Cardiac Pacers With Source Condition-responsive Rate

Abstract

In a pulse generating circuit, such as an implantable cardiac pacer having parallel connected batteries as its power supply, additional resistors are connected between the power supply and the pulse generating circuit of the pacer and form a part of the pulse generating circuit to indicate by a change in pulse rate when one or more cells of the batteries of the power supply fail prematurely.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to pulse generating devices, and more particularly to pulse generating devices where it is desired to determine the state of the batteries of the power supply of the device from the rate of operation of the device. The invention will here be described in most detail in association with battery powered electronic organ stimulation device or cardiac pacer since the apparatus according to the invention has been particularly developed for use with such pacers, however, the apparatus may be used in other battery powered pulse generating devices. It might perhaps be used in conjunction with stimulators for the brain, bladder and other organs as well, and with pacers other than the type hereinafter described without departing from the scope of the invention.

2. Description of the Prior Art

It may be explained that cardiac pacing has become the standard mode of therapy for heart block and its complications. Briefly, heart block is the reduction or complete lack of coordination in the beating of the atria and ventricles of the heart. In the human body, blood is pumped primarily by the contractions of the ventricles of the heart which are triggered by natural electrical signals originating in the atrium of the heart. Physiological conditions which weaken or eliminate these natural signals result in a lack of coordination between the atria and ventricles and consequently, the natural pumping action of the heart is affected, at times resulting in death.

In order to overcome this condition, cardiac pacer have been developed to artifically stimulate the contraction of the ventricles with electrical pulses generated by the pacer. These pacers are implanted in the body of a patient along with batteries for powering the pacer and electrical leads attached to the heart.

At present, most commercial cardiac pacers utilize a set of five or six primary mercury cells connected in series as their power source. With such pacers, a change in pace or pulse rate reflects a change in battery voltage, for example, a rate decrease of several pulses per minute will indicate a drop in battery voltage, suggesting pacer replacement.

Accordingly, a pulse rate decrease in pacers that use five or six cells connected in series is an indication of battery exhaustion. A disadvantage of such a configuration is that should an individual cell of the power source fail prematurely, it is possible for the failed cell to be driven by the other cells of the battery into potential reversal and thus generate internal gas pressure from the electrolysis of the electrolyte in the failed cell. Furthermore, performance and reliability of such pacers are also affected as a result of reduced battery voltage. Consequently, when one or two cells fail in a conventional pacer utilizing a power supply of five or six series connected cells, future operation of the pacer becomes doubtful and the unit is preferably replaced. Pacer longevity, therefore, is not necessarily determined by the average cell life but instead it can and often times is limited by the performance of the weakest cell in the battery forming the power supply of the pacer.

Because battery failure or premature battery exhaustion has been determined to be the principal cause of failure in cardiac pacers, it has become desirable to use several batteries connected in a parallel redundant configuration as the pacer power source.

The parallel connection of the cells or batteries is generally made through diodes or transistors such that when a cell or battery in one parallel branch fails prematurely the diode associated with that battery reverse biases thereby automatically disconnecting that battery from the load. Thus, the voltage of the power supply supplying the pacer remains relatively unchanged and the operation of the pacer remains virtually unchanged. As with the type pacers having a single battery comprised of series connected primary cells as the power source, pacers having parallel connected batteries as a power source will, near the end of life of the batteries of the power source, manifest a change in operation due to lower voltage which is reflected in a change in the pulse rate.

There is, however, a major disadvantage of using redundant batteries in that when one of the batteries or cells in one of the parallel branches prematurely fails there is no outward indication of such failure. In pacers having a battery comprised of series connected cells, as set forth above, the usual indication of battery failure is reduced pulse rate which, generally is proportional to battery terminal voltage. Consequently, with a three-cell redundant system as for example, the pacer could be operating on one third power very shortly after implant if cells of two parallel branches fail, with no outward indication to either patient or attending physician. The danger of this situation is obvious since it is assumed that after implantation the pacer will operate safely for a predicted period of time when, in fact, in only one third or less of that time the pacer will certainly fail.

