Title:
APPARATUS FOR STIMULATING MUSCLES CONTROLLED BY THE SAME MUSCLES
United States Patent 3628538


Abstract:
Apparatus for stimulating a muscle, using an E.M.G. signal sensed in the muscle, is described. The signal sensed is amplified, filtered and rectified before being applied to a monostable circuit which, if the E.M.G. signal is greater than a threshold value, enters its quasi-stable state. A stimulator circuit applies a voltage to electrodes adjacent to the muscle while the monostable circuit is in its quasi-stable state but the apparatus reverts to its sensing mode when the monostable circuit is in its stable state, allowing the E.M.G. signal to initiate further stimulation, if required. The apparatus is particularly useful in overcoming incontinence.



Inventors:
Vincent, Samuel Anderson (Belfast, EK)
Monds, Fabian Charles (Newtownabbey, EK)
Armstrong, David Roger (Belfast, EK)
Application Number:
04/855281
Publication Date:
12/21/1971
Filing Date:
09/04/1969
Assignee:
NATIONAL RESEARCH DEVELOPMENT CORP.
Primary Class:
Other Classes:
607/41, 607/63, 607/70, 607/74, 623/24
International Classes:
A61N1/36; (IPC1-7): A61N1/36
Field of Search:
128/2
View Patent Images:



Foreign References:
DE1007469B1957-05-02
Primary Examiner:
Kamm, William E.
Claims:
We claim

1. Apparatus for controlling muscles in living animals, including man, comprising

2. Apparatus according to claim 1 wherein the control means causes the stimulation means to terminate the stimulus signal after a predetermined interval to allow the sensing means to sense a further electromyographic signal.

3. Apparatus according to claim 2 including a pair of electrodes adapted to be mounted on an animal's body adjacent to a muscle to be stimulated, the electrodes being coupled to the sensing means as a part thereof to pick up electromyographic signals, and to the stimulation means as a part thereof to receive a voltage and apply the voltage to stimulate the muscle.

4. Apparatus according to claim 3 wherein the sensing means includes a differential amplifier with two input terminals one connected to each electrode.

5. Apparatus according to claim 2, wherein the sensing means is most sensitive to a signal of frequency between 100 to 200 Hz., and is least sensitive to signals having frequencies below 100 Hz. and above 1,000 Hz.

6. Apparatus according to claim 2, wherein the control means include a monostable circuit and the stimulation means include a pulse oscillator, the monostable circuit being coupled to the sensing means to take up its quasi-stable state when the electromyographic signal reaches the predetermined level, and the monostable circuit being coupled to the oscillator to allow oscillation only when the monostable circuit is in its quasi-stable state.

7. Apparatus according to claim 6 wherein the sensing means includes a rectification for providing a DC signal from the sensed signal for application to the monostable circuit, the magnitude of the DC signal determining whether or not the monostable circuit enters its quasi-stable state.

8. Apparatus according to claim 6 including a transformer having primary and secondary windings and including a pair of electrodes adapted to be mounted on an animal's body adjacent to a muscle to be stimulated, the electrodes being coupled to the sensing means as a part thereof to pick up electromyographic signals, and to the stimulation means as a part thereof to receive a voltage and apply the voltage to stimulate the muscle, where the oscillator output is connected to the primary winding of said transformer whose secondary winding is connected to the electrodes.

9. Apparatus according to claim 3 wherein the sensing means is constructed to sense levator ani muscles, including the anal sphincter, and other muscles of the pelvic floor.

Description:
The present invention relates to apparatus for stimulating muscles, particularly, but not exclusively, those muscles which control the bladder.

When a muscle contracts, this is the result of control information reaching the muscle from the brain via the nervous system. A nerve impulse, originating in the central nervous system, depolarizes a membrane enveloping a small group of muscle fibers known as a motor unit. The motor unit contracts sharply, and then relaxes again while other similar units are "fired." A smooth contraction of muscle is a continuous cyclic process of many motor units firing and relaxing.

This electrical activity gives rise to the electromyographic signal (E.M.G.) which can be sensed by electrodes placed on or close to the muscle. Thus the E.M.G. signal is associated only with muscle. Monitoring the E.M.G. emanating from a muscle indicates the activity of that muscle.

