Differential sensitivities of mammalian nerve fibers to local anesthetic agents.

The differential sensitivities of mammalian nerve fibers to various local anesthetic agents were investigated. Lidocaine, tetracaine, etidocaine, and bupivacaine demonstrated a consistent pattern of conduction blockade in which the large fast-conducting A fibers were blocked at the lowest drug concentration, the intermediate B fibers were blocked at a higher drug concentration, and the smallest, slowest-conducting C fibers required the highest drug concentration for conduction blockade. A comparison of procaine, chloroprocaine, cocaine, tetrodotoxin and saxitoxin on B and C fibers showed similar effects. These findings indicate that local anesthetic agents are similar to other biological stress modalities in terms of their differential effects on nerve fibers of various sizes and conduction velocities, i.e., the large fast-conducting fibers are more susceptible to conduction blockade than are the smaller, slower-conducting fibers. Discrepancies between results of this study and previous reports in the literature are discussed.