Anesthetic Potency of Remifentanil in Dogs

BACKGROUND: Remifentanil is an opioid that is rapidly inactivated by esterases in blood and tissues. This study examined the anesthetic potency and efficacy of remifentanil in terms of its reduction of enflurane minimum alveolar concentration (MAC) in dogs. METHODS: Twenty-five dogs were anesthetized with enflurane. One group received incremental infusion rates of remifentanil from 0.055 to 5.5 micrograms x kg(-1). A second group received constant rate infusions of remifentanil of 1.0 micrograms x kg(-1) x min(-1) for 6-8 h. Enflurane MAC was measured before, hourly during remifentanil infusion, and at the end of the experiment after naloxone administration. A third group received alternating infusions of 0.5 and 1.0 micrograms x kg(-1) x min(-1) with MAC determinations made 30 min after each change in the infusion rate. Heart rate, mean arterial pressure, and remifentanil blood concentrations were measured during MAC determinations. RESULTS: Enflurane MAC was reduced up to a maximum of 63 +/- 10.4% (mean +/- SD) in a dose-dependent manner by remifentanil infusion. The dose producing a 50% reduction in the enflurane MAC was calculated as 0.72 micrograms x kg(-1)x min(-1) and the corresponding blood concentration was calculated as 9.2 ng/ml. Enflurane MAC reduction remained stable during continuous, constant rate infusions for periods of 6-8 h without any signs of tolerance. Recovery of enflurane MAC to baseline occurred in 30 min (earliest measurement) after stopping the remifentanil infusion. CONCLUSIONS: Remifentanil is equally efficacious and about half as potent as fentanyl, judging from the blood concentrations causing equivalent reductions in enflurane MAC in the dog. The characteristics of MAC reduction are similar to those of other opioids, including the ceiling effect. Recovery from remifentanil anesthesia is much more rapid than for any other opioid studied to date, especially after continuous infusions maintained for 6 or more h.