Background: The power spectrum of the electroencephalogram (EEG) may be analyzed to provide quantitative measures of EEG activity (e.g., spectral edge, which defines the highest EEG frequency at which significant activity is found). The current study tested the hypothesis that spectral edge and similar measures distinguish different functional depths of anesthesia in humans. Methods: Three groups were studied. Group 1 consisted of 34 surgical patients (ASA physical status 1 or 2) who received 0.6, 1.0 and 1.4 MAC isoflurane anesthesia. A subgroup (group 2) of group 1 was tested during 1.0 MAC isoflurane anesthesia at surgical incision. Group 3 consisted of 16 volunteers who listened to an audiotape while receiving 0.15, 0.3, and 0.45 MAC isoflurane or 0.3, 0.45, and 0.6 MAC nitrous oxide in oxygen. The audiotape contained information designed to test implicit and explicit memory formation. We tested the ability of six EEG parameters (spectral-edge, 95th percentile power frequency, median power, and zero crossing frequencies and total power in the alpha- [8-13 Hz] and delta- [<4 Hz] power ranges) to predict movement after surgical incision, purposeful response to command, or memory of information presented during anesthetic administration. Results: Isoflurane decreased EEG activity in group 1 in a dose-related fashion. The 55% of group 2 who made purposeful movements in response to incision did not differ in their EEG from nonresponders (e.g., spectral edge 19.8 +/- 3.1 vs. 19.3 +/- 2.6 Hz, mean +/- SD). In group 3, memory of the information presented did not correlate with values of any EEG parameter. Response to verbal command was associated with lower anesthetic concentrations and with smaller alpha- and delta-band power (298 +/- 66 vs. 401 +/- 80 watts; and 75 +/- 20 vs. 121 +/- 49 watts, mean +/- SD), but there was no difference in values for other parameters. Conclusions: We conclude that our EEG measures do not predict depth of anesthesia as defined by the response to surgical incision, the response to verbal command or the development of memory.
