Cardiovascular dynamics after large doses of fentanyl and fentanyl plus N2O in the dog.

The effects of large doses of fentanyl (0.05 to 2 mg/kg) and fentanyl plus N2O on cardiovascular dynamics were determined in 10 unpremedicated dogs breathing 100% O2. Using computer analysis of the central aortic pulsepressure curve, stroke volume (SV), cardiac output, heart rate (HR), peripheral vascular resistance (PVR), and systolic, diastolic, and mean arterial blood pressures (BP) were determined while fentanyl was being given at a rate of 0.3 to 0.44 mg/min. Fentanyl caused a dose-related decrease in HR, which was significant at 0.05 mg/kg. Cardiac output, PVR, and systolic, diastolic, and mean arterial BP were also decreased and SV increased. The latter changes became significant at 0.1 mg/kg for diastolic BP; 0.15 mg/kg for cardiac output and mean BP; 0.25 mg/kg for sv and systolic BP; and at 1.25 mg/kg for peripheral vascular resistance. Addition of N2O after fentanyl did not significantly change any parameter, although SV, cardiac output, and HR were usually increased and PVR decreased. These data demonstrate that, while large doses of fentanyl or fentanyl plus N2O do alter cardiovascular dynamics in dogs, the changes appear to be less profound than those produced by equianalgesic doses of morphine. Our findings suggest that large doses of fentanyl-O2 may be an attractive alternative to morphine-O2 anethesia in critically ill patients.