Normal bladder control is maintained by the action of the pelvic floor muscles, which are constantly in a state of active contraction, except during the voluntary act of evacuation. The effect of this muscular contraction is to support the pelvic and abdominal contents, and this maintains a constant closure of the pelvic openings. Elevation of the normal bladder neck is sufficient to ensure that it remains closed.

In many incontinent persons, their condition is due to an inability to raise the bladder neck far enough to ensure closure, and, in many cases, this has been successfully treated by fitting an appliance which mechanically raises the perineal region. Such an appliance is described in United Kingdom Specification No. 910,837.

According to the present invention there is provided apparatus for controlling muscles in living animals, including man, comprising sensing means for sensing an electromyographic signal in a muscle, and stimulation means for automatically stimulating the same muscle in accordance with the characteristic of he signal sensed.

Preferably the means for sensing the signal includes means for monitoring the amplitude of the E.M.G. signal.

Where the apparatus is for use by incontinent persons, the means for sensing the E.M.G. signal senses the E.M.G. of the levator ani muscles, particularly the anal sphincter, and/or other muscles of the pelvic floor, and the stimulation means stimulates the same muscles causing them to contract elevating the bladder neck and holding the bladder closed.

This is a more sophisticated device than the mechanical appliance mentioned above, and it functions automatically.

Several electronic stimulation devices have been developed and are proving successful, but these must all be operated manually when required or left running continuously. The advantage of the present invention is that stimulation is applied automatically as the need for it arises.

The sensing means preferably includes two electrodes coupled to an amplifier whose frequency response peaks between 100-200 Hz., and falls off sharply below about 100 Hz. and above about 1,000 Hz., this frequency range corresponding with the frequency spectrum of the E.M.G. signal. The required frequency response may be obtained by using a filter which is coupled to the amplifier.

The amplifier may be coupled to control means which, when the amplitude of the amplifier output signal is greater than a predetermined value, causes an oscillator connected thereto to pass signals beck to the electrodes to stimulate the muscle. The control means may be a monostable circuit which prevents the amplifier from working when it is in its quasi-stable state but allows the oscillator to pass the stimulation signal to the electrodes. When the monostable circuit reverts to its stable state after a period of, typically, 10 seconds, stimulation ceases and the amplifier is allowed to work normally.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a first embodiment of apparatus according to the invention,

FIG. 2 is a circuit diagram of a second embodiment of the invention, and

FIG. 3 is a circuit diagram of a third embodiment of the invention.

During E.M.G. investigations of the muscles of the pelvic floor, in incontinent patients, it has been observed that there is a large increase in E.M.G. amplitude when the patient feels urgency, and is unable to prevent urination, as well as during coughing or under similar conditions of strain. It is at these times that external muscle stimulation is effective, and this increase in E.M.G. signal activity may be used as the control signal to initiate stimulation.

In FIG. 1 two electrodes 10 and 11 are placed on the levator ani muscles. The electrodes are flat plates fitted to a form of anal plug, or vaginal pessary, or to the perineal skin. If the electrodes are placed in the anal canal the E.M.G. signal picked up is from the anal sphincter and this muscle is then stimulated. This also applies to the muscle surrounding the vagina, and to the perineal muscle. The choice of electrode site depends upon the type of patient. Suitable anal plugs, vaginal pessaries including electrodes, and surface electrodes are commercially available. The electrodes are coupled to a variable gain amplifier 12, whose output is coupled to a filter 13, having a frequency response which peaks between 100 and 200 Hz., and falls off below 100 Hz. and above 1,000 Hz. As has already been pointed out, this frequency response corresponds with the frequency spectrum of the E.M.G. signal. The filter is coupled to a rectifying and smoothing circuit 13, whose output is passed to a monostable circuit 15. When the DC level from the smoothing circuit 14 exceeds a reference voltage developed in the monostable circuit, the monostable circuit is triggered to its quasi-stable state for about 10 seconds. Smoothing is necessary so that the monostable circuit is insensitive to random spikes which sometimes appear in the E.M.G., but is triggered only by a persistent increase in the amplitude of the E.M.G. When in the quasi-stable state, the monostable circuit 15 switches on a stimulating oscillator 16, and at the same time cuts off the amplifier 12. The oscillator 16 is a multivibrator providing 1 to 8 volt square wave pulses at a repetition frequency of 20 Hz. or more, and thus pulses at this frequency are passed back to the electrodes 10 and 11 by means of connections 18 and 19. AT the end of the 10 seconds in which the monostable circuit is in its quasi-stable state, the circuit reverts from its stimulation mode back to its sensing mode, until it is triggered again.

