Anesthesiology
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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.