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NEUROSURGICAL ANESTHESIA

A Comparison of Myogenic Motor Evoked Responses to Electrical and Magnetic Transcranial Stimulation During Nitrous Oxide/Opioid Anesthesia

Leon H. Ubags, MD, PhD^*, Cor J. Kalkman, MD, PhD^*, Henk D. Been, MD, PhD, Johannis H. Koelman, MD, and Bram W. Ongerboer de Visser, MD, PhD

Departments of *Anesthesiology, Orthopedics, and Clinical Neurophysiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands

Address correspondence and reprint requests to Cor J. Kalkman, MD, PhD, Department of Anesthesiology, Academic Hospital, University of Amsterdam, P.O. Box 22700, 1100 DE Amsterdam, The Netherlands. Address e-mail to c.j.kalkman{at}amc.uva.nl

Transcranial motor evoked potentials (tc-MEPs) are used to monitor spinal cord integrity intraoperatively. We compared myogenic motor evoked responses with electrical and magnetic transcranial stimuli during nitrous oxide/opioid anesthesia. In 11 patients undergoing spinal surgery, anesthesia was induced with IV etomidate 0.3 mg/kg and sufentanil 1.5 µg/kg and was maintained with sufentanil 0.5 µg · kg^-1 · h^-1 and N₂O 50% in oxygen. Muscle relaxation was kept at 25% of control with IV vecuronium. Electrical stimulation was accomplished with a transcranial stimulator set at maximal output (1200 V). Magnetic transcranial stimulation was accomplished with a transcranial stimulator set at maximal output (2 T). Just before skin incision, triplicate responses to single stimuli with both modes of cortical stimulation were randomly recorded from the tibialis anterior muscles. Amplitudes and latencies were compared using the Wilcoxon signed rank test. Bilateral tc-MEP responses were obtained in every patient with electrical stimulation. Magnetic stimulation evoked only unilateral responses in two patients. With electrical stimulation, the median tc-MEP amplitude was 401 µV (range 145-1145 µV), and latency was 32.8 ± 2.3 ms. With magnetic stimulation, the tc-MEP amplitude was 287 µV (range 64–506 µV) (P < 0.05), and the latency was 34.7 ± 2.1 ms (P < 0.05). We conclude that myogenic responses to magnetic transcranial stimulation are more sensitive to anesthetic-induced motoneural depression compared with those elicited by electrical transcranial stimulation.

Implications: Transcranial motor evoked potentials are used to monitor spinal cord integrity intraoperatively. We compared the relative efficacy of electrical and magnetic transcranial stimuli in anesthetized patients. It seems that myogenic responses to magnetic transcranial stimulation are more sensitive to anesthetic-induced motoneural depression compared with electrical transcranial stimulation.

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