Differential effect of halothane on motor evoked potentials elicited by transcranial electric or magnetic stimulation in the monkey

Objective: Halothane (HAL) is known to depress motor evoked potentials produced by transcranial magnetic (tcMMEP) or transcranial electric (tcEMEP) stimulation. This study was designed to determine if differences existed between tcEMEP and tcMMEP with increasing HAL concentra-tions. Methods: tcMMEP and tcEMEP were characterized during 0-2% inspired HAL in 10 adult cynomologous monkeys during a baseline anesthesia with a continuous ketamine infusion. tcEMEP and tcMMEP were assessed by measuring the onset latency (time from stimulation to the initial response), amplitude of the thenar compound action potential response and threshold (relative power required to elicit a response). Cortical stimulation was accomplished using a Cadwell MES-10 (tcMMEP) and Digitimer Dl80 (tcEMEP). Results: The baseline (no HAL) onset latency for tcEMEP (10.68 ms) was significantly shorter than that of tcMMEP (12.28 ms) (P < 0.05). The amplitudes (7,916, 4,858 μV, respectively) were not significantly different (P > 0.112). The onset latency increased and amplitude decreased for both techniques (no significant difference between tcEMEP and tcMMEP) with increase in HAL. All animals lost their responses below 2% HAL. In each animal the tcMMEP was lost at a HAL concentration below or equal to that for tcEMEP. T The ED₅₀ (where 50% of the animals lost the response) was significantly different between tcMMEP (0.66% HAL) and tcEMEP (1.04% HAL) (P < 0.05). The relative threshold gradually increased for tcEMEP and abruptly increased above 0.4% HAL for tcMMEP. Conclusions: These differences in sensitivity to HAL are consistent with other studies with intravenous anesthesia and are consistent with the known difference in the physiological mechanisms by which magnetic and electrical stimulation activates the motor cortex.