PROPOFOL INFUSION AND THE SUPPRESSION OF CONSCIOUSNESS - THE EEG AND DOSE REQUIREMENTS

We have used Median Power Frequency (MPF) to study changes in the elec troencephalogram during propofol infusions in 52 women about to underg o 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 differen t doses and concentrations of propofol. Using probit analysis, we deri ved 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 venepun cture (5.7 Hz and 3.0 Hz) stimuli. Likewise, we obtained dose and conc entration requirements for propofol to suppress these stimuli. The mea n (95 % confidence intervals) ED(50) (5.8 (3.5-6.8) mg kg(-1) h(-1)) a nd ED(95) (8.3 (7.1-16.9) mg kg(-1) h(-1)) propofol doses for suppress ion 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 k g(-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 % supp ression 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 st able anaesthesia, the mean MPF was 5.6 (4.5-6.3) Hz.