Atrial And Ventricular Demand Pacer Having Wide-range Atrial Escape Interval

Abstract

There is disclosed an atrial and ventricular demand pacer in which the atrial escape interval can be set to be less than 50 percent of the ventricular escape interval without multiple stimulations of the atria between ventricular beats. The atrial pulse generating circuit is triggered upon the occurrence of each ventricular beat, a single atrial stimulating pulse then being generated unless another ventricular beat is first detected within the atrial escape interval. Because an atrial stimulating pulse requires a trigger (ventricular beat), only one atrial stimulating pulse can be generated after each ventricular beat.

Description

This invention relates to atrial and ventricular demand pacers, and more particularly to such pacers in which at most one atrial stimulating pulse is generated for each ventricular beat.

In my co-pending application Ser. No. 214,218 filed on Dec. 30, 1971, which application is hereby incorporated by reference, there is disclosed an improved synchronized atrial and ventricular pacer. Each of the atrial and ventricular pulse generating circuits functions to generate a stimulating pulse a respective predetermined time interval after the last ventricular beat. The ventricular escape interval (the time period between successive ventricular beats) is longer than the atrial escape interval (the time period between successive ventricular and atrial beats) so that an atrial stimulating pulse, if one is required, will be generated before a ventricular stimulating pulse, if one is required, Each detected ventricular beat controls the resetting of both pulse generating circuits. In the presence of 60-Hz noise, both pulse generating circuits operate in a continuous mode, as opposed to a demand mode. In such a case, whenever a ventricular stimulating pulse is generated not only does the ventricular pulse timing circuit restart, but so does the atrial pulse timing circuit. Thus, even in the presence of noise the atrial pulse generating circuit is synchronized to the ventricular pulse generating circuit.

In the pacer disclosed in the aforesaid application, as well as in other prior art atrial and ventricular pacers, each of the pulse generating circuits is free-running in that respective atrial or ventricular stimulating pulses are generated at fixed intervals in the absence of the resetting of the pulse generating circuit. The pacer timing is synchronized to the natural heartbeats by controlling the resetting of the two circuits upon the detection of each ventricular beat. While systems of this type are satisfactory for implantable pacers, a problem has been encountered when they are used in connection with external pacers, that is, pacers which are external to the patient except for electrical leads.

In an atrial and ventricular demand pacer, each of the pulse generating circuits includes a potentiometer which can be adjusted for setting the respective escape interval. Since the pulse generating circuits are free-running in the absence of a resetting pulse, it is apparent that if the atrial escape interval is less than 50 percent of the ventricular escape interval, then two or more atrial stimulating pulses may be generated between each pair of ventricular stimulating pulses. The detection of a ventricular beat, or the generation of a ventricular stimulating pulse, causes both pulse generating circuits to be reset. If the ventricular escape interval is N milliseconds and the atrial escape interval is less than N/2 milliseconds, since both pulse generating circuits are free-running, it is apparent that if a spontaneous ventricular beat is not detected prior to the expiration of the ventricular escape interval, then at least two atrial stimulating pulses will be generated. That is because atrial stimulating pulses are generated continuously and at fixed time intervals in the absence of the detection of a spontaneous ventricular beat or the generation of a ventricular stimulating pulse. Multiple atrial stimulating pulses, of course, can only produce deleterious effects since the atria should not be stimulated too close in time to the ventricular beat.

In the case of implantable pacers, where the potentiometer settings are established on the production line and are not subject to change, the problem is not severe because satisfactory production procedures can be employed for ensuring that multiple stimulating pulses are not generated. However, in the case of an external pacer, medical and hospital personnel adjust the various control knobs to vary the two escape intervals. Especially with poorly trained personnel, the atrial escape interval can be set to be less than 50 percent of the ventricular escape interval, in which case multiple atrial stimulation is possible.

It is a general object of my invention to provide an atrial and ventricular pacer in which multiple atrial stimulation is precluded even with very short atrial escape intervals.

