Abstract:
An improved nerve stimulator needle which allows for improved ease of use and efficacy in the performance of targeted drug delivery to nerve. A variable control mechanism is contained in a housing to which a needle is attached and several electrical conductors are attached. The housing contains an embedded fluid path through which a syringe is attached to the needle. The variable control mechanism and the housing allows for positioning the needle and applying current to the needle with the same hand. The needle may also contain a linear resistive coating that enables the determination of the depth of the needle by determining the resistance of the length of the needle above the skin surface of a patient. Another embodiment of the needle may include an optical variable control mechanism.

Description:
[0001]    This application claims priority to U.S. provisional application 60/253,064 filed on Nov. 24, 2000, which is herein incorporated by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to a nerve-stimulating needle with fingertip control of stimulator current output and visual as well as electrical depth determination ability.  
         BACKGROUND OF THE INVENTION  
         [0003]    Anesthesiologists commonly use nerve stimulators and insulated needles for the purpose of locating peripheral nerves, or nerve plexuses, for the performance of regional anesthesia procedures. This procedure is discussed in Vloka J D et al., “A National Survey On Practice Patterns In The Use Of Peripheral Nerve Stimulators In Regional Anesthesia,” The Internet Journal of Anesthesiology, Vol. 3, No. 4, 1999. In addition to targeting local anesthetic delivery for regional anesthesia, the use of nerve stimulators reduces the potential for nerve injury since direct contact with the nerve is not required for performance of the anesthetic. This is in contrast to the alternative method of seeking paresthesias to confirm needle position. Paresthesias are provoked by directly disturbing the nerve. If such needle to nerve contact can be avoided, direct needle trauma should be reduced.  
           [0004]    The available nerve stimulators have differing output characteristics. The shape of the pulse is typically square or nearly so. The pulse widths vary from 40 microseconds (μs) to 2 milliseconds (ms). Frequency selections for these stimulators range from 1.0 Hertz (Hz) to 100 Hz, in step gradations rather than continuously. Stimulators that are manufactured specifically for regional anesthesia applications typically do not have frequency options greater than 5 Hz. The stimulators that serve as both nerve stimulators and neuromuscular blockade monitors offer higher frequency choices, typically 50 and 100 Hz. These stimulators are discussed in Barthram C N, “Nerve Stimulators For Nerve Location—Are They All The Same?,” Anaesthesia, Vol. 52, 1997, pp. 761-764.  
           [0005]    The needles used for stimulator assisted regional anesthesia typically have a molded plastic hub that contains both a connection to plastic tubing and a wire attached to the metal needle imbedded in the hub. This wire, when connected to a source, supplies electrical current to the needle with appropriate output characteristics for generation of action potentials in axons. Use of this equipment requires a means for holding the needle assembly, adjusting the output current, and injecting medication. This is awkward for a single user to accomplish and usually requires the presence of an assistant.  
           [0006]    The commercially available nerve stimulators offer two methods of controlling the current output from the nerve stimulator instrument to a nerve stimulator needle. The first method is by hand-operated dials on the face of the nerve stimulator instruments. In this method, it is difficult for a single operator to insert and position the needle in the patient, control the current supplied the nerve stimulator needle, and inject the medication in the patient.  
           [0007]    The second method is by a foot-operated pedal connected via a cable to the nerve stimulator instrument. An output source with foot-pedal control, such as described in U.S. Pat. No. 5,830,151 to Hadzic, necessitates a multiplicity of wires connecting pieces of equipment together. In the environment of an operating room where a multiplicity of electrical cables already exists, any equipment that adds additional cables spread out across the floor or tables and carts represents increased hazard for stumbling and consequent injury. Also, efficiency of movement is highly prized in the operating room environment. When an anesthetic procedure is completed, the operating room personnel move rapidly to begin the surgical positioning and prepping. Often, the anesthesiologist is in the position of gathering up equipment used for a procedure, and either disposing of it or placing it on a cart for subsequent storage. Tangles of cables and wires complicate this process and have a tendency to increase the clutter surrounding anesthesia machines and carts.  
