Differential peripheral axon block with lidocaine: unit studies in the cervical vagus nerve

The differential susceptibility of large and small axons to lidocaine was studied on units in the rabbit vagus nerve. The results classified the units into three groups: 1) myelinated, conduction velocity 37.5-5 m/s, which were blocked by lidocaine 0.4-0.8 mM; 2) slow, unmyelinated axons, conduction velocity 1.2-0.5 m/s, and these axons were not blocked by 0.2, 0.4, or 0.6 mM lidocaine but usually were blocked by 0.8 mM lidocaine; and 3) Axons of intermediate conduction velocity, between 1.2 and 4 m/s. The last group of axons was the most sensitive: some were blocked by as little as 0.2 mM lidocaine. No size-related trend was detected within the groups.