Pacer With Self-adjusting Output

Abstract

There is disclosed an implantable pacer, the characteristics of whose output pulses change automatically after the pacer has generated a predetermined number of pulses. The pacer includes a charge-dependent resistor whose impedance increases in accordance with the total current flow through it. After a predetermined number of pulses have been generated, the impedance increases relatively abruptly. The increase in impedance controls a decrease in the amplitude and/or decrease in the width of subsequent pulses.

Description

This invention relates to implantable pacers, and more particularly to a pacer whose output pulse characteristics change automatically shortly after the pacer is implanted in a patient.

It has been established that the stimulus required from a heart pacer is greatest during approximately the second week following implantation. The excitation threshold (the current magnitude required to stimulate the patient's heart) increases during the first week, remains relatively constant during the second week and then drops during the third week. The excitation threshold after 3 or 4 weeks is only approximately 25% of the maximum value. Prior art pacers are designed to stimulate at the level required during the interval of minimum excitability.

There are several disadvantages in providing stimulating pulses of excess magnitude after approximately the first 3 or 4 weeks of pacer operation. Not only is there more tissue damage at the site of stimulation than would otherwise be necessary, but the increased power consumption necessarily decreases the pacer life. Also, in the case of a pacer which is operated in the continuous mode (or in the case of a demand pacer in which the sensing amplifier has failed), that is, where stimulating pulses are generated at periodic intervals regardless of whether spontaneous beats are occurring, there is a greater possibility of competition between natural heartbeats and pacerstimulated heartbeats than there would be were the pacer pulses of smaller magnitude; this, in turn, increases the possibility of ventricular fibrillation.

It is a general object of my invention to provide a pacer in which the pulse characteristics change automatically at some time following implantation. More specifically, it is an object of my invention to provide a pacer in which the magnitude of the pacer pulses and/or the width of the pacer pulses decrease automatically following the generation of the predetermined number of such pulses.

Briefly, in accordance with the principles of my invention, I provide a coulometer device such as an "E-cell" whose impedance changes in accordance with the total current which flows through it. The current which flows through the coulometer device incorporated in the pacer of my invention equals the sum of a fixed fraction of the current pulses delivered to the patient's heart. As more and more current flows through the device, eventually there is a relatively abrupt change in impedance. This change in impedance is used to control a decrease in the magnitude of the pacer pulse. The change in impedance can also be used to control other characteristics of the pacer or the pacer pulses, e.g., a decrease in the pacer pulse width since a narrower pulse can usually be tolerated after the pacer has been implanted for several weeks.

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 pacer circuit;

FIG. 2 depicts the manner in which the pacer of FIG. 1 is modified in accordance with the principles of my invention;

FIG. 3 is a plot of the heart pacing threshold current as a function of time; and

FIG. 4 is a plot of the pacer pulse amplitude as a function of time for the pacer of FIG. 2.

FIG. 1 depicts a conventional prior art demand pacer of the type which can be modified in accordance with the principles of my invention. Only those parts of the pacer circuit which are required for an understanding of the present invention are shown in detail. A pacer of this type is disclosed in detail, for example, in Berkovits application Ser. No. 214,218 filed on Dec. 30, 1971, and issued on Sept. 11, 1973 as U.S. Pat. No. 3,757,791, and entitled "Synchronized Atrial And Ventricular Pacer." (Although the pacer disclosed in the Berkovits application provides a atrial stimulation as well as ventricular stimulation, if the circuitry required for atrial stimulation is omitted, what is left is a demand pacer for ventricular stimulation of the type depicted in FIG. 1.) Although the invention is described in the context of a demand pacer, it will be apparent that the invention is equally applicable to a pacer of the continuous type.

The pacer is powered by a power source 14 (typically, 5 cells connected in series). Following the generation of a pacer pulse, current flows from the source through resistor 32, capacitor 38, electrode E1, the patient's heart, and electrode E2 to "ground" conductor 18. The current is low in magnitude and has no effect on the heart. Current flows only until capacitor 38 has fully charged. In order to generate a pacer pulse, transistor T4 is turned on, as will be described below. At this time, capacitor 38 discharges through the transistor, the two electrodes and the patient's heart in order to stimulate a ventricular beat. When transistor T4 turn off, the capacitor charges once again in preparation for another cycle.

