Passive Pacer Refractory Circuit

Abstract

There is disclosed an implantable pacer having a one-shot multivibrator refractory circuit which does not require the use of any active elements other than those normally found in an implantable pacer. The last stage of the heartbeat detector and amplifier is capacitively coupled to the base of the discharging transistor which is in parallel with the conventional timing capacitor. Positive resistive feedback is provided between the collector of the discharging transistor and the input terminal of the last stage of the amplifier. While the last stage of the amplifier and the discharging transistor both perform their usual functions, together they also function as a one-shot multivibrator refractory circuit.

Description

This invention relates to refractory circuits for heart pacers, and more particularly to a passive refractory circuit for an implantable pacer.

During a short refractory period following a heartbeat, the human heart cannot be stimulated once again. Even if a stimulus is provided, the heart does not respond. When a patient is provided with a heart pacer, the pacer is often designed to exhibit a refractory period. For approximately 200 milliseconds following each heartbeat, the heart activity is not sensed, or if it is sensed no action is taken in response to the detection of a beat. The reason for this is that any heart activity during the refractory period is a reflection of the previous beat; there should be no heart signals (ventricular beat) which can be detected during the refractory period.

In a demand pacer, a timing circuit is usually provided. The timing circuit includes a capacitor which is discharged whenever a heartbeat takes place. The capacitor then starts to charge toward a firing level, and when this level is reached a stimulating pulse is generated. In the case of a natural heartbeat which occurs prior to the capacitor reaching the firing level, the detection of the heartbeat results in the discharge of the capacitor and the start of a new timing cycle.

There are two reasons for providing a refractory circuit in a pacer. First, if a reflection signal were to be detected and to result in the discharge of the capacitor, it is apparent that in the absence of a natural heartbeat, the next stimulating pulse would occur after the lapse of too long a time interval. This is because upon the occurrence of the previous heartbeat the capacitor started to charge toward the firing level, but the charging was interrupted and the capacitor was discharged upon the detection of the reflection signal. Since the capacitor must start to charge all over again, an extra time interval elapses between the previous heartbeat and the next stimulating pulse.

The second reason for providing a refractory circuit in a pacer relates to the particular refractory circuit which is usually provided. The circuit is usually a one-shot multivibrator. In the absence of the multivibrator, marginal heartbeats which are detected might control only the partial discharging of the timing capacitor. In such a case, the capacitor would start to charge from an intermediate level rather than from a minimum level, and the next stimulating pulse would be generated too early. In fact, the next stimulating pulse might even be generated during the natural T wave, and this can be dangerous to the patient. For this reason, a one-shot multivibrator is used because once it is triggered, even following the detection of a marginal heartbeat, it can be designed to fully discharge the timing capacitor. The output pulse of the multivibrator controls the discharge of the capacitor to the minimum level following the detection of even a marginal heartbeat. This same one-shot multivibrator functions as a refractory circuit because any reflection signals which are detected during the refractory period (the multivibrator pulse period) have no effect on the multivibrator and are effectively ignored.

Thus it is known to be desirable to utilize a one-shot multivibrator to insure the full discharge of the timing capacitor upon the detection of a heartbeat, and to provide for a pacer refractory period. Nevertheless, such refractory one-shot multivibrators, while used in external pacers, are not usually incorporated in implantable pacers. The reason for this is that they add to the volume of the unit, they draw current and thus reduce the life of the batteries, and they represent two additional active devices which may fail. In general, only absolutely necessary active devices are included in implantable pacers.

It is a general object of my invention to provide a one-shot multivibrator refractory circuit for an implantable pacer which does not require any additional active elements.

Briefly, in accordance with the principles of my invention, I interconnect two transistors which are already included in an existing implantable pacer in such a way that in addition to performing their usual functions, they further function as a one-shot multivibrator refractory circuit. In some pacers, a discharge transistor is placed across a timing capacitor. Ordinarily, the capacitor charges from a potential source, and when it reaches a firing level it discharges through a stimulating pulse forming circuit. The discharging transistor which is placed in parallel across the timing capcitor is normally non-conductive, and thus allows the timing capacitor to charge and then discharge through the stimulating pulse forming circuit. However, whenever a natural heartbeat is detected, the discharging transistor is turned on to provide an alternate discharge path for the timing capacitor. The pulse for turning on the discharging transistor is usually derived from the last transistor stage in the heartbeat detecting circuit. This circuit responds to the appearance of an electrical signal on the ventricular electrodes --which signal indicates the occurrence of a heartbeat-- amplifies the signal, and then applies the pulse to trigger the discharging transistor.

