pediatrics
Hypothermia in Neonates
Pathophysiology of Hypothermia in Neonates
Thermal equilibrium is affected by relative humidity, air flow, direct contact with cool surfaces, proximity to cool objects, 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. There are several mechanisms for heat loss: Radiant heat loss: Bare skin is exposed to an environment containing objects of cooler temperature. Evaporative heat loss: Neonates are wet with amniotic fluid. Conductive heat loss: Neonates are placed in contact with a cool surface or object. Convective heat loss: A flow of cooler ambient air carries heat away from the neonate. 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 oxygen consumption 2- to 3-fold. Thus, in neonates with respiratory insufficiency (eg, the preterm infant with Respiratory Distress Syndrome in Neonates ), 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 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. Theneutral thermal environment(thermoneutrality) is the optimal temperature zone for neonates; it is defined as the environmental temperature at which metabolic demands (and thus caloric expenditure) to maintain body temperature in the normal range (36.5 to 37.5° C rectal) are lowest. The specific environmental temperature required to maintain thermoneutrality depends on whether the neonate is wet (eg, after delivery or a bath) or clothed, its weight, its gestational age, and its age in hours and days. 