Surgical stimulation induces changes in brain electrical activity during isoflurane/nitrous oxide anesthesia: A topographic electroencephalographic analysis

Background: The aim of this study was to investigate topographic changes in electroencephalographic (EEG) power and frequency induced by abdominal surgery during anesthesia with 0.6% or 1.2% isoflurane in 66% nitrous oxide. Methods: Forty-six patients (aged 41 ± 13 yr) scheduled for elective abdominal surgery were studied. The trachea of each patient was intubated and the lungs ventilated. Patients were randomly assigned to one of four groups: anesthesia was maintained with 0.6% (group 1, n = 12; group 2, n = 11) or 1.2% end-tidal isoflurane (group 3, n = 12; group 4, n = 11) in 66% nitrous oxide. Data were recorded over 20 min. Groups 1 and 3 were studied without surgery (as controls). In groups 2 and 4 recording was started 6 min before skin incision. The EEG was acquired via 17 scalp electrodes placed in standard International 10-20 locations (reference C(z)). Absolute and relative power densities were calculated in selected frequency bands. EEG maps of spectral power densities were coded according to a continuous color spectrum. Results: During baseline recordings, alpha activity was dominant at frontal areas in groups 1 and 2. In comparison, in groups 3 and 4, delta and theta activities were dominant at frontal leads. In group 2, the start of surgery resulted in increases in delta activity and decreases in alpha activity that were most dominant frontally (delta +181% and alpha -61%, F₃). The delta shift was attenuated at 1.2% isoflurane (group 4, delta +44%, F₃), but decreases in alpha activity (-53%, F₃) were comparable to those in group 2. The EEG response in all frequency bands was attenuated at parietotemporal recording sites at both isoflurane concentrations. Conclusions: The current data demonstrate graded EEG responses induced by abdominal surgery during anesthesia with 0.6% or 1.2% isoflurane in 66% nitrous oxide. Spatial heterogeneities in absolute spectral power densities were reflected by color changes in the EEG maps. The topographic EEG analysis indicates that these changes were most dominant at frontal areas. The increases in delta and decreases in alpha activities may be related to intraoperative 'paradoxical' electrophysiologic arousal phenomena.