Cardiac Pacer With Voltage Doubler Output Circuit

Abstract

A pulse generating circuit suitable for use in an implanted cardiac pacer is described. In its simplest form, the invention comprises a long life, direct current energy source which is connected to an astable multivibrator which produces output pulses of a predetermined duration at a prescribed frequency. Connected to the output of the multivibrator is a voltage multiplying circuit which serves to increase the amplitude of the output pulses from the multivibrator. By employing a novel constant current source in the multivibrator, extremely stable operation is obtained over the useful life of the battery. Further, because of the novel design of the voltage multiplier circuit, the overall efficiency of the pulse generating circuit exceeds that of known prior art devices designed for a similar purpose.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to electronic heart pacing apparatus, and more specifically to the novel design of a semi-conductor pulse generator circuit which is efficient in terms of energy source drain, yet extremely stable in terms of frequency and amplitude of the output heart stimulating pulses.

Use of electronic pacer systems are indicated for certain conditions, and are frequently used, for example, in cases of heart disease where the nerve bundle linking the artrium and the ventricle deteriorates or becomes damaged with the result that spontaneous signals from the atrium which normally stimulate the pumping action of the ventricle are either not received, or are received on an irregular basis. A device of the type described herein is usually implanted within the body of the patient and electrodes are coupled from the implanted circuit to the heart muscle so that artificially generated pulses may be applied to the ventricle as if they were originated at the atrium.

Because the circuit is implanted surgically within the body of the patient, it is most desirable that the pacer operate reliably over extended periods of time. As such, the electronic design of the pulse generator must be such that the circuits operate with low current drain. The circuit is particularly adapted for use with lithium-iodide power sources, which source provides for delivery of power over extended periods of time without requiring venting for any gases which may be generated as a result of power generation.

SUMMARY OF THE INVENTION

The foregoing desirable features are satisfied by the pulse generating circuit of the present invention through the use of a novel constant current source which feeds the astable multivibrator pulse generator. The output from the pulse generator is supplied to a voltage multiplier network of unique design which produces at the cardiac electrodes impulses of an amplitude approximately twice that of the potential supplied by the implanted lithium-iodide battery power source.

It is accordingly a primary object of the present invention to provide an improved cardiac pacer of efficient design which may be implanted surgically within the body of the patient.

Another object of the invention is to provide an improved pulse generator network which efficiently utilizes power from the battery source utilized therein.

Still another object of the invention is to provide an improved pulse generating circuit having voltage multiplier means at the output thereof which operates extremely efficiently.

These and other objects of the invention will become more apparent from the following detailed description when considered in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit schematic of the preferred embodiment of the invention;

FIG. 2 illustrates the waveform of the signal existing at the output from the constant current source in the circuit of FIG. 1;

FIG. 3 illustrates the waveform of the signal appearing at the output of the multi-vibrator stage in the circuit of FIG. 1; and

FIG. 4 illustrates the waveshape of the output signal appearing at the cardiac electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the pulse generator of the present invention is seen to consist of three main sections, namely, the power supply section including a constant current source enclosed by dashed line box 10, an astable multivibrator contained within dashed line box 12, and a voltage multiplier output circuit shown enclosed by dashed line box 14.

Contained within the power supply section 10 is a lithium-iodide battery 16 which provides energization for the remainder of the circuit. A lithium-iodide cell is most desirable as the energy source because of its long life, the absence of gas generation during discharge, and the absence of a highly corrosive liquid electrolyte. Such a battery may be hermetically sealed and implanted in the body of a patient along with the electronic circuits which it serves.

Connected in parallel with battery 16 between terminals 18 and 20 is a capacitor 22. Also connected in parallel with battery 16 is a constant current network which includes a resistor 23, transistor 24, 26 and 28, and a voltage divider including resistors 30 and 32. More specifically, a conductor 34 connects junction 18 to a junction 36 to which is connected one terminal of the resistor 23. The other terminal of resistor 23 is connected by a conductor 38 to a junction 40 to which is connected the emitter electrode of the PNP transistor 24. The junction 40 is also connected by a conductor 42 to junction 44. A resistor 46 of relatively small ohmic value is disposed between junction 44 and the emitter electrode of PNP transistor 28. The base electrodes of transistors 24 and 28 are connected to one another. The collector electrode of transistor 24 connects to he emitter electrode of PNP transistor 26 and to the base of transistor 24. The base and collector electrodes of transistor 26 are connected together and to a terminal point 47 by means of a conductor 48. The resistors 30 and 32 are serially connected between the collector electrode of transistor 26 and a return conductor 50. Completing the constant current circuit 10 is a capacitor 49 which is connected between the junction 40 and the return conductor 50.

