Blood-brain equilibration rate constant ( k_e0 ) is derived from either pharmacokinetic and pharmacodynamic modeling ( k _{e0 _model}) or a model-independent observed time to peak effect ( k _{e0 _tpeak}). Performance in bispectral index (BIS) prediction was compared between k _{e0 _model} and k _{e0 _tpeak} for microemulsion or long chain triglyceride (LCT) propofol.

Time to peak effect (t_peak, time to a maximally reduced BIS value) of microemulsion propofol after an intravenous bolus (1 mg/kg) was measured in 100 patients (group A_micro). An observed t_peak of 1.6 min for LCT propofol was obtained from an earlier study. Another 40 patients received a target controlled infusions of microemulsion propofol ( k _{e0 _model} = 0.187/min, group B_micro = 20) or LCT propofol ( k _{e0 _model} = 0.26/min, group B_LCT = 20) and remifentanil. The k _{e0 _tpeak}'s in group B_micro and B_LCT were calculated using the observed t_peak value obtained from group A_micro and 1.6 min, respectively. Effect-site concentrations of propofol were recalculated using the amounts of propofol infused over time and k _{e0 _tpeak}'s. Predicted BIS values calculated by sigmoid Emax equations with k _{e0 _model} and k _{e0 _tpeak} were compared with observed BIS values during induction and emergence for both formulations of propofol.

Observed t_peak of microemulsion propofol was 1.68 min. The median performance errors of BIS in group B_micro were -1.83% (-24.8 to 18.9, k _{e0 _model}) and -2.42% (-26.1 to 36.2, k _{e0 _tpeak}), while 8.01% (-20.5 to 30.1, k _{e0 _model}) and 7.37% (-27.0 to 49.1, k _{e0 _tpeak}) in group B_LCT. The median absolute performance errors of BIS in group B_micro were 11.87% (2.2-31.1 k _{e0 _model}) and 14.38% (-0.6 to 44.6, k _{e0 _tpeak}), while 17.31% (5.54-36.0, k _{e0 _model}) and 18.28% (-0.1 to 56.0, k _{e0 _tpeak}) in group B_LCT.

The k _{e0 _model} showed better performance in BIS prediction than the k _{e0 _tpeak}.