FAILURE OF MEPERIDINE TO ANESTHETIZE HUMAN MEDIAN NERVE - A BLINDED COMPARISON WITH LIDOCAINE AND SALINE

Background and Objectives. Although meperidine safely produces clinica l spinal anesthesia, the responsible mechanism is unknown. This study was undertaken to test the possibility that this drug acts as a local anesthetic by investigating its ability to inhibit conduction in a hum an peripheral nerve. Methods. In a blinded fashion, the abilities of 5 -mL injections of meperidine (0.5% and 1.5%), lidocaine (0.25%), and s aline to produce median nerve block were tested in eight volunteer sub jects, and these four solutions were compared with standard local anes thetic solutions that had been tested in previous studies. The extent of local anesthesia was measured objectively by eletrodiagnostic tests , namely, compound motor action potentials (CMAPs) and sensory nerve a ction potentials (SNAPs), as well as by qualitative tests of sensation . Results. Lidocaine (0.25%) prolonged median SNAP latency from 3.1 ms to 3.3 ms (P < .015) and prolonged mean CMAP latency from 4.1 ms to 4 .7 ms (P < .002). The SNAP amplitude trended downward after lidocaine (0.25%), but the decrease did not reach statistical significance (35 m u V to 25 mu V, P < .19). Neither meperidine solution (0.5% or 1.5%) n or saline inhibited SNAP or CMAP amplitudes or prolonged SNAP or CMAP latencies. Also, in contrast to previous findings with more potent loc al anesthetic solutions (eg, lidocaine 1%, mepivacaine 1%, and bupivac aine 0.33%), none of the four solutions tested in this study altered. subjective sensations of hot, cold, or pinprick. Meperidine 1.5% produ ced systemic side effects, including vertigo, nausea, and flushing, in all subjects. Conclusions. Meperidine produced no signs of local anes thesia, even when given at a dose (75 mg) and concentration (1.5%) tha t consistently produced systemic side effects. Thus, the coequivalent ability of meperidine and lidocaine to produce spinal anesthesia contr asts with their discordant ability to produce local anesthesia. This d isparity suggests that meperidine may produce spinal anesthesia throug h mechanisms other than inhibition of sodium channel function.