Body Organ Stimulator With Voltage Converter

Abstract

An organ stimulator is provided having a power supply, a pulse generating circuit, a voltage converter and output terminals. The voltage converter includes at least one capacitor which is arranged such that, during the interpulse interval between pulses from the pulse generator, the capacitor charges to approximately the voltage of the power supply and upon application of a pulse to the voltage converter, the combined voltages of the power supply and the charged capacitor are supplied to the output terminals. A novel current limiting circuit is also provided which regulates the output pulse current of the pacer.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to pulse generating circuits, and more particularly it relates to battery powered pulse generating circuits such as used in electronic organ stimulation devices and provides an implantable device of simple, but reliable, construction which will function efficiently from a low source voltage. The invention will here be described in most detail in association with a 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 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 or 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. Psysiological conditions which weaken or eliminate these natural signals result in a lack of coordination between the atrium 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 pacers have been developed to artifically stimulate the contractions of the ventricles with electrical pulses generated by the pacer. Cardiac pacers of the type here contemplated are implanted electronic devices which cause the heart to beat by stimulating it with short pulses of electric currents applied directly to the heart muscle by metal electrodes. Since their initial clinical use in about 1960, fully implanted pacer systems have gained wide acceptance in the chronic treatment of heart block.

The term pacer implies two components, namely, the pulse generator circuit and the electrode system. The pulse generator circuit includes the energy source or power supply and the electronic pulse forming circuit. The electrode system includes the catheter or lead which conducts the electrical stimulus from the generator to the heart and the catheter electrode in contact with the heart.

The threshold, i.e., the intensity at which electrical stimulus will produce a response in the heart muscle, is related to the catheter electrode area and it has been determined that as the electrode area is reduced, the pulse current requirement for threshold diminishes proportionately. Chronically, the threshold of stimulation, after implant of the pacer, will increase by an average factor of 3 times and occasionally as high as 5 times the original threshold. Therefore, a criterion for safe and effective chronic stimulation can be established by adjusting the pacer output pulse current to approximately ten times the acute threshold sensitivity of the specific electrode system used. As for example, with a catheter electrode having an area of 0.08 cm.sup.2 and with an acute threshold sensitivity of about 0.5 mA, the pulse current output of the pacer can be adjusted to about 5.0 mA to about 5.5 mA, which will satisfy the criterion of approximately 10 times the acute threshold sensitivity. A typical impedance for an electrode having an area of 0.08 cm.sup.2 made of eligiloy is approximately 900 ohms, consequently, a pacer utilizing such an electrode requires an output voltage of at least 5 volts to supply a specified 5.5 mA pulse current. An output voltage of 7.5 to 10 volts would be preferable to provide an adequate impedance margin.

As a rule, an output voltage of at least 5 volts from the pacer presents no problem as most commercial pacer typically utilize a power supply comprising a set of five or six series connected primary electrochemical cells which will provide the required voltage. There has been a tendency in recent times, however, to use several primary batteries connected in a parallel redundant configuration as the pacer power source in an attempt to increase the longevity of pacers. This is because battery failure or premature battery exhaustion has been determined to be one of the principal causes of failure in cardiac pacers. As a result of the tendency to employ a redundant battery configuration, and the resultant lower supply voltage as provided by such a power supply, it is no longer quite so easy to provide the required output voltage from the pacers.

SUMMARY OF THE INVENTION

It is an overall object of the present invention to provide a cardiac pacer that will function efficiently from a low voltage source as provided by the parallel connection of a plurality of low voltage primary electrochemical cells or as provided by a rechargeable low voltage secondary battery.

Briefly, and in accordance with the invention, an organ stimulator is provided having a power supply and a pulse generating circuit. The pulse generating circuit is operatively connected to the power supply for producing a sequence of electrical pulses with an interpulse interval of preassigned duration. Output terminals are provided that are operatively connected to the stimulator and connectible to the organ to be stimulated for applying organ stimulating electrical impulses thereto. At least one capacitor is provided. First circuit means are provided operatively connecting the capacitor in parallel with the power supply during the interpulse interval between adjacent ones of the pulses generated by the pulse generating circuit to permit the capacitor to charge to approximately the voltage of the power supply during each interpulse interval. Second circuit means are provided operatively connected to the pulse generating circuit and being responsive to pulses therefrom for connecting the capacitor in series with the power supply and in circuit with the output terminals whereby to supply the combined voltages of the power supply and the charged capacitor to the output terminals.