SUMMARY OF THE INVENTION

Briefly, the invention provides apparatus that will indicate when one or more of the electro-chemical cells powering a pulse generating devices fails.

In accordance with the invention, a pulse generating device is provided having a power supply comprising a plurality of batteries connected in parallel wherein each battery is comprised of at least one electrochemical cell. A pulse generating circuit is provided operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses. Means are provided adapting the pulse generating circuit to generate pulses at a first frequency and, upon failure of at least one of the electrochemical cells of the power supply, to generate pulses at a second frequency different from the first frequency.

The apparatus, in accordance with the invention, is essentially free of the defects of the prior art devices having parallel connected batteries in that it provides an outward indication of cell or battery failure in the power supply when such failure occurs. The apparatus of the invention is an improvement over devices having five or six series connected cells as a power supply in that if a cell or battery of the power supply of the apparatus of the invention fails, there is an ample reserve of energy to permit continued operation of the pacer.

A more complete understanding of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings which form a part of this specification.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art pacer having a power supply comprising a plurality of cells connected in parallel;

FIG. 2 is a schematic diagram of a pacer is accordance with the invention;

FIG. 3 is a bar graph illustrating the effect of cell failure in the pacer of FIG. 2; and

FIG. 4 is a perspective view of the encapsulation container in which the circuit of FIG. 2 is housed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention may be best understood from the following detailed analysis of a prior art pacer shown in FIG. 1 and the pacer of the invention shown in FIG. 2. In the drawings, like reference characters refer to like parts throughout the several drawings.

Referring now to FIG. 1, the numeral 10 generally designates a heart stimulator or cardiac pacer which is adapted to provide electrical impulses of predetermined duration and rate for application to the human heart. The pacer 10 includes a power supply 12 comprising three parallel branches 14, 16 and 18 and the pacer circuitry shown at 19. Each branch of the power supply comprises a battery preferably consisting of two electrochemical primary cells connected in series. Each cell is preferably a 1 ampere-hour, medically certified cell having a terminal voltage of approximately 1.38 volts at 25.degree. C. Other suitable batteries may, of course, be utilized. The power supply described stores sufficient energy to operate the pacer 10 for a projected 5 year life.

Each parallel branch of the power supply includes a diode 20. Preferably, the diodes 20 are germanium diodes to minimize the forward voltage drop across them. The diodes 20 effect the parallel connection of the batteries such that if any one or two of the three batteries should fall, the voltage at the cathode side of the diodes would be relatively unchanged since the particular diode or diodes connected to the failed battery or batteries would reverse bias thereby disconnecting the defective battery from the power supply. Consequently, the operation of the pacer will be essentially unchanged if one or two of the three batteries of the power supply fails. This is because the pacer circuitry 19 requires very low currents in terms of the capacity of the batteries of the power supply and because each of the batteries of the power supply has a very low internal resistance.

The pacer circuitry 19 includes the pulse generating circuit, shown generally at 22. The pulse generating circuit 22 is connected across the power supply 12 to translate the power supplied by the power supply into electrical impulses.

In considering the operation of the pulse generating circuit 22, the transistors 25 and 26 are connected as a two stage rate adjustable complementary astable blocking oscillator which produces pulses of fixed duration, as for example, a 1ms pulse approximately once every 833ms. The pulse rate of the oscillator is proportional to the voltage of the power supply 12 and this feature provides an indication of battery voltage which can be used to predict normal pacer failure or normal battery exhaustion. The transistor 25 is of the NPN type and the transistor 26 is the PNP type. Both transistors have the usual emitter, collector and base electrodes. When the power supply charges capacitor 28 via resistors 30 and 32 sufficiently to forward bias the emitter base junction of the transistor 25, this transistor conducts. The resistor 31 is connected between the emitter of transistor 25 and point 35. Since the collector of transistor 25 is connected to the base of transistor 26, it in turn causes transistor 26 to conduct. As transistor 26 conducts, current flows through the primary winding 33 of transformer 34 which induces a voltage in the secondary winding 36 of transformer 34. The secondary winding is so connected that the induced voltage further increases the base current supplied to transistor 25. This regenerative action causes a rapid pulse rate which continues until both transistors 25 and 26 are in saturation. Transistors 25 and 26 remain saturated for the duration of the pulse during which capacitor 28 is partially discharged. The width of the pulse thus produced is controlled primarily by the inductance of the transformer 34 and the capacitance of capacitor 28 with little dependence upon supply voltage.

