"A SET OF NEEDLE FOR ELECTROESTIMULUS OF A NERVE"BACKGROUND OF THE INVENTION1. Field of Invention The present invention relates to an apparatus for efficiently locating a nerve and subsequently supplying an anesthetic to the nerve.2. Description of the Prior Art. Many medical procedures require that a patient be anesthetized at least locally. A regional anesthesia or nerve block offers advantages in relation to general anesthesia for many medical procedures. For example, a regional anesthesia or nerve block is typically less traumatic to the patient undergoing surgery, and often allows a shorter post-operative recovery. A regional anesthetic or nerve block necessarily requires the location of the nerve to which the anesthetic agent will be administered. The prior art includes methods for locating the nerve. In most of these methods of the prior art, the doctor typically uses general knowledge of physical anatomy to locate approximately the reference nerve. In accordance with a method of the prior art, an electrically conductive pad is placed on the skin in a portion of the patient's body at a distance from the reference nerve. For example, if the reference ridge is in the shoulder, the electrically conductive pad can be secured at a distant portion of the arm. The electrically conductive pad is connected by a wire with a stimulator box of the prior art which is capable of generating an electric current, as will be further explained herein. An electrically isolated needle cannula with a non-isolated conductive tip is then pushed through the skin and the subcutaneous tissue, in the general direction of the nerve to be anesthetized. The needle of the prior art is connected by a wire to the electrical stimulator box of the prior art. The stimulator box of the prior art is electrically energized and is capable of operating to produce an adjustable current pulse over a duration of about 100 to 200 microseconds ("uS"). The current pulse is initially graded to a level of approximately 1.0 to 5.0 milliamperes ("mA"). This level of current is typically sufficient to stimulate the reference nerve when the needle has been placed within the tissue in the approximate area of the reference nerve.
The stimulus can cause a perceptible muscular shock in the areas of the body controlled by the reference nerve (eg, the fingers). The current then slowly decreases until the shaking disappears. The needle of the prior art is then advanced slowly towards the reference nerve until the shaking reappears. This iterative procedure continues until the needle of the prior art is capable of generating perceptible muscle shocks at a current level of about 0.2 to 0.3 millia. At this point, the needle of the prior art is considered to be sufficiently close to the reference nerve for administration of the anesthetic agent. The anesthetic agent is then delivered directly through the needle, while the needle continues to produce current pulses. The cessation of muscle jerks is typically considered as indicating satisfactory nerve localization. The electrolocation method of the prior art is intended to ensure the exact placement of a needle for the anesthetic delivery. However, the prior art device and the prior art method for electrolocation of a reference nerve have several inconveniences. For example, the electrolocation device of the prior art, including the stimulator box, is a rather large piece of equipment., expensive and reusable that is not easily sterilized. Therefore, there are problems with the use of a prior art electro location device in the sterile environment of an operating room. It is typically necessary to employ two technicians to carry out this prior art procedure, namely, a first technician operating under sterile conditions and handling the needle, and a second technician separated from the first technician and operating under non-sterile conditions to decrease incrementally the level of the current. The use of two technicians necessarily requires rather high costs and requires considerable coordination and communication between the two technicians. Second, the electrolocation device of the prior art does not provide a definitive indication of when the needle is properly positioned to inject the anesthetic. The doctor has to rely on judgment and experience to determine when the needle is in the optimal position. Third, the considerable distance between the isolated needle and the conductive pad of the prior art requires the generation of a relatively high voltage to achieve the desired current level. A voltage of at least 25 volts ("V") is common in the electrolocation apparatus of the prior art. These relatively high voltage levels limit the use of the prior art apparatus. For example, high voltage levels can affect the operation of spacers and other implanted electronic devices. Therefore, the electrolocation device of the prior art, generally can not be used in patients with implanted electronic devices. In addition, relatively high energy creates the risk of arcing. Therefore, the electro-localization apparatus of the prior art can not be used in many surgical environments, such as those where oxygen is being used, due to the risk of fire or explosion. High current levels can also create the potential for tissue damage in proximity to the needle.
