Quantitative EEG analysis during anaesthesia with isoflurane in nitrous oxide at 1.3 and 1.5 MAC

In 14 patients undergoing elective surgery the EEG was studied during anaesthesia with isoflurane and nitrous oxide (in oxygen) at 1.3 and 1.5 MAC. The distribution of spectral EEG indices of the baseline EEG, during the intraoperative and recovery periods were established and compared. Median frequency exhibited the most clear separation between the distributions during recovery and the intraoperative period. During anaesthesia, the median values were found to be lower than 5 Hz; when the patient was conscious, the EEG median frequency values were greater than 6 Hz. Time to recovery was 13.4 +/- 2.9 min and 30.0 +/- 8.5 min for the groups treated with 1.3 and 1.5 MAC, respectively. Burst suppression was observed during the loading period in all patients treated with 1.5 MAC and in five patients out of seven receiving 1.3 MAC. The average duration of the period of burst suppression was markedly greater in the group receiving 1.5 MAC than in the group receiving 1.3 MAC. It is concluded that devices designed for EEG trend monitoring during anaesthesia should preferably depict a frequency measure, and allow for burst suppression recognition before spectral analysis.