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Hypothermia is a core temperature < 35 to 35.5° C. The condition may be purely environmental or represent intercurrent illness. Treatment is rewarming.
Thermal equilibrium is affected by relative humidity, air flow, proximity of cold surfaces, and ambient air temperature. Neonates are particularly prone to rapid heat loss and consequent hypothermia because of a high surface area to volume ratio, which is particularly high in low-birth-weight neonates. Radiant heat loss occurs when bare skin is exposed to an environment containing objects of cooler temperature. Evaporative heat loss occurs when neonates are wet with amniotic fluid. Conductive heat loss occurs when the neonate is placed in contact with a cool surface or object. Hypothermia also may be caused by pathologic conditions that impair thermoregulation (eg, sepsis, intracranial hemorrhage).
Pathophysiology
Prolonged, unrecognized cold stress may divert calories to produce heat, impairing growth. Neonates respond to cooling by sympathetic nerve discharge of norepinephrine in the brown fat. This specialized tissue of the neonate, located in the nape of the neck, between the scapulae, and around the kidneys and adrenals, responds by lipolysis followed by oxidation or re-esterification of the fatty acids that are released. These reactions produce heat locally, and a rich blood supply to the brown fat helps transfer this heat to the rest of the neonate's body. This reaction increases the metabolic rate and O2 consumption 2- to 3-fold. Thus, in neonates with respiratory insufficiency (eg, the preterm infant with respiratory distress syndrome), cold stress may also result in tissue hypoxia and neurologic damage. Additionally, hypothermia can result in hypoglycemia, metabolic acidosis, and death.
Treatment
Hypothermia is treated by rewarming in an incubator or under a radiant warmer. The neonate should be monitored and treated as needed for hypoglycemia, hypoxemia, and apnea. Underlying conditions such as sepsis or intracranial hemorrhage require specific treatment.
Prevention
Hypothermia can be prevented by immediately drying and then swaddling the neonate (including the head) in a warm blanket. A neonate exposed for resuscitation or observation should be placed under a radiant warmer. Sick neonates should be maintained in a neutral thermal environment to minimize the metabolic rate. The proper incubator temperature varies depending on the neonate's birth weight and postnatal age. Alternatively, heating can be adjusted with a servomechanism set to maintain skin temperature at 36.5° C.
Last full review/revision March 2007 by James W. Kendig, MD
Content last modified March 2007
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