Another important aspect of the invention is the provision of novel current limiting means which regulate the output pulse current of the pacer to an effective chronic stimulation level over a wide range of catheter impedances and the anticipated load impedance as presented by the heart.

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 pacer in accordance with the invention; and

FIG. 2 is a perspective view of the encapsulation container in which the circuit of FIG. 1 is housed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, the pacer in accordance with the invention is shown in FIG. 1. The pacer shown in FIG. 1 comprises four basic elements; a primary battery power source 10 which stores sufficient energy to operate the pacer for a projected 5-year life; a pulse generating circuit 12 which determines the width and repetition rate of the heart stimulus; a voltage converter shown generally at 14 which steps up the voltage by a factor of approximately four times the voltage of the power supply 10; and a current limiter 16 which regulates the output pulse current to an effective chronic stimulation level over a wide range of catheter impedance.

Output pulse current is coupled directly to the endocardium of the heart via a unipolar transvenous catheter, shown in FIG. 1 as lead 18. A large corrosion resistant metal anode plate functions as the catheter antipode and is shown in the FIG. 1 as lead 20. The lead 20 is preferably located on an outer wall surface of the pacer and takes the form of a plate as is shown in FIG. 2. Saline body fluids complete the current path between catheter (cathode) and anode plate when the pacer is implanted in a body.

The power supply 10 comprises three parallel branches 22, 24 and 26. Each branch of the power supply comprises a battery preferably consisting of two electrochemical primary cells connected in series. Each cell, B1 - B6, 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 for a projected 5-year life. With appropriate modification of the pacer circuitry the batteries of the power supply 10 may be replaced with a low voltage rechargeable secondary battery.

Each parallel branch of the power supply 10 includes a diode, D1 - D3. Preferably, the diodes D1 - D3 are germanium diodes to minimize the forward voltage drop across them. Typically this voltage drop is approximately 0.26 volts. Accordingly, the initial power source voltage is 2.76 volts (two series connected cells of 1.38 volts each) minus the voltage drop across the diodes or approximately a terminal voltage at the terminals a, b or 2.5 volts. This specific value of 2.5 volts is given only by way of example and will be used hereinafter in the description, however, the circuitry to be described is designed to operate from a power supply or battery whose voltage is in the range of about 1 to about 3 volts.

The diodes D1-D3 effect the parallel connection of the batteries such that if any one or D1 - D3 of the three batteries should fail, 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 in accordance with the invention 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 capacitor C1 is connected across the power supply 10 to maintain a low AC source impedance and thus preclude any variations in pacer characteristics which would result if the battery impedance were to increase significantly after implantation.

In considering the operation of the pulse generating circuit 12, the transistors Q1 and Q2 are connected as a two stage rate adjustable complementary astable blocking oscillator. The transistor Q1 is of the NPN type and the transistor Q2 is of the PNP type. Both transistors have the usual emitter, collector and base electrodes. When the power supply charges the capacitor C2 via resistors R1-R6 sufficiently to forward bias the emitter base junction of the transistor Q1, this transistor conducts. Since the collector of transistor Q1 is connected to the base of transistor Q2, it in turn causes transistor Q2 to conduct. As transistor Q2 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 Q2. This regenerative action causes a rapid pulse rise which continues until both transistors Q1 and Q2 are in saturation. Transistors Q1 and Q2 remain saturated for the duration of the pulse during which capacitor C2 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 C2 with little dependence upon supply voltage.

Continuing with the operation of the pulse producing circuit 12 of FIG. 1, when the induced voltage in the secondary winding 36 begins to diminish, the current flowing into the base of transistor Q1 also diminishes which in turn reduces the base current of transistor Q2. As transistor Q2 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 Q1, Q2 from saturation to cutoff and the pulse terminates. The voltage which was developed across capacitor C2 during the pulse now reverse biases the emitter-base junction of transistor Q1 to the voltage level at which the capacitor C2 was partially discharged. The cycle repeats with the charging of capacitor C2 through resistors R1 - R6.

The pulse generating circuit 12 is characterized by the fact that the pulse repetition rate thereof is determined by the rate at which capacitor C2 charges to the base potential at which transistor Q1 will conduct current. The current that charges capacitor C2 is determined by the resistors R1 - R6. Accordingly, the values of the resistors R1 - R6 and the value of the capacitor C2 may be considered as a RC timing means or circuit which determines the interpulse interval between pulses, i.e., the pulse repetition rate.

