Relative Analgesic Potencies

Abstract

Relative analgesic potencies (APs) of an opioid can be defined as the inverse ratios of the amounts of opioids that produce equal effects in a specific test procedure. For example, 3.21 mg morphine and 0.011 mg fentanyl are required intravenously to produce the same endpoint in the rat tail flick withdrawal test from 55° C water. The relative intravenous fentanyl analgesic potency, therefore, calculated as dose ratio of intravenous morphine to intravenous fentanyl, is 292 (Van Beever et al. 1976). The relative intravenous fentanyl analgesic potency to morphine is 480 when the rabbit tooth-pulp test is used (Kutter et al. 1970). In anesthetized dogs, in which selected hemodynamic parameters are observed under standard conditions, the relative intravenous fentanyl to morphine analgesic potency is 125 (DeCastro et al. 1979). After abdominal operations patients require a mean of 0.083 mg fentanyl/h, 2.7 mg morphine/h and 26 mg meperidine/h during the first hours of patient-controlled intravenous analgesia (Gourlay et al. 1988; Tamsen et al. 1982). This indicats that with this type of treatment of postoperative pain the average relative analgesic potency of fentanyl is 30 and 2506 against the reference opioids morphine and meperidine, respectively. Mean hourly postoperative epidural meperidine requirements average 10.7 mg (Zaren et al. 1984). The dose ratio of intravenous meperidine and epidural meperidine is 2.4, indicating that epidural meperidine is about 2 1/2 times more potent than intravenous meperidine. As the above examples demonstrate, for AP values to be meaningful it is important that the test procedure used, the routes of opioid administration and the reference opioid all be specified unambiguously.