The multivibrator section of the circuit enclosed by box 12 includes a timing network which comprises a capacitor 52 and a resistance voltage divider including resistors 54 and 56. Also included in the multivibrator are the semi-conductor switches 58, 60 and 62. Specifically, the output from the constant current source is obtained at the collector electrode of transistor 28 which is tied to junction 64. The timing capacitor 52 has one terminal thereof connected to junction 64 and the remaining terminal connected by conductor 66 to junction 68. Connected between junction 68 and the return conductor 50 are the series resistors 54 and 56. The base electrode of transistor 62 is coupled through a resistor 70 to junction 68 by conductor 72. The collector electrode of transistor 62 is coupled by means of a diode 74, a resistor 76, and a conductor 80 to junction point 64. The emitter electrode of transistor 62 is directly connected to the return conductor 50.

Transistor 60, here shown as being of the NPN type, has its base electrode connected to junction 64 by means of a resistor 78 and conductor 80. The emitter electrode of transistor 60 is connected by conductor 82 to the junction point 84 between timing circuit voltage divider resistors 54 and 56. The collector electrode of transistor 60 is connected to the base electrode of transistor 58. The collector electrode of transistor 58 is connected to junction 68 by the conductor 86. The emitter electrode of transistor 58 is connected to the emitter electrode of transistor 28 by conductor 42 and resistor 46.

The voltage multiplier portion of the circuit enclosed in box 14 includes a capacitor 90 and first and second semi-conductor switches, here shown as NPN transistor 92 and PNP transistor 94. Transistor 92 has its base electrode coupled through a resistor 96 to the junction point 68 at which the output from the multivibrator appears. The emitter electrode is connected by conductor 98 to the return conductor 50. The collector electrode of transistor 92 is coupled through a resistor 100 to a junction 102 on conductor 34.

The base electrode of PNP transistor 94 is coupled through resistor 104 to the collector electrode of transistor 92. The emitter electrode of transistor 94 is also connected to the junction point 102 on conductor 34. The collector electrode of transistor 94 is coupled through a resistor 106 to the return conductor 50.

A first terminal of capacitor 90 is connected to the junction point 108 between the collector electrode of transistor 94 and the resistor 106. Its other terminal is connected to the positive output terminal 110. The negative output terminal 112 is connected by a conductor 114 to the collector electrode of transistor 92. A diode 116 is connected directly across the output terminals 110 and 112. The load 118, which in the present application, is the heart muscle to be stimulated, is connected across the output terminals 110 and 112.

Now that the details of the circuit layout have been described, consideration will now be given to its mode of operation.

OPERATION

As mentioned, the circuit shown enclosed by box 12 is an astable multivibrator, i.e., it is a free running multivibrator having two astable states. In one state, the transistors 58, 60 and 62 are non-conducting, and in the other state, these transistors are all simultaneously conducting. The operation of the circuit can best be understood by considering the voltage appearing at the junction 64. The waveform of this voltage is illustrated in FIG. 2.

Let it be assumed that operation begins with transistors 58, 60 and 62 each non-conducting and with the voltage appearing at junction 64 at its zero value. The current flowing out of the constant current source which includes transistors 24, 26 and 28 and the high impedance resistors 30 and 32, serves to charge the timing capacitor 52 until such time as the base-emitter threshold of transistor 60 is reached. At this point, conduction is initiated in transistors 58 and 62. As the collector of transistor 58 assumes a more positive value, so does the emitter of transistor 60 and the junction 64. Since the rate of voltage change at the emitter of transistor 60 is reduced by the voltage divider comprised of resistors 54 and 56, the base current in transistor 60 increases in a regenerative manner until transistors 58 and 62 reach saturation.

With transistor 62 fully conducting, a discharge path through conductor 80, diode 74, resistor 76 and the collector to emitter path of transistor 62 is established. Because of the relative values of the various resistors and resulting circuit parameters, the rate of discharge of the capacitor 52 through this last-mentioned path is greater than the rate at which charge is being added to the capacitor 52 by way of the constant current source connected to junction 64, conductor 66 and the resistors 54 and 56. Thus, the voltage at junction 64 now decreases until the point is reached that there is insufficient base current to hold transistor 58 in its saturated condition. As a result, transistors 62, 60 and 58 revert to their non-conducting state in a regenerative manner. The voltage appearing at junction 64 is thereby decreased to a negative value. With transistor 62 non-conducting, the capacitor 52 again begins to charge in a positive direction from the constant current source connected to junction 64 through conductor 66 and resistors 54 and 56, thus repeating the cycle.