A wearer can hold the stimulator oscillator 15 in operation for as long as he wishes by maintaining his E.M.G. signal above the threshold level, that is the level at which the amplifier output signal is sufficient to switch the monostable circuit to its quasi-stable state. Under these conditions when the monostable circuit reverts to its stable state, the apparatus is in the sensing mode for a period of less than 50 milliseconds before returning to its stimulating mode, so that only one pulse in the 20 Hz. pulse train is missed. This will not be noticed by the wearer. As soon as his E.M.G. signal drops below the threshold level the apparatus reverts to sensing at the end of the quasi-stable state of the monostable circuit until triggered again.

In FIG. 2 the same designations have been used as in FIG. 1 for corresponding parts of the circuit, the blocks of FIG. 1 being indicated by dotted lines.

The amplifier 12 includes a Fairchild μA702C integrated amplifier 21 biased in known way, coupled to an NPN-transistor T1 connected in the common emitter mode. The amplifier characteristic is modified in the required way by a capacitor 26 (0.15μF.) connected to pin 6 of the integrated amplifier, and a capacitor 27, both of these capacitors providing high frequency suppression. The collector of the transistor T1 is connected by way of a coupling capacitor 22, which suppresses some low frequencies, to a rectifying circuit 13 comprising diodes 23 and 24. Smoothing is obtained by a capacitor 25 which is coupled to the monostable circuit 15 which comprises transistors T2 and T3. The transistor T3 conducts in the stable state of the circuit 15, the transistor T2 being cut off. The earth voltage applied at the emitter of the transistor T2 can be regarded as a reference voltage which when exceeded sufficiently by the output from the smoothing circuit 14 will switch on the transistor T2, and thence cut off the transistor T3, causing the circuit 15 to enter its quasi-stable state.

When the transistor T2 is switched on, two transistors T4 and T5 in the stimulator oscillator 16 are able to conduct. The transistors T4 and T5 are connected in a multivibrator circuit which, when its transistors are allowed to conduct, passes pulses by way of a further amplifying transistor T6 to the electrode 10. The pulses which are positive at the input of the amplifier 12 appear inverted at its output but are partially suppressed by the capacitor 26. They are then integrated by the capacitor 27, holding the base of the transistor T1 negative and cutting it off. Thus the monostable circuit 15 reverts to its stable state at the end of the quasi-stable period.

In FIG. 3, as in FIG. 2, the same designations have been used as in FIG. 1 for corresponding parts of the circuit, the blocks of FIG. 1 being indicated by dotted lines as far as this is possible since parts of some blocks are detached. To aid in obtaining a high signal-to-noise ratio a differential input configuration, matched for good common mode rejection, is advantageous at the input of the amplifier 12.

The electrodes 10 and 11 are therefore connected by way of two differential amplifying stages, including transistors T7 to T10 to the integrated circuit amplifier 21 which again consists of a Fairchild μA702C operational amplifier connected to have a differential input configuration. The numbers 1, 2, 3, 4, 6, 7 and 8 on FIG. 3 relate to the designations given by the maker to the pin connections of the amplifier.

The differential stages are protected against high-input voltages by a back-to-back diode pair 30, and similar protection is provided across the inputs to the amplifier 21 by another back-to-back diode pair 31. Common mode rejection can be adjusted, that is the inputs balanced to reject signals of the same polarity, by varying a potentiometer. A capacitor 44 connected across the input is employed to suppress radiofrequency interference. The overall amplifier gain may be varied by adjusting a variable logarithmic resistor 41. This stage is coupled to the amplifier 21 by way of coupling capacitors 42 and 43.

The amplifier 21 is connected to have a single-ended output, which is coupled to an NPN-transistor, T1, connected in the common emitter mode. The amplifier 12 is able to detect voltages down to 50μv. and can attain a voltage gain of approximately 50,000.

