ANESTHETIC POTENCY OF REMIFENTANIL IN DOGS

Background: Remifentanil is an opioid that is rapidly inactivated by e sterases in blood and tissues, This study examined the anesthetic pote ncy and efficacy of remifentanil in terms of its reduction of enfluran e minimum alveolar concentration (MAC) in dogs. Methods: Twenty-five d ogs were anesthetized with enflurane. One group received incremental i nfusion rates of remifentanil from 0.055 to 5.5 mu g . kg(-1). min(-1) . A second group received constant rate infusions of remifentanil of 1 .0 mu g . kg(-1). min(-1) for 6-8 h, Enflurane MAC was measured before , hourly during remifentanil infusion, and at the end of the experimen t after naloxone administration, A third group received alternating in fusions of 0.5 and 1.0 mu g . kg(-1). min(-1) with MAC determinations made 30 min after each change in the infusion rate, Heart rate, mean a rterial pressure, and remifentanil blood concentrations were measured during MAC determinations. Results: Enflurane MAC was reduced up to a maximum of 63.0 +/- 10.4% (mean +/- SD) in a dose-dependent manner by remifentanil infusion, The dose producing a 50% reduction in the enflu rane MAC was calculated as 0.72 mu g . kg(-1). min(-1) and the corresp onding blood concentration was calculated as 9.2 ng/ml. Enflurane MAC reduction remained stable during continuous, constant rate infusions f or periods of 6-8 h without any signs of tolerance, Recovery of enflur ane MAC to baseline occurred in 30 min (earliest measurement) after st opping the remifentanil infusion. Conclusions: Remifentanil is equally efficacious and about half as potent as fentanyl, judging from the bl ood concentrations causing equivalent reductions in enflurane MAC in t he dog, The characteristics of MAC reduction are similar to those of o ther opioids, including the ceiling effect, Recovery from remifentanil anesthesia is much more rapid than for any other opioid studied to da te, especially after continuous infusions maintained for 6 or more h.