We have studied in 12 patients the effect of desflurane in nitrous oxide on the electroencephalogram (EEG) and the early cortical auditory evoked response (AER). After induction with desflurane, patients' lungs were ventilated to maintain three different end-expiratory concentrations of desflurane (1.5, 3 and 6%) during four consecutive 10-min periods before surgery. As the end-expiratory concentration of desflurane was increased, Pa and Nb (AER) amplitudes decreased and their latencies increased, and spontaneous EEG showed an increase in amplitude and a slowing of frequency. A linear relationship was demonstrated between log10 concentration of desflurane and all variables (P = 0.001). Pa amplitude showed the greatest linearity followed by the derived variable F95 of the EEG. From regression slopes, mean percentage changes of each variable were calculated for a 1 MAC change in desflurane concentration, Pa amplitude showed the largest change (mean 49% (95% confidence interval 40-56%) decrease for a 1 MAC increase). This was greater than that of F95 for a similar confidence interval, indicating better resolution. 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 anaesthesia.

We have studied the arousal effect of suxamethonium on the auditory evoked response (AER) of the electroencephalogram (EEG) in 40 ASA I and II patients during isoflurane anaesthesia. After induction of anaesthesia, the patient's lungs were ventilated for 20 min with 0.6 MAC end-expiratory isoflurane (0.59-0.77% depending on the age of the patient), and 50% nitrous oxide in oxygen. The patients were then allocated randomly to one of two groups: 21 received suxamethonium 1 mg kg-1, while 19 were given saline. The AER before and after administration of suxamethonium or saline was compared to determine the changes in Pa and Nb amplitudes and latencies. Pa amplitude after suxamethonium increased by 53% (95% confidence interval (CI) 15, 104%) compared with a reduction in Pa amplitude in the saline group of 19% (95% CI, -41, 12%) (P = 0.004) suggesting an arousal effect. Similarly, Nb amplitude increased in the suxamethonium group by 47% (95% CI, 3, 110%) and decreased in the saline group by 11% (95% CI, -33, 19%) (P = 0.03). We conclude that suxamethonium caused arousal according to the AER and postulate that this may have been caused by increased muscle afferent activity after stimulation of muscle spindles, although further studies are required to confirm this.

Adopting the revised treatment threshold in the U.S. would result in a significant cost, yet provide an unclear clinical benefit.