A capacitor 26, at the base of the output transistor of the integrated circuit helps in achieving the required high frequency fall off. Low frequencies are attenuated by coupling capacitors 42, 43 and 33.

Half-wave rectification is achieved both by biasing the transistor T1, and with the diodes 23 and 24. It has been found that smoothing is not required in this embodiment.

Transistors T2 and T3 are connected in a monostable circuit which has a stable state in which the transistor T3 is conducting and the transistor T2 is cut off. The earth voltage applied at the emitter of the transistor T2 can be regarded as a reference voltage which, when exceeded sufficiently by the output from the rectifier circuit will switch on the transistor T2, and thence cut off the transistor T3, causing the monostable circuit to enter its quasi-stable state. This state has a duration of approximately 5 seconds, dependent upon the value of a capacitor 34 and a variable resistor 45, before the circuit reverts to its stable state. The duration can be adjusted by varying the resistor 45.

When the monostable circuit 15 is in its quasi-stable state a transistor T11 conducts and current is passed to allow two transistors T13 and T14 connected as a multivibrator oscillator to provide stimulus pulses. The repetition frequency and duration of the pulses can be varied by adjusting resistors 46 and 47, these controls being interdependent.

The stimulus pulses are amplified by an output stage comprising a pair of transistors T15 and T16 connected in the Darlington configuration. The transistors feed the primary winding of an isolating transformer 36 which is in series with a variable resistor 48 which acts as a stimulus strength control.

The output pulses from the stimulator circuit are inductively coupled back to the electrodes at the amplifier inputs by the transformer 36 which improves isolation between the wearer and the stimulator circuit.

There are three inhibitor features which prevent the stimulator pulses from activating the sensing circuit, even though they appear at the amplifier inputs.

The pair 30 of protective diodes at the input to the first differential stage reduces the pulses to an amplitude corresponding to the forward voltage drop across the diodes. The frequency compensating capacitor 26 bypasses most of the remaining pulse signal to earth, but nevertheless, a pulse of approximately 1.4 volts still reaches the triggering point at the base of transistor T2, which is sufficient to induce the monostable circuit to remain in its quasi-stable state. The diode 50 is reverse biased except during the transition of the monostable circuit from its quasi-stable state to its stable state. It then acts as a steering diode providing a path to ground for the pulses at the base of transistor T2, allowing it to switch fully out of saturation. These inhibitor features are sufficient to ensure that the stimulus pulses do not themselves hold the circuit in its stimulating mode.

A stimulator manufactured by Devices Implants Limited (Bladder Stimulator Transmitter S2780-2) may be used in conjunction with their Bladder Implant diode detector circuit as the stimulator circuit both in the circuits of FIGS. 2 and 3 if these circuits are suitably modified, and in other embodiments of the invention.

The wearer of this device will be able, by a muscular effort, to activate a stimulator which will contract his levator ani muscles for as long as he required. By maintaining his E.M.G. above the threshold level, the monostable circuit will remain it its quasi-stable stimulating state, except for 0.5 second in every, say, 10 seconds, during which the circuit will revert to its stable sensing mode. This short interruption of the stimulus will be beneficial in maintaining contraction of the muscles. Once he allows his E.M.G. to fall below the threshold level, the circuit will complete its current quasi-stable state and then revert to sensing until triggered again.

The apparatus specifically described may be improved by providing a stimulator oscillator which has bipolar pulses instead of unipolar pulses to overcome problems of electrolysis associated with implanted electrodes.

Flat plate electrodes in an anal plug have been mentioned, but other electrodes, such as needle electrodes, implanted close to the anal canal, may be used.

The apparatus described specifically provides stimulation pulses of square wave shape but other wave shapes may be used.

Although this specification specifically describes an embodiment of the invention for the aid of the incontinent, the invention can be applied to the stimulation of many muscles or groups of muscles.

The invention may be applied to cases of incomplete nerve injury, by allowing the circuit to oscillate between its sensing and stimulating modes at a fast, physiologically acceptable rate, so that a stimulus proportional to the sensed E.M.G. would be applied after each period of sensing. This would, in effect, amplify the natural E.M.G. in a muscle, an otherwise impossible closed loop phenomenon.