           [0008]    U.S. Pat. No. 4,515,168 to Chester et al. discloses to clamp a nerve stimulator and locating device onto a syringe. As the entire nerve stimulator device is clamped onto the syringe, the unit is a long and clumsy assembly, which is difficult to maneuver. Moreover, the device disclosed by Chester does not allow for one-handed operation of needle advancement and current control. Particularly, the needle is advanced by one hand while the current must be controlled by turning the knob  27  with the other hand, which is an extremely awkward operation for the user. Additionally, the nerve stimulator of the Chester patent restricts the size of the syringe upon which it may be mounted, thus, requiring the operator to change the syringe on the needle. This combination makes it very difficult to stabilize the needle within 1-2 mm of a nerve as desired for a regional block.  
           [0009]    U.S. Pat. No. 5,306,236 to Blumenfeld et al. discloses a handle  36  to which the syringe, the needle and a conductor for carrying an electrical signal are attached. The control mechanism for controlling the application of current to the needle is located remotely from the needle at a stimulator device. Like the Chester patent, the system of Blumenfeld also does not allow for one-handed operation of both needle advancement and current control. Accordingly, the system of the Blumenfeld patent also provides a clumsy operation for the user.  
           [0010]    It is frequently useful, during and after the performance of a regional anesthetic procedure, to know the depth at which the nerve structure was located. The consideration of needle tip depth is valuable both for medical record purposes as well as a check on needle position during the performance of a procedure. Accurate initial needle position may be obtained, but then undergo alteration by displacement during the injection portion of the procedure. Displacement may be due to inadvertent pressure applied by the operator, or the tendency of the injected fluid to force the needle back along its tissue track. The presence of visual guides on the needle itself, or a read out of needle tip depth on the nerve stimulator device, would provide feedback to the operator so as to prevent needle displacement. At present, there is no mechanism for providing such information with the currently available needles designed for use with nerve stimulators.  
         SUMMARY OF THE INVENTION  
         [0011]    In accordance with the present invention, an output control needle comprising an insulated hypodermic needle having a means for fingertip control of the stimulator output current and a means of depth determination. The invention will provide a means for a single operator, the anesthetist, to accurately position a needle and perform a regional anesthetic technique without need of an assistant or excessive cabling. This further reduces the costs associated with an assistant&#39;s time while maintaining operating room hazards at a minimum.  
           [0012]    It is an object of the present invention to provide a nerve stimulator needle with fingertip control of output current, which avoids the need for an assistant or foot pedal to adjust the output source during the performance of a regional anesthetic procedure.  
           [0013]    It is an object of the present invention to provide a nerve stimulator needle with fingertip control of output current allowing the second hand to perform medication injection.  
           [0014]    It is an object of the present invention to provide a nerve stimulator needle with a means of electronically measuring the depth at which a nerve structure is encountered.  
           [0015]    It is an object of the present invention to provide a nerve stimulator needle with a means of visually measuring the depth at which a nerve structure is encountered.  
           [0016]    In the present invention a nerve stimulator needle apparatus may comprise: a needle capable of carrying an electric current; a variable control mechanism which is operable to variably control the amplitude of an application of electric current to the needle; a plurality of electrical connectors connected to the variable control mechanism and the needle which allows the variable control mechanism to remotely connect to a nerve stimulation device which is operable to provide a voltage to the variable control mechanism and to provide a current pulse to the needle having an amplitude which is controlled by the variable control mechanism; and a housing which holds the variable control mechanism, the plurality of electrical connectors and the needle.  
           [0017]    The needle may be an insulated hypodermic needle, and the apparatus may further comprise: an injection tube operably connected to the needle to provide a fluid to said needle. The housing may include a fluid path, and the injection tube may be connected to one end of the fluid path and the needle may be connected to another other end of said fluid path.  
           [0018]    The variable control mechanism of the needle apparatus may include a pressure sensitive switching mechanism which changes the amplitude of the current applied to the needle in relation to the amount of pressure applied to the pressure sensitive switching mechanism.  
           [0019]    The housing of the needle apparatus may further include conductive traces that connect the variable control mechanism and the needle to said electrical connectors.  
           [0020]    The needle unit of the needle apparatus may include an electrically resistive layer covering the needle, wherein the resistance of the resistive layer changes with the length of the resistive layer, and a conductor associated with the housing may provide a voltage to the electrically resistive layer. The needle unit may further include an insulating layer between the electrically resistive layer and the needle.  
