Atrial And Ventricular Pacer Having Independent Rate Controls And Means To Maintain A Constant Av Delay

Abstract

There is disclosed an atrial and ventricular pacer having independent rate and AV delay controls. An adjustable timing circuit is provided to control the rate of the ventricular stimulating pulses. The occurrence of each ventricular beat triggers an atrial pulse generating circuit, an atrial stimulating pulse being generated following a variable time interval. The ventricular rate control circuit includes one potentiometer, and the atrial pulse generating circuit includes two potentiometers connected in series, one of which is ganged to the ventricular rate potentiometer. If the impedance of the ventricular rate potentiometer is increased to increase the ventricular escape interval, then as a result of the ganging of the two potentiometers the atrial pulse delay is similarly increased; thus the AV delay remains constant even as the pacer rate is varied. The second potentiometer in the atrial pulsing circuit is used to vary the AV delay.

Description

This invention relates to atrial and ventricular demand pacers, and more particularly to such pacers in which the rate and AV delay controls are independent of each other.

In prior art atrial and ventricular pacers, two separate pulse generating circuits are provided to generate stimulating pulses at respective predetermined time intervals following 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 is generated before a ventricular stimulating pulse. Each detected spontaneous ventricular beat controls the resetting of both pulse generating circuits.

Each of the pulse generating circuits in some prior art pacers is free-running; 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 freerunning 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 a 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 stimulations are possible.

In the improved atrial and ventricular pacer disclosed in my application Ser. No. 233,135, filed on Mar. 9, 1972, now U.S. Pat. No. 3,768,486, and entitled "Atrial and Ventricular Demand Pacer Having Wide-Range Atrial Escape Interval" (which application is hereby incorporated by reference), multiple atrial stimulations are precluded even with very short atrial escape intervals. 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.

The ventricular pulse generator includes a potentiometer for varying the ventricular escape interval; the atrial pulse generator includes a potentiometer for varying the atrial escape interval. This latter interval is the time period between a ventricular beat and the following atrial stimulating pulse and is referred to herein as the VA interval. The interval between each atrial stimulating pulse and the following ventricular stimulating pulse is known as the AV delay. Also, the term VV interval as used herein refers to the time interval between successive ventricular stimulating pulses. In the pacer disclosed in my aforesaid application, it is apparent that adjustment of the ventricular pulse generator potentiometer controls not only the VV interval, but also the AV delay. If the VV interval is increased, for example, and the atrial pulse generator potentiometer setting is not changed so that the VA interval remains the same, it is apparent that the AV delay increases as well. Very often, however, it is necessary for a physician to vary the VV interval without changing the AV delay. In the pacer disclosed in my aforesaid application, after the physician adjusts the pacer rate, he must necessarily adjust the atrial pulse generator potentiometer if he desires to maintain the same AV delay.

It is a general object of my invention to provide an atrial and ventricular pacer in which two independent controls are provided for adjusting the pacer rate and the AV delay. With the pacer of my invention, the physician can make one adjustment to the pacer rate without affecting the AV delay, and/or he can make another adjustment to change the AV delay without affecting the pacer rate.

In accordance with the principles of my invention, the ventricular pulse generator includes a single potentiometer; the larger the impedance setting, the longer the VV interval. The atrail pulse generator includes two serially connected potentiometers; the larger the series impedance, the longer the VA interval. One of the atrial pulse generators potentiometers is ganged to the ventricular pulse generator potentiometer; as the latter potentiometer is turned to increase its impedance, the ganged potentiometer in the atrial pulse generator is similarly turned to increase its impedance.

Suppose, for example, that the physician desires to increase the VV interval. By turning the shaft which gangs the two potentiometers together, the VV interval is extended as a result of the increase in the impedance of the ventricular pulse generator potentiometer. But, at the same time, because the ganged atrial pulse generator potentiometer is turned so that its impedance increases as well, the VA interval also increases. The two ganged potentiometers have the same maximum impedances and function so that the same change in impedance in each potentiometer affects the VV interval or the VA interval to the same degree. Consequently, if the VV interval is increased by turning the shaft which gangs the two potentiometers together, then the VA interval increases to the same extent. Since the AV delay is equal to the difference between the VV interval and the VA interval, it is apparent that the AV delay remains constant. In order to adjust the AV delay, all that is necessary is to change the setting of the other (independent) potentiometer in the atrial pulse generator. This potentiometer affects only the VA interval, and thus the AV delay can be adjusted independently.

It is a feature of my invention to provide two potentiometers in the atrial pulse generator, and one potentiometer in the ventricular pulse generator which is ganged to one of the atrial pulse generator potentiometers.

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 FIG. 2 in my above-identified application and depicts an atrial and ventricular pacer in which the rate and AV delay controls are not independent of each other; and

FIG. 2 depicts the illustrative embodiment of my invention in which independent rate and AV delay adjustments may be made.

