Electromyographic assessment of ulnar nerve motor block induced by lidocaine

Study Objective: To determine the differences in the onset time and duratio n of motor block produced by lidocaine 1% and lidocaine 2% via a quantitati ve and objective method, the measurement of compound muscle action potentia ls (CMAPs). Study Design: Prospective study. Setting: Main operating rooms of a Univers ity hospital. Patients: 20 consecutive patients undergoing surgery not requ iring intraoperative muscle relaxation. Interventions: General anesthesia with unilateral ulnar nerve block was adm inistered. In patients' nondominant (expermental) arms, an Insulated block needle was placed adjacent to the ulnar nerve at the wrist while continuous nerve stimulation was delivered to ensure its proper placement. Though thi s needle, lidocaine 1 % or lidocaine 2 % was injected. The dominant (contro l) arm received no injection. Measurements and Main Results: Monitoring of ulnar nerve-evoked CMAPs was p erformed simultaneously on both arms. Ulnar nerve function was assessed at baseline and then at 10-second intervals by automatically measuring the amp litude of the evoked CMAPs on a two-channel electromyogram. The mean (+/- S EM) baseline CMAP amplitude prior to injection of lidocaine 1 % was 3.10 +/ - 0.87 mV and 3.06 +/- 0.89 mV for the experimental and control ulnar nerve s, respectively (p = NS);for lidocaine 2 %, baseline CMAP amplitude was 3.5 8 +/- 1.39 mV and 3.70 +/- 1.46 mV, respectively (p = NS). Over the course of the study, the control CMAP amplitude varied by < 12 %. At the experimen tal ulnar nerve 90% CMAP decrease after injection of lidocaine 1% and lidoc aine 2 % occurred 7.5 +/- 2 minutes and 5 +/- 1.5 minutes, respectively (p = NS), whereas maximal blockade was achieved after 15 +/- 3 minutes and 11 +/- 5 minutes, respectively (p = NS). Recovery of CMAP to 90 % of baseline occurred 184 +/- 31 minutes after injection of lidocaine I % and 248 +/- 30 minutes following lidocaine 2% (p = NS). Conclusion: The present study describes a technique that can be used in viv o to objectively measure the speed of onset and duration of local anestheti c-induced motor blockade. Although statistically not different, lidocaine 2 % demonstrated a faster onset and longer duration of ulnar nerve motor bloc k than lidocaine 1 %. (C) 1998 by Elsevier Science Inc.