The interaction of nitrous oxide and fentanyl on the minimum alveolar concentration of sevoflurane blocking motor movement (MACNM) in dogs

The study objective was to determine the effects of 70% nitrous oxide (N₂O) and fentanyl on the end-tidal concentration of sevoflurane necessary to prevent movement (MAC_NM) in response to noxious stimulation in dogs. Six healthy, adult, intact male, mixed-breed dogs were used on 3 occasions in a randomized crossover design. After induction of anesthesia with sevoflurane, each of the following treatments was randomly administered: fentanyl loading dose (Ld) of 15 μg/kg and infusion of 6 μg/kg per hour [treatment 1 (T1)], 70% N₂O (T2), or fentanyl (Ld of 15 μg/kg and infusion of 6 μg/kg per hour) combined with 70% N₂O (T3). Each dog received each of the 3 treatments once during the 3-week period. Determination of MAC_NM was initiated 90 min after the start of each treatment. The values were compared using the baseline MAC_NM, which had been determined in a previous study on the same group of dogs. Data were analyzed using a mixed-model analysis of variance (ANOVA) and Tukey-Kramer tests, and expressed as least squares mean ± SEM. The baseline MAC_NM decreased by 36.6 ± 4.0%, 15.0 ± 4.0%, and 46.0 ± 4.0% for T1, T2, and T3, respectively (P < 0.05), and differed (P < 0.05) among treatments. Mean fentanyl plasma concentrations did not differ (P ≥ 0.05) between T1 (3.70 ± 0.56 ng/mL) and T3 (3.50 ± 0.56 ng/mL). The combination of fentanyl and N₂O resulted in a greater sevoflurane MAC_NM sparing effect than either treatment alone.