THE EFFECT OF KETAMINE ANESTHETIC INDUCTION ON MUSCLE RESPONSES TO TRANSCRANIAL MAGNETIC CORTEX STIMULATION STUDIED IN MAN

In man, an anesthetic agent that induces surgical anesthesia with mini mal influence on descending pyramidal tract activity remains to be fou nd. Anesthesia with ketamine allows recording of stable compound muscl e action potentials (CMAPs) to single transcranial magnetic stimulatio ns of the motor cortex (CortStim) in monkeys. This report describes th e findings in 5 patients, where CMAPs to CortStim were recorded from t he right hypothenar during anesthesia induction with ketamine. The age nt was injected intravenously every 90 s in 6 steps of 0.5 mg up to a maximum of 3 mg/kg body weight (BW). Surgical anesthesia was achieved after ketamine injection of 1.5 (n=4 patients) or 2.0 mg/kg BW (n=l). In the five individuals tested, CMAP amplitudes and latencies (mean; r ange) were 2.6 (1.6-5.8) mV and 22.8 (20.4-24.6) ms before induction, and 1.6 (0.3-4.7) mV and 23.5 (21.7-24.5) ms after administration of t he maximum dose. The paired differences (mean +/- 1 S.D.) were 0.8 +/- 0.6 mV and 1.0 +/- 0.8 ms and were statistically not significant (n=5 , P=0.1, Wilcoxon-test). With ketamine as a single anesthetic inductio n agent CMAPs to single CortStim remain easily recordable even in dosa ges higher than those necessary to induce surgical anesthesia. All oth er previously tested anesthetic agents suppress CMAPs to CortStim as s oon as the patient is unconscious.