The effects of epidural and intravenous lidocaine on somatosensory evoked potentials after stimulation of the posterior tibial nerve.

The measurement of somatosensory evoked potentials (SEPs) after stimulation of the posterior tibial nerve (PTN-SEPs) has been proposed as an objective indicator of the quality of lumbar epidural block. It is unclear whether peak latency increases after epidural application of local anesthetics may be due in part to systemic effects of the drug absorbed from the epidural space. In this clinical study, we compared PTN-SEPs after intravenous and epidural administration of lidocaine to those of a control group who did not receive lidocaine. Plasma concentrations of lidocaine remained within expected ranges for epidural and intravenous administration. No subjects developed signs for overdose or toxicity. After epidural application of 2% lidocaine, mean latencies of peaks P1, N1, and P2 increased significantly in comparison to baseline values. In 3 of 10 patients, latency changes were not observed. Intravenous lidocaine did not produce statistically significant changes in latencies, although a trend toward increasing latencies appeared to be present. In the control group without lidocaine, no statistically significant changes occurred during the 1-h study period. No correlation was found between peak latency changes and plasma concentrations of lidocaine. We conclude that latency increases observed after epidural application of lidocaine are due primarily to local, not systemic, effects of the local anesthetic.