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

Abstract

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 (CRI) of remifentanil (0.15, 0.30, 0.60, and 0.90 μg/kg/min), and 80 minutes after remifentanil infusion. After a 1-week washout period, dogs received a CRI of remifentanil (0.15 μg/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 μg/kg/min) decreased ISOMAC by 43 ± 10%, 59 ± 10%, 66 ± 9%, and 71 ± 9%, respectively. The ISO MAC values determined during the 0.30, 0.60, and 0.90 μg/kg/min infusion rates did not differ from each other, but these values were significantly lower, compared with the 0.15 μg/kg/min infusion rate. The ISOMAC recorded after remifentanil infusion (1.09 ± 0.18%) did not differ from baseline ISOMAC. There was no change in ISO MAC 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 μg of remifentanil/kg/min resulted in nearly maximal isoflurane-sparing effect in dogs; a ceiling effect was observed at higher infusion rates.