after an artificial stimulus is made easier because the heart is electrically stable soon after stimulation, in view of no recharging current flowing during the critical detection period. The natural depolarization potentials of the heart are more easily detectable because of the high impedance of the new output circuit as compared with any known stimulator. In addition, the balanced reverse currents used are believed to minimize metallic ion migration from the electrodes into the tissue to a greater extent than any heart pulsing sequence practiced heretofore.
Although the new output circuit has been described in connection with a stand-by or demand type of cardiac Pacemaker, it will be understood by those versed in the art that the circuit may be advantageously employed in fixed rate Pacemakers and in stimulators for other organs too. Moreover, even though specific embodiments have been described, such description is to be considered illustrative rather than limiting for the inventive concepts presented herein may be variously embodied and are to be limited only by construing the claims which follow.
ll. An electric organ stimulator comprising:
a. terminals which are adapted to be connected to a dc source, 1
b. a capacitor,
c. a first output terminal connected to one side of said capacitor and adapted to be connected to an organ load,
d. a second output terminal connected to a source terminal and adapted to be connected to an organ load,
e. first and second substantially unidirectionally conductive means each having main conductive paths connected in series with each other between said d-c source terminals, at least the first conductive means including a control element for selectively controlling conduction thereof in response to a signal on said element, the other side of said capacitor being connected to a point between said conductive means,
f. means for applying a control signal to said control element to cause said first conductive means to conduct for a predetermined interval at a time when stimulation of an organ is desired, conduction by said first conductive means causing current flow through said capacitor and between said organ load output terminals to thereby alter the initial state of charge on the capacitor rapidly, said second unidirectionally conductive means remaining substantially nonconductive when said first means is conducting,
g. said second conductive means responding to termination of conduction of said first conductive means by conducting and causing current flow through said capacitor and between said organ terminals to thereby substantially reestablish the initial state of charge on said capacitor rapidly,
h. a high value resistor means connected between one source terminal and to said capacitor, said resistor means continuing to conduct after said second conductive means stops conducting whereby to complete reestablishing the charge on said capacitor to its initial state.
2. The stimulator in accordance with claim 1 wherein:
a. said first unidirectionally conductive means is a semi-conductor means having load terminals serially connected in said conductive path and a control terminal to which said control signal is applied to cause said first conductive means to conduct, and
b. said second unidirectionally conductive means is a diode.
3. The stimulator in accordance with claim 1 wherein:
a. said second unidirectional conductive means is a diode means and said high value resistor means is also connected in parallel with said diode means to complete reestablishment of said charge as aforesaid.
4. An electric organ stimulator comprising:
a. terminals adapted to be connected to a d-c source,
b. a capacitor and a first output terminal connected to one side thereof and adapted to be connected to an organ load,
c. a second output terminal connected to a d-c source terminal and adapted to be connected to an organ load,
d. diode means and a controllable switch means having load terminals and a control means, said diode means and said load terminals being connected in series between said d-c source terminals, said control means selectively controlling conduction between said load terminals in response to a control signal on said control means for a predetermined interval when stimulation of an organ is desired,
e. means for applying a control signal to said control means,
the other side of said capacitor being connected to a point between said diode means and controllable switch means, whereby to enable said capacitor to charge to an initial state from said source through said diode means when said switch means is nonconductive,
g. a high value resistor means connected between the d-c source terminal to which said diode means is connected and said other side of said capacitor, said resistor thereby shunting said diode means,
h. conduction of said controllable switch means in response to said control signal causing current flow through said capacitor and between said organ load output terminals to thereby alter the initial state of charge on said capacitor rapidly, termination of said signal being followed by altered current flow through said diode, said capacitor and between said organ load terminals to substantially reestablish the initial state of charge on said capacitor rapidly, reestablishment of said charge being completed by ensuing conduction through said high value resistor means.
5. The stimulator in accordance with claim 4 including:
a. a resistor means connected in series with said diode, said resistor means having a substantially lower value than said high value resistor means.
6. An electric organ stimulator comprising:
a. terminals which are adapted to be connected to a d-c source,
b. output terminals adapted to be connected to an organ,
c. a capacitor having one and another sides the one of which is connected to one of said output terminals,
d. first and second unidirectionally conductive means, the first of said conductive means being connected between one of said source terminals and said other capacitor side and the second of said conductive means being connected between said other capacitor side and another of said source terminals, said conductive means being connected in series with each other between said d-c source terminals, the other of said output terminals being connected to said one source terminal,
e. at least one of said unidirectionally conductive means having a control means for selectively controlling said one conductive means to conduct in response to a signal on said control means, means for applying a control signal to said control means to cause said one conductive means to conduct for a predetermined interval at a time when an organ is to be stimulated, conduction by said one conductive means causing current flow in one direction across said output terminals and through said capacitor to thereby alter the initial charge on said capacitor,
said other conductive means responding to said one conductive means conducting by remaining substantially nonconducting, and said other conductive means responding to said one conductive means terminating conduction by immediately conducting to cause current flow in an opposite direction across said output terminals and through said capacitor to thereby reestablish the initial state of charge on said capacitor,
g. means for sensing the appearance of natural organ signals on said output terminals,
h. pulse signal generator means controlled by said sensing means to produce said control signal in response to said sensing means sensing no natural signal for a predetermined interval, and
i. high value resistor means connected between said one source terminal and to said capacitor, said resistor means continuing to conduct after said second conductive means stops conducting whereby to complete charging said capacitor to its initial state.
7. The invention set forth in claim 6 wherein:
a. said first and second unidirectionally conductive means are transistor means each having two load terminals and a control terminal,
b. a diode means connected between said other side of said capacitor means, one of said load terminals of said first transistor means and said diode means also being jointly connected between one of said load terminals of said second transistor means and said control terminal of said second transistor means whereby to establish said second transistor control terminal and its one said load terminal at substantially the same potential when said first transistor means conducts to thereby prevent said second transistor means from conducting upon occurrence of said control signal, the other of said load terminals of said second transistor means being connected to one d-c source terminal and the other of said load terminals of said first transistor means being connected to another of said d-c source terminals.
8. The invention set forth in claim 7 wherein:
a. said transistor means each have an emitter and a collector comprising said load terminals and a base terminal comprising said control terminal,
b. said second transistor having its collector connected to said another source terminal and its emitter connected to the anode of the diode means,
c. said first transistor means having its collector connected to the cathode of said diode means and its emitter connected to said one of said source terminals,
d. the said other side of said capacitor being connected to said anode of the diode means,
e. means for applying control signals to said control terminal of said first transistor means said signals being applied in a sequence corresponding with the desired responses of the organ, the said first transistor responding to said signals by conducting and thereby discharging said capacitor sufficiently to stimulate the organ after which said second transistor means conducts in response to termination of conduction by said first transistor means to recharge said capacitor to its initial state.