COMPENDIUM OF THE INVENTIONThe present invention is directed to an electro-localization apparatus for accurately and efficiently locating a nerve to which an anesthetic agent can be administered. The apparatus employs energy levels low enough to avoid potential tissue damage and to allow the use of the apparatus in situations where the patient has an implanted electronic device. The apparatus is also small and economical enough to be manufactured for single use and can be manufactured sterile enough to be used in the sterile field of an operating room. In addition, the apparatus can be used by only one technician. As mentioned above, the voltage required for an electrolocation device is a function of the distance between two conductors and the contact resistance towards the patient. To minimize the distance considerably, the present invention provides both conductors in the needle cannula. More particularly, the electro-location apparatus of the present invention can employ a needle assembly having a pair of coaxially placed conductors. An internal conductor of the pair of coaxial conductors can be defined by the needle. A non-conductive liner or tube can then be mounted above the inner conductor and can be galvanized, coated, co-extruded or otherwise provided with an electrically conductive material that functions as the external conductor. A bevel or chamfer can be defined at the distal end of the non-conductive tube. The bevel can be defined by a non-conductive adhesive at the distal end of the tube. The bevelled adhesive works to hold the tube in place and also facilitates the entry of the needle assembly into the patient. The spacing between the conductors of the electro-localization device is defined by the distance from the distant edge of the bevel to the conductive lining, which is preferably slightly more than 1.0 millimeter ("mm"). In view of this very small distance, a very low voltage can be used to generate the required current. It is believed by the present inventors that this aspect of the invention makes the present electro-localization apparatus suitable for use with patients having implanted electronic devices, such as spacers. In addition, the low energy level allows the present electro-localization apparatus to be used virtually in all environments of an operating room, including those in which the prior art electrolocation apparatus has created the combustion potential. In addition, the low voltage allows a simple electronic circuit to be conveniently provided in a small package. As mentioned above, the electrolocation device of the prior art had required two technicians, namely a first technician to carefully handle the needle, and a second technician to carefully vary the level of the current. The present electro-localization apparatus employs an entirely different structure that operates under entirely different principles, and allows the use of the electro-localization device present by a single technician. The electrolocation device takes advantage of the determination that the threshold electrical parameter for generating a muscle jolt is more accurately measured in terms of the electrical load rather than the electrical current. The electric charge is the product of current and time, and the charge can be varied by changing either the level of the current or the length of time. In a first preferred embodiment, the present electro-localization apparatus generates constant current pulses; however, the pulses in sequence alternate between a relatively long duration and a relatively short duration. In this way the constant current pulses in sequence alternate between the relatively high load and a relatively low load. In a second modality, the electrolocation apparatus is capable of operating to alternatively supply relatively high current pulses (e.g., 0.5 mA) with relatively low current pulses (e.g., 0.1 to 0.2 mA). Each pulse can be of the same duration (eg, 0.1 to 0.2 millisecond ("mS")) and the pulses can be generated at uniform intervals (eg, from 0.25 to 2.0 seconds). An approach for using the electro-localization device of the present invention may include pushing the needle into the patient and into the reference nerve. Relatively high load impulses will generate muscle jerks at a distant site of the nerve after the skin has been penetrated (e.g., when the tip of the needle is at a distance of approximately 1.0 centimeter from the reference nerve). The relatively low charge impulses, however, will not produce a sufficient load to generate muscle jerks at this initial distance. The pulses can be separated, for example, by approximately half (then "1/2" or "0.5") second. Therefore, the doctor will initially observe the muscle jerks at intervals of approximately one second, coinciding with the high load impulses. As the needle moves toward the reference nerve, the doctor may notice a slight increase in the magnitude of the initial observed muscle jerks, caused by high load impulses. Simultaneously, the doctor will begin to observe small muscle jerks in response to a low load impulse following each high load impulse. In this way, using the example above, the physician may observe a large shake in response to a high load impulse followed by 0.5 second later by a smaller shake in response to a low load impulse and followed by 0.5 second later by another larger shake in response to a high load impulse. The jerks generated in response to the high load impulses will quickly reach a maximum such that further movement of the needle towards the reference nerve will not significantly increase the magnitude or seriousness of the jerks resulting from the high load impulses. The jerks generated in response to the low load impulses will gradually increase in magnitude of intensity as the needle continues to approach the reference nerve. These changes in the magnitude and intensity of the low load shocks can easily be observed by the doctor inserting the needle. As the tip of the needle approaches the reference nerve, the main and small shakes will become essentially indistinguishable, and the physician will only observe almost identical muscle jerks at intervals of approximately 0.5 second or twice the initially observed range. This will indicate to the doctor that the tip of the needle is properly placed for - Il ¬the administration of the specified anesthetic. The anesthetic agent can then be pushed through the needle and into the reference nerve. The anaesthetized nerve will then stop the shaking, thus providing the doctor with a clear indication that the reference nerve has been reached and that the anesthetic has produced the proposed effect. The physician can then simply activate a switch on the small control of the electro-localization device to terminate the flow of current to the needle. Although primarily described herein with the concept of generating alternate charge pulses in high and low sequence, it will be appreciated by a skilled person that the construction of the electrolocation device and the components described herein can be configured to produce a pulse repetition graduated load pattern, depending on the desired application. For example, depending on the anatomy of the region surrounding the nerve being sought, it may be beneficial to have a gradual repetition pattern of the load impulse as it approaches the nerve, rather than an alternative series. of high and low absolute charge impulses as the nerve approaches. That is, the apparatus and the associated components can be configured in such a way that instead of providing an alternative series of high and low level charge pulses, it will provide a repetition pattern of graduated charge pulses, with the graduation in each pattern decreasing from a selected maximum level charge pulse to a selected minimum level charge pulse. Thus, for certain anatomies, the physician is provided with a larger scale of clinical observations with respect to the nerve reaction, to the load impulses, thus providing a more accurate knowledge to the physician of the location of the device with respect to to the nerve. Other patterns are also possible.
