Effects of Nitrous Oxide on Motor Evoked Potentials Recorded from Skeletal Muscle in Patients under Total Anesthesia with Intravenously Administered Propofol

The effect of nitrous oxide (N2O) on motor evoked potentials (MEPs) recorded from human subjects under total intravenous anesthesia with propofol (2,6-diisopropylphenol) was studied. MEPs were recorded from the 1st dorsal interosseous muscle of the foot in nine subjects; in two of these, simultaneous recordings were made from the 2nd dorsal interosseous muscle of the hand and from the deltoid muscle. Single transcranial electrical stimuli were used in recording the MEPs. The effects of N2O were studied at concentrations from 20 to 70%. Increasing concentrations of N2O caused a progressive increase in onset latency and a fall in the peak-to-peak amplitude of the MEPs recorded from the foot. Latency values showed a significant increase above the baseline at concentrations of N2O greater than 20% (Pvalues, 0.05-0.005). The response amplitude showed a significant decrease from the baseline at concentrations of N2O greater than 50% (P values, 0.05-0.005). The 2nd dorsal interosseous muscle of the hand demonstrated a pattern of sensitivity to N2O similar to that of the 1 st dorsal interosseous muscle of the foot. The onset latency and initial peak-to-peak amplitude of the deltoid muscle were insensitive to N2O at the concentrations used. We conclude that N2O can be used as an anesthetic adjunct without a significant deleterious effect on MEPs during intraoperative monitoring in patients under propofol anesthesia, providing concentrations are maintained below 50%.