Spectral edge frequency of the electroencephalogram to monitor "depth" of anaesthesia with isoflurane or propofol.

To determine threshold values, sensitivity and specificity of the spectral edge frequency (SEF) of the electroencephalogram (EEG) that indicate intraoperative movements, we studied 49 patients undergoing, elective laparotomy. Extradural analgesia was used in all patients. To maintain general anaesthesia, patients in group 1 (n = 23) received 0.4-1.2 vol% isoflurane and patients in group 2 (n = 24) propofol 3-5 mg kg-1 h-1 i.v. During operation and emergence from anaesthesia, spontaneous purposeful movements were documented. The EEG was recorded continuously in the awake state until the end of anaesthesia. Power spectral analysis calculated the SEF and power in the delta, theta, alpha and beta bands and the delta ratio. Adequate anaesthesia caused a statistically significant decrease in SEF from 16 to 12 Hz. Power in the beta band decreased and power in the theta band and total power increased compared with the awake state. Before and during movements observed in the intraoperative period or during emergence from general anaesthesia, SEF increased from 12 to 18 Hz, the power in beta band increased and theta power decreased compared with the state of adequate anaesthesia. A threshold value of SEF 14 Hz to predict movements during anaesthesia had a sensitivity of 72% and specificity of 82%.