To ensure that the source connected to junction 64 is a constant current source, transistors 24 and 28 are selected to have matched characteristics, thereby ensuring that the collector currents of these two transistors will be approximately equal or at least proportional to one another. If, in practice, it is found that the parameters of transistors 24 and 28 vary, stabilization in circuit operation can be obtained by interposing the small resistor 46 in the emitter circuit of the transistor 28.

The constant current source 10 is designed to provide a stable oscillator drive current, yet one which will yield an external indication of the condition of the battery 16. More specifically, the resistors 30 and 32 are trimmed during manufacture to provide a six-beat per minute decrease in output pulse rate when the output voltage across the load 118 drops to approximately 3.5 volts. This change in rate becomes noticeable to the patient and allows him to seek medical attention prior to the time that the battery fails completely.

Referring now to FIG. 2, there is shown the waveform of the voltage observed at junction 64. When the preferred embodiment was constructed using the component values set forth in the table at the end of this specification, the voltage swings were as illustrated in FIG. 2. No attempt has been made to calibrate the abscissa and accordingly this parameter is not to scale.

As can be seen, with transistors 58, 60 and 62 non-conducting and the voltage at junction 64 at the zero level, current from the constant current source comprised of transistors, 24, 26 and 28 flows through capacitor 52 and resistors 54 and 56 causing a voltage to build up on the capacitor 52. When the voltage at junction 64 reaches approximately 0.4 volt, base current flows through resistor 78 and the base to emitter path of transistor 60 and through the resistor 56 causing transistor 60 to begin conducting. As was explained above, when transistor 60 is rendered slightly conductive, transistors 58 and 62 are rapidly switched from their non-conducting to their saturated state in a regenerative manner, thereby causing the voltage appearing at junction 64 to swing to a positive value of 2.9 volts. With transistor 62 fully conducting, a low impedance path is presented to the charge on capacitor 52 and the capacitor begins to discharge through conductor 80, diode 74, resistor 76 and the collector to emitter path of transistor 62. (The time constant R.sub.76 .times. C.sub.52 for this portion of the waveform is indicated on FIG. 2.) This causes the voltage at junction 64 to decay exponentially until such time that transistor 58 is driven out of its saturated condition by transistor 60. This occurs when the voltage at junction 64 reaches approximately 1.2 volts. Once transistor 58 ceases to be saturated, transistors 60 and 62 are rapidly driven out of conduction in a regenerative manner and the voltage appearing at junction 64 snaps down to approximately -1.3 volts. At this point, the discharge path through diode 74 and resistor 76 is blocked by the high impedance now presented between the collector and emitter of non-conducting transistor 62, and the timing capacitor 52 again begins to charge up from the constant current source until junction 64 again reaches 0.4 volt where the foregoing cycle is repeated. The rate of change of voltage with respect to time for this re-charge portion of the wave-form is approximately equal to the collector current, I.sub.C, of transistor 28 divided by the capacitance of capacitor 52. The thresholds at which switching takes place in the foregoing discussion can be controlled by proper selection of the ratio of resistors 54 and 56.

The waveform of FIG. 3 represents the voltage appearing at junction 68, the output terminal of the multivibrator. It can be seen from this waveform that the voltage at this point remains fairly constant in spite of the partial discharge of capacitor 52 through diode 74, resistor 76 and transistor 62. This is due to the fact that the voltage across capacitor 49 is applied to this junction by way of the low impedance emitter to collector path of transistor 58. However, when transistor 58 (along with transistors 60 and 62) is turned off, the voltage at junction 68 approaches zero volt as a reference.

During the time that the voltage at junction 68 is close to its zero level, transistors 92 and 94 will be non-conducting. As a result, the capacitor 90 will be charged by way of a current flowing from terminal 18 of the battery 16, through conductor 34, through resistor 100, conductor 114, diode 116, resistor and conductor 50 back to the terminal 20 of battery 16. The charging current for capacitor 90 passes through diode 116 only when the voltage drop across the heart load 118 exceeds the diode threshold voltage. The diode 116 is included primarily to prevent erosion of the cardiac electrodes in the event a non-nobel metal is used in forming the electrodes.

