Cooling potentiates lidocaine inhibition of median nerve sensory fibers.

To determine the effect of cooling on lidocaine potency, nine consenting volunteers underwent bilateral median nerve blocks using 1% lidocaine HCl solution. Room-temperature and ice-cold lidocaine were injected into either dominant or nondominant wrists. Subjects were blinded to the temperature of the anesthetic. Inhibition of A alpha sensory and motor fibers was assessed as the decline in sensory nerve action potentials and compound motor action potentials, respectively. Inhibition of C fibers was measured as an increase in skin temperature and a decline in galvanic skin potentials. All indices of nerve function demonstrated profound (P less than 0.001) time-related changes after injection of local anesthetic. When ice-cold lidocaine was injected, inhibition of sensory nerve action potentials was significantly greater at all time points (P = 0.001) than when room-temperature lidocaine was injected. Inhibition of C fibers as assessed by galvanic skin potentials was marginally faster (P = 0.07) when ice-cold lidocaine was used compared with room-temperature lidocaine. No differences between room-temperature and ice-cold lidocaine were observed in inhibition of compound motor action potentials, or in the increase in skin temperature. We conclude that inhibition of median sensory fibers may be increased by cooling 1% lidocaine HCl in an ice bath before injection.