Effect of alkalinization of lidocaine on median nerve block

Median nerve blocks were performed in 10 volunteers in a randomized, double-blind, crossover study to compare the effects of 1% plain lidocaine with 1% lidocaine in sodium bicarbonate 0.1 mmol litre-1. Sensations of hot, cold, pinprick and light touch, compound motor and sensory nerve action potentials, and skin temperature were assessed at 2-min intervals. pH was 6.4 +/- 0.1 for plain lidocaine and 7.7 +/- 0.2 for alkalinized lidocaine (P < 0.001). Alkalinized lidocaine produced more rapid inhibition of compound motor action potentials than plain lidocaine (median 4 (range 2-6) vs 9 (2-14) min) (P = 0.039). Alkalinized lidocaine also produced more rapid onset of inhibition of compound motor than sensory nerve action potentials (4 (2-6) vs 8 (4-12) min) (P = 0.0039). There was no significant difference in any other sensory modality between alkalinized and plain lidocaine. These data suggest that addition of bicarbonate to lidocaine for median nerve block significantly increased the rate of motor block without changing the onset or extent of sensory block.