Hypothermia is a core temperature < 35 to 35.5° C. The condition may be purely environmental or represent intercurrent illness. Treatment is rewarming and correction of the cause.
Normal rectal temperature in term and preterm infants is 36.5 to 37.5° C. Although hypothermia is a core temperature < 35 to 35.5° C, there is cold stress at higher temperatures whenever heat loss requires an increase in metabolic heat production.
Thermal equilibrium is affected by relative humidity, air flow, proximity of cold surfaces, and ambient air temperature. Neonates are prone to rapid heat loss and consequent hypothermia because of a high surface area to volume ratio, which is even higher 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 neonates are placed in contact with a cool surface or object. Convective heat loss occurs when a flow of cooler ambient air carries heat away from the neonate.
Prolonged, unrecognized cold stress may divert calories to produce heat, impairing growth. Neonates have a metabolic response to cooling that involves chemical (nonshivering) thermogenesis 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. Activation of glycogen stores can cause transient hyperglycemia. Persistent hypothermia can result in hypoglycemia and metabolic acidosis and increases the risk of late-onset sepsis and mortality.
Despite their compensatory mechanisms, neonates, particularly low-birth-weight infants, have limited capacity to thermoregulate and are prone to decreased core temperature. Even before temperature decreases, cold stress occurs when heat loss requires an increase in metabolic heat production. The neutral thermal environment (thermoneutrality) is the optimal temperature zone for neonates; it is defined as the environmental temperature at which metabolic demands (and thus calorie expenditure) to maintain body temperature in the normal range (36.5 to 37.5° C rectal) are lowest. The neutral thermal environment has a narrow range from 36.7° to 37.3° C.
Hypothermia may be caused by environmental factors, disorders that impair thermoregulation (eg, sepsis [see Neonatal Sepsis], intracranial hemorrhage [see Intracranial Hemorrhage], drug withdrawal [see Prenatal Drug Exposure]), or a combination. Risk factors for hypothermia include maternal hypertension (see Hypertension in Pregnancy), cesarean delivery (see Cesarean Delivery), and low Apgar scores (see Assessment).
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, drug withdrawal, or intracranial hemorrhage require specific treatment.
Hypothermia can be prevented by immediately drying and then swaddling the neonate (including the head) in a warm blanket to prevent evaporative, conductive, and convective losses. Preterm very-low-birth-weight infants also benefit from a polyethylene occlusive wrapping at the time of delivery. A neonate exposed for resuscitation or observation should be placed under a radiant warmer to prevent radiant losses. 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, and humidity in the incubator. Alternatively, heating can be adjusted with a servomechanism set to maintain skin temperature at 36.5° C.
Last full review/revision January 2015 by James W. Kendig, MD; Ursula Nawab, MD
Content last modified January 2015