MAGNETICALLY COMPATIBLE PERIPHERAL NERVE STIMULATOR
CROSS REFERENCE TO A RELATED APPLICATION
This application claims priority of earlier filed provisional patent application Serial No. 60/063,996, filed on October 15, 1997, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to externally applying an electrical pulse to the central nervous system, and more particularly to a new and improved peripheral nerve stimulator.
2. Background of the Invention
Peripheral nerve stimulators are used for monitoring depth of anesthetic during a surgical procedure performed on a patient. The current state of technology of the peripheral nerve stimulator is mature and well understood. A step-up transformer amplifies the signal in the conventional peripheral nerve stimulator. The transformer core makes the instrument sufficiently magnetic to be unsafe for use in the intense magnetic field of an MRI (Magnetic Resonance Imaging) unit. This is becoming an issue with the advent of interventional MRI, where surgical procedures are performed under real time MRI, and the patient is operated upon in the magnetic field of the MRI unit. The magnetic forces applied to the magnetically susceptible nerve stimulator could interfere with the actions of personnel in the MRI suite. The present invention solves this problem by providing a magnetically compatible peripheral nerve stimulator for providing pain control during a surgical procedure performed on a patient while inside an MRI suite .
SUMMARY OF THE INVENTION
The present invention provides a peripheral nerve stimulator of low magnetic susceptibility that can be used in the high magnetic field of an MRI suite. Non-magnetic lithium batteries power the peripheral nerve stimulator. A stack of capacitors and diodes (hereinafter "components") forming a voltage multiplier replace the conventional step-up transformer. Each Component is carefully chosen for low content of the magnetic materials. To minimize the number of
Components, and enable the magnetically compatible peripheral nerve stimulator to be a hand-held instrument, a full H-bridge supplies power to the multiplier. The power is an AC signal with voltage equal or plus or minus to the supply voltage. This provides a peak to peak voltage value of two times the available battery voltage. This approach reduces the number of capacitors and diodes to half, compared to the half bridge solution. To increase efficiency of the diodes, miniature surface mount Shotky Diodes are used. To reduce the size of the capacitors, a high switching frequency (1MHz) mode of operation is employed.
The following detailed description of the invention, when read in conjunction with the accompanying drawings wherein the same reference numerals denote the same or similar parts throughout the several views, is in such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains, or with which it is mostly nearly connected, to make and use the invention. BRIEF DESCRIPTION OF THE DRAWING FIGURES
Fig. 1 is a schematic circuit diagram of the magnetically compatible peripheral nerve stimulator according to the present invention;
Fig. 2 is a schematic circuit diagram of another embodiment of the voltage multiplier in the circuit of Fig. 1 ;
Figs . 3A and 3B are graphs including waveforms illustrating operation of the voltage multiplier of Fig. 2;
Figs . 4A - 4D are graphs including waveforms illustrating various modes of operation of the stimulator of Fig. 1 ; Fig. 5 is a graph including waveforms further illustrating operation of the stimulator of Fig. 1; and
Fig. 6 is an elevational view of the magnetically compatible peripheral nerve stimulator apparatus according to the prevent invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a magnetic resonance imaging (MRI) compatible peripheral nerve stimulator. The stimulator is used for pain control during a surgical procedure performed on a patient while inside an MRI apparatus. The MRI compatible nerve stimulator substitutes the conventional step-up transformer with a diode-capacitor voltage multiplier array. Thereby, the device reduces its magnetic susceptibility. The trigger circuit for the device includes common 555 timers. Further refinements include the use of a full H-bridge power supply, a 1 Mhz switching frequency to the diode-capacitor voltage multiplier, the use of the surface-mount Shotky diodes, and a lithium battery pack. These refinements reduce the number of components, overall size, and further reduce the magnetic susceptibility of the nerve stimulator.
Referring to Fig. 1 which is a schematic circuit diagram of the magnetically compatible peripheral nerve stimulator 10 of the present invention, the diode-capacitor voltage multiplier is shown at 12 and the trigger circuit is designated 14. The stimulator output pulses are provided across terminals 16 and 18. Fig. 2 is a schematic circuit diagram of an alternative form of diode-capacitor voltage multiplier 20 which can be utilized in the circuit of Fig. 1. The waveforms 22 and 24 in Figs. 3A and 3B, respectively, illustrate the operation of the voltage multiplier 20. Figs. 4A-4D show different output pulse trains corresponding to four operational modes of the stimulator of Fig. 1. Pulses 30 in Fig. 4A are a train of four mode and pulses 32 in Fig. 4B are a tetanus mode. Pulses 34 in Fig. 4C are a twitch mode and pulses 36 in Fig. 4D are a double burst mode. These modes will be described in further detail presently. The curves 40, 42 and 44 in Fig. 5 illustrate output impedance characteristics for the stimulator of Fig. 1 in response to variation in operational parameters. Fig. 6 is an elevational view of the stimulator 10 of Fig. 1 contained in a rectangular housing 50. Output ball electrodes 52 and 54 are connected to terminals 16 and 18, respectively, of the circuit of Fig. 1. The apparatus includes a battery L.E.D. indicator 58 and an output stimulator pulse
L.E.D. indicator 60. An on-off switch for the apparatus is designated 62. The four modes of operation are controlled by twitch key 64, train of four key 66, double burst stimulation key 68 and tetanus key 70. The apparatus is intended for monitoring the effects of skeletal muscle relaxants in an MR or interventional MR environment, not to exceed a 1.5 Telsa field strength. It has been tested and shown to be safe and effective for normal use in a 1.5T GE Signa System. The apparatus is not intended to be used in the magnet bore while imaging due to potential distortion of the MR image . The four pulse stimulation modes previously described have the following characteristics. DOUBLE BURST is two groups of three 200uS impulses, pulses every 20mS, and groups separated by 1 second. TWITCH is 200uS pulses every 1 second continuous. TENANUS is 200 uS pulses every 20mS continuous. TRAIN-OF-FOUR is four 200uS pulses every 1/2 second.
