Lack of size-related differential sensitivity to equilibrium conduction block among mammalian myelinated axons exposed to lidocaine

This study sought to evaluate the sensitivity of individual, relatively thick myelinated axons of mammalian nerve to equilibrium conduction block by lidocaine, and to compare this to the incidence of conduction block previously measured in individual thinner myelinated axons. The incidence of conduction block by lidocaine 0.3 and 0.6 mM (8.1-16.2 mg/dl) was determined on 35 individual axons in dissected filaments of rabbit recurrent laryngeal nerve (RLN) in which the control conduction velocity ranged from 28 to 77 m/sec. Thirty-four axons (97%) remained excitable in lidocaine 0.3 mM; 2 axons (6%) remained excitable in lidocaine 0.6 mM. These proportions did not differ significantly (P greater than 0.2) from those in thinner axons of vagus, either in comparisons with previous data from extracellular recordings or with new data from filament recordings. The results imply that differential blocks observed under clinical conditions probably depend on factors other than a size-related difference in the minimal equilibrium blocking concentration among myelinated axons. Equilibrium depression of the amplitude of RLN compound action potentials by lidocaine 0.3 and 0.6 mM was disproportionately great relative to the incidence of equilibrium conduction block in individual axons, confirming that depression of the compound action potential is not a reliable measure of nerve conduction block.