           [0021]    The needle unit may further includes depth measurement marks that indicate the insertion depth of the needle. The variable control mechanism of the needle apparatus may comprise a variable optical switching device. The variable optical switching device may include a partially colored plate and a fiber optic cable which directs light from a light source to impinge on the partially colored plate, and direct reflected light from the partially colored plate to a sensor. The partially colored plate preferably alters color or intensity component of the light impinged on it from said fiber optic cable in response to an application of pressure.  
           [0022]    The variable optical switching device may include a graduated reflective plate and a fiber optic cable that directs light from a light source to impinge on the graduated reflective plate, and direct reflected light from the graduated reflective plate to a sensor. The graduated reflective plate preferably alters color or intensity component of the light impinged on it from the fiber optic cable in response to an application of pressure.  
           [0023]    The invention may also include a nerve stimulator apparatus comprising: a nerve stimulator device comprising: a voltage source; a controller; and a current source operable to produce an electrical current in response to said controller; and a needle unit remotely located from said nerve stimulator device and connected to said nerve stimulator device only by at least one electrical conductor, said needle unit comprising: a variable control mechanism which is operable to receive a voltage from said voltage source and to provide instructions to said controller to variably control the amplitude of said electrical current provided by said current source; a needle capable of carrying said electric current from said current source having the amplitude controlled by said variable control mechanism; and a housing which holds said variable control mechanism and said needle.  
           [0024]    The variable control mechanism may include a pressure sensitive switching mechanism that changes the amplitude of the current applied to the needle in relation to the amount of pressure applied to the pressure sensitive switching mechanism. The current source is preferably operable to increase or decrease current in response to the operation of the variable control mechanism, and the variable control mechanism is operable to increase or decrease current to the needle. The current source preferably changes the amplitude of the current in response to the operation of the variable control mechanism and the variable control mechanism is operable to control the rate of change of the current.  
           [0025]    The needle unit may include an electrically resistive layer covering the needle, wherein the resistance of the resistive layer changes with the length of the resistive layer, and an electrical trace from an electrical connector provides a voltage from the voltage source, so as to maintain a constant current, to the electrically resistive layer. The microprocessor preferably determines the insertion depth of the needle according to the equation:  
             L   b =( R   t   −R   a )/ r   L    
           [0026]    where R a  is the resistance of the portion of the needle protruding above a skin surface of a subject; R t  is the resistance of the total length of the needle; r L  is the resistance per unit length of the needle; and L b  is the insertion depth of the needle. Wherein the value R a  is calculated from the ratio of the voltage of a signal detected by a return electrode attached to the surface of the skin of a subject divided by the applied current of the resistive layer on the needle.  
           [0027]    The invention further comprises a method of locating nerves may comprise the steps of: (A) inserting a needle mounted on a housing through a surface of skin of a subject; (B) activating a variable control mechanism mounted on the housing to provide a variably controlled current to the needle; (C) monitoring a detected current signal from a return electrode attached to the surface of skin of a subject using a nerve stimulator device,  
           [0028]    wherein steps A and B are performed by the same hand of an operator.  
           [0029]    The method of locating nerves may further comprise the step of determining the needle insertion depth by providing a voltage to a resistive layer, completing a circuit through a return electrode on the skin surface.  
           [0030]    The step of determining the needle insertion depth is preferably performed according to the equation:  
             L   b =( R   t   −R   a )/ r   L    
           [0031]    where R a  is the resistance of the portion of the needle protruding above a skin surface of a subject; R t  is the resistance of the total length of the needle; r L  is the resistance per unit length of the needle; and L b  is the insertion depth of the needle.  