Continuing with the operation of the pulse producing circuit 22 of FIG. 1, when the induced voltage in the secondary winding 36 begins to diminish, the current flowing into the base of transistor 25 also diminishes which in turn reduces the base current of transistor 26. As transistor 26 turns off, the current in primary winding 33 decreases which further reduces the induced secondary voltage of winding 36. This latter regenerative action rapidly switches transistors 25, 26 from saturation to cut-off and the pulse terminates. The voltage which was developed across capacitor 28 during the pulse now reverse biases the emitter-base junction of transistor 25 to the voltage level at which the capacitor 28 was partially discharged. The cycle repeats with the charging of capacitor 28 through resistors 30 and 32.

Continuing with the operation of the pacer 10 of FIG. 1, the pulse appearing across the primary winding 33 of transformer 34 is coupled through resistor 40 to the base of transistor 42. Transistor 42 acts as a switch which connects the voltage of the power supply, less the voltage drop across the diodes 20, via capacitor 43 to a simulated heart load, shown here as resistor 44, which is connected across the output terminals. The resistor 46 discharges capacitor 43 between pulses.

The pacer circuit of FIG. 1 is characterized by the fact that the pulse repetition rate of the pulse generating circuit 22 is determined by the rate at which capacitor 28 charges to the base potential at which transistor 25 will conduct current. The current that charges capacitor 28 is determined by the resistors 30 and 32. Accordingly, the values of the resistors 30, 32 and the value of the capacitor 28 may be considered as a RC timing means or circuit which determines the length of time between pulses, i.e., the pulse repetition rate. As described above, as the voltage of the power supply 12 decreases, the pulse rate will also decrease. The batteries of the power supply 12, however, are of the type that maintain a substantially constant voltage throughout their lives and then suddenly run down. Therefore, a change or decrease in pulse rate is normally not had with the pacer of FIG. 1 unless the batteries of the power supply are near the end of their useful lives. The pacer of FIG. 1, therefore, has an inherent disadvantage in that there will be no outward indication, i.e., decrease in pulse rate, when one or two of the batteries of the power supply fails since the terminal voltage of the power supply is virtually unchanged due to the redundant configuration of the batteries of the power supply. Consequently, pulse rate proportional to voltage, as provided by the circuit 22 of FIG. 1, is only useful when all three batteries of the power supply fail simultaneously, generally, near the end of the expected life of the batteries. If one or two of the batteries of the power supply fail shortly after implantation of the pacer, the pacer could be operating on one-third original capacity and there would be no way of determining this fact with the pacer of FIG. 1.

In order to have an outward indication of the premature failure of one or two of the batteries of the power supply, the pacer of FIG. 1 was modified as shown in FIG. 2.

Comparing FIG. 1 and FIG. 2, it will be seen that they differ in that FIG. 2 has included therein three resistors 50, 52, and 54. Each of the resistors is connected at one end to the junction point 56 which in turn is connected to the junction point 58 between resistors 30 and 32. The opposite end of each resistor 50, 52 and 54 is connected to one of the batteries of the power supply 12; the resistor 50 being connected to the point 60 between the battery of and the diode 20 of parallel branch 18; the resistor 52 being connected to the point 62 between the battery and the diode 20 of parallel branch 16; and the resistor 54 being connected to the point 64 between the battery and the diode 20 of parallel branch 14.

