Relationship between, bispectral index, auditory evoked potential index and effect-site EC50 for propofol at two clinical end-points

Background. Many anaesthetists are deterred from using total i.v. anaesthesia because of uncertainty over the concentration of propofol required to prevent awareness. We predicted blood and effect-site concentrations of propofol at two clinical end-points: loss of consciousness and no response to a painful stimulus. Methods. Forty unpremedicated Caucasian patients were anaesthetized with i.v. propofol delivered by a Diprifusor target-controlled infusion (TCI). Bispectral index (BIS) and auditory evoked potential index (AEPex) were measured and blood and effect-site propofol concentrations were predicted. Logistic regression was used to estimate population values for predicted blood and effect-site propofol concentrations at the clinical end-points and to correlate these with BIS and AEPex. Results. The effect-site EC50 at loss of consciousness was 2.8 mum ml(-1) with an EC05 and an EC95 of 1.5 and 4.1 mum ml(-1), respectively. The predicted EC50 when there was no response to a tetanic stimulus was 5.2 mum ml(-1) with an EC05 and an EC95 of 3.1 and 7.2 mum ml(-1), respectively. Conclusions. Unconsciousness and lack of response to a painful stimulus occur within a defined range of effect-site concentrations, predicted by Diprifusor TCl software.