Cardiovascular and respiratory effects of two doses of fentanyl in the presence or absence of bradycardia in isoflurane-anesthetized dogs.

Abstract

OBJECTIVE: To compare the cardiopulmonary effects of low and high doses of fentanyl before and after the correction of bradycardia in isoflurane-anesthetized dogs. STUDY DESIGN: Prospective, randomized crossover trial. ANIMALS: Eight healthy male Beagle dogs weighing 11.1 ± 1.3 kg [mean ± standard deviation (SD)] and aged approximately 1 year. METHODS: The dogs were anesthetized with isoflurane [1.3 × minimum alveolar concentration (MAC)] on two occasions and fentanyl was administered intravenously; either low-dose fentanyl, loading dose (33 μg kg) and infusion (0.2 μg kgminute) or a high-dose, loading dose (102 μg kg) and infusion (0.8 μg kgminute). Cardiopulmonary variables were measured at three time points in equipotent isoflurane concentrations (1.3 MAC): before fentanyl administration (ISO), during fentanyl-induced bradycardia (ISO-F) and after administration of glycopyrrolate normalized heart rate (ISO-F). Data are mean ± SD. RESULTS: Heart rate and cardiac index (CI) decreased and systemic vascular resistance index (SVRI) increased at ISO-F in both treatments. Bradycardia and vasoconstriction at ISO-F were greater in high than in low-dose fentanyl (42 ± 7 versus 57 ± 15 beats minuteand 3457 ± 1108 versus 2528 ± 968 dyne second cmm), respectively. Oxygen delivery index (DOI) decreased only during high-dose fentanyl. CI and DOI were higher in both treatments at ISO-Fthan at ISO-F; however, they were higher only during the high-dose fentanyl than at ISO. SVRI was higher at ISO-F than at ISO and ISO-Fin both treatments, and was higher at ISO-F in the high than in the low-dose treatment. CONCLUSIONS AND CLINICAL RELEVANCE: An overall improvement in cardiovascular function of dogs anesthetized with equipotent isoflurane doses (1.3 MAC) was observed after the treatment of bradycardia only with the high-dose fentanyl.