The chance of survival of neonates is markedly enhanced by the successful prevention of excessive heat loss. The newborn infant must be kept under a neutral thermal environment. This is defined as the external temperature range within which metabolic rate and hence oxygen consumption are at a minimum while the infant maintains a normal body temperature (Figures 8–1 and 8–2 and Table 8–1). The normal skin temperature in the neonate is 36.0°C to 36.5°C (96.8–97.7°F), and the normal core (rectal) temperature is 36.5°C to 37.5°C (97.7–99.5°F). Axillary temperature may be 0.5°C to 1.0°C lower (95.9–98.6°F). A normal body temperature implies only a balance between heat production and heat loss and should not be interpreted as the equivalent of an optimal and minimal metabolic rate and oxygen consumption.
I. HYPOTHERMIA AND EXCESSIVE HEAT LOSS
Preterm infants are predisposed to heat loss because they have a high ratio of surface area to body weight (3–5 times more than the adult), little insulating subcutaneous fat, and reduced glycogen and brown fat stores. In addition, their hypotonic (“frog”) posture limits their ability to curl up to reduce the skin area exposed to the colder environment.
Mechanisms of heat loss in the newborn include the following:
Radiation. Heat loss from the infant (warm object) to a colder nearby (not in contact) object. It is the major source of daily heat loss (40% or more without clothing/blanket and low room air movement and room air temperature of 24–26°C).
Conduction. Direct heat loss from the infant to the surface with which he or she is in direct contact such as lying on a cold table or under an x-ray plate.
Convection. Heat loss from the infant to the surrounding air proportional to the surrounding air temperature and movement velocity.
Evaporation. Heat loss by water evaporation from the skin of the infant. Immediately after delivery, evaporative heat loss may contribute to >50% of all heat loss. Thereafter, its magnitude is inversely proportional to the degree of immaturity. The underdeveloped stratum corneum results in higher skin permeability for the extremely low birthweight infant (<1000 g body weight). Transepidermal water loss as high as 6 to 8 mL/kg/h may be seen in the most immature infants during the first weeks of life.
Consequences of excessive heat loss. In contrast to adults, the newborn is unable to compensate for heat loss by increasing production through shivering and increased muscular activity. Heat production is thus performed through a nonshivering process (nonshivering thermogenesis) that involves oxidation of fatty acids from brown fat. Brown adipocytes are found in a discrete location in the cervical-supraclavicular area (the most common) and in perirenal/adrenal and paravertebral regions around the major vessels. They contain a large number of mitochondria, hence their brown coloration. Temperature receptors sensing a low temperature stimulate increased sympathetic output from the central nervous system, resulting in norepinephrine release, which in turn stimulates β-adrenergic receptors in brown fat, increasing cyclic adenosine monophosphate production. The release of cytoplasmic stores of triglycerides and fatty acids increases metabolism, resulting in increased oxygen consumption and heat ...