Electrophysiologic effect of injectates on peripheral nerve stimulation

Background and Objectives: A small volume of local anesthetic or normal saline abolishes the muscle twitch induced by a low current (0.5 mA) during electrolocation. This study examines the hypothesis that the mechanism of this phenomenon is primarily the electrophysiologic effect of the injectate on the electrical current density at the needle tip. Methods: Five pigs were studied. An insulated Tuohy needle was inserted in each pig toward the left and right brachial plexuses and the left and right femoral nerves. The needle was advanced until corresponding motor responses were observed at each site, using a current of 0.5 mA. The effect of injecting 1 mL each of normal saline and 5% dextrose in water (NS and D5W) on muscle twitch was investigated at all 20 needle insertion sites. Changes in the conductive area induced by the injectates were also demonstrated using gel electrophoresis. Results: In all cases, the muscle twitches were abolished immediately after the injection of NS and recovered instantaneously after a subsequent injection of D5W. The electrical resistance between the needle and the ground electrodes decreased instantly after the NS injection. The resistance not only recovered but also increased after the injection of D5W. In the gel electrophoresis experiment, the results demonstrated that the expanded conductive area induced by the saline column surrounding the insulated needle was similar to that observed with the uninsulated needle. Conclusion: The injection of a conducting solution (i.e., NS) rendered the current that was previously sufficient to elicit a motor response (0.5 mA) ineffective. The most likely reason for this change is that the conductive area surrounding the stimulating needle expanded after the injection and dispersion of the conducting solution (i.e., NS), thereby reducing the current density at the target nerve. This effect can be reversed by injecting a nonconducting solution (i.e., D5W) via the stimulating needle.