EFFECTS OF PROPOFOL, ETOMIDATE, MIDAZOLAM, AND FENTANYL ON MOTOR EVOKED-RESPONSES TO TRANSCRANIAL ELECTRICAL OR MAGNETIC STIMULATION IN HUMANS

The effects of propofol, etomidate, midazolam, and fentanyl on motor evoked responses to transcranial stimulation (tc-MERs) were studied in five healthy human volunteers. Each subject, in four separate sessions, received intravenous bolus doses of propofol 2 mg.kg-1, etomidate 0.3 mg.kg-1, midazolam 0.05 mg.kg-1, and fentanyl 3-mu-g.kg-1. Electrical tc-MERs (tc(e)-MERs) were elicited with anodal stimuli of 500-700 V. Magnetic tc-MERs (tc(mag)-MERs) were elicited using a Cadwell MES-10 magnetic stimulator at maximum output. Compound muscle action potentials were recorded from the tibialis anterior muscle. Duplicate tc(e)-MERs and tc(mag)-MERs were recorded before and up to 30 min after drug injection. Reproducible baseline tc(e)-MERs (amplitude 4.7 +/- 0.43 (SEM) mV, latency 29.4 +/- 0.35 ms) and tc(mag)-MERs (amplitude 3.7 +/- 0.43 mV, latency 31.1 +/- 0.39 ms) were obtained in all subjects. Pronounced depression of tc(e)-MER amplitude to 2% of baseline values (P < 0.01) was observed 2 min after injection of propofol. Thirty minutes after injection of propofol, amplitude depression to 44% of baseline (P < 0.05) was still present, despite an apparent lack of sedation. Midazolam caused significant (P < 0.01) amplitude depression, e.g., tc(mag)-MER to 16% of baseline values 5 min after injection. Significant depression persisted throughout the 30-min study period. Fentanyl did not cause any statistically significant amplitude changes in this small population. Etomidate caused significant but transient depression of tc-MER amplitude. However, there was considerable intersubject variability. Latency did not change significantly after any drug. The magnitude of drug-induced MER changes was similar for magnetic and electrical stimulation, although in two instances, at the peak of drug-induced depression, tc(mag)-MERs were absent when tc(e)-MERs were recordable. Since amplitude depression after etomidate was less pronounced and of shorter duration, etomidate may be preferable to propofol as an induction agent when tc-MER monitoring is indicated. Similarly, fentanyl may be preferable to midazolam as an intravenous supplement.