Figure 31-18 A, Relationship between the plasma concentration of alfentanil and response/no response at three specific events of short duration. The quantal data are characterized by logistic regression in the lower panel, ±SE for the Cp₅₀ , which is the alfentanil plasma concentration associated with a 50% probability of no response. (From Ausems ME, Hug CC Jr, Stanski DR, et al: Plasma concentrations of alfentanil required to supplement nitrous oxide anesthesia for general surgery. Anesthesiology 65:362, 1986.) B, Plasma concentration of alfentanil versus the probability of no response for each of 34 patients during the intra-abdominal phase of lower abdominal surgery. Dots represent the Cp₅₀ values, and the heavy dark line represents the average response of the 34 patients. (From Ausems ME, Vuyk J, Hug CC Jr, et al: Comparison of computer-assisted infusion versus intermittent bolus administration of alfentanil as a supplement to nitrous oxide for lower abdominal surgery. Anesthesiology 68:851, 1988.)