We have used Median Power Frequency (MPF) to study changes in the electroencephalogram during propofol infusions in 52 women about to undergo gynaecological surgery. Patients were allocated to receive propofol by one of nine different manually-controlled infusion schemes designed so achieve and maintain a stable blood propofol concentration between 1.0 and 6.0 mu g ml(-1), covering a range of states between conscious sedation and full anaesthesia. We recorded the changes in MPF and the response to clinical signs of loss of consciousness at these different doses and concentrations of propofol. Using probit analysis, we derived MPF values corresponding to 50 % and 95 % suppression of response to verbal (9.3 Hz and 6.8 Hz), eyelash (8.9 Hz and 6.7 Hz) and venepuncture (5.7 Hz and 3.0 Hz) stimuli. Likewise, we obtained dose and concentration requirements for propofol to suppress these stimuli. The mean (95 % confidence intervals) ED(50) (5.8 (3.5-6.8) mg kg(-1) h(-1)) and ED(95) (8.3 (7.1-16.9) mg kg(-1) h(-1)) propofol doses for suppression of consciousness were similar to the values for suppression of She eyelash reflex (6.2 (5.3-6.8) mg kg(-1) h(-1) and 8.6 (7.8-10.8) mg kg(-1) h(-1), respectively). The EC(50) for loss of consciousness was a propofol concentration of 2.3 (1.8-2.7) mu g ml(-1) and for 50 % suppression of MPF was 3.1 (2.7-3.5) mu g ml(-1). The dose required for 50 % suppression of MPF was 7.1 (6.2-8.0) mg kg(-1) h(-1) After 30 min, at blood propofol concentrations > 4.0 mu g ml(-1), consistent with stable anaesthesia, the mean MPF was 5.6 (4.5-6.3) Hz.
