Intraoperative EEG changes in relation to the surgical procedure during isoflurane-nitrous oxide anesthesia: Hysterectomy versus mastectomy

Study Objective: To investigate topographical changes in electroencephalographic (EEG) frequencies and spectral power density in relation to different surgical procedures (abdominal hysterectomy versus mastectomy) during steady-state isoflurane- nitrous oxide (N₂O) anesthesia. Design: Prospective, nonrandomized, open study. Setting: University hospital. Patients: 34 ASA status I and II patients scheduled for elective abdominal hysterectomy or mastectomy. Interventions: 12 patients were studied without surgery (Group I, control), 22 patients were studied for the first 14 minutes following skin incision during hysterectomy (Group 2, n = 11) or mastectomy (Group 3, n = 11). Measurements and Main Results: Anesthesia was maintained with 0.6% isoflurane in 66% N₂O in oxygen (O₂). EEG was recorded via 17 channels followed by calculation of spectral power densities in selected frequency bands for each recording site. In addition, heart rate, mean arterial pressure (MAP), end-tidal carbon dioxide tensions, and isoflurane concentration were recorded. Total observation time was 20 minutes in all groups. At baseline, EEC variables were comparable in all groups. The EEC demonstrated slow wave activity superimposded with alpha waves. Start of surgery resulted in increases of slower waves and decreases in alpha activity. In both surgical groups, these EEC changes were most pronounced at frontal recording sites (p < 0.05) with differences in the frequency content. In Group 2 (hysterectomy), delta activity became dominant, whereas on Group 3 (mastectomy), a shift to theta waves was observed. During surgery MAP was increased by 40% (Group 2; p < 0.05) and 21% (Group 3; p < 0.05), respectively. Conclusions: These results show that specific surgical procedures may induce EEG slow wave activity to a different degree. The EEG response varied in relation to the surgical procedure and/or the intensity of noxious stimulation. Mastectomy resulted in the appearance of theta activity whereas, during laparotomy, the EEG frequency content was shifted to delta waves. The topographical analysis indicates spatial inhomogeneities in the EEG responses with a dominance at frontal areas. From this finding, it may be concluded that the electrode montage used for intraoperative EEG recordings has to be carefully selected.