Differential effect of nerve fiber structure on block by local anesthetic.

The incidence of conduction block by lidocaine 0.3 mmol/l (8.1 mg/dl) in several successive lengths of individual afferent axons of rabbit was compared. The conduction velocity of the axons was either "slow," "intermediate" (1.3-4 m/s), or "fast." The "intermediate" group showed a higher incidence of proximal acceleration of conduction (P less than 0.001) and a greater incidence of block (P less than 0.001) than the "slow" and "fast" fiber groups. The results were interpreted as indicating that the fibers of the "intermediate" group had an unmyelinated peripheral and a myelinated proximal length, with a junctional heminodal region that was the seat of the high sensitivity to block. The potential clinical significance of the observation is discussed in terms of the known distribution of heminodes in the peripheral nervous system.