Propofol infusion and the suppression of consciousness: the EEG and dose requirements

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 to achieve and maintain a stable blood propofol concentration between 1.0 and 6.0 micrograms 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) ED50 (5.8 (3.5-6.8) mg kg-1 h-1) and ED95 (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 the 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 EC50 for loss of consciousness was a propofol concentration of 2.3 (1.8-2.7) micrograms ml-1 and for 50% suppression of MPF was 3.1 (2.7-3.5) micrograms 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 micrograms ml-1, consistent with stable anaesthesia, the mean MPF was 5.6 (4.5-6.3) Hz.