In some prior art implantable pacers, there is provided a capacitor for coupling the pulse from the output stage of the amplifier to the trigger terminal of the discharging transistor. But there is generally no feedback from the discharging transistor back to the last stage of the amplifier. In accordance with the principles of my invention, I provide regenerative resistive feedback. This feedback, together with the coupling capacitor, converts the last stage of the amplifier and the discharging transistor to a one-shot multivibrator. Even though the last stage of the amplifier still performs its amplification function, and the discharging transistor still performs its discharging function, the two of them together further function as a refractory circuit. In effect, a refractory circuit for the pacer is achieved without requiring any additional active elements other than those usually found in an implantable pacer; the additional elements required for the refractory circuit are passive only.

Further objects, features and advantages of my invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 depicts a prior art implantable pacer; and

FIG. 2 depicts the modifications required thereto in accordance with the principles of my invention to provide a one-shot multivibrator refractory circuit without requiring the use of additional active elements.

FIG. 1 is the same as the drawing in U.S. Pat. application Ser. No. 214,218, filed Dec. 30, 1971, of Barouh V. Berkovits entitled "Synchronized Atrial and Ventricular Pacer," which application is hereby incorporated by reference. Although the pacer of FIG. 1 provides stimulating pulses for a paient's atria as well as his ventricles, it will be apparent that the principles of the present invention are applicable to pacers without an atrial stimulating capability. The atrial stimulating circuit of FIG. 1 includes those elements in the bottom half of the drawing, and if they are omitted from the circuit, tgether with resistors 89 and 38, capacitor 54 and diodes 36 and 40 (with diode 40 being replaced by a short circuit), there results a prior art type ventricular pacer. It is this ventricular pacer which will now be described briefly.

Capacitor 57 is the timing capacitor. It is bridged by transistor T6 which is normally off. The capacitor charges from batteries 1-5 through potentiometers 35 and 37, and when the voltage across it reaches the firing level, the capacitor discharges tnrough transistors T7 and T8, and a stimulating pulse is applied to ventricular electrodes E1 and E2.

THe electrodes are coupled to the base and emitter terminals of transistor T1, this transistor being the first in a 3-stage amplifier for detecting and amplifying heartbeat signals which appear on the electrodes. Whenever a heartbeat is detected, a negative pulse is applied at the base of transistor T4 to turn this transistor on. As a result of current flowing through transistor T4, the junction of resistors 34, 45 and 32 rises in potential. The positive pulse is extended through capacitor 53 to the base of transistor T6 to turn the latter on. When the transistor turns on, capacitor 57 discharges through it; the capacitor is not allowed to charge to the firing level because a natural heartbeat has been detected. Instead, the capacitor is discharged and a new timing cycle begins. The capacitor must charge from a minimum level up to the firing level before a stimulating pulse is next generated.

It is thus apparent that in the pacer of FIG. 1 the last stage T4 of the heartbeat detector and amplifier extends a pulse directly to the base of discharging transistor T6. There are two problems with this configuration. First, reflection signals which occur during the heart's refractory period can cause transistors T4 and T6 to conduct and to thus discharge capacitor 57, with the result that the next stimulating pulse, if one is required, may be delayed unnecessarily. Second, a marginal heartbeat which is detected may not result in the full discharge of capacitor 57 and thus it is possible for the next stimulating pulse to be generated prematurely.

FIG. 2 depicts the changes required in the prior art circuit of FIG. 1 to provide a one-shot multivibrator refractory circuit without the inclusion of any addtional active elements in the circuit. In the circuit of FIG. 2, two elemenets have been added -- resistor 91 connected between the collector of transistor T6 and the base of transistor T4, and diode 93 connected between the upper end of capacitor 57 and the collector of transistor T6. (In the event that a prior art pacer does not include capacitor 53, such a capacitor should be added to the circuit.)

