Abstract
A reliable and safe peripheral nerve blockade can only be assured by a nerve locater. But its usage is hindered by high cost. We have a cost- effective solution.

Since ancient times peripheral nerve blockade has attracted all the medicinemen. The methods they used mostly was physical, e.g. Nerve compression, thermal injury etc. it was not only unreliable, but also risky, resulting many a times in nerve damage. Since then, the technique of PNB has progressed a lot. Much of the success goes to discovery of newer, safer local anaesthetic drugs and hypodermic needle. The physician irritates the nerve by hypodermic needle. When the patient complains of paraesthesia, the drug is injected in the vicinity of the nerve to achieve PNB. Still failed block was a constant threat. While employing paraesthesia method, physician had to depend on patient’s interpretation of paraesthesia. This produced high interpersonal variability. Also iatrogenic nerve damage was a real risk1. The knowledge of nerve-muscle physiology taught us the solution. Electrical stimulation of nerve has solved most of the problems.

The ability of nerve locater (NL) to produce an evoked motor response (EMR) depends on current strength, current duration and needle to nerve distance. By definition rheobase is the minimum strength of current just enough to produce an excitation. Chronaxie is the minimum duration for which double the rheobase strength current can elicit excitation. Chronaxie is an useful index of relative excitability.2 Most anaesthesiologists recommend “ a brisk EMR at 0.5 mA current strength, 0.1mS current duration” as optimal condition for injecting local anaesthetic to produce high success rate of nerve blockade. Under these conditions needle tip is neither in the nerve (no risk of nerve damage), nor too far away from the nerve (less risk of failed block). But even the use of NL demands one to know surface anatomy, muscle innervation and cutaneous distribution of target nerve.

Today PNB is more of science than art. But only hindrance to general application is high cost of NL. We have a solution. We would like to share our experience with you. The ‘spark generating unit ‘of ordinary cigarrette lighter gives a low intensity current of very short duration. We used a veinflow to locate the nerve (Fig. 1). The proximal end of the metallic stylet, since insulated by external sheath, if shocked by this device carries electricity to the nerve, generating an adequate EMR. Current duration is extremely short, matching chronaxie of A fibre (0.1 mS), not C fibre (0.3-1mS). So patient discomfort was minimal, even without the use of sedatives. We have achieved more than a hundred PNBs through various approaches, using this device. Each time analgesia was excellent, though, motor blockade was not so consistent. Even in uncooperative patients the success rate was high.

So, to conclude, this device assures cost effective solution (cost Rs.10/-) for PNB to the anaesthesiologists and general practitioners even in remote corners of the country, till availability of NL (cost Rs.40,000/-) becomes reality.

References

1. Selander D, Edshage S, Wolff T. Paresthesiae or no paresthesiae? Nerve lesions after axillary blocks. Acta Anaesthesiol Scand 1979 Feb; 23 (1) : 27-33.
2. Best and Taylor: Physiological basis of medical practice, 11th edition, pp- 36, Baltimore, Williams and Wilkins, 1985.