Effect of fentanyl on the induction dose and minimum infusion rate of alfaxalone preventing movement in dogs

Objective: To determine the effect of fentanyl on the induction dose and minimum infusion rate of alfaxalone required to prevent movement in response to a noxious stimulus (MIR_NM) in dogs. Study design: Experimental crossover design. Animals: A group of six healthy, adult, intact female mixed-breed dogs, weighing 19.7 ± 1.3 kg. Methods: Dogs were randomly administered one of three treatments at weekly intervals: premedication with 0.9% saline (treatment A), fentanyl 5 μg kg^–1 (treatment ALF) or fentanyl 10 μg kg^–1 (treatment AHF), administered intravenously over 5 minutes. Anesthesia was induced 5 minutes later with incremental doses of alfaxalone to achieve intubation and was maintained for 90 minutes in A with alfaxalone (0.12 mg kg^–1 minute^–1), in ALF with alfaxalone (0.09 mg kg^–1 minute^–1) and fentanyl (0.1 μg kg^–1 minute^–1) and in AHF with alfaxalone (0.06 mg kg^–1 minute^–1) and fentanyl (0.2 μg kg^–1 minute^–1). The alfaxalone infusion was increased or decreased by 0.006 mg kg^–1 minute^–1 based on positive or negative response to antebrachium stimulation (50 V, 50 Hz, 10 ms). Data were analyzed using a mixed-model ANOVA and presented as least squares means ± standard error. Results: Alfaxalone induction doses were 3.50 ± 0.13 (A), 2.17 ± 0.10 (ALF) and 1.67 ± 0.10 mg kg^–1 (AHF) and differed among treatments (p < 0.05). Alfaxalone MIR_NM was 0.17 ± 0.01 (A), 0.10 ± 0.01 (ALF) and 0.07 ± 0.01 mg kg^–1 minute^–1 (AHF) and differed among treatments. ALF and AHF decreased the MIR_NM by 44 ± 8% and 62 ± 5%, respectively (p < 0.05). Plasma alfaxalone concentrations at MIR_NM were 5.82 ± 0.48 (A), 4.40 ± 0.34 (ALF) and 2.28 ± 0.09 μg mL^–1 (AHF). Conclusions and clinical relevance: Fentanyl, at the doses studied, significantly decreased the alfaxalone induction dose and MIR_NM.