Fig. 1. Loss of consciousness (LOC): isobolographic analysis of propofol and sevoflurane interaction. LOC was defined as loss of response to loud voice with mild prodding (Observer’s Assessment of Alertness/Sedation score < 2). The 50% response was determined by averaging crossover pairs following the method of Dixon. 32,Cp50LOC(predicted plasma concentration, ±95% confidence interval) of propofol and LOC50(percent end-tidal concentration) of sevoflurane alone (intercept points) were 2.90 μg/ml (2.68–3.12 μg/ml) and 0.88% (0.83–0.93%), 10 respectively. The line connecting these values graphically represents the line of simple additivity (±95% confidence interval). Our experimental method examined only a single point on the predicted line of additivity for a single end-tidal sevoflurane concentration (0.46%; 0.45–0.47%). The observed Cp50LOCfor propofol at this sevoflurane concentration was 1.19 μg/ml (95% confidence interval, 1.09–1.29 μg/ml) and was not significantly different than the predicted value of 1.45 μg/ml (95% confidence interval, 1.23–1.67 μg/ml).