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) 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 (treatment ALF) or fentanyl 10 μg kg (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 minute), in ALF with alfaxalone (0.09 mg kg minute) and fentanyl (0.1 μg kg minute) and in AHF with alfaxalone (0.06 mg kg minute) and fentanyl (0.2 μg kg minute). The alfaxalone infusion was increased or decreased by 0.006 mg kg minute 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 (AHF) and differed among treatments (p < 0.05). Alfaxalone MIR was 0.17 ± 0.01 (A), 0.10 ± 0.01 (ALF) and 0.07 ± 0.01 mg kg minute (AHF) and differed among treatments. ALF and AHF decreased the MIR by 44 ± 8% and 62 ± 5%, respectively (p < 0.05). Plasma alfaxalone concentrations at MIR were 5.82 ± 0.48 (A), 4.40 ± 0.34 (ALF) and 2.28 ± 0.09 μg mL (AHF).

Conclusions And Clinical Relevance: Fentanyl, at the doses studied, significantly decreased the alfaxalone induction dose and MIR.