Susceptibility of Nerve Fibers to Local Anesthesia:“Size Principle” Challenged. Gokin et al. (page 1441)

Gokin et al.  studied features of conduction blockade in different classes of rat sciatic nerve fibers after injection of lidocaine via  procedures resembling those used inhumans. Thirty adult male Sprague-Dawley rats were initially anesthetized with either urethane or pentobarbital. Longitudinal skin incisions were made to rats’ posterior right hind legs. In each rat, the sciatic nerve and its main branches (posterior tibial, common peroneal, and sural nerves) were exposed. Impulses in different classes of sensory axons (large, Aα, Aα/β; small, Aδ myelinated fibers and unmyelinated C fibers) and motor axons (large, Aα; small, Aγ myelinated fibers) were recorded and classified by conduction velocity. The sciatic nerve was stimulated distally, and impulses were recorded from small filaments teased from the L4–L5 dorsal and ventral roots sectioned from the spinal cord. Lidocaine, in concentrations ranging from 0.05 to 1%, was injected at the sciatic notch. The team then assessed both tonic and use-dependent impulse inhibition by lidocaine.

The minimal effective lidocaine concentrations (capable of blocking conduction in 10% of fibers) ranged from 0.03 to 0.1% for sensory and 0.03 to 0.05% in motor fibers. The overall order of fiber susceptibility (ranked by the concentration needed to block conduction in 50% of fibers) was Aγ > Aδ= Aα > Aα/β > C. Compound action potentials recorded simultaneously from both large myelinated sensory and motor sciatic nerve fibers showed no difference in the degree and rate of tonic block at relatively high lidocaine concentrations (0.25–1%). However, at lower concentrations of lidocaine, large motor fibers were suppressed significantly more than large sensory fibers. Faster-conducting C fibers were more susceptible to lidocaine block than slower ones. The researchers concluded that susceptibility to lidocaine does not always follow the generally accepted size principle, i.e. , that susceptibility to local anesthesia depends inversely on fiber diameter.