Figure 3-38 Influence of fentanyl on the minimum alveolar concentration (MAC) of isoflurane associated with a 50% probability of movement on incision. Modest amounts of fentanyl greatly reduce the MAC of isoflurane, but a ceiling in MAC reduction is quickly reached. Even with large opioid doses, some isoflurane is required to ablate the movement response. (Adapted from McEwan AI, Smith C, Dyar O, et al: Isoflurane minimum alveolar concentration reduction by fentanyl. Anesthesiology 78:864–869, 1993.)