A comparison of myogenic motor evoked responses to electrical and magnetictranscranial stimulation during nitrous oxide opioid anesthesia

Transcranial motor evoked potentials (tc-MEPs) are. used to monitor spinal cord integrity intraoperatively, We compared myogenic motor evoked response s with electrical and magnetic transcranial stimuli during nitrous oxide/op ioid anesthesia. In 11 patients undergoing spinal surgery, anesthesia was i nduced with IV etomidate 0.3 mg/kg and sufentanil 1.5 mu g/kg and was maint ained with sufentanil 0.5 mu g.kg(-1).h(-1) and N2O 50% in oxygen. Muscle r elaxation was kept at 25% of control with IV vecuronium. Electrical stimula tion was accomplished with a transcranial stimulator set at maximal output. (1200 V). Magnetic transcranial. stimulation was accomplished with a trans cranial stimulator set at maximal output (2 T). Just before skin incision, triplicate responses to single stimuli with both modes of cortical stimulat ion were randomly recorded from the tibialis anterior muscles. Amplitudes a nd latencies were compared using the Wilcoxon signed rank test. Bilateral t c-MEP responses were obtained in every patient with electrical stimulation. Magnetic stimulation evoked only unilateral responses in two patients. Wit h electrical stimulation, the median tc-MEP amplitude was 401 mu V (range 1 45-1145 mu V), and latency was 32.8 +/- 2.3 ms. With magnetic stimulation, the tc-MEP amplitude was 287 mu V (ranee 64-506 mu V) (P < 0.05), and the l atency was 34.7 +/- 2.1 ms (P < 0.05). We conclude that myogenic responses to magnetic transcranial stimulation are! more sensitive to anesthetic-indu ced motoneural depression compared with those elicited by electrical transc ranial stimulation. Implications: Transcranial motor evoked potentials are used to monitor spinal cord integrity intraoperatively. We compared the rel ative efficacy of electrical and magnetic transcranial stimuli in anestheti zed patients. It seems that myogenic responses to magnetic transcranial sti mulation are more sensitive to anesthetic-induced motoneural depression com pared with electrical transcranial stimulation.