ABSENCE OF NONSHIVERING THERMOGENESIS IN ANESTHETIZED ADULT HUMANS

Background. Typically, core temperature rapidly decreases after induct ion of anesthesia, but reaches a stable plateau after several hours. T his plateau typically occurs in conjunction with the onset of thermore gulatory vasoconstriction. Decreased heat loss, caused by vasoconstric tion, may not be sufficient to establish thermal steady state without a concomitant increase in heat production. Accordingly, the authors te sted the hypothesis that nonshivering thermogenesis contributes to the rmal steady state during anesthesia. Rewarming from hypothermia is oft en associated with an afterdrop (a further reduction in core temperatu re, despite cutaneous warming). Because total body heat content increa ses during cutaneous warming, heat storage during afterdrop must refle ct increased temperature and heat content of the peripheral tissue mas s. Thermal balance was measured during rewarming to estimate the therm al capacity of the peripheral tissues. Methods: Five volunteers were a nesthetized with isoflurane and paralyzed with vecuronium. Oxygen cons umption was measured during cooling to a core temperature at least 1-d egrees-C less than that which triggered vasoconstriction. Volunteers w ere subsequently rewarmed using a circulating-water blanket and forced -air warmer. Oxygen consumption and cutaneous heat flux were measured to assess thermal balance and peripheral tissue heat storage during re warming. Results. The core temperature threshold for vasoconstriction was 35.2 +/- 0.8-degrees-C. Oxygen consumption decreased 9 +/- 5%/degr ees-C during active cooling before vasoconstriction and 9 +/- 3%/degre es-C after vasoconstriction. After the start of rewarming, core temper ature continued to decrease for an additional 32 +/- 8 min. The magnit ude of this afterdrop was 0.6 +/- 0.1-degrees-C. Peripheral tissue hea t storage measured from the start of re-warming until the first net in crease in core temperature was 144 +/- 60 kcal, which approximately eq uals 2 h of resting metabolic heat production. Conclusions: The author s concluded that nonshivering thermogenesis is not an important thermo regulatory response in adults anesthetized with isoflurane. Afterdrop and delayed core temperature recovery during rewarming reflect the lar ge heat storage capacity of peripheral tissues.