BRIEF DESCRIPTION OF THE DRAWINGSFigure 1 is a perspective view of a bipolar electro-localization apparatus in accordance with the present invention. Figure 2 is a cross-sectional view of a needle in accordance with the present invention. Figure 3 is a functional diagram of a set of circuit components operating to produce appropriate load pulses through the needle of Figure 2, in accordance with the present invention.
Figure 4 illustrates an example of a combination of operating circuit components within the blocks of Figure 3. Figure 5 is a graph showing a pulse generation pattern according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED MODALITYAn electrolocation apparatus in accordance with the present invention is generally identified by the number 10 in Figure 1. The apparatus 10 includes a needle assembly 12, a stimulator 14 and a tube 16 for delivering a dose of anesthetic through the assembly 12. of needle The needle assembly 12, as shown more clearly in Figure 2, includes an elongated needle cannula 20 having opposite proximal and distant ends 22 and 24 and a lumen 26 extending continuously therebetween. The needle cannula 20 is formed of an electrically conductive material and preferably of stainless steel. The proximal proportions of the needle cannula 20 are mounted securely on the stimulator 14 with the proximal and distal ends of the needle cannula 20 remaining on opposite sides of the stimulator 14. The distal end 24 of the needle cannula 20 is beveled to a point that facilitates the perforation of the tissue to have access to the reference nerve. The needle assembly 12 further includes a thin-walled tube 28 positioned coaxially above the needle cannula 20. The tube 28 has opposite proximal and far ends 30 and 32 respectively and is formed of a non-conductive material such as polyimide. The proximal end 30 of the tube 28 is placed in the stimulator 14 as further explained herein. The distal end 32 of the tube 28 is spaced proximally from the beveled distal end 24 of the needle cannula 20. The tube 28 is dimensioned to be tightly coupled against the outer cylindrical surface of the needle cannula 20. Nevertheless, the secure retention of the tube 28 in the needle cannula 20 is achieved by a non-conductive epoxy 34 or other adhesive extending between the distal end 32 of the plastic tube 28 and the outer cylindrical surface of the needle cannula 20. Epoxy 34 is chamfered to facilitate entry of needle assembly 12 into a patient. The chamfering preferably defines a length of 1.0 millimeter. The tube 28 includes a conductive layer 36 on its external cylindrical surface which can be applied by electroplating or coating. The layer 36 is preferably gold and extends continuously from the proximal end 30 to the distal end 32 of the tube 28, to a thickness of approximately 550 angstroms. The needle assembly 12 functions effectively as a pair of coaxial conductors as explained hereinabove. In particular, the stainless steel needle cannula 20 functions as an internal conductor, while the gold layer 36 in the tube 28 functions as an external conductor. The tube 28 defines a non-conductive insulating material that separates the inner and outer conductors defined respectively by the stainless steel needle cannula 20 and the gold layer 36. As mentioned above, the stainless steel needle cannula 12 extends continuously through the stimulator 14 such that the proximal end 22 of the needle cannula 20 is placed on one side of the stimulator 14, while the Distant end 24 is placed on the opposite side thereof. The proximal end 30 of the plastic tube 28 is placed inside the stimulator 14. As a result, both the stainless steel needle cannula 12 and the gold layer 36 are exposed for electrical contact within the stimulator 14. The stimulator 14 includes a housing 38 generally rectangular which may have dimensions of length and width of for example, about 19.84 millimeters and a thickness dimension, for example, of about 9.53 millimeters. The housing 38 can be formed of two molded thermoplastic housing halves 40 and 42 that are welded or adhered to one another. The upper and lower walls respectively may include concave regions for ease of attachment by the digits of the hand. The housing 38 performs multiple functions, including providing structural support for the needle assembly 12, providing a convenient handle for handling the needle assembly 12, and securely enclosing the electronic components of the electro-location