In accordance with the principles of my invention, the ventricular pulse generator is free-running as in prior art pacers. However, the atrial pulse generating circuit is not; instead, it includes a one-shot multivibrator. The detection of a spontaneous ventricular beat, or the generation of a ventricular stimulating pulse, triggers an atrial timing circuit. At the end of the pre-set atrial escape interval, the multivibrator is fired and an atrial stimulating pulse is generated. Thereafter, the multivibrator returns to the quiescent state; no further atrial stimulating pulses are generated. In order for the multivibrator to be triggered once again, a new timing period must be initiated, and this takes place only when the next spontaneous ventricular beat is detected or the next ventricular stimulating pulse is generated. Even if the atrial escape interval is set to be less than 50 percent of the ventricular escape interval, the multivibrator is triggered only once for each ventricular beat. Consequently, it is not possible for there to be multiple atrial stimulations between ventricular beats.

It is a feature of my invention to provide in the atrial pulse generating circuit a mechanism for generating only a single atrial stimulating pulse after a predetermined time interval has elapsed following the detection of the last spontaneous beat or the generation of the last ventricular stimulating pulse.

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 is the same as the drawing in my above-identified application and discloses the circuitry of an atrial and ventricular pacer which can result in multiple atrial stimulations if care is not taken in insuring that the atrial escape interval is greater than 50% of the ventricular escape interval; and

FIG. 2 depicts an illustrative embodiment of the present invention.

Only those parts of the circuit of FIG. 1 will be explained which are required for an understanding of the present invention. The pacer includes a pair of electrodes, E1 and E2, which are used for ventricular stimulation and a pair of electrodes, E3 and E4, which are used for atrial stimulation. A spontaneous ventricular beat causes a signal to appear on electrodes E1 and E2, and this signal is processed and results in a pulse being applied through capacitor 53 to the base of transistor T6 and through capacitor 54 to the base of transistor T10. (If switch S is closed, then the pacer operates in a continuous mode and no pulses are extended to the bases of transistors T6 and T10. Similarly, in the presence of 60-Hz noise, transistors T3 and T4 function to prevent the application of pulses to the bases of transistors T6 and T10 so that the pacer can operate in the continuous mode.) Capacitor 57 normally charges from batteries 1-5 through potentiometer 35, poteniometer 37, and resistors 61 and 63. When the voltage across the capacitor is sufficient to fire transistors T7 and T8, these transistors conduct and a large current flows through them to raise the potential across resistor 63. At this time transistor T9 fires and capacitor 65 discharges through the transistor, the electrodes and the heart tissue to stimulate the ventricles. After transistor T9 turns off, capacitor 65 recharges in preparation for the generation of another stimulating pulse. The setting of potentiometer 35 determines the magnitude of the charging current for capacitor 57. This, in turn, determines the ventricular escape interval. Each time that the voltage across capacitor 57 is high enough to cause transistors T7 and T8 to fire, the capacitor discharges through them so that another timing cycle can begin. The duration of the discharge is determined by the setting of potentiometer 37. Since a large current flows through resistor 63 whenever transistors T7 and T8 conduct, it is apparent that the setting of potentiometer 37 determines the width of the ventricular stimulating pulse. If a spontaneous ventricular beat is detected before the expiration of the ventricular escape interval, then transistor T6 conducts and capacitor 57 discharges through it. In such an event, the ventricular stimulating pulse which would otherwise have been generated when the voltage across capacitor 57 would have reached the firing level is not generated. Instead, a new timing cycle begins, with a ventricular stimulating pulse being generated only if the ventricular escape interval elapses before the detection of another spontaneous ventricular beat.