Capacitor 24 controls the timing of the pacer pulses. Current flows from the source through resistors 28 and 26, the capacitor, and resistors 34 and 36. Transistors T2 and T3 are arranged in a standard configuration, with the base of transistor T2 being connected to a reference voltage. The reference voltage is shown as being derived from a reference source 30; the details of the reference source are not important for the purposes of the present invention. When capacitor 24 has charged to a level sufficient to control conduction in transistors T2 and T3, these transistors turn on and the capacitor discharges through the transistors and resistor 26. The transistors turn off as soon as the capacitor has discharged. While the transistors remain on, current flows from source 14 through resistor 28, transistors T2 and T3, and resistors 34 and 36. The positive potential developed across resistor 36 causes transistor T4 to turn on so that a stimulating pulse can be generated.

In the absence of the turning on of transistor T1, capacitor 24 charges and discharges continuously, with pacer pulses being generated at a fixed rate. The inter-pulse interval is controlled by the resistors in the charging current path. Frequently, resistor 28 is a variable impedance which is set prior to implantation. As the impedance of this resistor is increased, it takes longer for capacitor 24 to charge to the firing level of transistors T2 and T3, and consequently pacer pulses are generated at a slower rate. Since capacitor 24 discharges through resistor 26 and transistors T2 and T3, and transistor T4 remains on only while transistors T2 and T3 conduct, it is apparent that the width of each pacer pulse depends upon the magnitude of resistor 26. Frequently, resistor 26 is also a variable impedance which is set prior to implantation to control the pacer pulse width.

Electrodes E1 and E2 are coupled via conductors 18 and 20 to QRS detector 10. This unit, as well as interference rejection circuit 12, is powered by potential source 14. The QRS detector detects an electrical signal on the electrodes during the spontaneous beating of the patient's heart. When such a condition is detected, a pulse is extended to the input of interference rejection circuit 12. Normally, this circuit simply passes the pulse on to the base of transistor T1, the pulse appearing across bias resistor 22. The pulse causes transistor T1 to turn on, at which time capacitor 24 discharges through it. Since the capacitor discharges, the next stimulating pulse which would otherwise have been generated is inhibited, and a new timing cycle begins as soon as transistor T1 turns off and capacitor 24 starts to charge once again. The function of interference rejection circuit 12 is to prevent the detected pulses from being transmitted to the base of transistor T1 if such pulses occur at too fast a rate. If they do, it is an indication that what is being detected is some kind of "noise," such as stray 60 Hz, in which case pacer pulses should not be inhibited. As long as such noise is detected, transistor T1 is not turned on and the pacer operates in a continuous mode.

FIG. 3 depicts a plot of the heart pacing threshold current (in milliamperes) as a function of time -- with the time axis not being drawn to scale. As is apparent from the plot, the minimum current required to stimulate a ventricular beat increases immediately following implantation, remains approximately constant during the second week, and then decreases to a minimum level. The minimum level is approximately 25% of the maximum level. The plot varies from patient to patient, but it is clear that after approximately 4 weeks the current threshold drops to the minimum value.

FIG. 4 depicts the amplitude of the current pulses generated in accordance with the principles of my invention as a function of time -- in the case of continuous pacing. The initial current level is approximately 15 milliamperes but after approximately 4 weeks of continuous pacing, the current level drops to approximately 4 milliamperes. The fact that current pulses with magnitudes greater than necessary are generated during the first week of pacer operation is of little concern; during only one week of operation, there can be little tissue damage and little needless power consumption. What is important, of course, is that after approximately 4 weeks of continuous pacing, the current amplitude decreases and remains at the lower level during the succeeding years of operation. Also, it should be noted that the plot of FIG. 4 is based on the case of continuous pacing. As will become apparent when the circuit of FIG. 2 is considered, the switching in current level is dependent upon the total number of pulses which are generated following implantation. If it is assumed that there are approximately 100,000 pulses/day in the case of continuous pacing, then the pacer is designed such that after approximately 2,800,000 pulses, the lower current level is achieved. In the case of demand pacing, 2,800,000 pulses may not be generated until after more than 4 weeks have gone by, but while the switch-over may occur later, it still occurs after the same number of pulses have been generated and all of the advantages of the invention are still achieved. It should also be noted that the plot of FIGS. 3 and 4 are only approximate. To be on the safe side, the pacer may be designed to switch current levels after approximately 3,500,000 pulses have been generated, and the minimum current level may be greater than 25% of the maximum current level, e.g., 35% of it.