           [0032]    The determination of the needle depth may comprise the steps of: continuously applying a constant current to a resistive layer on the surface of the needle; periodically applying a current pulse to the needle while applying the constant current; waiting for a period of time to elapse after applying the current pulse to the needle and determining the resistance of the resistive layer of the needle exposed from the surface of skin of the subject; and calculating the depth of the needle inserted in the surface of skin of the subject.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the various embodiments of the present invention, and together with the description, serve to explain the principles of the invention. In the drawings:  
         [0034]    [0034]FIGS. 1A and 1B illustrate a first embodiment of stimulator needle assembly of the present invention;  
         [0035]    [0035]FIG. 2 illustrates the stimulator needle assembly of the first embodiment in operation;  
         [0036]    FIGS.  3 A- 3 C illustrate a second embodiment of a stimulator needle assembly of the present invention;  
         [0037]    [0037]FIG. 4 illustrates the stimulator needle assembly of the second embodiment in operation;  
         [0038]    [0038]FIG. 5 illustrates a modified version of the needle shaft of the second embodiment;  
         [0039]    [0039]FIG. 6 illustrates a third embodiment of a stimulator needle assembly of the present invention;  
         [0040]    [0040]FIG. 7 is a flow diagram that illustrates the determination of the needle depth;  
         [0041]    [0041]FIG. 8 illustrates a fourth embodiment of a stimulator needle of the present invention; and  
         [0042]    [0042]FIG. 9 illustrates the operation of the variable control mechanism of the fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings.  
         [0044]    A preferred embodiment of the present invention comprises a nerve stimulator function that allows control of the current output through a fingertip control on the stimulating needle. A preferred embodiment of the needle assembly of the present invention is illustrated in FIG. 1A (front view) and FIG. 1B (side view). A needle assembly consists of a housing unit  102 . The housing unit may be made of any suitable material such as molded plastic. The housing unit  102  preferably contains a plurality of electrical pin connectors  103 ,  104  and  105  that electrically connect the housing unit  102  to an external nerve stimulator device or a plurality of external devices. Electrical traces  139 ,  140 , and  143  are preferably embedded within the housing unit  102  and are electrically connected to a corresponding electrical pin connector.  
         [0045]    The housing unit  102  also preferably contains an embedded fluid path  110  through which fluid from a tube  109  may flow to a needle  107 . The tube  109  may be any tube suitable for carrying fluids such as a plastic injection tube. The tube  109  may be formed within the housing unit  102  or may mate with the housing unit  102  via known techniques of mating tubes. Needle  107  is preferably inserted into a cavity (not shown) in the housing unit  102  so that it mates with the embedded fluid path  110 . Needle  107  may be detachable from housing  102  or may be permanently affixed to housing  102 . Those of skill in the art will appreciate that the tube  109  and the needle  107  should be attached to the embedded fluid path  110  according to techniques known in the art in a manner that avoids leakage of the fluid and also avoids contamination of the fluid. The hypodermic needle preferably consists of stainless steel, and is preferably coated with an insulation layer  108 , although a needle without the insulation layer may be used but may provide less efficient current transfer to the nerve. The needle tip is preferably not coated with the insulation layer and the needle tip may be of any type of bevel, such as a short or a long bevel. The insulation layer  108  is preferably a biocompatible insulation layer and preferably comprised of Teflon, polyethylene, PVC, polypropylene, or any other suitable material.  
         [0046]    A variable control mechanism  101  for variably controlling the current applied to needle  107  may include any type of finger actionable switch, such as a rocker switch, pressure switch, slider switch or any other known finger actionable switch, attached to the housing unit  102 . A voltage may be applied from a voltage source  180  to the variable control mechanism  101  via an electrical pin connector  103  and electrical trace  140  in the housing unit  102  and reduced or increased depending on the operation of the variable control mechanism  101 . The output voltage on the variable control mechanism  101  is directed to a control device  181 , via a second electrical pin connector  104 , for processing and determining the output current to be delivered from a current source  182  via the third electrical pin connector  105  to the embedded hypodermic needle  107 . The current provided by the current source  182  is preferably in the form of a pulse train as known in the art.  
         [0047]    [0047]FIG. 2 illustrates the operation of the stimulator needle assembly. As shown in FIG. 2, a syringe  132  is connected to housing unit  102  via tube  109 . A hypodermic needle  107  is also attached to the housing unit  102 , and advanced through the skin surface  136 . A nerve stimulator device  131  is electrically connected to electrical pin connectors  103 - 105  via electrical cables  126 ,  142 , and  122  through electrical pin connectors  129 ,  130  and  127 , respectively. The source  180 , control device  181  and the current source  182  are contained in the nerve stimulator  131 .  