The atrial pulse generating circuit is very similar to the ventricular pulse generating circuit. Capacitor 58 is the timing capacitor, with potentiometer 62 being used to adjust the atrial escape interval and potentiometer 60 being used to adjust the atrial stimulating pulse width. Whenever the voltage across capacitor 58 reaches the triggering level transistors T11 and T12 conduct, and a large current flows through resistor 66. At this time transistor T14 conducts and (through a circuit different from that used in the ventricular pulse generator) an atrial stimulating pulse flows through electrodes E3 and E4. If a spontaneous ventricular beat is detected after the previous ventricular beat and prior to the expiration of the atrial escape interval, then the pulse transmitted through capacitor 54 to the base of transistor T10 controls the discharge of capacitor 58. In such a case, an atrial stimulating pulse is not generated and a new atrial timing period begins. Since it is possible for a premature ventricular beat to occur prior to the expiration of the atrial escape interval, the detection of a spontaneous ventricular beat controls the resetting of the atrial pulse generator along with the resetting of the ventricular pulse generator so that the two pulse generating circuits are synchronized to each other and to the natural heart rhythm. Furthermore, in the presence of noise when spontaneous ventricular beats cannot be detected reliably, the generation of a ventricular stimulating pulse causes the atrial pulse generating circuit to be reset; when transistors T7 and T8 conduct, a pulse is transmitted through diode 36 and resistor 38 to the base of transistor T10. Thus, at the same time that capacitor 57 discharges through transistors T7 and T8 to control the generation of a ventricular stimulating pulse and the start of a new ventricular escape timing interval, capacitor 58 discharges through transistor T10 to control the start of a new atrial escape timing interval. In this manner the two pulse generating circuits are synchronized to each other even when spontaneous ventricular beats are not detected (either as a result of noise, or the failure of the ventricular beat detecing circuit).

The problem with using the circuit of FIG. 1 for an external pacer in which the settings of potentiometers 35 and 62 can be manually adjusted by medical or hospital personnel is that it is possible for the atrial escape interval to be set to be less than 50 percent of the ventricular escape interval. In such a case, consider what happens following the detection of a spontaneous ventricular beat or the generation of a ventricular stimulating pulse. In the former situation, capacitor 57 discharges through transistor T6 and in the latter the capacitor discharges through transistors T7 and T8. But in either case, the ventricular escape timing interval begins at this time. Also, in both situations the base of transistor T10 is pulsed (in the former situation, through capacitor 54, and in the latter situation, through diode 36 and resistor 38) and capacitor 58 discharges through transistor T10. Thus the atrial escape timing interval also begins at this time. Sometime before half of the ventricular escape timing interval has elapsed the potential across capacitor 58 is great enough to fire transistors T11 and T12. At this time an atrial stimulating pulse is generated and capacitor 58 discharges through the two transistors. The capacitor then starts to charge once again through potentiometer 62 and after another atrial escape interval has elapsed, a second atrial stimulating pulse is generated. Depending of how low the setting of potentiometer 62, two or more stimulating pulses may be generated before another spontaneous ventricular beat is detected or another ventricular stimulating pulse is generated. In either case, the atrial escape timing interval in progress is interrupted when the next ventricular beat (either spontaneous or stimulated) occurs so that the two pulse generators are synchronized to each other. But it is apparent that multiple atrial stimulating pulses have been generated to the detriment of the patient.

In accordance with the principles of my invention, the circuit of FIG. 2 has a considerably different atrial pulse generator. Capacitor 58 charges through potentiometer 62, the setting of the potentiometer determining the atrial escape interval. That end of the capacitor connected to the emitter of transistor T10 is returned directly to conductor 9, rather than through resistors 64 and 66 as in the circuit of FIG. 1. Potentiometer 60 is not provided in the circuit of FIG. 2 because it is potentiometer 95 which determines the width of the generated pulse.