The multivibrator parameters are such that it remains in its "off" condition sufficiently long for the full battery potential to be stored on capacitor 90. When the output at junction 68 swings positive as shown in FIG. 3, transistors 92 and 94 are simultaneously rendered fully conductive and capacitor 90 discharges through the load connected between terminals 110 and 112 (the heart), through conductor 114, from the collector to emitter path of transistor 92, through conductors 98 and 50, through the low impedance of the battery 16 and capacitor 22, through conductor 34 and the emitter to collector path of transistor 94. As such, at the moment that discharge begins, the voltage across the load will be approximately equal to the sum of the voltage across capacitor 90 and the potential of the battery 16 which is also stored in capacitor 22. Since, when the output pulse is applied to the heart muscle, the only impedance (other than the low impedance of the heart itself) is the forward impedance of the transistor 94, substantially twice the battery voltage is initially impressed.

By way of example, the following circuit parameters may be used in the embodiment of the cardiac pacer circuit shown in FIG. 1:

Table ______________________________________ R.sub.100 -- 4.7K ohms R.sub.106 -- 4.7K ohms R.sub.104 -- 10K ohms R.sub.76 -- 2.0K ohms R.sub.96 -- 10K ohms R.sub.56 -- 10K ohms R.sub.78 -- 22K ohms R.sub.70 -- 10K ohms R.sub.30 -- variable C.sub.22 -- 100 microfarads R.sub.32 -- variable C.sub.52 -- 0.47 microfarads R.sub.54 -- 22K ohms C.sub.90 -- 18 microfarads R.sub.23 -- 51K ohms C.sub.49 -- 18 microfarads Transistors 24, 26, 28 and 58 -- 2N2605 Transistor 94 -- 2N2905 Transistor 92 -- 2N2222 Transistors 60 and 62 -- 2N2484 Diode 116 -- 400 MW Battery source; lithium-iodide cell -- 2.8 volts ______________________________________

The sum of resistors 30 and 32 should be approximately 3 megohms, but are normally adjusted to a value such that the multi-vibrator circuit produces 72 pulses per minute. However, with resistor 30 shorted, the circuit will oscillate at 120 pulses per minute if resistors 30 and 32 are properly trimmed.

Although a specific embodiment of the invention has been shown and described, it should be apparent to those skilled in the art that minor changes and modifications can be made thereto without departing from the spirit and scope of the invention. For example, by reversing the supply connections, NPN type transistors may be used where PNP transistors are illustrated. Hence, the invention is to be determined by the scope of the accompanying claims.

Claims

We claim:

1. A cardiac pacer circuit comprising in combination:

a. a constant current source including

(1) a source of direct current potential of a predetermined voltage value having first and second terminals,

2. a series circuit connected between said first and second terminals including first and second resistors and first and second transistors, each having an emitter electrode, a collector electrode and a base electrode, the emitter electrode of said first transistor being connected to the collector and base electrode of said second transistor, the base and collector electrodes of said first transistor being coupled through said first resistor of said first terminal of said source and the emitter electrode of said second transistor being coupled through said second resistor to said second terminal of said source,

3. a third transistor having an emitter, a collector and a base electrode, the base electrode of said third transistor being connected to the emitter electrode of said first transistor and the emitter electrode of said third transistor being coupled to the emitter electrode of said second transistor,

b. an astable multivibrator including:

1. a third resistor and a first capacitor connected in series between said collector electrode of said third transistor and said first terminal of said source,

2. fourth, fifth and sixth transistors each having a base, emitter and collector electrodes, the emitter electrode of said fourth transistor being connected to the emitter electrode of said second transistor, the collector electrode of said fourth transistor being connected to the junction between said third resistor and first capacitor and coupled to the base electrode of said fifth transistor, the emitter electrode of said fifth transistor being connected to the first terminal of said source and the collector electrode of said fifth transistor being coupled through fourth resistor means to the collector electrode of said third transistor and the base electrode of said sixth transistor, said collector electrode of said sixth transistor being connected to the base electrode of said fourth transistor, said emitter electrode of said sixth transistor being coupled to said first terminal of said source,

c. a voltage doubler circuit including:

1. seventh and eighth transistors, each having an emitter electrode, a collector electrode a base electrode, said base electrode of said seventh transistor being coupled to said junction, the emitter electrode of said seventh transistor being connected to said first terminal of said source, said collector electrode of said seventh transistor being coupled to said second terminal of said source by means of a fifth resistor and coupled to said base electrode of said eighth transistor,

2. said emitter terminal of said eighth transistor being coupled to said second terminal of said source, a sixth resistor coupling said collector terminal of said eighth transistor to said first terminal of said source, and

d. a pair of output electrodes

1. a first of said pair of electrodes being connected to the collector electrode of said seventh transistor,

2. the second of said pair of output electrodes being connected in series with second capacitor means of said collector electrode of said eighth transistor.