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% bupivacaine in different nerve fibre types in 10 volunteers undergoing median nerve blocks, in a randomized, double-blind, crossover study. Hot, cold, pinprick and light touch sensations, compound motor action potentials (CMAP), sensory nerve action potentials (SNAP) and skin temperature were recorded at 2-min intervals. Hot, cold, pinprick, light touch sensations, SNAP and CMAP were significantly inhibited, and skin temperature 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 inhibited SNAP and CMAP with a similar time delay to steady-state. Bupivacaine produced steady-state inhibition of hot and cold sensations significantly 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 agents over time.