Figure 3-40 Response surfaces for potential pharmacodynamic interactions of anesthetic drugs. A, Additive interaction between two agonists that have the same mechanism of action (e.g., fentanyl and alfentanil). B, Supra-additive interaction between two agonists (e.g., isoflurane and fentanyl). C, Infra-additive interaction between two agonists (i.e., reported for cyclopropane and nitrous oxide). D, Partial agonist and full agonist (e.g., hypothetical interaction of nalbuphine and fentanyl). E, Competitive antagonist and full agonist (e.g., naloxone and fentanyl). F, Inverse agonist and full agonist (e.g., R0 19-4063 and midazolam).