The use of ketamine or etomidate to supplement sufentanil/N₂O anesthesia does not disrupt monitoring of myogenic transcranial motor evoked responses

Intraoperative monitoring of myogenic transcranial motor evoked responses (tc-MERs) requires an anesthetic technique that minimally depresses response amplitudes. Acceptable results have been obtained during opioid/N₂O anesthesia, provided that the concentration of N₂O does not exceed 50%. However, this technique may necessitate supplementation with additional agents to achieve adequate depth of anesthesia. Etomidate and ketamine have been reported anecdotally or in nonsurgical situations to produce little tc- MER depression. We investigated the effects on tc-MER amplitude and latency of supplementation of a sufentanil/N₂O anesthetic with etomidate or ketamine in patients undergoing spinal instrumentation. Anesthesia was induced with etomidate 0.3 mg/kg and sufentanil 1.5 mg/kg and maintained with sufentanil 0.5 mg/kg/h and N₂O 50%. Muscle relaxation was kept at 25% of control. Paired transcranial electrical stimulation was performed. Each patient randomly received either ketamine (0.5 mg/kg) or etomidate (0.1 mg/kg) as a single bolus intravenously, during stable surgical conditions. Triplicate tc- MERs were recorded from the tibialis anterior muscles before and 2, 5, 10, and 15 min after drug administration. Administration of ketamine did not significantly change tc-MER amplitudes, whereas etomidate resulted in a transient amplitude depression to 72% of control (p < 0.05) at 2 min after injection. Latency remained unchanged with both drugs. In conclusion, the data suggest that both ketamine (0.5 mg/kg) and etomidate (0.1 mg/kg) can be used to supplement sufentanil/N₂O anesthetic without disrupting tc-MER monitoring.