         [0048]    As the needle is advanced through the skin surface  136 , the nerve stimulator  131  is activated and controlled by the variable control mechanism  101  via the current output control cable  142  attached to the nerve stimulator  131  by electrical pin connector  130 . The output current is supplied through electrical pin connector  127  via an electrical cable  122  to electrical pin connector  105  on the housing unit  102 . Voltage is output through electrical pin connector  129  via electrical cable  126  to electrical pin connector  103  on the housing unit  102  for output current control. The electrical return electrode  135  bears a connector  134  that attaches via an electrical cable  123  to an electrical pin connector  128  on the nerve stimulator  131 . The return electrode  135  is typically a silver-silver chloride electrocardiographic electrode. The operator is able to determine the proximity of the needle tip to the nerve by observation of a visible twitch stimulated in the muscle supplied by the target nerve. In a nerve stimulator operation, current is supplied to the hypodermic needle  107  at a level of 1-2 mA and the needle advanced until muscle twitch is achieved. By following appropriate muscle twitches, the amplitude of the current output may be gradually decreased by the use of the variable control mechanism  101  until twitch is observed at less than 0.5 mA as shown on display  141 . When the needle tip position is an appropriate distance from the nerve, e.g., 1-2 mm at a current output of less than 0.5 mA, the operator injects the solution in the syringe. This process, using a nerve stimulator needle of the prior art, is generally described in U.S. Pat. No. 5,830,151 to Hadzic et al. A second embodiment of the present invention is illustrated in FIGS. 3A, 3B and  3 C. The second embodiment is similar to the first embodiment except that the second embodiment includes the ability to determine the depth of the needle inserted in the skin. The same components are numbered with the same reference numbers. FIG. 3A illustrates a frontal view of the second embodiment, FIG. 3B illustrates a side view, and FIG. 3C illustrates a cross sectional view of the needle of the second embodiment.  
         [0049]    The needle  107  shown in FIGS. 3A, 3B and  3 C is able to determine depth of hypodermic needle penetration beneath the skin by using a linear resistance coating  106  connected to the voltage source  180  via an electrical pin connector  103  and an electrical trace  111 . The preferred material for this linear resistance coating is a conductive polymer coating such as a polyaniline (Ormecon™), although any suitable linear resistive material may be used, such as a normally nonconductive polymer that has been doped with a conductive material. For example, the nonconductive polymer silastic can be doped with carbon to become conductive. By controlling the amount of dopant, the resistance of the polymer may be adjusted to suitable levels. The linear resistance coating  106  is separated from the stainless steel hypodermic needle  107  by an insulation layer  108 , shown as a hatched area surrounding the needle in the figures.  
         [0050]    The needle of the second embodiment also may be used with the nerve stimulator  131 ′ shown in FIG. 4. The needle of the second embodiment locates a nerve in the same manner as described in the first embodiment, except that the needle of the second embodiment has the ability to determine the insertion depth of the needle in the skin of a subject. For this reason, nerve stimulator  131 ′ in FIG. 4 contains a constant current voltage source  190 .  
         [0051]    Existing nerve stimulators are designed to deliver constant current pulses for nerve stimulation purposes. These devices do not provide constant current (a constant non pulsed current). However, one of skill in the art is able to readily design a constant current source  190 , and the circuit design may be analogous to that for producing the constant current pulse for stimulation. The nerve stimulator  131 ′ described herein preferably contains two separate current sources, one of which, the constant current source  182 , is adjustable by the user for stimulation pulse generation, i.e. pulse current i. The other of which, the constant current voltage source  190 , is not adjustable by the user and provides a constant current (I) to the linear resistance coating  106 .  
         [0052]    By way of example, as shown in FIG. 4, the return electrode  135  for depth determination by resistance measurement is located remotely on the skin surface and is the same return electrode as that for the current output of the nerve stimulator  131 . In operation, a voltage signal from the constant current voltage source  190  is applied to the linear resistance coating  106  via electrical pin connector  103  and electrical trace  111 . The circuit is completed through the linear resistance coating  106  as it penetrates the skin, thus the return electrode  135  detects the signal and provides the detected signal to the nerve stimulator  131 ′ via electrical cable  123  and electrical pin connector  128 . The nerve stimulator  131 ′ detects the voltage of the detected signal using a voltmeter  145 .  
