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

Objective To compare the cardiopulmonary effects of low and high doses of fentanyl before and after the correction of bradycardia in isofluraneanesthetized 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 x minimum alveolar concentration (MAC)] on two occasions and fentanyl was administered intravenously; either low-dose fentanyl, loading dose (33 mu g kg(-1)) and infusion (0.2 mg kg(-1) minute(-1)) or a high-dose, loading dose (102 mg kg(-1)) and infusion (0.8 mg kg(-1) minute(-1)). 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-NHR). 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 minute(-1) and 3457 +/- 1108 versus 2528 +/- 968 dyne second cm(-5) m(-2)), respectively. Oxygen delivery index (DO2I) decreased only during high-dose fentanyl. CI and DO2I were higher in both treatments at ISO-F-NHR than 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-F-NHR in 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 brady-cardia only with the high-dose fentanyl.