The yellow pulse LED 60 flashes each time a pulse is generated. The green battery LED 58 indicates that the power is on, and the battery voltage is sufficient. The apparatus is powered by one Greatbatch Scientific Battery Pack No. GN-7B0194. Housing 50 can be high impact ABS plastic, 2.75" x 4.50" x 1.50". The apparatus has a weight of 11.3 oz . , including battery. The stimulator apparatus is controlled by the four push buttons or keys 64, 66, 68 and 70 on the case 50, and by the combination potentiometer control knob and switch 62 also on the case 50. The green LED 58, labeled BATTERY, indicates the power is on, and the battery voltage is sufficient. If the green BATTERY LED 58 does not light when the switch is turned on, the battery should be replaced. The yellow LED 60, labeled PULSE, indicates output impulses, and will flash each time an output stimulus pulse is generated. An audio indicator located inside the case 50 gives audio indication of output impulses along with the yellow LED 60. The device is activated by turning the potentiometer control knob 62 until a light click is heard. The maximum counter-clockwise position of the control knob is power off. The Potentiometer adjusts the amplitude of the impulses from zero to 400 Volts. The ball electrodes 52 and 54 are placed in direct contact with the patient.
The Double Burst Stimulation (DBS) button 68 delivers two groups of three impulses each. The impulses are 200 microseconds long, spaced 20 milliseconds apart within the group, with the groups spaced 1 second apart. The yellow PULSE LED 60, indicates the impulses being generated. Before the next group of impulses can be generated, the DBS button 68 must be released. Additional sets of pulses can be generated by depressing the DBS button 68.
The TWITCH button 64 delivers a continuous waveform of impulses 200 microseconds each, spaced 1 second apart as long as the button is depressed. The yellow PULSE LED 60 indicates the impulses are being generated.
The TETANUS button 70 delivers a continuous waveform of impulses 200 microseconds each, spaced 20 milliseconds apart as long as the button is depressed. The yellow PULSE LED 60 indicates the impulses are being generated.
The Train-of-Four (TOF) button 66 delivers a train of four impulses 200 microseconds each, spaced 500 milliseconds apart. The yellow PULSE LED 60 indicates the impulses are being generated. Depressing the button will deliver only a single train of four impulses.
Before the next group of impulses can be generated the TOF button 66 must be released. The series of train-of- four pulses can be repeated as often as one wishes by repeated depressing of the TOF button 66. The following is a brief description of clinical use of the magnetically compatible peripheral nerve stimulator of the present invention. One use involves the neuromuscular junction. The impulse passing down a motor nerve is transmitted to a muscle across the motor end plate or neuromuscular junction. The transmission of this impulse is medicated via acetylcholine which becomes attached to specific receptors sites on the motor end plate. In general terms, it is the blockade of transmission across the neuromuscular junction by muscle relaxants that produce muscle relaxations. Although all muscle relaxants act on the neuromuscular junction, the mode and duration of action and intensity of blockade differ for different agents. These specific aspects of the action of neuromuscular agents can be differentiated by nerve stimulation using the magnetically compatible peripheral nerve stimulator of the present invention.
Another use relates to depolarizing and non- depolarizing blockade. Physiological conduction across the neuromuscular junction is by depolarization of the motor end plate by acthycholine . This depolarization is of exceedingly short duration. Depolarizing relaxants such as succinylcholine chloride cause depolarization of the end plate but the recovery period is much greater and extends to minutes rather than milliseconds. Prolonged depolarization causes neuromuscular blockade. Non-depolarizing relaxants such as d-tubocurarine chloride do not cause depolarization; rather they occupy the specific receptor sites on the motor end plate. This prevents acetylcholine from attaching to the receptor sites. The duration of action of these drugs is probably dependent on how long they occupy the receptor sites. The difference in the type of blockade produced by the depolarizing or non-depolarizing relaxants accounts for their differing response to nerve stimulation, such as that provided by the magnetically compatible peripheral nerve stimulator of the present invention.
The magnetically compatible peripheral nerve stimulator of the present invention can be used in detection of the type of neuromuscular blockade. The response to a brief train of stimuli is a more sensitive index of receptor blockade by a competitive neuromuscular blocking agent than is the response to a single twitch. Also, the Train-of-Four stimulus may produce less discomfort in a conscious patient than a tetanic stimulus. Double Burst Stimulation (DBS) is a pattern of stimulation developed to reveal residual neuromuscular blockade. DBS was developed with the specific aim of allowing manual (tactile) detection of small amounts of residual neuromuscular blockade under clinical conditions.