It can be seen that as portions of the current that charge capacitor C2 are provided through the resistors R1 - R3 in addition to the current provided through R4 - R6 should a battery in one parallel branch fail prematurely, the portion of capacitor current provided by the specific resistor (R1, R2 or R3) associated with the failed battery will be reduced which in turn will reduce the pulse rate of the pacer. This feature can be used as an outward indication of battery failure which is reflected in a reduction in pulse rate. This indication can be used to inform the patient or physician of impending pacer failure. For a more complete explanation of the desirability of this feature, see the copending application, Ser. No. 97,254 which is incorporated by references herein.

To complete the description of this portion of the pacer, diode D4 is required to suppress the large negative voltage spike which develops across the windings of transformer 34 at the termination of each pulse as the result of the energy stored in the inductance of transformer 34. If this voltage spike is not suppressed, damage to transistors Q1 or Q2 would ultimately occur. The capacitor C3 serves to reduce interference by suppressing extraneous high frequency magnetic signals which can be picked up directly by the transformers magnetically permeable core and cause premature triggering of the pulse generating circuit 12. Finally, the resistor R7 serves to bleed off the Icbo leakage currents of transistors Q1 and Q2. If these currents were not bled off during the interpulse period, these currents would be reflected in the collector of transistor Q2 and magnified by the Beta of the transistor. For the specific transistors chosen, this could increase the total average current drain of the circuit by several percent at the normal pacer operating temperature of 37.degree. C.

Continuing with the operation of the pacer of FIG. 1 and with specific reference to the voltage converter section thereof shown generally at 14, pulses generated by the pulse generating circuit of 12 are coupled to the voltage converter 14 via resistor R8. Before continuing further with the specific operation of the voltage converter section 14, it may be appropriate at this point to generally set forth the constructional details and the overall operation of this portion of the pacer circuit.

During the interpulse interval the capacitor C9 is connected in parallel with the power supply 10 with one end of the capacitor C9 being connected to the lead 20 via resistor R10 and therefore is connected to the positive side of the power supply via resistor R10. The other end of the capacitor C9 is connected to the negative side of the power supply 10 via diode D5. The capacitor C10 and C11 are similarly connected in parallel with the power supply 10; the resistor R12 and diode D6 connecting the capacitor C10 to the positive and negative sides of the power supply 10 as shown, and the resistor R14 and diode D9 connecting the capacitor C11 to the positive and negative sides of the power supply 10 as shown. Between the pulses supplied to the voltage converter 14 from the pulse generating circuit 12, the capacitors C9, C10 and C11 are charged to approximately power supply voltage, 2.4 volts, through resistors R10, R12 and R14, respectively, with the polarity of the charge on the capacitors as shown in FIG. 1. When a pulse from the pulse generating circuit 12 is coupled to the voltage converter 14, the switching transistors Q3, Q4, Q5 and Q6 simultaneously turn on and the diodes D5, D6 and D9 reverse bias. This action has the effect of connecting the capacitors C9, C10 and C11 in series with the power supply 10 and in circuit with the output terminals 46 and 48. This series circuit can be traced as follows: beginning with the negative side of the power supply, through the lead 50 to the point 52; from point 52 through lead 54; through the emitter-collector of transistor Q3; through capacitor C9; through the emitter-collector of transistor Q4; through capacitor C10; through the emitter-collector of transistor Q5; through capacitor C11; through resistor R17; through the emitter-collector of transistor Q6; through coupling capacitor C12; through the simulated heart load R; and through lead 20 to the positive side of the power supply 10.

When the charged capacitors C9, C10 and C11 are connected in series with the power supply due to the turning on of the transistors Q3-Q6, the effect is to supply approximately the combined voltages of the power supply 10 and the voltage on each of the capacitors C9, C10, and C11 to the output terminals 46 and 48. The combined voltages of the power supply and the voltage on the capacitors C9, C10, and C11 sum up to approximately 9.3 volts which in in effect approximately a four fold multiplication of the voltage of the power supply. There is not an exact four fold multiplication due to the small voltage drops across the various circuit elements. This output voltage of approximately 9.3 volts is adequate, as for example, to supply 5.5 mA which is a pulse current output from the pacer that satisfies the criterion of approximately ten times as acute threshold sensitivity of 0.5 mA. From the foregoing, it will be understood to those skilled in the art, that identical additional stages of components (e.g., Q3, R10, C9, D5) can be added to the voltage converter 14 to yield any practical multiplication of the power supply voltage. Further, it will be understood that a voltage doubler can be realized with only a single stage comprising the transistors Q3 and Q6 and the components R10, C9 and D5. Voltage converter stages, in accordance with the invention, enables the conversion of low voltage batteries to high pulse voltages utilizing either conventional semiconductor components or hybrid microminiaturized modules.

