Figure 12-11 Relationship between a response surface and a standard isobologram. Conventional "isobolographic" analysis, whether for doses or concentrations, describes only the concentrations of both drugs that yield a 50% drug effect and thus fails to capture the entire response surface. (From Minto CF, Schnider TW, Short TG, et al: Response surface model for anesthetic drug interactions. Anesthesiology 92:1603–1616, 2000.)