         [0053]    The resistance of the linear resistance coating  106  is constant per unit length and significantly higher than that of tissue, which is on the order of 1.0 megaohm per mm. Tissue impedance is typically in the range of 0.1-1.0 kilohms. Since the tissue impedance is less than the resistance of any portion of the needle by orders of magnitude, the resistance of the circuit is approximately that of the needle coating alone. The total resistance (R t ) of the needle linear resistance coating  106  is the product of the resistance per unit length (r L ) and the length of the needle (L t ). As the needle is advanced through the skin, R t  may be represented as the sum of the length protruding above the skin (L a )  137  multiplied by r L  and of the length beneath the skin (L b )  138  multiplied by r L , as given in equation 1.  
           R   t   =r   L   ×L   t =( r   L   ×L   a )+( r   L   ×L   b )  Eq. 1  
         [0054]    Therefore, since the resistance determined by the nerve stimulator  131 ′ in this circuit is directly related to the length of the coating that protrudes above the skin surface (R a =r L ×L a ), the length of the needle below the skin surface may be determined by equation 2.  
           L   b =( R   t   −R   a )/ r   L   Eq. 2  
         [0055]    where R a  is the resistance of the portion of the needle protruding above the skin. Since R t  and r L  are known and R a  is calculated directly from the ratio of the measured voltage to the applied current (Ohm&#39;s Law: V=IR), L b  may be calculated. If the needle is inserted to the point that the housing unit  102  contacts the skin surface, L a =0, the measured resistance is that of the tissue alone, and thus L b =L t  in that situation.  
         [0056]    The return electrode  135  is used for both stimulator current control and for needle depth determination through the measurement of resistance. FIG. 7 illustrates an exemplary method of determining the depth of the needle which is preferably carried out by a microprocessor in the nerve stimulator  131 ′. As shown in step SO, the nerve stimulator  131 ′ illustrated in FIG. 4 provides a constant DC current I, by way of a variable voltage, to the linear resistive coating  106  on the nerve stimulator needle  107  via electrical pin connector  103  and electrical trace  111 . The nerve stimulator  131 ′ provides a periodic current pulse i to the needle  107 , having its amplitude controlled by variable control mechanism  101 , via electrical trace  139  (step S 2 ). The constant current I is preferably continuously provided, and is not interrupted by the periodic current pulse i provided to the needle  107 . The resistance R a  is determined from the DC current I after the current pulse i decays in the skin of the subject, i.e. during the portion of the interpulse interval when no current pulse from the stimulator function is occurring. The resistance measurement cannot occur early in the interpulse interval due to capacitive functions of the skin which result in a discharge voltage according to the relationship V=V i ×e (−t/RC) , where V is the observed voltage, V i  is the applied voltage of the current pulse i on needle  107 , t is the elapsed time, and RC is the product of the resistance and the capacitance of the skin. Consequently, the resistance measurements for depth determination are collected in a timed fashion, rather than continuously. For example, in the preferred embodiment, the current pulse is less than or equal to 5 msec in duration and, since the frequency of the pulse signal provided to the nerve stimulator needle  107  is a maximum of 5 Hz, the interpulse duration is 45 msec.  
         [0057]    In step S 4 , the microprocessor determines the decay time t of the current pulse in the skin of the subject as t=5RC. Particularly, for a circuit containing a parallel resistance (R) and capacitance (C), such as biological tissue, a tissue time constant may be calculated from the product of R and C. When subjected to a current pulse, the capacitive element of such a circuit charges over a defined time interval. On termination of the current pulse, this capacitive element discharges over a characteristic time interval, which follows an exponential decay curve, which takes about five tissue time constants (5RC) to reach 99% of the final value. This is described in more detail by Nunn (Nunn J F, Applied Respiratory Physiology, Butterworths, London, 1977, pg. 464-469) and Horowitz and Hill (Horowitz P and Hill W, The Art of Electronics, Cambridge University Press, Cambridge, Mass., 1986, pg.20-21). Since tissue time constants are in the range of 1 msec or less, allowing greater than 5 msec to pass following the termination of the stimulating pulse provides adequate time for the measured voltage to approximate baseline values of the voltage applied to the resistive layer. Alternatively, t may set to a value greater than or equal to 5 msec.  
