Figure 4. The propofol-alfentanil concentration isobologram for a 50% probability of no response (movement) to tetanic stimulation of the ulnar nerve at 15 min. The equations obtained by logistic regression for the probability of a response versus no response are shown in the figure (solid line = EC⁵⁰, dashed line = EC⁹⁰). The EC⁵⁰for propofol and nitrous oxide alone (without alfentanil) was 6.1 [micro sign]g/ml. The predicted EC⁵⁰for alfentanil alone with nitrous oxide (without propofol) was 194 ng/ml. The plasma concentrations utilized in the analysis were those measured in samples taken at 15 min just before nerve stimulation. If both 10- and 15-min samples were obtained, a mean concentration was used for the two samples. If only one sample was obtained (either 10- or 15-min sample), only one value was used in the determination. Blood samples were not obtained at 15 min if patient movement necessitated bolus administration of propofol within the preceding 10 min. The regression obtained assuming additivity (represented by the dotted line) had an R²value of 0.47, P = 0.0003. The regression obtained by acknowledging propofol-alfentanil interaction (synergy, represented by the solid line) had an R²value of 0.55, P = 0.0002.