Returning now to a specific description of the operation of the voltage converter 14, the positive pulses 60 generated by the pulse generating circuit 12 are coupled to the base of transistor Q3 through resistor R8. The value of this resistor is so chosen to provide sufficient base current to saturate transistor Q3 under full load conditions. Resistor R9 effectively returns the base potential of transistor Q3 to battery negative potential between pulses thus assuring that this transistor is completely turned off during the interpulse interval. The capacitor C8 shunts to battery negative any high frequency pulses which may enter the voltage converter and ultimately stimulate the heart.

When Q3 saturates, it switches the positive terminal of capacitor C9 which was at power supply positive or anode ground potential, to approximately -2.4 volts with respect to ground. Since the voltage across C9 was charged to nearly the battery potential (2.4) volts, the emitter of transistor Q4 is depressed to approximately -4.8 volts. This transition reverse biases diode D5 and causes sufficient current to flow into the base of transistor Q4 via resistor R11 to saturate this transistor. When Q4 saturates it switches the positive terminal of capacitor C10 from ground potential to approximately -4.7 volts (4.8 minus Vce of Q4). Furthermore, since the voltage charged across C10 was 2.4 volts, the emitter of transistor Q5 is depressed to approximately -7.1 volts which reverse biases diode D6 and causes sufficient current to flow into the base of transistor Q5 via resistor R13 to saturate this transistor.

When transistor Q5 saturates, the positive terminal of C11 switches from ground potential to approximately -7.0 volts. Again, since the voltage across C11 was charged to 2.4 volts, the negative terminal of this capacitor is depressed to approximately -9.4 volts. If the voltage drop across resistor R17 is zero, which is the case under no load (RL=.delta.), the emitter of Q6 is also depressed to -9.4 volts. As in the previous transistor, Q5, sufficient current will flow into the base of transistor Q6 via resistor R15 to cause this transistor to saturate (no load). Consequently, the pulse voltage appearing at the collector of transistor Q6 is approximately -9.3 volts. This is shown in FIG. 1 as the negative output pulse 62. Capacitor C12 couples the negative output pulse to the heart thereby to initiate contractions thereof. The capacitor C12 prevents the flow of DC current, which would be potentially dangerous should transistor Q6 fail. The Zener Diode D10, shunts any extraneous high-voltage signals that might be introduced to the circuit by external defibrilation or other high voltage electromedical equipment applied to the patient.

Another feature or aspect of the present invention is the provision of the novel current limiter 16 which comprises the components resistor 17 and Diodes D7 and D8. The pacer as thus far described would function without these latter components, however, current limitation is desirable for the reasons that it diminishes the total battery drain, and with some electrode materials there is a risk of corrosion with currents that are too high.

The operation of the current limiter is as follows: When the junctive point of capacitor C11, diodes D8 and D9, that is point 65, is depressed to -9.4 volts, as set forth above, positive current flows into the base of transistor Q6 from resistor R15 causing this transistor to conduct current. When Q6 turns on, current will flow from the anode plate 20 into the load impedance RL, then back through the catheter 18, through capacitor C12 into the collector of transistor Q6. It will continue through this transistor, through resistor R17, then return to the battery negative terminal b via the path of capacitor C11, transistor Q5, capacitor C10, transistor Q4, capacitor C9, transistor Q3, lead 54 and lead 50.

The magnitude of the current that flows through the load RL is controlled by the conductance of transistor Q6 which increases the current flow until the voltage drop across R17 is equal to the voltage drop across the diodes D7 and D8 (typically 1.2 volts) minus the emitter-base voltage of transistor Q6 (typically 0.6 volts). The resultant voltage drop across resistor R17 is thus 0.6 volts. Since transistor Q6 is chosen to have a current gain or Beta greater than 25, the collector current of transistor Q6 is approximately equal to the emitter current thereof. Consequently, the output or load current is determined by the ratio of 0.6 volts divided by the resistance of R17. Preferably, the value of this resistor R17 is chosen to adjust the output pulse current to 5.5 mA. If the load resistance is less than approximately 1,000 ohms, more voltage would be dropped across transistor Q6, and vice versa, to maintain this constant output current.

FIG. 2 shows the circuit of FIG. 1 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 tranvenous catheter, designated as 72 in FIG. 2 (designated as 18 in FIG. 1), 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 a wall of the encapsulation container. The anode plate is designated by the reference numeral 77 in FIG. 2 and corresponds to the lead 20 in FIG. 1. It will be understood, of course, that a bipolar electrode system may be utilized if desired. Saline body fluids complete the current path between the catheter electrode 48 and the anode plate 77 when the pacer is implanted in the body.

