Transcranial magnetic-evoked potentials under total intravenous anaesthesia and nitrous oxide

Magnetic stimulation of the cortex and recording of the motor-evoked potentials (MEPs) by electromyography (EMG) is a well proven method to assess the descending pathways of the spinal cord and detect neurological impairment. We have assessed, in 33 adult patients undergoing spinal surgery, the influence of four total i.v. anaesthesia regimens (TIVA) on this recording technique. In 20 patients, the effect of 50% nitrous oxide was also studied. MEP amplitudes, latencies and success rates of stimulation were obtained in the steady-state after induction of anaesthesia. Combinations of midazolam and ketamine, and alfentanil and etomidate had the least effect on MEPs. Propofol (in combination with alfentanil or ketamine) showed marked depression of the MEP amplitude and the lowest success rates of stimulation. The latencies did not change at all. The addition of nitrous oxide significantly depressed the registered MEPs and lowered the success rates.