apparatus 10. The electronic circuit of the stimulator 14 includes an on / off switch 48 and a light-emitting diode (LED) 50 both of which are accessible and / or visible from the outside of the housing 38. The on / off switch 48 operates to completing the circuit between a battery and other circuit portions, as will be described further below, and optionally allowing switching between high and low load levels. The LED 50 operates to generate a light pulse with each pulse of electrical energy so that the technician or physician can compare the energy pulses with the muscle jerks in the patient. Figure 3 is representative of the circuit that can be used in the stimulator 14. As an expert will appreciate, one way to implement this circuit is to digitize it using CMOS technology as active elements. Other implementations, such as customary integrated circuits ("ICs") are also possible. Here, the on / off switch 48 is connected to a 3 volt lithium cell battery 52. In the disconnected state, the quiescent current is under 1 microampere ("uA") providing a battery life in excess of eight years, thus ensuring adequate shelf life for the electrolocation apparatus 10. In the connected state, the oscillator and the counter which will be described below are capacitated and the battery will operate the stimulator 14 for approximately 100 hours. The modulation of the time duration pulse is achieved by a counter 54. Using the outputs of the counter 54, it is possible to generate a pulse as short as 122uS. Since the outputs of the counter 54 are periodic signals, the Timing Selection Gateway network 56 selects only one period of the output signal and applies the same to the current source network 58. In the embodiment shown in Figure 3, the gate network 56 may alternatively allow either a low load pulse or a high load pulse. As shown schematically in Figure 5, the stimulator 14 functions to alternatively generate pulses of short and prolonged duration. All the impulses will be of a constant current but of different durations. For example, stimulator 14 can operate to generate a pulse at a current level of 0.2mA for 122uS in order to produce a relatively low load of 24.4 nanocoulombs ("nC") followed by a current pulse of 0.2mA for approximately 488uS in order to produce a relatively high load of 97nC. It will be analyzed by an expert that depending on the components selected to generate the impulses, the duration of the impulses can vary within a time scale, eg, +/- 20 percent of the durations manifested in the present. Other pulses can be used in constant current pairs for different durations to produce alternative low and high loads. The circuit of Figure 3 is also designed to optionally provide pulses of constant duration with modulation of current amplitude. For example, a low current pulse of 0.2mA can be generated for 122uS to produce a relatively low load of 24.4nC, and can be followed by a high current pulse of O.dmA for 122uS to produce a relatively high load of 97nC . It will be noted that the loads produced by the modulation option of the current level is equal to the loads produced by the modulation option of time duration. Figure 3 is a functional diagram of a set of circuit components in the stimulator 14 that function to produce appropriate load pulses through the bipolar needle 12, and Figure 4 illustrates an example of a combination of operating circuit components within of the blocks of Figure 3. As seen in Figure 3, an on / off control 51 operated by the switch 48 has an output that manipulates an oscillator 53 to activate a counter 54 and another output that enables and incapacitates the counter 54. A third output is supplied to a control circuit 55 which receives an output of the counter 54 and activates a constant current sink 58 coupled with an electrode (20 or 36) of the bipolar needle 12. The indicator circuit 57 that drives the LED 50 receives the inputs of the oscillator 53, the counter 54 and the current source V +, which through a load limiter 59 is coupled with the other electrode (36 or 20) of the needle 12 bipolar. The synchronization and magnitude of the charge pulses are modulated by a component 56 of the synchronization selection gate that is coupled to the control circuit 55. Returning to the details of the circuit in Figure 4, the on / off control 51 may consist of the on / off switch 48 which couples the voltage V + of the battery 52 to the circuit including a vascular circuit A1B and an RC combination (R1). , C3). When the apparatus 10 is to be used, the switch 48 is placed in the connected position and remains connected to avoid any of the transient high currents in the needle. The vascular circuit A1B controls the synchronization of the oscillator 53 and may comprise a Schmitt A3A trigger, and capacitate and disable the counter 54 which may be in the form of a 12-bit counter A2 and the dissipation control circuit 55 which may comprise a vascular circuit ALA. When A1B is connected, the output line 12 is low or zero, so that the readjustment of the counter A2 is off and therefore, it is free to count, and the AlA reset is off, therefore, it is free. to change state. Concomitantly, the output line 13 of A1B is high or positive so that the oscillator A3A operates, v, gr, at 4,096 kilohertz ("kHz") to cause the counter A2 to count, after which the pin 1 of A2 is caused to change state every half second and pin 15 goes to the positive state every half second. In this way the pin 15 changes state to twice the speed of the pin 1. When A1B is disconnected, line 13 goes to a low state stopping the output of A3A, and line 12 goes to a raised state by resetting A2 and AlA. When the pin 15 of A2 goes towards the positive state, the clock signal towards Al causes the output line 1 to go towards a high state through the voltage V +, supplying the base current to the transistor Q3, through the resistors R4 and R5. Q3 is thus caused to drive by closing a current path so that current flows through needle 12 from battery V +, through capacitor C4 and through resistor R7 to ground. If the voltage in R7 rises to more than 0.55 V, the base of transistor Q2 will be driven through resistor R6 to connect Q2, which in turn decreases the base current to Q3, thereby maintaining the voltage across the R7 to 0.55 V. Therefore, the current through the needle 12 remains essentially constant. In the event of a short circuit or a fault in the path of the needle current, capacitor C4 acts as a load limiter charging a preselected maximum load and limiting the current level.
The synchronization and shape of the current pulses is determined through the use of the synchronization selection gate component 56 comprising three gates A3B, A3C and A3D receiving inputs from the oscillator A3A and the counter A2, which provide an output to the vascular circuit ALA of the dissipation control circuit 55. The gate A3B controls the short pulses shown in the timing diagram of Figure 5. It will be seen that the input pin 10 to the counter A2 works in negative pulses so that when the output A3A on the pin 3 goes to the negative state , the output pin 15 of A2 goes towards positive drive A1 to connect the current through the path of the needle as explained above. The output on the pin 3 of A3A is also supplied to the entry pin 6 of the gate A3B, the other input pin 5 from which it receives the output of the pin 1 of A2. If the signal on the pin 1 and in turn on the pin 5 is raised, A3B can operate when the pin 6 goes towards the raised state. If the pin 1 goes to the low state or is zero, then the pin 5 is low and A3B can not work. The operation of A3B can be used to control the alternation of short and long charge pulses. When pin 1 is in an elevated state, short pulses will occur.
More particularly, when the A3A pin 3 goes to the low state, the counter 54 will go to its next state. The pin 15 goes to the raised state so that the current begins to flow through the needle and the pin 1 which is raised so that the pin 5 of A3B is in the raised state, while the pin 6 is in the state under zero along with pin 3. The output of A3B on pin 4 will be 1, which is the input on pin 13 to gate A3D. With an elevated entry in the pin 12, the output of the gate A3D in the pin 11 will be zero. Now, when the oscillator A3A sends a high current in the pin 3, the counter A2 does not change its state, but the pins 5 and 6 of A3B will both be in a raised state so that the output on the pin 4 will go towards zero causing the input to pin 13 is zero. If the entry in the pin 12 is still high, the output of A3D in the pin 11 goes towards the raised state. The signal raised on the pin 11 is coupled through the capacitor C2 with the readjustment of the vascular circuit ALA causing its output on the pin 1 to go to the zero state, disconnecting the constant current suppressor 58 and the current through the needle 12. A short current pulse will then have been produced for a duration of 122uS.
