INFLUENCE OF ISOFLURANE ON MYOGENIC MOTOR EVOKED-POTENTIALS TO SINGLEAND 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 conside rably depress amplitudes of tc-MEPs. This study was undertaken to dete rmine whether multipulse stimulation might overcome this depressant ef fect. METHODS: In 10 patients undergoing spinal surgery, incremental d oses of isoflurane were added to a nitrous oxide/opioid anesthetic reg imen and maintained constant at 0.2, 0.4, and 0.6% end tidal for at le ast 15 minutes. tc-MEP responses to single-pulse and trains of three a nd 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 sing le-pulse stimulation and in all patients with three and five successiv e stimuli. At 0.4% isoflurane, responses were recordable in only one p atient using single-pulse stimuli and in all patients using three and five stimuli. With 0.6% isoflurane, tc-MEPs to trains of three and fiv e 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 w as significantly smaller than that of control responses (P < 0.05). CO NCLUSION: These data suggest that despite the powerful depressant effe cts of isoflurane on myogenic motor responses, tc-MEP monitoring durin g isoflurane anesthesia may be feasible, provided that multipulse stim ulation paradigms are used and the concentration of isoflurane does no t exceed 1 minimal anesthetic concentration unit.