DIFFERENTIAL ONSET OF MEDIAN NERVE BLOCK - RANDOMIZED, DOUBLE-BLIND COMPARISON OF MEPIVACAINE AND BUPIVACAINE IN HEALTHY-VOLUNTEERS

We have compared the delay in onset of 1% mepivacaine and 0.33% bupiva caine in different nerve fibre types in 10 volunteers undergoing media n nerve blocks, in a randomized, double-blind, crossover study. Hot, c old, pinprick and light touch sensations, compound motor action potent ials (CMAP), sensory nerve action potentials (SNAP) and skin temperatu re were recorded at 2-min intervals. Hot, cold, pinprick, light touch sensations, SNAP and CMAP were significantly inhibited, and skin tempe rature was significantly increased after administration of both agents . The first noticeable reduction in cold sensation was detected later after bupivacaine compared with mepivacaine, but after a delay similar to that of other nerve functions. Bupivacaine and mepivacaine inhibit ed SNAP and CMAP with a similar time delay to steady-state. Bupivacain e produced steady-state inhibition of hot and cold sensations signific antly later than mepivacaine; nevertheless, the sequence that sensory modalities failed, with few exceptions, and the extent of anaesthesia at 40 min were similar for both agents. Our technique provides a novel , multi-modal method of comparing local anaesthetics and related agent s over time.