Effect of fentanyl, with or without treatment of bradycardia, on the minimum alveolar concentration of isoflurane and cardiovascular function in dogs.

Abstract

OBJECTIVE: To determine the effect of fentanyl on the minimum alveolar concentration of isoflurane (MAC) and cardiovascular variables in dogs, and how the treatment of bradycardia affects them. STUDY DESIGN: Prospective, randomized crossover-controlled trial. ANIMALS: A total of six male Beagle dogs weighing 9.9 &#xb1; 0.7 kg (mean &#xb1; standard deviation) and aged 13 months. METHODS: To each dog, two treatments were assigned on different days: fentanyl (FENTA) or fentanyl plus glycopyrrolate (FENTA) to maintain heart rate (HR) between 100 and 132 beats minute. Determinations of MACwere performed with 10 plasma fentanyl target concentrations ([Fenta](0, 0.16, 0.32, 0.64, 1.25, 2.5, 5.0, 10.0, 20.0 and 40.0 ng mL) for FENTA and 5 [Fenta](0, 1.25, 2.5, 5.0, 10.0 ng mL)) for FENTA. During each MACdetermination, cardiovascular variables [mean arterial pressure (MAP), HR and cardiac index (CI)] were measured, and systemic vascular resistance index (SVRI) calculated. Pharmacodynamic models were used to describe the plasma fentanyl concentration [Fenta]-response relationship for the effect on MACand cardiovascular variables. A mixed-model analysis of variance followed by Dunnett's or Tukey's test, and the Bonferroni adjustment were used for comparisons within and between each treatment, respectively. Significance was set as p < 0.05. RESULTS: Fentanyl decreased MACby a maximum of 84%. The [Fenta] producing 50% decrease in MAC, HR and CI were 2.64, 3.65 and 4.30 ng mL(typical values of population model), respectively. The prevention of fentanyl-mediated bradycardia caused no significant effect on MAC, but increased HR, MAP and CI, and decreased SVRI when compared with isoflurane alone. CONCLUSIONS AND CLINICAL RELEVANCE: Fentanyl caused a plasma concentration-dependent decrease in MAC, HR and CI and an increase in SVRI. Cardiovascular improvements associated with fentanyl in isoflurane-anesthetized dogs only occurred when the fentanyl-mediated bradycardia was prevented.