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.