Auditory evoked response, median frequency and 95% spectral edge during anaesthesia with desflurane and nitrous oxide

We have studied in 12 patients the effect of desflurane in nitrous oxide onthe electroencephalogram (EEG) and the early cortical auditory evokedresponse (AER). After induction with desflurane, patients' lungs wereventilated to maintain three different end-expiratory concentrations ofdesflurane (1.5, 3 and 6%) during four consecutive 10- min periods beforesurgery. As the end-expiratory concentration of desflurane was increased,Pa and Nb (AER) amplitudes decreased and their latencies increased, andspontaneous EEG showed an increase in amplitude and a slowing of frequency.A linear relationship was demonstrated between log10 concentration ofdesflurane and all variables (P = 0.001). Pa amplitude showed the greatestlinearity followed by the derived variable F95 of the EEG. From regressionslopes, mean percentage changes of each variable were calculated for a 1MAC change in desflurane concentration, Pa amplitude showed the largestchange (mean 49% (95% confidence interval 40-56%) decrease for a 1 MACincrease). This was greater than that of F95 for a similar confidenceinterval, indicating better resolution. This study confirms that the earlycortical AER is affected by desflurane in a similar manner to that of otheranaesthetic agents and as such remains the most promising EEG derivedmeasure of depth of anaesthesia.