In a practical embodiment of the invention, the components of the described apparatus can have the values as shown in Table 1.

TABLE 1

Resistance (ohms): Diodes: R1 10M 1N3287 R2 10M D2 1N3287 R3 10M D3 1N3287 R4 560K D4 1N456 R5 560K D5 1N3287 R6 1.1M D6 1N3287 R7 470K D7 1N456 R8 1K D8 1N456 R9 47K D9 1N3287 R10 47K D10 1N766 R11 10K Capacitors (.mu.) R12 47K C1 15 R13 10K C2 .235 R14 47K C3 .0047 R15 10K C8 0.1 R16 47K C9 15 R17 75 C10 15 R18 10 C11 15 Transistors: C12 15 Q1 2N718A Transformer: Q2 2N2844 34 United Transformer Corp. Part No. BIT - 250 - 48 Q3-Q6 MQ2219A

when connected as shown in FIG. 1, these components, at 37.degree. C., deliver 72 .+-. pulses per minute; the pulse width is 1.0 .+-. 0.1 milliseconds; the output pulse current is 5.5 .+-. 0.5 miliamps; Biphasic Regulation: .+-. 10 percent, 1,500 ohm load; the output pulse voltage depends upon load impedance and it approximately 9 volts at open circuit; and the total average battery current drain is 40.+-. 5 to 10 microamps at 72 pulses/minute, 5.5 mA pulse current and 1,000 ohm load.

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 and scope of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

Claims

I claim:

1. A body implantable artificial cardiac pacer comprising the combination,

a. a body compatible implantable insulating encapsulation container,

b. a power supply in the encapsulation container,

c. a pulse generating circuit in the encapsulation container operatively connected to the power supply for producing a sequence of electrical pulses with an interpulse interval of preassigned duration.

d. a first output terminal forming at least a portion of a wall of the encapsulation container,

e. a second output terminal comprising an electrode lead extending from the encapsulation container,

f. a plurality of capacitors in the encapsulation container,

g. a plurality of first circuit means in the encapsulation container operatively connecting each of the capacitors in parallel with the power supply during the interpulse interval between adjacent ones of the pulses generated by the pulse generating circuit to permit each capacitor to charge to approximately the voltage of the power supply during each interpulse interval, and,

h. second circuit means in the encapsulation container operatively connected to the pulse generating circuit and to the capacitors and being responsive to pulses from the pulse generating circuit for connecting the charged capacitors in series with each other, in series with the power supply and in series circuit with the first and second output terminals whereby to supply the combined voltages of the power supply and each of the charged capacitors to the output terminals.

2. A body implantable artificial cardiac pacer as set forth in claim 1 wherein the plurality of first circuit means each comprise a charging resistor operatively connected to one side of an associated capacitor and a diode operatively connected to the other side of an associated capacitor such that each charging resistor, capacitor and diode combination is connected in parallel across the power supply, and wherein the second circuit means comprise a plurality of transistors with there being one more transistor than there are capacitors, the transistors being operatively connected to the pulse generating circuit and with the capacitors and being responsive to pulses from the pulse generating circuit to connect the capacitors in series with each other, in series with the power supply and in series circuit with the output terminals.

3. A body implantable artificial cardiac pacer as set forth in claim 1 wherein said power supply is characterized as having a terminal voltage in the range from about 1 to about 3 volts.

4. A body implantable artificial cardiac pacer as set forth in claim 1 and further including means operatively associated with the second circuit means to limit the output current of the pacer.

5. In a body organ stimulator, the combination comprising:

a power supply,

a pulse generating circuit operatively connected to the power supply for producing a sequence of electrical pulses with an interpulse interval of preassigned duration,

output terminals operatively connected to the pulse generating device,

a plurality of capacitors,

a plurality of first circuit means operatively connecting each of the capacitors in parallel with the power supply during the interpulse interval between adjacent ones of the pulses generated by the pulse generating circuit to permit each capacitor to charge to the voltage of the power supply during each interpulse interval, and

second circuit means operatively connected to the pulse generating circuit and to the capacitors and being responsive to pulses from the pulse generating circuit for connecting the capacitors in series with the power supply and in series circuit with the output terminals whereby to supply the combined voltages of the power supply and the charged capacitors to the output terminals.

6. In a body organ stimulator as set forth in claim 20 wherein the power supply comprises a plurality of batteries connected in parallel.