In the circuit of FIG. 1, the level to which the voltage across capacitor 58 must rise before transistors T11 and T12 turn on is determined by the potential of the base of transistor T11. This potential, or firing level, is derived by resistors 70, 72 and 74, transistor T13 and diode 76. Capacitor 68 functions as a filter to prevent variations in the firing level as various transients are generated in the overall circuit. In the circuit of FIG. 2, transistor T13 and diode 76 are not provided, and the firing level is determined by the relative magnitudes of resistors 70, 72 and 74, the base of transistor T11 being connected to the junction of resistors 72 and 74. To minimize the effect of transients, filter capacitor 98 is connected between the junction of resistors 70 and 72, and conductor 9.

when capacitor 58 fires through transistors T11 and T12, an atrial stimulating pulse is generated. Transistors T11 and T12 do not then turn off. In the steady state, a small current flows from the batteries through potentiometer 62, transistors T11 and T12, resistor 92, potentiometer 95, and resistors 96 and 97. The current is large enough to keep transistors T11 and T12 on, but it is small enough so that the potential developed across resistors 97 does not forward bias the base-emitter junction of transistor T14. The voltage across capacitor 58 is equal to the battery voltage minus the drop across potentiometer 62. When transistors T11 and T12 first conduct, as will be described below, current also flows through capacitor 93 and resistor 94. Capacitor 93 charges and then discharges until the potential across it is equal to the potential drop across potentiometer 95, and resistors 96 and 97 connected in series. At this time a steady-state condition is reached in which there are no voltage variations in the circuit.

As soon as a spontaneous ventricular beat is detected or a ventricular stimulating pulse is generated, transistor T10 conducts and capacitor 58 fully discharges through it. At this time, the voltage across transistors T11 and T12 is negative (relative to the direction of current flow through them) since capacitor 93 is charged slightly. Prior to the discharge of capacitor 58, transistors T15 and T16 are held off because no current flows through capacitor 93 and resistor 94 to forward bias the base-emitter junction of transistor T16; with transistor T16 being held off, no base current can flow in transistor T15. As soon as transistors T11 and T12 turn off and no longer deliver current through potentiometer 95 and resistors 96 an 97, capacitor 93 discharges through these elements and resistor 94. Transistors T15 and T16 still remain off because the direction of current flow through resistor 94, as a result of the discharge of capacitor 93, is in a direction to reverse bias the base-emitter junction of transistor T16. With transistors T10, T11 and T12 off, capacitor 58 begins to charge through potentiometer 62 from a near-zero level as a result of its heavy discharge through transistor T10. As soon as the capacitor voltage rises sufficiently to forward bias the base-emitter junction of transistor T11 (the time interval for this taking place being determined by the setting of potentiometer 62, that is, the atrial escape interval), transistors T11 and T12 conduct and current flows through resistor 92, capacitor 93 and the base-emitter junction of transistor T16. The transistor turns on and controls the conduction of transistor T15. When the latter transistor turns on, the additional current through potentiometer 95 contributes to the charging of capacitor 93. Current from transistor T15 also flows through resistors 96 and 97, and the resulting pulse across resistor 97 turns on transistor T14 to control the application of an atrial stimulating pulse to the atrial electrodes.

As soon as transistors T11 and T12 fire, capacitor 93 starts to charge from both transistor T12 and transistor T15. Eventually, the current through resistor 94 is so small that transistors T16 and T15 turn off. Capacitor 93 then starts to discharge through potentiometer 95, and resistors 94, 96 an 97. The capacitor stops discharging when the voltage across it equals the combined drop across potentiometer 95, and resistors 96 and 97 (produced by current flow from transistor T12). The combined drop is determined by the potentiometer and the resistor magnitudes. Capacitor 58 no longer can fire transistors T11 and T12 because these transistors are already on. The capacitor voltage remains equal to the battery voltage drop across potentiometer 62. For another atrial stimulating pulse to be generated, it is necessary to completely discharge the capacitor through transistor T10 so that transistors T11 and T12 can turn off and capacitor 93 can fully discharge. Thereafter, capacitor 58 starts to charge toward the firing level and when it reaches the firing level an atrial stimulating pulse is generated. Thus, only one atrial stimulating pulse can be generated for each ventricular beat trigger (turning on of transistor T10).

