The effect of remifentanil on the minimum alveolar concentration (MAC) and MAC derivatives of sevoflurane in dogs

Remifentanil is an ultra-short-acting µ-opioid receptor agonist. The purpose of this study was to determine the relationship of the minimum alveolar concentration (MAC) of sevoflurane and other MAC derivatives, including the MAC for blocking adrenergic responses (MAC-BAR) and the MAC at which tracheal extubation is occurred (MAC-extubation), with or without remifentanil infusion. Six healthy adult beagle dogs were randomly anesthetized three times for determining the MAC-BAR (SEV(MAC-BAR)), MAC (SEV(MAC)), and MAC- extubation (SEV(MAC-extubation)) of sevoflurane under infusion of saline and remifentanil at rates of 0.15, 0.30, 0.60, 1.20, and 2.40 µg/kg/min. The ratio of the SEV(MAC-BAR) and SEV(MAC) and that of the SEV(MAC-extubation) and SEV(MAC) were not significantly different at baseline and during treatment. The MAC-BAR(95) and MAC(95) decreased in a dose-dependent manner until reaching 1.20 µg/kg/min, and the MAC-extubation(5) decreased in a dose-dependent manner until reaching 0.60 µg/kg/min. The percentage reduction of SEV(MAC-BAR), SEV(MAC), and SEV(MAC-extubation) increased in a dose-dependent manner during remifentanil infusion. The heart rate significantly decreased in the MAC-BAR and MAC groups, and the systolic and mean arterial pressures increased after remifentanil infusion compared with the baseline values. Remifentanil infusion caused reduction of the SEV(MAC-BAR), SEV(MAC), and SEV(MAC-extubation) in a dose-dependent manner, and ceiling effects were observed in the dogs. Higher doses of remifentanil and sevoflurane were necessary for blocking the sympathetic response to the supramaximal stimulus to prevent movement and extubation in dogs.