Effects of remifentanil on the minimum alveolar concentration of isoflurane in dogs

Objective-To evaluate the effects of remifentanil on isoflurane minimum alveolar concentration (ISOMAC) in dogs. Animals-6 adult mixed-breed dogs. Procedures-Dogs were anesthetized with isoflurane on 2 occasions. During the first set of experiments, ISOMAC was determined before remifentanil infusion (baseline), during constant rate infusion (M of remifentanil (0.15, 0.30, 0.60, and 0.90 mu g/kg/min), and 80 minutes after remifentanil infusion. After a 1-week washout period, dogs received a CRI of remifentanil (0.15 mu g/kg/min) and ISOMAC was redetermined 2, 4, and 6 hours after commencing the infusion. Results-Mean +/- SD baseline ISOMAC was 1.24 +/- 0.18%. Remifentanil infusion (0.15, 0.30, 0.60, and 0.90 mu g/kg/min) decreased ISOMAC by 43 +/- 10%, 59 +/- 10%, 66 +/- 9%, and 71 +/- 9%, respectively. The ISOMAC values determined during the 0.30, 0.60, and 0.90 mu g/kg/min infusion rates did not differ from each other, but these values were significantly lower, compared with the 0.15 mu g/kg/min infusion rate. The ISOMAC recorded after remifentanil infusion 0.09 +/- 0.18%) did not differ from baseline ISOMAC. There was no change in ISOMAC throughout the 6-hour period of a CRI of remifentanil. Conclusions and Clinical Relevance-Remifentanil decreased ISOMAC in a dose-related fashion; the reduction in ISOMAC was stable over the course of a prolonged CRI (6 hours). A dose of 0.30 mu g of remifentanil/kg/min resulted in nearly maximal isoflurane-sparing effect in dogs; a ceiling effect was observed at higher infusion rates. (Am J Vet Res 2010;71:150156)