IL31356A - Peripheral nerve stimulator - Google Patents

Description

PERIPHERAL HE WJ STIMULATOR THE V/ELLCO I3 FOTODATION LIMITED This invention relates to an improved, preferably portable, electronic peripheral nerve stimulator for applying stimuli to a patient.

The stimulator in accordance with the present invention (as hereinafter defined) is adapted, inter alia, to monitor the effects of a muscle relaxant drug on the neuromuscular junction of a muscle or muscle group in a patient, or to enable the effect of such a drug to be detected and reversed or counteracted, or to determine whether the cause of muscular relaxation is due to the presence of muscle relaxants or to an effect on or by the central nervous system.

During surgery it is often necessary to know the type, duration oi action and intensity of neuromuscular blockade present in a patient; i.e., for example, whether the blockade is produced by a depolorising or non-depolorising relaxant drug. These specific aspects of the action of the relaxants can be determined by nerve stimulation, the determining features being referred to in fo pondi g; ftrm. U al-Luii Pireont ■¾ .y, I.1 l7—6«¾£7/fiC;f fl,l?f-| ιΐρ Π On the other hand it may, in the course of surgery, be necessary that the voluntary muscles in the area where a surgeon is working be relaxed or paralysed so that they cannot contract or move as the surgeon works. This is achieved by administering a neuromuscular blocking agent (muscle relaxant drug) and, thereafter, continued monitoring of a neuromuscular junction enables incremental doses of relaxant to be administered in time to prevent unwanted movement of the muscles occuring.

Similarly, it may be required to counteract the effect of the muscle relaxant drug to facilitate study of a patient's condition during a post-operative period.

Furthermore it may be desirable to identify, during the course of surgery, nerves or nervous tissues by stimulation and thereby obtaining a response from associated muscle groups.

Reference may be made to the /Patent /Vppl i rn i f -above mentioned for a fuller description of the types of neuromuscular blockade agents, of the effect of such agents, and of the types and effects of stimulation required for various conditions.

The present invention has for its objects to provide an electrical nerve stimulator for the purposes abovementioned and which is capable of providing either continuous or intermittent output pulsea over a wide (variable) frequency range, which has a low power source drain (this being an important consideration for surgery use where it is imperative that it should not fail during prolonged operating periods) , which preferably has a variable voltage output, and which is safe for use in operating theatres where highly inflammable gases may be encountered. 31356/2 In accordance with th© presen invention there is provided a portable electronic peripheral nerve stimulator comprising a power supply, a transistorlzed'oscillator circuit coupled to said power supply and including a single transistor and resistance-capacitance network defining the oscillation frequency of the said oscillator, said resistance-capacitance network and the emitter-collector path of said transistor each being coupled in parallel circuit across said power supply, means to vary the effective impedance of said resistance-capacitance network to provide a selective output pulse repetition rate from said oscillator circuit withini the range of one pulse per ten seconds to 50 pulses per, second* and a transformer,, said transformer having a first primary1 winding, meane coupling said first primary winding in circuit between the collector of the transistor and said power supply, a second primary winding, means coupling said second primary winding in circuit between the base of the transistor and a point on said resistance-capacitance network, said last-mentioned means defining a positive feed-back loop through which a "reflected" current is passed to said transistor base, a secondary winding from which an output pulse is obtained, and an indicator lamp connected in ciroui jwith the secondary winding of the transformer* Preferably, means are preferably connected in circuit with said indicator lamp and said tertiary winding for adjusting the voltage of said output pulses to a desired level.

The invention will be better understood from the following description of a preferred embodiment thereof taken in con 31356/2 From tho drawing, the peripheral nerve stimulator generally comprises a power supply, an oscillator circuit employing a single transistor VTl in conjunction with a pulse transformer (or ringing choke) Tl and the pulse repetition rate of which is controlled by a R-C network, and an output ciralt connected to the secondary winding of said transformer.

The fundamental operation of the circuit is based upon a modified "blocking oscillator".

Capacitor CI initially has zero voltage across it bu charges to a negative potential through the timing resistor network Ri~R ^or through a single timing resistor as the case may be) until suc time as the voltage across CI exceeds the base-emitter forward conduction voltage of the transistor VT1. The transistor base then conducts and collector current is drawn through winding Wl of the transformer Tl. The current through winding Wl is reflected into winding W2 and, through positive feed-back, turns the transistor "on" hard. This gives a short rise time to the base, and therefore collector, current p lr.^-.

The additional base bias supplied from the reflected pulse permits the base current to discharge capacitor CI to a very low level. Base current is desirably limited to a safe value by introduction of a series limiting resistor R6.

After the pulse from the collector circuit has decayed transistor VT1 is cut-off, since the voltage across CI again approaches zero, and the circuit can no longer ring. One pulse only is thus generated.

The cycle then repeats as capacitor CI charges through resistor network - R^, a train for as long as the power supply is "switched-on" , in this case by switch SI ganged to a potentiometer RV1 hereinafter referred to.

The pulse repetition rate is dependent upon the time constant of the R.C. network, this, as shown in the drawing, being made variable by the resistor switching arrangement comprising switch S2 and push-buttons S3 and S4, but, in a much simplified embodiment, being fixed by a single timing resistor. The valve of capacity of CI must be selected such that transistor VT1 is initially fully "turned on", the resistor(s) being the main timing element(s).

The collector current pulse in winding Wl is also reflected into the secondary winding W3 and across which a high voltage pulse is developed, the value of which is determined by the primary to secondary winding ratio.

This high voltage pulse feeds into a series connection of a gas-discharge (neon)lamp L3?l and potentiometer RV1. The gas-discharge lamp LP1 serves both as a visual indicator of the presence of an output pulse and to limit the voltage present across the potentiometer RV1.

