The Effect of Nitrous Oxide on Transcranial Magnetic-Induced Electromyographic Responses in the Monkey

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Abstract
Transcranial magnetic stimulation (TMS) to produce motor evoked potentials (MEPs) is a newly developed intraoperative modality to monitor functional integrity of the motor pathways. The present study was designed to examine the reliability of magnetic MEP recording under nitrous oxide (N2O) inhalation. Following ketamine injection (10 mg/kg i.m.) and endotracheal intubation, 16 monkeys were exposed to N2O:O2 mixture ratios 1:3, 1:1, and 3:1. Electromyographic (EMG) responses, evoked by pulsed magnetic fields applied extracranially to the scalp zone overlying motor cortex, were recorded from the contralateral fore- and hind limb flexor muscles. The scalp topography zone for contralateral muscle excitation was markedly reduced by 75 vol% N2O. Significant stimulation threshold elevation, latency prolongation, and amplitude depression were noted after inhalation of 75 vol% N2O (p <0.05) compared with </=50 vol% N2O inhalation. The mean range of brain stimulation rise was 1%, 7-8%, and 13-20%, and MEP latency delay was 3%, 3-6%, and 7-11% on exposure to 25%, 50%, and 75 vol% N2O, respectively. The mean amplitude response first increased by 20% and 7% with 25% and 50% N2O, respectively, and then reduced by 60% with 75 vol%. We conclude that, in a primate model, while magnetic MEPs could reliably be recorded under 25-75% N2O, the evoked motor responses were least affected by </=50 vol% inhalation. Knowledge of N2O-induced MEP alteration is essential when interpreting the response change while using this agent.

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