Thermal Regulation

Humans conserve body temperature by several mechanisms, including (1) shunting blood from the skin and periphery to the core, (2) increasing basal metabolic rate, (3) voluntary muscle activity, (4) shivering, and (5) nonshivering thermogenesis. With the exception of nonshivering thermogenesis, all of these mechanisms are less effective in the neonate. Although older children and adults can maintain normal core body temperature when subjected to a wide range of environmental conditions, neonates, particularly premature infants, are very limited in this regard. In addition, even under conditions in which a neonate can maintain normal body temperature, this is often accomplished at the expense of increased oxygen consumption and carbon dioxide production. These consequences are particularly onerous in infants with respiratory failure.

Neonates should be cared for in a neutral thermal environment in which core temperature remains normal and oxygen consumption is minimized. Such an environment is best provided by treating a neonate on or within a thermo-controlled bed specially designed for neonates. Thermo-controlled beds come in two varieties: an open platform heated with an overhead radiant heat source and a closed plastic incubator heated with a convection heater. Although not satisfactory for transport, open incubators with radiant heaters are ideal for the care of critically ill neonates in the emergency department because they permit access by several caretakers. An alternative is the use of a portable overhead heat lamp and a standard crib. These devices should be used with extreme caution because they do not usually include a servo-control mechanism. The patient's body temperature should be monitored frequently to avoid hyperthermia. The neutral thermal environment is presumed when the infant's body temperature is normal and there is a minimal gradient between the core and the skin temperature.

Humans lose body heat in four ways: (1) evaporation, (2) conduction, (3) convection, and (4) radiation. Neonates, especially preterm infants, are particularly susceptible to heat loss because they have a relatively large surface-to-body mass ratio when compared to older patients. Also, their skin is more permeable to water vapor, and they may have a paucity of subcutaneous tissue. In addition to providing a heat source, attempts to create a neutral thermal environment should include provisions to minimize heat loss:

1. Infants should be thoroughly dried to avoid evaporative heat loss. This is critical after an emergent delivery. Drying should not be delayed under any circumstances. If emergent procedures are necessary, such as intubation, another caretaker should simultaneously dry the infant.

2. Whenever possible, infants should be placed on a pre-warmed surface to avoid conductive heat loss. The temperature of these surfaces or auxiliary heat sources (e.g., hot water bottles) should not exceed 104°F (40°C) because of the risk of thermal injury.

3. When treating an infant in an open crib or platform warmer, the room temperature should be increased to avoid convective heat loss. The infant should be located away from drafts (e.g., heat and/or air conditioning vents).

4. An infant should be clothed to the extent that it does not interfere with patient care and should not be placed near cold surfaces (e.g., exterior windows) to avoid radiant heat loss. At a minimum, a hat should be placed on the infant's head.5

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