         [0058]    At step S 6 , the microprocessor waits for the time t to elapse since the end of the current pulse i (No in step  6 ). Once the time t has elapsed (Yes in step  6 ), the measured voltage from return electrode  135  via voltmeter  145  is used to calculate R a  (step S 8 ). The needle depth L b  is then calculate in step S 10  according to equation 2, and displayed in step S 12 . The calculated value of L b  may then be visually displayed on display  141 . Those of skill in the art will appreciate that the calculated value L b  may also be audibly displayed and/or may be printed by a printing device attached either directly or indirectly to nerve stimulator  131 ′.  
         [0059]    [0059]FIG. 5 illustrates a cross section of the needle that may be used with the second embodiment. As illustrated in FIG. 5, needle  107  is coated with insulation layer  108  that is coated with linear resistive coating  106  as in the second embodiment. In FIG. 5, indelible marks  113 , preferably comprised of biocompatible material, may be included on the resistance coating to provide visual reference of the depth to which the tip of the needle has been inserted. These depth measuring mechanisms allow accurate recording of the needle depth at which an injection was performed and allow for accurate maintenance of needle position during the injection procedure as well as for a period of time after injection when subsequent stimulation attempts are undertaken by the anesthetist. It will be appreciated that the indelible marks  113  may be applied to the insulation layer  108 , in the absence of the resistive coating  106 , to provide a visual depth reference.  
         [0060]    A third embodiment is shown in FIG. 6. In this embodiment, a strip of resistance material  112 , such as a tantalum wire, or ceramic strip resistance, may be substituted for the conductive polymer. This material preferably runs the length of the needle shaft and is separated from the hypodermic needle  107  by the insulation layer  108 . This material provides the same linear resistance characteristics as the conductive polymer for use in depth determination, and operates in the same manner as described in embodiment 2.  
         [0061]    A fourth embodiment of the invention is illustrated in FIGS. 8 and 9. In this embodiment, the variable control mechanism  101 ″ may be an optical device controlling the current output by a mixture of light wavelengths determined by the degree to which the variable control mechanism  101  is depressed. An example of such a control device is the Coldswitch™. In the case of optical control, the electrical pin connector  104  is replaced by a fiber optic cable  124 . The same elements in this embodiment as in the previous embodiments are numbered the same. Just as in the first embodiment, the variable control mechanism  101 ″ is preferably connected to a nerve stimulator  131  or  131 ′ that senses the direction of change mediated through the variable control mechanism  101 ″, as well as the rate of change of switching events.  
         [0062]    The operation of this embodiment is illustrated in FIG. 9. In FIG. 9, light from an LED source  116  contained in a photonic sensor and control  115  is directed via fiber optic cable  124  to an optical control mechanism  114 , contained in the variable control mechanism  101 ″. Optical control mechanism  114  contains a reflective plate  118  that is partially colored, such as by a color coating, or is a graduated reflective plate that has different reflective properties along the length of the plate. Optical control mechanism  114  also contains a pivot  119  upon which reflective plate  118  may pivot upon the application of pressure from an operator. The light from LED source  116  is reflected off of reflective plate  118  and transmitted back to the photonic sensor  117  via fiber optic cable  124 . The photonic sensor and control  115  detects the direction of change of variable control mechanism  101 ″, mediated through the optical control mechanism  114 , as well as the rate of change of switching events, by the reflected color or intensity mixing. Color or intensity mixing is accomplished by a reflective plate  118 . When the reflective plate  118  is rocked by finger pressure on its pivot  119 , the white incident light emitted from the fiber optic cable  124  is reflected back with altered color or intensity components. The intensity or color is detected by the photonic sensor  117 , electronically processed by the microprocessor  120  and converted to a corresponding current output via the constant current pulse generator  121 . The current output is directed to the stimulating needle electrical pin connector  105  by an electrical cable  122  with the return supplied by a second electrical cable  123  to the return electrode  135  located remotely on the skin surface. Just as in the first embodiment, commands to the nerve stimulator can be controlled through switching events, including, but not limited to sequential taps, sudden release or sudden depression.  
         [0063]    The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, although the method of FIG. 7 is described as being performed by a microprocessor, the method may be performed by a hard wired system or any other suitable processing system. Additionally, the terms electrical traces and electrical cables is are considered to encompass any type of known electrical conductors, such as, but not limited to metallic wires or non metallic electrical conductors, which may be embedded or non-embedded, and which may be coated with an insulator or non-coated.