LACK OF NONSHIVERING THERMOGENESIS IN INFANTS ANESTHETIZED WITH FENTANYL AND PROPOFOL

Background: Sweating, vasoconstriction, and shivering have been observ ed during general anesthesia. Among these, vasoconstriction is especia lly important because--once triggered--it minimizes further hypothermi a. Surprisingly, the core-temperature plateau associated with vasocons triction appears to preserve core temperature better in infants and ch ildren than adults. This observation suggests that vasoconstriction in anesthetized infants maybe accompanied by hypermetabolism. Consistent with this theory, unanesthetized infants rely on nonshivering thermog enesis to double heat production when vasoconstriction alone is insuff icient. Accordingly, the authors tested the hypothesis that intraopera tive core hypothermia triggers nonshivering thermogenesis in infants. Methods: With Ethics Committee approval and written parental consent, the authors studied six infants undergoing abdominal surgery. All were aged 1 day to 9 months and weighed 2.4-9 kg. Anesthesia was maintaine d with propofol and fentanyl The infants were mechanically ventilated and allowed to cool passively until core (distal esophageal) temperatu res reached 34-34.5 degrees C. Oxygen consumption--the authors' index of metabolic rate--was recorded throughout cooling. Because nonshiveri ng thermogenesis triples circulating norepinephrine concentrations, ar terial blood was analyzed for plasma catecholamines at approximate to 0.5 degrees C intervals. Thermoregulatory vasoconstriction was evaluat ed using forearm - fingertip, skin-surface gradients, with gradients e xceeding 4 degrees C, indicating intense vasoconstriction. The patient s were subsequently rapidly rewarmed to 37 degrees C. Regression analy sis was used to correlate changes in oxygen consumption and plasma cat echolamine concentrations with core temperature. Results: All patients were vasoconstricted by the time core temperature reached 36 degrees C. Further reduction in core temperature to 34-34.5 degrees C did not increase oxygen consumption. Instead, oxygen consumption decreased lin early. Hypothermia also failed to increase plasma catecholamine concen trations. Conclusions: Even at core temperatures approximate to 2 degr ees C below the vasoconstriction threshold, there was no evidence of n onshivering thermogenesis. This finding is surprising because all othe r major thermoregulatory responses have been detected during anesthesi a. Infants and children thus appear similar to adults in being unable to increase metabolic rate in response to mild intraoperative hypother mia.