To produce longer impulses, gate A3C is used and gate A3B becomes disabled. Since A3B can only function when the pin 1 of A2 is in high state, the signal in pin 1 is caused to go to the low state by disconnecting A3B. In this condition, the AlA reset function is controlled only by A3C. The output of A3C can be controlled according to the pulse ratio chart shown adjacent to A3C in Figure 4. By appropriately connecting A (8) and B (9) which are the A3C inputs with the enumerated combination of pins of the A2 counter, the time relationships between the short and prolonged pulses that are shown in the column to the left of the table can be achieved, thus obtaining pulse width modulation of the load impulses. To achieve pulse amplitude modulation, inputs A and B to A3C can both be connected to pin 10 of A2 to produce a pulse time ratio of 1 to 1, with pulses of 122uS. An RIO resistor in the constant current suppressor 58 is connected in the circuit between the pin 1 of A2 and the emitter of the transistor Q3, closing a switch S 1. When the pin 1 is in an elevated state, current flows through RIO and resistance R7 towards ground. The current in the current path through the needle 12 therefore decreases since the voltage across R7 remains constant and the current through R7 is made up of two sources. Consequently, the magnitude of the impulse of the current through the needle 12 becomes a comparatively low current pulse. When the pin 1 of A2 is in the low state, that is, it goes to earth, RIO is configured in parallel with R7 with respect to ground, so that the resistance through R7 and RIO decreases with respect to the current path. Since the 0.55V volt is maintained at its junction point, as explained above, a greater amount of current is needed through both resistors. Therefore, the magnitude of the current pulse through the needle 12 increases resulting in a comparatively high current pulse. Accordingly, you can achieve modulation of the pulse amplitude with this circuit. If desired, the modulation of both pulse width and pulse amplitude can be produced by selecting the pulse ratios in the pulse ratio frame and switching the RIO resistor to the circuit. Finally, the indicator circuit 57 is configured to be activated when an impulse has occurred, independently of the modulation, and to produce a simple connection or disconnection indication. Therefore, the LED 50 will turn on when a charging pulse occurs or the buzzer 60 will produce a sound in accordance with the synchronization and change of state of the outputs on the pin 3 of A3A and the pin 15 of A2. As mentioned above, the proximal end 22 of the stainless steel needle cannula 20 projects entirely through the housing 38 of the stimulator 14. As shown in Figure 1, the proximal end 22 of the cannula 20 of The stainless steel needle is connected to a flexible pipe 16 which extends to a socket which is capable of being connected with a syringe to deliver a selected dose of anesthetic. In an alternative embodiment, the proximal end 22 of the stainless steel needle cannula 20 can be mounted directly to a needle plug that is connectable with a syringe to deliver a dose of selected anesthetic. During use, an anesthesiologist or nurse inserts the beveled distant tip 24 of the stainless steel needle cannula 20 into a patient and into the reference nerve. No wires or a conductive pad are used. In the constant current mode described herein, the switch 48 on the stimulator 14 is then operated to generate the low constant current pulses of electrical energy. The proper functioning of the electro-location apparatus 10 is confirmed by the flashing of LED 50 which generates a light pulse simultaneously with each respective energy pulse. The respective impulses of energy are generated at intervals of 1/2 second. The high load impulses of 0.2mA during 488uS will generate a load of 97nC. The low charge impulses are of the same current of 0.2mA, but they last only 122uS and will generate a charge of only 24.4nC. Higher load impulses of 97nC will be sufficient to generate observable muscle jerks in an essentially superficial location after the skin has been penetrated by gold layer 34, while lower loads, 24.4nC impulses will not be sufficient to generate initially no observable muscle jerks at this distance from the nerve. Therefore, the anesthesiologist or nurse will observe the muscle jerks at intervals of approximately one second coinciding with the high load impulses. The needle assembly 12 is pushed further towards the reference nerve. This advance of the needle assembly 12 will show a gradual increase in the magnitude of the jerks that occur at one second intervals. Nevertheless, these shakes in response to a high load will reach the maximum in a short time. The anesthesiologist or nurse will then observe muscle shaking of small magnitude between shakes of larger magnitude. In this way, alternative shakes of small and large magnitude can easily be observed. As the needle assembly 12 is advanced further towards the patient, small-magnitude muscle jerks will increase in magnitude to approximate the magnitude of the large-magnitude muscle shocks that have reached the maximum generated by the high-load impulses. As the distal tip 24 of the stainless steel needle cannula 20 approaches the reference nerve, the muscle jerks generated in response to the low load impulses will essentially be indistinguishable from the muscle jerks generated in response to the impulses of high charge energy In this way, the anesthesiologist or nurse will observe almost identical muscle jerks at 0.5 second intervals. This readily observable response will indicate to the anesthesiologist or nurse that the beveled distal tip of the needle cannula 20 is sufficiently close to the reference nerve for administration of the anesthetic. The anesthetic is supplied in the conventional manner by operating the hypodermic syringe communicating with the proximal end 22 of the stainless steel needle cannula 20. The exact procedure can be carried out by the alternative mode that modulates the level of the current. Although the invention has been described in connection with a preferred embodiment, it is evident that changes can be made without departing from the scope of the invention as defined in the appended claims. For example, the stimulator may have switching mechanisms to change the current level or pulse width to vary the respective levels of the loads delivered to the patient. In addition, other indications of pulse generation may be provided by including an audible buzzer instead of or in addition to the LED described above.