AUDITORY-EVOKED RESPONSE, MEDIAN FREQUENCY AND 95-PERCENT SPECTRAL EDGE DURING ANESTHESIA WITH DESFLURANE AND NITROUS-OXIDE

We have studied in 12 patients the effect of desflurane in nitrous oxi de on the electroencephalogram (EEG) and the early cortical auditory e voked response (AER). After induction with desflurane, patients' lungs were ventilated to maintain three different end-expiratory concentrat ions of desflurane (1.5, 3 and 6%) during four consecutive 10-min peri ods before surgery. As the end-expiratory concentration of desflurane was increased, Pa and Nb (AER) amplitudes decreased and their latencie s increased, and spontaneous EEG showed an increase in amplitude and a slowing of frequency. A linear relationship was demonstrated between log,, concentration of desflurane and all variables (P=0.001). Pa ampl itude showed the greatest linearity followed by the derived variable F 95 of the EEG. From regression slopes, mean percentage changes of each variable were calculated for a 1 MAC change in desflurane concentrati on. Pa amplitude showed the largest change (mean 49% (95% confidence i nterval 40-56%) decrease for a 1 MAC increase). This was greater than that of F95 for a similar confidence interval, indicating better resol ution. This study confirms that the early cortical AER is affected by desflurane in a similar manner to that of other anaesthetic agents and as such remains the most promising EEG derived measure of depth of an aesthesia.