It can be seen that portions of the current that charge capacitor 28 in FIG. 2 are provided through the resistors 50, 52 and 54 in addition to the current provided through resistors 30 and 32, the latter resistors being the only ones in the charging path between the power supply 12 and the capacitor 28 in the circuit of FIG. 1. The resistors 50, 52 and 54, therefore, form part of the RC timing circuit of pulse generating circuit 22 of FIG. 2.

In the pacer circuit of FIG. 2, should a battery in one parallel branch fail prematurely, the portion of capacitor current provided by the specific resistor 50, 52, or 54 associated with the failed battery will be reduced which in turn will reduce the pulse rate of the pacer. The modified pacer shown in FIG. 2 will, therefore, provide an outward indication of battery failure reflected in a reduction in pulse rate. This indication can be used to inform the patient or physician of impending pacer failure.

FIG. 3 is a bar graph illustrating the effect of catastrophic or irregular cell failure in the pacer of FIG. 2. The failure mode in each situation was simulated by replacing a cell of the various batteries of the power supply 12 with a short circuit. The data given for three and four cells failed was obtained with one of the 2-cell batteries in one parallel branch intact.

FIG. 4 shows the circuit of FIG. 2 in an encapsulation container designated by the reference numeral 70. The components of the circuit are assembled on a circuit board, not shown, and encapsulated in a suitable epoxy resin or other suitable body implantable material. The container 70 may be surgically implanted in a body with the insulated unipolar transvenous catheter 72 connected to the heart at the desired location, preferably, the endocardium. A large corrosive resistant metal anode plate functions as the catheter antipode and is physically located on the outer surface of the encapsulation container as is shown at 77. Saline body fluids complete the current path between catheter electrode 78 and the anode plate 77 when the pacer is implanted in the body.

In a practical embodiment of this invention, the components of the described apparatus can have the following values.

Resistance (ohms) 50 10,000K 52 10,000K 54 10,000K 30 680K 32 680K 33 1K 46 47K 40 4.7K Capacitors (.mu.F) 28 2.3 43 15 Transistors: 25 2N718A 26 2N3217 42 2N2219A Transformer: 34 United Transformer Corp. No. BIT - 250 - 48 Diodes: 20 1N3287

while there has been described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and its operation may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

Claims

We claim:

1. In an artificial cardiac pacer having

a. a power supply comprising a plurality of batteries connected in parallel and wherein each battery is comprised of at least one electrochemical cell and,

b. a pulse generating circuit operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses, and being operable to provide said electrical impulses to electrodes operatively connected to a patient's heart, the improvement comprising, means for causing the pulse generating circuit to generate pulses at a first frequency and, upon failure of any of the batteries of the power supply, to generate pulses at a second frequency different from the first frequency; the power supply being further characterized as having at least two parallel branches, each parallel branch of the power supply having disposed therein at least one electrochemical cell, and at least one semiconductor means operatively connected with the electrochemical cell, the semiconductor means in each parallel branch being arranged to permit current flow therethrough from the electrochemical cell connected therewith to the pulse generating circuit and to prevent current flow therethrough from the electrochemical cells in other parallel branches of the power supply, said means for causing the pulse generating circuit to generate pulses at a first frequency and, upon failure of any of the batteries of the power supply, to generate pulses at a second frequency different from the first frequency including at least a pair of resistors, each resistor being operatively connected to the pulse generating circuit and forming a part thereof.

2. In an artificial cardiac pacer as set forth in claim 1 wherein each resistor is operatively connected to a parallel branch of the power supply at a point between the electrochemical cell and the semiconductor means disposed in each parallel branch of the power supply.