When transistor T15 first turns on, following the expiration of the atrial escape timing interval when transistors T11 and T12 turn on, collector current from transistor T15 flows through resistors 96 and 97 to turn on transistor T14. But collector current also flows through potentiometer 95 and capacitor 93 to contribute to the charging of the capacitor. Thus capacitor 93 is charged not only from the emitter current of transistor T12, but also from the collector current of transistor T15 flowing through potentiometer 95. Transistor T14 conducts only as long as transistor T15 is on; thus the atrial stimulating pulse width is determined by the setting of potentiometer 95 since the potentiometer impedance determines the time constant of the charging circuit which includes capacitor 93.

Not only does the circuit of FIG. 2 allow continuous adjustment of the two escape intervals and the pulse widths but in accordance with the principles of the invention the atrial escape interval can be made less than 50 percent of the ventricular escape interval without producing multiple atrial stimulations.

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. An atrial and ventricular pacer comprising ventricular pulse generating means for generating a ventricular stimulating pulse for extension to a patient's heart following the expiration of a ventricular escape timing interval, means for detecting a spontaneous ventricular beat of a patient's heart or the generation of a ventricular stimulating pulse for synchronizing said ventricular pulse generating means to the beating action of the patient's heart, timing means responsive to the operation of said detecting means for measuring an atrial escape timing interval, means for adjusting the width of said atrial escape timing interval, means responsive to the expiration of said atrial escape timing interval for generating one and only one atrial stimulating pulse for extension to a patient's heart for each single operation of said detecting means and independent of the duration of said atrial escape timing interval, said timing means including a capacitor, first means for discharging said capacitor responsive to the operation of said detecting means, means for controlling the charging of said capacitor following the discharge thereof, second means conductively connected to said first discharging means and responsive to the voltage across said capacitor reaching a firing level for discharging said capacitor and controlling the operation of said atrial stimulating pulse generating means, and means for maintaining said second discharging means conducting to hold the voltage across said capacitor below said firing level, said second discharging means turning off only after said capacitor is discharged by said first discharging means.

2. An atrial and ventricular pacer in accordance with claim 1 wherein said ventricular pulse generating means includes means for adjusting the width of the respective escape timing interval, and each of said pulse generating means includes means for adjusting the width of the respective generated stimulating pulse.

3. An atrial and ventricular pacer comprising ventricular pulse generating means for generating ventricular stimulating pulses for extension to a patient's heart, means for synchronizing said ventricular pulse generating means to the beating action of a patient's heart, timing means for operating in synchronism with the beating action of a patient's heart for measuring an atrial escape timing interval, means for adjusting the width of said atrial escape timing interval, means responsive to the expiration of said atrial escape timing interval for generating at most one atrial stimulating pulse for extension to a patient's heart for each venticular beating action of the patient's heart independent of the duration of said atrial escape timing interval, said timing means including a capacitor, first means for discharging said capacitor responsive to a beating action of a patient's heart, means for controlling the charging of said capacitor following the discharge thereof, second means conductively connected to said discharging means responsive to the voltage across said capacitor reaching a firing level for discharging said capacitor and controlling the operation of said atrial stimulating pulse generating means, and means for maintaining said second discharging means conducting to hold the voltage across said capacitor below said firing level, said second discharging means turning off only after said capacitor is discharging by said first discharging means.

4. An atrial and ventricular pacer in accordance with claim 3 wherein said atrial pulse generating means includes means for adjusting the width of the generated atrial stimulating pulse following the expiration of said atrial escape timing interval.

5. An atrial and ventricular pacer in accordance with claim 3 wherein said width adjusting means of said atrial escape timing interval is adjustable to produce an escape interval less than 50 percent of the time interval separating successive stimulating pulses generated by said ventricular pulse generating means.