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 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 mu g ml(-1) and increased by 1.0 mu g ml(-1) steps every 4 min up to 6.0 mu g ml(-1). A 4-min interval was chosen to ensure that steady-state effect-site 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) mu g ml(-1) and for LOC it was 2.8 (2.8-5.65) mu g 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 greater correlation with effect-site concentration and may be more useful than BIS in predicting both LVC and LOC.