Effects of fentanyl and nitrous oxide on contractility of blood-perfused papillary muscle of the dog

The effects of fentanyl, nitrous oxide, and their combination on myocardial contractility were investigated in the papillary muscle preparation perfused by a donor dog. With a conscious donor, fentanyl infused directly into the arterial blood perfusing the papillary muscle produced a dose-related depression of developed tension. However, blood concentrations of fentanyl required to obtain the depression were in the range of 30-120 micrograms/ml. The ED50 for fentanyl for suppression of papillary muscle contractility was 89 +/- 9 micrograms/ml. When the donor dog was given nitrous oxide (N2O,80% and O2,20%), the developed tension of the papillary muscle decreased 25 +/- 5%. Fentanyl administered during nitrous oxide anesthesia caused a decrease in developed tension that was not significantly different from that obtained without N2O anesthesia (18 +/- 4% vs 13 +/- 4% for 30 micrograms/ml, and 61 +/- 5% vs 58 +/- 4% for 100 micrograms/ml). The results suggest that fentanyl produces a direct negative inotropic effect only in concentrations that are 2-3 orders of magnitude higher than its blood concentrations in fentanyl-induced anesthesia. When fentanyl and nitrous oxide are used together their interaction is not significantly different from additive.