Relationship between bispectral index, electroencephalographic state entropy and effect-site EC50 for propofol at different clinical endpoints.

Background: State entropy (SE) is a newly available monitor for depth of anaesthesia. We investigated whether the relationship between predicted effect-site propofol concentration and both bispectral index (BIS) and SE values is useful for predicting loss of verbal contact and loss of consciousness during steady-state conditions.

Methods: Twenty unpremedicated patients undergoing elective major abdominal surgery were recruited. A target-controlled infusion of propofol was administered using Schneider's pharmacokinetic model. The propofol infusion was set at an initial site-effect concentration of 1.0 microg ml(-1), and increased by 1.0 microg ml(-1) steps every 4 min, up to 6.0 microg ml(-1). A 4-min interval was chosen to ensure that steady-state site-effect concentrations were obtained. Propofol site-effect concentrations and BIS and SE values were recorded at loss of verbal contact (LVC) and loss of consciousness (LOC). Population values for predicted effect-site concentrations at the clinical endpoints were estimated and correlated with BIS and SE values.

Results: For LVC, the effect-site concentration for 90% of patients was 1.1 (1.1-3.2) microg ml(-1) and for LOC 2.8 (2.8-5.65) microg ml(-1). LVC occurred in 90% of patients at a BIS value of 70.2 (70.2-90.2) and an SE value of 60.3 (60.3-75.5) and LOC occurred at a BIS value of 38.2 (38.2-70.4) and an SE value of 42.2 (42.2-60.4).

Conclusions: LVC and LOC occurred within a defined range of predicted effect-site concentrations. SE had a smaller range than BIS and higher correlation with effect-site concentration and may be more useful than BIS in predicting both LVC and LOC.