Implantable nerve stimulator

Abstract

An implantable stimulator for a nerve includes a voltage amplifier, a voltage regulator, a pulse generator, a pulse train generator, a pulse amplitude modulator, and at least one flexible conduit to a nerve, such stimulator being powered by the combination of a nucleonic heat generator, and a thermopile for converting thermal energy to electric energy. Each train of pulses is regulated so that the gradual variation in the amperage is from 25 per cent to 95 per cent more than the minimum amperage. A saw tooth pulse signal more closely simulates the biological stimulation of a nerve. The nerve stimulator is particularly effective as a nuclear-powered respiratory pacer for energizing a phrenic nerve for controlled breathing of a mammal.

Description

CROSS NOTATION TO CONVENTION APPLICATION

Reference is made to British Provisional application 37,151 of 1971 filed Aug. 6, 1971 on License No. 351,021 issued July 6, 1971, applicant claiming convention priority. All the disclosure of said 37,151 is deemed here reiterated. Reference is made to U.S. Pat. No. 3,818,304, derived from Ser. No. 171,383 and parent Ser. No. 827,187, filed May 23, 1969, assigned to common assignee, Atlantic Richfield Company, of which this is a continuation-in-part, disclosing some embodiments of nuclear batteries of sufficient power for a respiratory pacer.

FIELD OF THE INVENTION

This invention relates to prosthetic electrical devices which can be placed within the body to provide electrical signals effectively stimulating a nerve.

PRIOR ART

Some disfunction in mammals are attributable to the reliability of a controlled frequency of nerve stimulations for actuating appropriate muscles. Cardiac pacers have been helpful in dealing with certain disfunctions of the heart attributable to neurological problems. The breathing disfunction of a neurological nature are similarly dealt with by respiratory pacers stimulating one or more of the phrenic nerves at appropriate intervals. Significant greater wattage is required in a respiratory pacer than in a cardiac pacer.

An implantable cardiac pacer featuring a nuclear battery has heretofore been proposed in the commonly assigned patent application U.S. Ser. No. 109,857, (now abandoned and replaced by Ser. No. 325,473) all the disclosure of which is deemed here reiterated. A lecture providing background information concerning respiratory pacers was presented at the American Association for Thoracic Surgery by Seymour Furman et al, entitled "Transvenous Stimulation of the Phrenic Nerves" about Apr. 27, 1971, all the disclosure of which is deemed here reiterated. Although there has been a recognition that the phrenic nerve sometimes benefited from stimulation by a pacer, there had not been any answer to the problem of an implantable phrenic nerve stimulator prior to the development of the present invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, a nerve pacer comprises the combination of a nuclear battery, an electrical pulse train generator, and a modulator adapted to provide a signal having a maximum amperage at least 25 per cent greater but not more than 95 per cent greater than the minimum amperage of such signal, whereby the nerve is stimulated by a signal which varies in a manner achieving a desired biological effect. An implantable nerve stimulator features the combination of a nuclear battery and electronic circuitry providing signals consisting of a train of a controlled number of pulses of direct current. An implantable respiratory pacer having an output of a range of from about 1 to 5 milliwatts and powered by a nuclear battery stimulates one or more phrenic nerve to provide a controlled rate of breathing for the mammal.

DRAWING

A schematic plan for the apparatus is shown in the drawing. In the drawing, which is essentially self-explanatory, a respiratory pacer 10 features a nuclear battery 11. Within the nuclear battery, a radioisotope heat source 12 energizes a thermopile 13 to generate a voltage energizing the respiratory pacer. The battery voltage is modified by a voltage amplifier 14 and a constant voltage regulator 15. Electrical circuit means energized by the voltage from said regulator 15 energizes a pulse generator, the electrical circuitry for such pulse generator comprising an oscillator. A pulse train generator 17 assures the absence of current flow during significant intervals, with a train of pulses between intervals, for example, 18 intervals per minute, corresponding to a relaxed breathing rate. A signal amperage modulator 18 transforms each train of pulses into a saw tooth signal having an end peak from 125 per cent to 195 per cent to the initial portion of such signal, thus more nearly simulating the natural signal to a phrenic nerve. Conductors 19 and 20 transmit such saw tooth signal to left and/or right phrenic nerves. The electrical circuit means and nuclear battery are hermetically sealed in a casing 21.

