Figure 12-12 Response surface for each of the paired interactions between propofol and midazolam (A), alfentanil and midazolam (B), and alfentanil and propofol (C) on the probability of opening eyes to a verbal command. The isoboles for a 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% response are shown. (From Minto CF, Schnider TW, Short TG, et al: Response surface model for anesthetic drug interactions. Anesthesiology 92:1603–1616, 2000.)