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 spontaneous beat detection, 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, potentiometer 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 similarly includes a timing capacitor 58 and a potentiometer 62 which is used to control the VA interval. The detection of a spontaneous beat or the detection of a ventricular stimulating pulse results in the pulsing of the base of transistor T10. At this time capacitor 58 fully discharges through the transistor. Thereafter, the capacitor charges through potentiometer 62. When the charge across the capacitor is sufficient to control the firing of transistors T11 and T12, a pulse, whose duration is determined by the setting of potentiometer 95, appears across atrial stimulating electrodes E3 and E4. Even after the atrial stimulating pulse is generated, transistors T11 and T12 both remain on, and capacitor 58 remains charged. It is only the next firing of transistor T10 that controls the discharge of capacitor 58, the turning off of transistors T11 and T12, and the start of a new VA timing interval. Since an atrial stimulating pulse can only be generated after capacitor 58 is first discharged through transistor T10, and transistor T10 is only turned on when a spontaneous ventricular beat is detected or an atrial stimulating pulse is generated, it is apparent that only one atrial stimulating pulse can be generated following each ventricular beat.

It is possible with the pacer of FIG. 1 to change the AV delay without changing the pacer rate. If the setting of potentiometer 62 is increased, for example, the VA interval is increased (and the AV delay is thus decreased) but this has no effect on the ventricular pulse generating circuit and therefore the pacer rate remains the same. However, it is not possible to change the pacer rate without changing the AV delay. If the setting of potentiometer 35 is increased, for example, and no adjustment is made in the setting of potentiometer 62, it is apparent that the VV interval increases while the VA interval remains the same. This, in turn, results in an increase in the AV delay together with the decrease in the pacer rate.

With the pacer of FIG. 2, it is possible to change the pacer rate without affecting the AV delay. Potentiometer 35 in FIG. 1 is replaced by a fixed resistor 35b and a potentiometer 35a. Potentiometer 62 of FIG. 1 is replaced by two serially connected potentiometers 62a and 62b. The dotted lines 35a-62a represents a single shaft which gangs potentiometers 35a and 62a together. The settings of these two potentiometers are changed in the same way when the pacer rate adjusting shaft is turned. In all other respects the pacer of FIG. 2 operates as does the pacer of FIG. 1.

It is apparent that if the setting of potentiometer 62b is changed, the VA interval (and therefore the AV delay) is changed without affecting the pacer rate in any way. This is because potentiometer 62b affects the charging of only capacitor 58 in the atrial pulse generating circuit. (This is true also of the pacer of FIG. 1 in which a change in the setting of potentiometer 62 changes the AV delay without affecting the pacer rate, as described above.) But, unlike the pacer of FIG. 1, the pacer of FIG. 2 allows a change to be made in the pacer rate without any change being effected in the AV delay. When shaft 35a-62a is turned so as to increase the impedance of potentiometer 35a (and therefore the setting of potentiometer 62a as well), it takes longer for capacitor 57 to charge to the firing level of transistors T7 and T8. This results in an increase in the VV interval. But because the impedance of potentiometer 62a is also increased, it also takes longer for capacitor 58 to charge to the firing level of transistors T11 and T12 following its initial discharge through transistor T10. If both RC charging circuits are linear, and potentiometers 35a and 62a are identical, any increase or decrease in the VV interval results in an identical increase or decrease in the VA interval. Since the AV delay is equal to the difference between these two intervals, it is apparent that the AV delay is not affected by a change in the pacer rate.

Resistor 35b in the ventricular pulse generator is provided to insure a minimum VV interval, no matter how low the setting of potentiometer 35a. A similar fixed resistor can be included in series with potentiometers 62a and 62b if desired. Similarly, it is possible to provide another potentiometer in series with potentiometer 35a, although an adjustment to the setting of this potentiometer would necessarily affect the AV delay along with the pacer rate. It is the ganging of at least one potentiometer in the ventricular pulse generating circuit to at least one potentiometer in the atrial pulse generating circuit that allows the pacer rate to be changed without affecting the AV delay.

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, first potentiometer means for varying said 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, second potentiometer means for adjusting the width of said atrial escape timing interval, means responsive to the expiration of said atrial escape timing interval for generating an atrial stimulating pulse for extension to said patient's heart, means for controlling the automatic adjustment of said second potentiometer means when said first potentiometer means is adjusted, wherein said controlling means includes a shaft which is ganged to said first and second potentiometer means, and wherein said ventricular escape timing interval is a linear function of the setting of said first potentiometer means and the width of said atrial escape timing interval is a linear function of the setting of said second potentiometer means, and said controlling means functions to maintain a constant difference between said ventricular escape timing interval and said atrial escape timing interval.

2. An atrial and ventricular pacer in accordance with claim 1 further including third potentiometer means for adjusting the width of said atrial escape timing interval independent of the width of said ventricular escape timing interval.

3. An atrial and ventricular pacer comprising ventricular pulse generating means for generating ventricular stimulating pulses for extension to a patient's heart, first means for varying the rate at which ventricular stimulating pulses are generated, 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, second means for adjusting the width of said atrial escape timing interval, means responsive to the expiration of said atrial escape timing interval for generating an atrial stimulating pulse for extension to said patient's heart, means for controlling the automatic adjustment of said second means when said first means is adjusted, and wherein the rate at which ventricular stimulating pulses are generated is a linear function of the setting of said first means and the width of said atrial escape timing interval is a linear function of the setting of said second means, and said controlling means is operative to maintain constant the time interval between the generation of an atrial stimulating pulse and the generation of the next ventricular stimulating pulse independent of the setting of the said first means.

4. An atrial and ventricular pacer in accordance with claim 3 further including third means for varying the width of said atrial escape timing interval independent of said controlling means.