As seen in the flow pattern, the combination of radioisotope thermopile, and voltage amplifier to provide a source of useful voltage is consistent with prior teachings concerning nucleonic powered cardiac pacers. The combination of a constant voltage regulator and a pulse generator is also previously taught in connection with nerve stimulators.

The provision of a pulse train generator to provide a signal consisting of a controlled series of direct current pulses is a novel feature of the present nerve stimulator.

The step of modulating the amperage to provide a saw tooth signal having an end peak from 125 per cent to 195 per cent of the initial portion of a signal is a novel and significant forward step of the present invention.

Previous nerve stimulators have generally lacked the novel adjustable means for allocating electrical energy amongst a plurality of conductors, so that the provision of the load isolation and distribution regulator should be given special consideration.

This invention represents a medical break-through as the first device adapted for placement or implantation within the body and featuring the combination of nucleonic battery and a conduit stimulating the phrenic nerve. The unique nuclear battery makes feasible the first respiratory pacer sufficiently durable and compact to be carried within the body after surgical implantation.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the nerve stimulator serves to actuate the phrenic nerve to provide a respiratory pacer. A radioisotope provides a continuing radioactivity which is transformed into heat and the heat is transformed into a low voltage electric current. A voltage amplifier converts the DC voltage to a more useful direct current voltage which is maintained constant by a constant voltage regulator. A self-starting oscillator regulates the conversion of the constant direct current to a square wave series of direct current pulses having a frequency of about 90 pulses per second, each pulse having a duration of about 132 microseconds.

A significant difference from a conventional cardiac pacer concerns a pulse train generator adapted to provide a train of pulses followed by an interval each signal being a train of pulses. For a respiratory pacer, about 18 signals per minute are appropriate, the interval between the signals being about 24 per cent greater than the signal duration. Each signal can be about 1.49 seconds and the interval between signals about 1.85 seconds.

Important advantages are attained by reason of the provision of a signal consisting of a plurality of direct current pulses. Certain biochemical phenomena are apparently associated with a neurological electrical system. Although certain neurological effects can be achieved by a signal partaking of the nature of a single electrical pulse, better simulation of the neurological signal is achieved by providing the train of pulsation of direct current.

An important feature of the present invention is the provision of a modulator adapted to alter the train of pulses in a signal so that the peak amperage during a particular signal is at least 25 per cent greater than the minimum amperage during such signal. Various patterns of modulation of the signal are appropirate but particular advantages arise from the control of a saw tooth train having a relatively lower amperage at the beginning of the signal than at the end of the signal, with the amperage increasing during the continuation of the signal. It is sometimes desirable for a signal remote from this invention to have a saw tooth pattern involving an increase which is several hundred per cent over the minimum amperage but such a steep increase in amperage during the signal does not appear to closely simulate a neurological signal. The saw tooth increase modulates the signal so that the amperage of the terminal portion of the signal of the present invention is from about 125 per cent to about 195 per cent of the amperage of the initial signal, whereby the simulation of the natural neurological signal is more accurately achieved.

The disfunction impairing the operation of the phrenic nerves is often not the same for the right phrenic nerve and left phrenic nerve. Accordingly, a load isolation regulator is advantageously included in the respiratory pacer to permit adjustment of the intensity of the phrenic nerve signal for each of the two nerves; that is, the left phrenic nerve and the right phrenic nerve. A manually adjustable potentiometer can split the power between the two phrenic nerves. A conductor extends from the respiratory pacer to the phrenic nerve. Such conductor is advantageously detachably connected to the respiratory pacer.

A preferred embodiment of the invention can be further clarified by description of a respiratory pacer.

Nuclear Battery

Plutonium 238 has a half life of 86.4 years, and has numerous advantages for a nuclear battery. About 2 watts of power are generated, theoretically, by the amount of plutonium employed in the battery, but much of the energy is dissipated. The electrical output of the battery is of the general magnitude of the range of 0.001 to 0.005 watt, or about 1 to 5 milliwatts. The energy utilization efficiency of the battery and/or nerve stimulator is in the range from about 0.1 per cent to 1 per cent. A plutonium-type battery having a size of about 11/2 inches diameter and 1 1/16 inch thick weighs about 92 grams.