The prior art pacer of FIG. 1 is modified as shown in FIG. 2 in the following respects. First, instead of providing a single capacitor 38, two capacitors 38-1 and 38-2 are provided. Capacitor 38-2 is connected in series with diode 44, and these two elements are in parallel with capacitor 38-1. In parallel with diode 44 is another oppositely-poled diode 42 and an E-cell 40. Also, instead of providing a single resistor 26 for determining the pulse width, two such resistors 26-1 and 26-2 are provided. Resistor 26-1 is connected across an additional transistor T5 whose base terminal is coupled to the junction of diode 42 and E-cell 40.

It will be recalled that in the circuit of FIG. 1 capacitor 38 is charged by current which flows through resistor 32, the two electrodes and the patient's heart. In the circuit of FIG. 2, capacitor 38-1 charges in the same way. But capacitor 38-2 also charges at the same time, with current flowing through this capacitor and diode 44 in parallel with the current flow through capacitor 38-1.

Referring back to FIG. 1, it will be recalled that when transistor T4 is turned on, capacitor 38 discharges through the transistor, the two electrodes and the patient's heart. When transistor T4 is turned on in the circuit of FIG. 2, capacitor 38-1 discharges in the same way. But now capacitor 38-2 discharges as well, the current flowing through the transistor, the two electrodes and the patient's heart, diode 42 and E-cell 40. A fixed fraction of the sum of the pacer pulses thus flows through the E-cell.

The E-cell is a coulometer device; one of its characteristics changes in accordance with the total charge (current) which flows through it. In the case of an E-cell, the total current which flows through the cell is "remembered," and after a predetermined total current has passed through the cell, the cell impedance changes. The change is more or less abrupt, but even if the change is gradual, that has little effect on the circuit operation. As the impedance builds up (e.g., from a few ohms to a few hundred thousand ohms), capacitor 38-2 discharges to a lesser extent whenever a stimulating pulse is generated, because the time constant of the discharge path for capacitor 38-2 includes the E-cell. Eventually, the impedance of this device increases to a level for which capacitor 38-2 cannot discharge to any appreciable extent during the time that transistor T4 is on. Consequently, when the pacer is first implanted, both capacitors discharge through the transistor and a large-magnitude current pulse is achieved. As more and more pulses are generated, capacitor 38-2 is eventually taken out of the circuit operation.

The reason for providing two diodes 42 and 44 should be noted. If the E-cell is simply placed in series with capacitor 38-2, equal currents would flow through the E-cell during each complete charging/discharging cycle of capacitor 38-2. Since an E-cell is "reversible" and functions to subtract charge as well as to add it, the net effect would be that the characteristics of the cell would not change. If is for this reason that separate charging and discharging paths are provided; the two diodes steer the charging and discharging currents along two different paths. It is only when capacitor 38-2 discharges that any current flows through the E-cell.

It would be possible to include the E-cell in series with diode 44 rather than diode 42, in which case the E-cell characteristics would be changed in accordance with the charging current. However, this is not the preferred position for the cell because even when the cell has a very large magnitude, capacitor 38-2 would still eventually charge -- albeit at a very slow rate. This, in turn, would result in some of the pacer pulses being excessively large since capacitor 38-2 would discharge through transistor T4 and diode 42 occasionally. In order to prevent occasional large pulses, it is preferable to place the E-cell in series with diode 42; even though capacitor 38-2 can charge rapidly through diode 44, the capacitor cannot discharge to any significant extent during the short time interval (typically 0.5-4 milliseconds) that transistor T4 conducts.

