Figure 12-9 Interaction of isoflurane and fentanyl in preventing a somatic response at skin incision (i.e., minimum alveolar concentration [MAC] reduction of isoflurane). The solid line represents the concentrations of isoflurane and fentanyl, when administered together, that are required to prevent purposeful movement in 50% of patients at skin incision. The dashed lines represent the 95% confidence interval of the MAC at each combination of fentanyl and isoflurane. (From McEwan Al, Smith C, Dyar O, et al: Isoflurane MAC reduction by fentanyl. Anesthesiology 78:864–869, 1993.)