INTERACTION OF FENTANYL AND NITROUS-OXIDE ON PERIPHERAL AND CEREBRAL HEMODYNAMICS IN NEWBORN LAMBS

Background: The ability of opioids to produce complete general anesthesia is controversial. Nitrous oxide (N2O) is often added to fentanyl-based anesthetics to produce unconsciousness and amnesia. The addition of N2O may adversely affects fentanyl's hemodynamic stability and safety. The purpose of this study was to determine the physiologic consequences of combining N2O with fentanyl in newborn animals. Methods: The effects of 50% nitrous oxide in oxygen (O-2), and 50% N2O in O-2 combined with 3,000 mu g/kg fentanyl, on cerebral and peripheral hemodynamics were studied in seven unanesthetized newborn lambs, in whom catheters were previously inserted. After a control period, lambs were placed in a hood in which inspired gas concentrations were controlled and which minimized external stimuli. After 30 min of breathing room air, the lambs breathed 50% N2O in O-2 for an additional 30 min. The lambs were then given 3,000 mu g/kg fentanyl by intravenous bolus and by infusion (1,000 pg kg(-1) h(-1)) for 60 min while continuing to breathe 50% N2O in O-2. Results: All animals responded to pain (tail clamp) and alerted to sound when breathing room air or when N2O was used alone. Adding fentanyl to the N2O abolished ah responses to pain, but not to sound. Additionally, fentanyl produced immediate apnea necessitating tracheal intubation and mechanical ventilation. Mean arterial pressure and heart rate increased 27% and 23%, respectively, after fentanyl administration, intubation, and ventilation. It did not change over the course of the fentanyl infusion. Cerebral blood flow, O-2 consumption, and O-2 delivery did not change when N2O was administered alone or in combination with fentanyl. Splanchnic blood how was unaffected by treatment over time. Renal blood now decreased by 21% after fentanyl administration, but was unaffected by N2O alone. Right and left ventricular blood how increased (47% and 26%, respectively) after fentanyl administration, intubation, and ventilation, but not when N2O was administered alone. Conclusions: Fentanyl (3,000 mu g/kg) when combined with 50% N2O in O-2 produced a plane of general anesthesia in newborn lambs in which the behavioral responses to painful stimuli were abolished. The response to sound was never eliminated, nor was cerebral oxygen consumption decreased. The combination of 50% N2O in O-2, 3,000 mu g/kg fentanyl, tracheal intubation, and mechanical ventilation did not depress heart rate, blood pressure, or blood now to any of the major organs, except the kidneys.