Effect of thiopentone on motor evoked potentials induced by transcranial magnetic stimulation in humans

We have studied the effect of repeated doses of thiopentone on motor evoked potentials (MEP) after transcranial magnetic stimulation in 13 patients. Thiopentone was administered i.v. in an initial dose of 2 mg kg-1, followed by repeated doses (1 mg kg-1 every 3 min) until the appearance of burst suppression on the EEG. The total dose administered was mean 10.7 (SD 2.6) mg kg-1. The magnetic coil was placed over the MEP scalp stimulation region and evoked electromyographic responses were recorded from the contralateral abductor pollicis brevis. After an initial dose of thiopentone, reproducible MEP responses were recorded in all patients, but amplitudes were reduced to 42.8% of baseline values. Further administration of repeated doses of thiopentone produced a dose-dependent decrease in success rate of MEP recordings and a significant reduction in MEP amplitude (P < 0.01). Latency did not change significantly, although there was a tendency to increase. During burst suppression on the EEG, MEP was not recorded successfully in all patients. We conclude that MEP recording during the administration of thiopentone is feasible only at a minimum dose, with a marked reduction in MEP amplitude.