The function of resistor 91 is to provide regenerative feedback from the collector of transistor T6 to the base of transistor T4. When a natural heartbeat is detected, the detecting circuit applies a negative pulse to the base of transistor T4. This causes a positive pulse to be extended to the base of transistor T6 and this transistor to turn on for discharging capacitor 57. When transistor T6 turns on, its collector drops in potential, and this drop in potential is extended through resistor 91 to the base of transistor T4. Consequently, transistor T4 is held on even after termination of the negative pulse which is extended through resistor 30 to its base terminal. Both transistors remain on and capacitor 53 charges from current flowing through resistor 32 and the capacitor. After the capacitor has charged to an extent such that a sufficiently positive potential is no longer extended to the base of transistor T6, this transistor turns off. The increased potential which is now extended through resistor 91 to the base of transistor T4 turns the latter transistor off. Capacitor 53 then discharges through the various resistors connected across it preparatory to the next cycle of operation.

In effect, the combination of resistor 32 and capacitor 53 between the collector of transistor T4 and the base of transistor T6, and the regenerative feedback provided by resistor 91, convert transistors T4 and T6 to a one-shot multivibrator. Even though transistor T4 still functions as the last stage of the input amplifier, and even though transistor T6 still functions to discharge capacitor 57 when a natural heartbeat is detected, the two transistors together also function as a one-shot multivibrator. As such, they provide the known advantages of a one-shot multivibrator refractory circuit, without however requiring any additional active elements. Even marginal R waves which are detected result in the complete discharge of capacitor 57 because once the multivibrator is triggered, both of transistors T4 and T6 remain on until the termination of the multivibrator pulse period. Even though the input pulse to the base of transistor T4 may be only marginal, transistor T6 remains fully on for the multivibrator pulse period (typically, 50-500 milliseconds, depending upon the magnitudes of resistor 32 and capacitor 53). As for reflection signals which are applied to the base of transistor T4 during the refractory period, they have no effect because the one-shot multivibrator remains on for the entire refractory period, during which time the voltage across capacitor 57 remains at the minimum level. Charging of the capacitor begins only after termination of the multivibrator pulse, that is, at the end of the refractory period, and any reflection signals which are detected during this refractory period have no effect on the circuit.

It should be noted that without the refractory circuit, capacitor 57 is discharged only so long as a pulse is extended through resistor 30 to the base of transistor T4. The capacitor then starts to charge immediately. With the refractory circuit, however, the capacitor does not start to charge once again until after the refractory period is over. For this reason, the potentiometers in the charging circuit of capacitor 57 should be decreased in magnitude when the refractory circuit is used so that the capacitor, once it starts to charge, can reach the firing level by the time that the next stimulating pulse is required.

It should also be noted that with the addition of feedback resistor 91 in the circuit, and if diode 93 is not included, the charging current for capacitor 57 which flows through potentiometer 37 could be diverted through resistor 91. This would necessarily interfere with the charging of the capacitor. It is for this reason that diode 93 is provided. When transistor T6 is off, its collector is at a higher potential (as a result of the potential extended through resistor 91) than the upper end of capacitor 57. Thus, diode 93 is reverse biased and current through potentiometer 37 flows only into capacitor 57. It is only when transistor T6 is turned on that its collector potential drops and the diode is forward biased; at this time, capacitor 57 discharges through the diode and transistor T6.

Although the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

Claims

What I claim is:

1. In a pacer having a pair of ventricular electrodes, a timing capacitor, means for charging said capacitor, means responsive to the voltage across said capacitor reaching a predetermined value for discharging said capacitor and for supplying a stimulating pulse on said ventricular electrodes, means coupled to said ventricular electrodes for detecting and amplifying a heartbeat signal, said detecting and amplifying means including a transistor in the last stage thereof, transistor means for selectively discharging said timing capacitor, said last stage transistor and said discharging transistor means each having a base terminal and a collector terminal, means for capacitively coupling the collector terminal of said last stage transistor to said base terminal of said discharging transistor means for controlling the conduction thereof responsive to the detection of a heartbeat signal, the improvement comprising diode means connected between said timing capacitor and the collector terminal of said discharging transistor means poled in a direction to permit easy current flow from said timing capacitor to the collector terminal of said discharging transistor means, and regenerative resistive feedback means connected between the collector terminal of said discharging transistor means and the base terminal of said last stage transistor for controlling said last stage transistor and said discharging transistor to function as a one-shot multivibrator refractory circuit.