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 7–1 7–2 and Table 7–1). The normal skin temperature in the neonate is 36.0–36.5°C (96.8–97.7°F) and the normal core (rectal) temperature is 36.5–37.5°C (97.7–99.5°F). Axillary temperature may be 0.5–1.0°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.
APPROXIMATE NEUTRAL THERMAL ENVIRONMENT IN INFANTS WHO WEIGH >2500 G OR ARE >36 WEEKS' GESTATIONa
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Table 7–1. APPROXIMATE NEUTRAL THERMAL ENVIRONMENT IN INFANTS WHO WEIGH >2500 G OR ARE >36 WEEKS' GESTATIONa
|Age ||Temperature (°C) |
|0–24 hours ||31.0–33.8b |
|24–48 hours ||30.5–33.5 |
|48–72 hours ||30.1–33.2 |
|72–96 hours ||29.8–32.8 |
|4–14 days ||29.0–32.6 |
|>2 weeks ||Data not establishedb |
Neutral thermal environment during the first week of life (in °C), based on gestational age. (Reproduced, with permission, from Sauer PJJ, Dane HJ, Visser HK. New standards for neutral thermal environment of healthy very low birthweight infants in week one of life. Arch Dis Child. 1984;59:18.)
Neutral thermal environment from days 7 to 35 (in °C), based on body weight. (Reproduced, with permission, from Sauer PJJ, Dane HJ, Visser HK. New standards for neutral thermal environment of healthy very low birthweight infants in week one of life. Arch Dis Child. 1984;59:18.)
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 (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.
Conduction. Direct heat loss from the infant to the surface with which he or she is in direct contact.
Convection. Heat loss ...