RELATION BETWEEN FUNCTIONAL DEFICIT AND INTRANEURAL LOCAL-ANESTHETIC DURING PERIPHERAL-NERVE BLOCK - A STUDY IN THE RAT SCIATIC-NERVE

Background: During peripheral nerve block, local anesthetic (LA) penet rates within and along the nerve to produce the observed functional de ficits. Although much is known about the kinetics and steady-state rel ation for LA inhibition of impulse activity in vitro in isolated nerve , little is known about the relation between functional loss and intra neural LA content in vivo. This study was undertaken to investigate th e relation of functional change to intraneural LA. Methods: A sciatic nerve block was performed in fats with 0.1 ml 1% lidocaine radiolabele d with C-14. The total intraneural uptake of LA was determined at diff erent times after injection, and the distribution of lidocaine along t he nerve was assayed at different stages of functional block. Drug con tent was also compared with equilibrium lidocaine uptake in the isolat ed rat sciatic nerve.Results: Total intraneural lidocaine in vivo incr eased to near steady-state in about 3 min, stabilizing at approximatel y 14.3 nmol/mg wet tissue for about 12 min before decreasing to zero a t 70 min after injection, Although intraneural Lidocaine was 1.6% of t he injected dose during full block, only 0.3% was left when deep pain sensation returned and 0.065% was still detected when functions fully recovered. Despite these large differences in total lidocaine content, the longitudinal distribution remained constant. Intraneural lidocain e concentrations obtained at full block and partial recovery could be achieved in vitro by equilibration in 0.7-0.9 and 0.2-0.3 mM lidocaine , respectively. Conclusions: During peripheral nerve block only a smal l amount of injected LA penetrates into the nerve, The intraneural con tent of LA correlates with the depth of functional block.