Effects of droperidol, pentobarbital, and ketamine on myogenic transcranial magnetic motor-evoked responses in humans

MYOGENIC MOTOR-EVOKED RESPONSES to transcranial magnetic stimulation of the motor cortex (tc^mag-MERs) may become clinically useful for the noninvasive assessment of motor pathway conduction during surgery. However, application is hindered because most anesthetic regimens result in severe depression of tc^mag-MER amplitudes. As part of our systematic attempts to identify anesthetic agents and supplements suitable for use during tc^mag-MER recording, we studied the effect of bolus doses of pentobarbital (1.5 mg/kg), droperidol (0.07 mg/kg), or ketamine (1 mg/kg), administered intravenously, on compound muscle action potentials to transcranial magnetic stimulation in five healthy volunteers. The doses were chosen to be comparable with doses that might be suitable for supplementation of a nitrous oxide/opioid anesthetic technique. Droperidol administration resulted in sustained amplitude depression of both tibialis and adductor pollicis tc-MERs to 30 ± 9% and 39 ± 14% of baseline (P < 0.01). Tc^mag-MER amplitude changes after pentobarbital were variable, ranging from no change to substantial amplitude depression (to 20% of baseline) in two subjects. In contrast, ketamine administration did not result in significant amplitude depression. In three subjects, tibialis anterior amplitude increased to 150 to 220% of control values in the first 10 minutes after ketamine. Onset latency was unchanged after any drug. These data indicate that tc^mag-MERs are moderately depressed after droperidol and pentobarbital but well preserved after ketamine. Ketamine may be a more suitable supplement to opioid/nitrous oxide anesthesia than droperidol or pentobarbital.