Influence of isoflurane on myogenic motor evoked potentials to single and multiple transcranial stimuli during nitrous oxide/opioid anesthesia

OBJECTIVE: Transcranial motor evoked potentials (tc-MEPs) are used to monitor the spinal cord intraoperatively. Volatile anesthetics considerably depress amplitudes of tc-MEPs. This study was undertaken to determine whether multipulse stimulation might overcome this depressant effect. METHODS: In 10 patients undergoing spinal surgery, incremental doses of isoflurane were added to a nitrous oxide/opioid anesthetic regimen and maintained constant at 0.2, 0.4, and 0.6% end tidal for at least 15 minutes. tc-MEP responses to single-pulse and trains of three and five (interstimulus interval, 2 ms) transcranial electrical stimuli were recorded from the tibialis anterior muscles. RESULTS: Before the addition of isoflurane, tc-MEPs were recordable in all patients, even with single-pulse stimuli (median amplitude, 428 mu V). With 0.2% end-tidal isoflurane, tc-MEPs were recordable in eight patients with single-pulse stimulation and in all patients with three and five successive stimuli. At 0.4% isoflurane, responses were recordable in only one patient using single-pulse stimuli and in all patients using three and five stimuli. With 0.6% isoflurane, tc-MEPs to trains of three and five stimuli were recordable in all patients except one. The amplitude of the responses obtained with 0.2, 0.4, and 0.6% end-tidal isoflurane was significantly smaller than that of control responses (P < 0.05). CONCLUSION: These data suggest that despite the powerful depressant effects of isoflurane on myogenic motor responses, tc-MEP monitoring during isoflurane anesthesia may be feasible, provided that multipulse stimulation paradigms are used and the concentration of isoflurane does not exceed 1 minimal anesthetic concentration unit.