Interaction of fentanyl and pentobarbital on peripheral and cerebral hemodynamics in newborn lambs

The effects of 3.0 mg · kg^-1 fentanyl on cerebral and peripheral hemodynamics, alone and when combined with subanesthetic doses of pentobarbital (4.0 mg · kg^-1), were studied in 11 unanesthetized, newborn lambs, in whom catheters had been previously inserted. After a control period, drugs were administereed at 20-min intervals by intravenous bolus injection. Group 1 animals (n = 5) received fentanyl, pentobarbital, and naloxone (0.01 mg · kg^-1), whereas Group 2 animals (n = 6) had the order of fentanyl and pentobarbital reversed. All animals responded to pain (withdrawal to tail clamping) and appeared conscious (eyes open, alert to sound) when either fentanyl or barbiturate was given alone. The combination of drugs, however, produced complete unresponsiveness. All of these effects were reversed by naloxone. Cardiac output did not change after either fentanyl or pentobarbital was administereed individually but decreased significantly (29% in Group 1, 21% in Group 2) after administration of the combination of both. Mean arterial pressure and heart rate were unchanged. Cerebral blood flow, oxygen (O₂) transport, and O₂ consumption did not change after either administration of fentanyl or pentobarbital alone but decreased significantly after both (22%, 30%, 19%, respectively, in Group 1 and 35%, 40%, 38%, respectively, in Group 2). The decrease in cerebral O₂ transport nearly paralleled the decrease in cerebral O₂ consumption such that the ratio, the fractional O₂ extraction, increased slightly. Fentanyl decreased kidney blood flow alone (24%) and in combination with pentobarbital (25%), although pentobarbital did so only when combined with fentanyl. Neither drug affected blood flow to the stomach or to the small or large intestines when given alone but did decrease flow significantly when combined (27%, 27%, 34%, respectively, in Group 1; and 36%, 32%, 21%, respectively, in Group 2). Thus, blood flow to major organs may not be sustained at normal levels when fentanyl is combined with subanesthetic doses of pentobarbital.