Output connections SKI and SK2 are taken from the moving arm and one side of the potentiometer RV1 respectively, the voltage across the load being regulatory by adjustment of the potentiometer aim.

To facilitate improved operation of the circuit a ove described, a resistor R5 is connected in parallel with capacitor CI, a-τ«ΐ*£« ί ?*5~^Γε3 a resistor R7 is connected in parallel with the gas-discharge lamp LP1, a resistor R8 is "connected in parallel with the potentiometer and a resistor R9 is connected in series with the moving arm of the potentiometer* From a practical viewpoint, resistor R5 has the effect of stabilising the firing point of transistor VT1 and helps to compensate for variations in the capacitance of capacitor CI. Resistor R7 stabilises the firing point of the lamp LP1 and prevents the occurrence of large peak voltages across winding WJ during the instant before gas ionization in the lamp LP1. The resistor R8 is employed purely to act as an output voltage adjustment, the value being chosen such that the no-load output voltage is of a desired value and thus compensates for variations in component parameters.

The value of resistor R9 is chosen so as to provide optimum compensation for variations in load resistance, so as to achieve an approximately linear law for output voltage against potentiometer rotation for all values of load resistance within a working range.

Many modifications may be made in the circuit as described above and as illustrated without departing from the scope of the invention. However. the following are given by way of exemplification as components and component values which may he employed to construct such a device, the elements referred to being those shown in the drawing: Battery, 6 volt (4 x 1.5 volt size AA pencil cell) Resistor Rl, 470K ohms Resistor R2, 270K ohms Resistor R3, 56 ohms Resistor R4, 1000 ohms Resistor R5, 100 ohms Resistor R6, 22 ohms Resistor R7, 330K ohms Resistor R8, 33K ohms Resistor R9, 1000 ohms Potentiometer RV1, 2 K ohms, linear Capacitor Clt 200 micro Farade (6 Volt working) Transistor, Germanium PUP type 2N217 Transformer, "Ferguson" type TSW112.

All of the resistors abovem With the various discussed components having values as above indicated, the following results have been found obtainable: a) Output voltage, - 0 to 375 volts (no load, open circuit condition) - 0 to 20 volts (1,000 ohms load resistance) b) Pulse rise time, - less than 1.0 microsecond c) Pulse width, - approx. 1.3 milli-seconds (overall) d) Pulse repetition rate, - 1 pulse/10 sees. (slide switch S2 to left) - 1 pulse/5 sees, (slide switch S2 to right) - 1 pulse/sec. (push button S3 operated) - 50 pulses/sec. (push button S4 operated).

From the foregoing it will be appreciated that the two position switch S2 permits the selection of continuous automatically cycling "twitch stimulus" frequencies of one pulse per 5 or 10 second intervals. Operation of push-button S3 effects the automatic selection of a continuous "twitch stimulus" frequency of one pulse per second but this will revert to the 5 or 10 second pulse rate immediately the push-button is released. Similarly, a maximum "tetanic stimulus" rate of 50 pulses per second will be obtained for as long as the push-button S4 is depressed.

A container for the components afore described may be formed of any suitable plastic material, the cover therefor being apertured to permit the mounting of external projection of the lamp IP1, switch S2, push-buttons S3 and S4 and a control knob for the ganged potentiometer/switch SI. The cover may additionally be marked with suitable indicia and the container apertured to take a connectable probe or electrode lead plug.

Elactrodes for use in conjunction with the device in accordance with the present invention may take the form of plate electrodes, needle electrodes or, for convenience, probes. 31356/2

Claims (7)

1. A portable electronic peripheral nerve stimulator comprising a power suppl , a traneistoriaed oscillator circuit coupled to said power supply and including a single transistor and resistance-capacitance network defining the oscillation frequency of the said oscillator, said resistance-capacitance network and the emitter-collector path of said transistor each being coupled in parallel circuit across said power supply, means to vary the effective Impedance of said resistance^ capacitance network to provide a selective output pulse repetition rate from said oscillator circuit within the- range of one pulse per ten seconds to 50 pulses per second, and a transformer, said transformer having a first rio y, winding, means coupling said first primary winding in circuit between the collector of the transistor and said power supply, a second primary winding, means coupling said second primary winding in circuit between the base of the transistor and a point on said resistance-capacitance network; said last-mentioned means defining a positive feed-back loop through which a "reflected" current is passed to said transistor base, a secondary winding from which an output pulse is obtained, and an indicator lamp connected in circuit \ith the secondary winding of the transformer*

2. « An electronic peripheral nerve stimulator as claimed in claim 1, wherein said resistance-capacitance network comprises a single capacitor and four series connected resistors, three Qf said resistors being adapted to be selectively switched out of circuit to provide the variable pulse repetition rate.

3. · An electronic peripheral nerve stimulator as claimed in claim 2, wherein by selectively switching said resistors into or out of the circuit, pulse repetition rates of 1 pulse/10 sees.,

4. « An electronic peripheral nerve stimulator as claimed in any one of the preceding claims having a potential divider connected in circuit with said secondary winding for adjusting the voltage of said output pulses to a desired level.

5. An electronic peripheral nerve stimulator as claimed in any one of the preceding claims, wherein the oscillator circuit comprises a "modified" blocking oscillator cirouit, substantially as shown in the accompanying circuit diagram and substantially as hereinbefore described with reference thereto*

6. » An electronic peripheral nerve stimulator as claimed in any one of the preceding claims, wherein the power supply comprises a low voltage dry-cell type battery.

7. An electronic peripheral nerve stimulator substantially as hereinbefore described with reference to the accompanying drawing. For the Applicants TNERS