In the actual manufacture of a pacer, a value for capacitor 38-1 should be selected which provides the proper level pulses after the coulometer device has changed state. Capacitor 38-2 should then be chosen so that the additional current provided by it insures that the maximum level pulses are achieved during the first few weeks of pacing. It is important that the E-cell not change state before approximately 2,800,000 pulses have been generated. Rather than to rely on published specifications for coulometer devices, it is preferable to charge the E-cell until it is in a state of high impedance. Thereafter, pulses from a capacitor equivalent to capacitor 38-2 can be passed through the E-cell in a direction opposite to that in which diode 42 is poled. This can be accomplished by placing a pulsing source directly across the E-cell. Each such pulse subtracts from the total charge accumulation in the cell, and after 2,800,000 pulses have been generated, 2,800,000 pacer pulses will have to be generated before the E-cell switches from the low impedance state to the high impedance state. This, however, is merely a preferred manufacturing technique, and is not essential to the method of making pacers.

The combined impedance of resistors 26-1 and 26-2 equals the impedance of resistor 26 in the circuit of FIG. 1. Initially, transistor T5 is held off and capacitor 24 discharges through the two resistors in series. Thus, the initial pacer pulse width in the circuit of FIG. 2 is the same as the pacer pulse width in the circuit of FIG. 1. The gate of transistor T5 is connected to the junction of the E-cell and diode 42, and since initially the E-cell has no voltage across it, transistor T5 remains off.

But as pacer pulses are generated and the charge on the E-cell accumulates, the voltage across the cell rises. The voltage reaches the maximum level when the E-cell reaches the high impedance state. At this time, the increased potential on the gate of transistor T5 causes this transistor to turn on. Consequently, the pacer pulse width is determined solely by the magnitude of resistor 26-2, and since this resistor is lower in magnitude than the magnitude of the two resistors in series, the pacer pulse width is shortened. If the magnitude of resistor 26-1 is several times greater than the magnitude of resistor 26-2, the pacer pulse width may be decreased, for example, from 2 milliseconds to 0.5 milliseconds. The use of the coulometer device to decrease the pacer pulse width, at the same time that the pulse current level is reduced, is only one example of the manner in which a pacer characteristic may be changed automatically after a predetermined number of pulses have been generated or after a predetermined time period has elapsed following implantation.

Although the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is only illustrative of the application of the principles of the invention. For example, pacer pulse width shortening may be provided without significant amplitude reduction by making capacitor 38-2 much smaller than capacitor 38-1. Thus it is to be understood that numerous modifications may be made in the illustrative embodiment of the invention and other arrangements may be devised without departing from the spirit and scope of the invention.

Claims

What I claim is:

1. A heart pacer comprising electrode means for connection to a patient's heart, pulser means for generating and applying stimulating pulses to said electrode means, timing means for controlling the generation of stimulating pulses by said pulser means in timed sequence, means for measuring the number of stimulating pulses generated by said pulser means, means responsive to said measuring means for controlling said pulser means to change a characteristic of subsequently generated pulses, said measuring means including a coulometer device having a characteristic which is a function of the total current which flowed through it, means for causing a current to flow through said coulometer device whenever said pulser means generates a stimulating pulse, said pulser means comprising two capacitors connected in parallel for furnishing a current pulse to said electrode means, said coulometer device being connected in series with one of said capacitors, and said measuring means including means for allowing a current flow in only one direction through said coulometer device.

2. A heart pacer in accordance with claim 1 wherein said coulometer device includes means for inhibiting current by said one capacitor to said electrode means after a predetermined total current has flowed therethrough.

3. A heart pacer in accordance with claim 2 wherein said controlling means includes means for causing the amplitude of the stimulating pulses generated by said pulser means to decrease as the total number of stimulating pulses generated by said pulser means increases, until the amplitude reaches a predetermined magnitude.

4. A heart pacer in accordance with claim 3 wherein said controlling means includes further means for decreasing the width of the stimulating pulses generated by said pulser means as the total number of stimulating pulses generated by said pulser means increases.

5. A heart pacer in accordance with claim 1 wherein said controlling means includes means for causing the amplitude of the stimulating pulses generated by said pulser means to decrease as the total number of stimulating pulses generated by said pulser means increases, until the amplitude reaches a predetermined magnitude.

6. A heart pacer in accordance with claim 5 wherein said controlling means includes further means for decreasing the width of the stimulating pulses generated by said pulser means as the total number of stimulating pulses generated by said pulser means increases.

7. A heart pacer in accordance with claim 1 wherein said controlling means includes further means for decreasing the width of the stimulating pulses generated by said pulser means as the total number of stimulating pulses generated by said pulser means increases.