The voltage resulting from a thermocouple is amplified by a DC voltage amplifier to provide the electric current suitable for the electronic circuitry. The oscillator modifies the current by a square wave series of pulses of direct current having a frequency of about 89 pulses per second. Each pulse has a width or duration of about 132 microseconds or 0.132 milliseconds.

A pulse train generator transforms the pulses into signals at a range of about 18 signals per minute, the intervals between signals being about 24 per cent greater than the duration of the signals. The signal is about 1.49 seconds and the interval between signals about 1.85 seconds. The maximum pulse amplitude is about 9.6 milliamps. The pulse amplitude modulation is about 54 per cent. The maximum amperage is within the range from 125 per cent to 195 per cent of the minimum amperage by reason of the control of the modulation of the signal.

Casing

The respiratory pacer is hermetically sealed in a flat cylindrically shaped titanium can having a diameter of about 23/4 inches and about 11/8 inches thick. Such hermetic sealing protects the contents from adverse effects of liquids between organs within the body, whereby it may be implanted within a living body. The respiratory pacer weighs about 198 grams, thus being similar in weight and dimensions to a battery powered cardiac pacer having an output of about one-twentieth of the power of a phrenic nerve stimulator.

A signal having a saw tooth shape can have a substantially uniform increase of amperage during the 1.49 seconds of the signal. The modulation circuitry is adapted to provide up to about 54 per cent modulation of the maximum signal and to achieve saw tooth wave signal. In the development of the present invention, it has been established that the maximum amperage of the signal should be at least 25 per cent greater than the minimum amperage of the signal, and should be less than 95 per cent of such minimum.

It should be noted that a train of pulses which has not been modulated tends to stimulate staccato breathing. Such unnatural staccato breathing is harmful to the health. The modulation of the signal to the phrenic nerve so that the amperage during the terminal portion of the signal is significantly greater (for example, about 50 per cent) than the initial amperage promotes a breathing pattern which closely simulates that which the autonomic nervous system would stimulate in a healthy individual.

Various modifications of the invention are appropriate without departing from the scope of the appended claims.

Claims

The invention claimed is:

1. In a respiratory pacer, the combination of:

a nuclear battery providing several years of useful life;

electrical means converting electrical power from said battery to a series of signals having a frequency per minute corresponding approximately to the desired breathing rate, each signal comprising a plurality of direct current pulses, one output terminal being a connector and the other output terminal for said electrical means being the hereinafter recited casing adapted to contact liquids between organs within the body, some such liquids being where the pacer is implanted, said electrical means including means for modulating the amperage of a signal so that its maximum amperage is within the range from about 125 per cent to about 195 per cent of the minimum amperage of such signal;

at least one conductor transmitting signals from said connector toward a phrenic nerve; and

a casing containing said battery, electrical means, and said connector for said conductor, said casing hermetically sealing its contents from adverse effects attributable to said liquids, whereby the respiratory pacer may be implanted within a living body.

2. In an implantable nerve stimulator comprising at least one conductor transmitting signals toward a nerve, the improvement which includes:

a casing;

hermetic sealing means for said casing adapted to protect the contents of the casing from adverse effects attributable to liquids between organs;

a nucleonic battery within the casing;

means conducting current from said battery to the hereinafter recited circuit means; and

electrical circuit means converting electrical power from said battery to signals consisting of a controlled train of direct current pulses transmitted by said conductor, said circuit means including means for modulating the amperage of a signal so that its maximum amperage is within the range from about 125 per cent to about 195 per cent of the minimum amperage of such signal.

3. The nerve stimulator of claim 2 in which the train of pulses constituting said signal has a sawtooth pattern with maximum amperage at about the terminal portion of the signal and a minimum amperage at about the initial portion of the signal.

4. The nerve stimulator of claim 2 in which there are a plurality of conductors and manually adjustable distribution means allocating the signal energy flowing respectively toward the right phrenic nerve and the left phrenic nerve.

5. A nuclear powered respiratory pacer in accordance with claim 2 in which the output flowing toward at least one phrenic nerve is within the range from about 1 to about 5 milliwatts.

6. A nuclear powered respiratory pacer in accordance with claim 5 in which the nuclear battery weighs about 92 grams and contains plutonium 238 as the radioisotope heat source.