Heat is produced as a by-product of cell metabolism. The basal metabolic rate is the lowest obligatory rate of heat production which occurs when an individual is starved, quiet and resting. In the newborn this is usually taken as the minimal rate of oxygen consumption in an infant who is lying still and asleep, at least 1 h after a feed in a neutral thermal environment. Values depend on gestation and postnatal age. Heat is lost from the skin surface to the surrounding air by convection. Loss is high if there is rapid movement of cold air over the exposed skin. A naked baby in a cold drafty room has a high convective heat loss.
Heat is lost from the skin to the nearest surface facing the baby. Radiative heat loss varies with the temperature of that surface and its distance from the skin, but is independent of the temperature of the intervening air. A naked infant can radiate large amounts of heat to the cool walls of an incubator even if the air is warm. An overhead warmer provides heat by radiation.
Heat is lost as water evaporates from the surface of the skin (560 cal/ml of water). A newly born infant wet with amniotic fluid loses heat as the skin dries. Evaporative heat loss is low in mature infants unless they are sweating in response to heat stress. Losses are high in preterm infants who have a high transepidermal water loss (TEWL) due to passive diffusion of water through the thin, poorly keratinized immature epidermis. TEWL is related to both gestational and postnatal age.
Newborn infants are not usually in direct contact with a structure of high thermal capacity, so conductive losses are small. Heat can be gained by conduction if the baby is lying on a heated gel- or water-filled mattress. Heat production must be balanced by that lost from convection, radiation, evaporation and conduction. The more immature the infant, the greater the difficulty in temperature control. Heat loss is related to surface area which is relatively high in the smaller babies. There is poor insulation due to lack of subcutaneous tissue, and the poorly developed stratum corneum results in high TEWL and evaporative heat loss. The ability to conserve heat by vasoconstriction is limited and heat production, which is related to mass, is low. This poor response to relatively high heat losses means that hypothermia is a common problem in the immature baby.
Source: Forfar and Arneil’s Textbook of Pediatrics, 7E