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 microV). 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).


CONCLUSIONS
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.
Ubags
L H
LH
Department of Anesthesiology, Academic Hospital, University of Amsterdam, The Netherlands.
Kalkman
C J
CJ
Been
H D
HD
D000293:Adolescent; D000328:Adult; D000768:Anesthesia, General; D000769:Anesthesia, Inhalation; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D019054:Evoked Potentials, Motor / Q000187:drug effects*; D005260:Female; D006801:Humans; D007530:Isoflurane; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D018482:Muscle, Skeletal / Q000294:innervation; D009609:Nitrous Oxide; D013116:Spinal Cord / Q000187:drug effects / Q000601:surgery; D017409:Sufentanil
D017409:Sufentanil; D007530:Isoflurane; D009609:Nitrous Oxide