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 respon ses (tc-MERs) requires an anesthetic technique that minimally depresse s response amplitudes. Acceptable results have been obtained during op ioid/N2O anesthesia provided that the concentration of N2O does not ex ceed 50%. However, this technique mag; necessitate supplementation wit h additional agents to achieve adequate depth of anesthesia. Etomidate and ketamine have been reported anecdotally or in nonsurgical situati ons to produce little tc-MER depression. We investigated the effects o n tc-MER amplitude and latency of supplementation of a sufentanil/N2O anesthetic with etomidate or ketamine in patients undergoing spinal in strumentation. Anesthesia was induced with etomidate 0.3 mg/kg and suf entanil 1.5 mg/kg and maintained with sufentanil 0.5 mg/kg/h and N2O 5 0%. Muscle relaxation was kept at 25% of control. Paired transcranial electrical stimulation was performed. Each patient randomly received e ither ketamine (0.5 mg/kg) or etomidate (0.1 mg/kg) as a single bolus intravenously, during stable surgical conditions. Triplicate tc-MERs w ere recorded from the tibialis anterior muscles before and 2, 5, 10, a nd 15 min after drug administration. Administration of ketamine did no t significantly change tc-MER amplitudes, whereas etomidate resulted i n a transient amplitude depression to 72% of control (p < 0.05) at 2 m in after injection. Latency remained unchanged with both drugs, In con clusion, the data suggest that both ketamine (0.5 mg/kg) and etomidate (0.1 mg/kg) can be used to supplement sufentanil/N2O anesthetic witho ut disrupting tc-MER monitoring.