3. A body implantable artificial cardiac pacer comprising

a. a body compatible implantable insulating encapsulation container,

b. a power supply in the encapsulation container comprising at least two parallel branches, each parallel branch of the power supply having disposed therein at least one electrochemical cell and at least one semiconductor means in series circuit with the electrochemical cell,

c. a semiconductor pulse generating circuit in the encapsulation container operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses and being operable to provide said electrical impulses to electrodes operatively connected to a patient's heart, the semiconductor means in each parallel branch being arranged to permit current flow therethrough from the electrochemical cell in series circuit therewith to the pulse generating circuit and to prevent current flow therethrough from the electrochemical cells in other parallel branches of the power supply, and

d. timing means in the encapsulation container operatively connected to the power supply and forming a part of the pulse generating circuit to govern the pulse repetition rate of the pulses generated by the pulse generating circuit and, upon failure of any of the electrochemical cells of the power supply to vary the pulse repetition rate of the pulse generating circuit; said timing means including at least a pair of resistors, each of the resistors being operatively connected to a different parallel branch of the power supply at a point between the electrochemical cell and the semiconductor means disposed in each parallel branch of the power supply.

4. In an artificial cardiac pacer having

a. a power supply comprising a plurality of batteries connected in parallel and wherein each battery is comprised of at least one electrochemical cell and,

b. a pulse generating circuit operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses, and being operable to provide said electrical impulses to electrodes operatively connected to a patient's heart, the improvement comprising, means including circuit means operatively connected to each of the batteries and to the pulse generating circuit and forming a part of the pulse generating circuit for causing the pulse generating circuit to generate pulses at a first frequency and, upon failure of any of the batteries of the power supply, to generate pulses at a second frequency different from the first frequency.

5. A body implantable artificial cardiac pacer comprising

a. a body compatible implantable insulating encapsulation container,

b. a power supply in the encapsulation container comprising at least two parallel branches, each parallel branch of the power supply having disposed therein at least one electrochemical cell and at least one semiconductor means in series circuit with the electrochemical cell,

c. a semiconductor pulse generating circuit in the encapsulation container operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses and being operable to provide said electrical impulses to electrodes operatively connected to a patient'heart, the semiconductor means in each parallel branch being arranged to permit current flow therethrough from the electrochemical cell in series circuit therewith to the pulse generating circuit and to prevent current flow therethrough from the electrochemical cells in other parallel branches of the power supply, and

d. timing means including means operatively connected to each branch of the power supply and to the pulse generating circuit and forming a part of the pulse generating circuit in the encapsulation container for governing the pulse repetition rate of the pulses generated by the pulse generating circuit and, upon failure of any of the electrochemical cells of the power supply to vary the pulse repetition rate of the pulse generating circuit.

6. In an artificial cardiac pacer having

a. a power supply comprising a plurality of batteries connected in parallel and wherein each battery has substantially the same terminal voltage and is comprised of at least one electrochemical cell and,

b. a pulse generating circuit operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses, and being operable to provide said electrical impulses to electrodes operatively connected to a patient's heart, the improvement comprising, means for causing the pulse generating circuit to generate pulses at a first frequency and, upon failure of any of the batteries of the power supply, to generate pulses at a second frequency different from the first frequency.

7. A body implantable artificial cardiac pacer comprising

a. a body compatible implantable insulating encapsulation container,

b. a power supply in the encapsulation container comprising at least two parallel branches, each parallel branch of the power supply having disposed therein at least one electrochemical cell and at least one semiconductor means in series circuit with the electrochemical cell, each electrochemical cell having substantially the same terminal voltage,

c. a semiconductor pulse generating circuit in the encapsulation container operatively connected across the power supply to translate the power supplied by the power supply into electrical impulses and being operable to provide said electrical impulses to electrodes operatively connected to a patient's heart, the semiconductor means in each parallel branch being arranged to permit current flow therethrough from the electrochemical cell in series circuit therewith to the pulse generating circuit and to prevent current flow therethrough from the electrochemical cells in other parallel branches of the power supply, and

d. timing means in the encapsulation container operatively connected to the power supply and forming a part of the pulse generating circuit to govern the pulse repetition rate of the pulses generated by the pulse generating circuit and, upon failure of any of the electrochemical cells of the power supply to vary the pulse repetition rate of the pulse generating circuit.