A newborn infant who is cold stressed rapidly depletes essential stores of fat and glycogen.

• Up to 10% of the birth weight of an appropriately grown term infant is fat, but not all sources of fat (eg, brown fat) are capable of rapid conversion into energy and heat during epinephrine-mediated stress.

• The liver in a term infant contains adequate glycogen for up to 12 hours of postnatal stress if no exogenous glucose supply is provided.

• Normal glucose utilization rates of 5 to 6 mg/kg per minute can be increased markedly by conditions that raise the metabolic rate (eg, cold stress, infection, anemia) or the consumption of oxygen.

• Factors that impair heat retention and tolerance to cold stress in a newborn:

  • inability to shiver (absent "shivering thermogenesis")

  • impaired peripheral vasoconstriction

  • limited brown fat stores

  • a high surface area-to-weight ratio

• Cold-stressed infants demonstrate mild acidemia and may have clotting abnormalities, disseminated intravascular coagulation, or scleremic skin changes. The acidemia diminishes the likelihood of hypocalcemia.

• An elevation of blood urea nitrogen is more suggestive of maternal/placental disease, dehydration, or renal abnormalities than of cold stress.

• Similarly, hypernatremia in a newborn indicates enormous insensible free water losses (eg, preterm infants who have transparent skin).

• Hyperkalemia usually follows severe hemolysis (eg, Rh incompatibility, sepsis), acute renal failure, tissue necrosis (eg, necrotizing enterocolitis), or adrenal insufficiency.

• Early neonatal hypocalcemia typically develops in low-birth-weight and sick infants who are 1 to 4 days old. A serum calcium level of 5.5 mg/dL would be unusual in a term infant who had been cold-stressed for fewer than 24 hours. All of these conditions are less likely to be present in a 24-hour-old cold-stressed infant than is hypoglycemia.

Recognize the hazards and benefits associated with the use of radiant warmers for neonates.

• servocontroller probe must be attached firmly to the infant's skin over the midepigastrium.

• If the probe loses contact with the skin, it will read the ambient temperature, which is lower than the infant's core temperature. The radiant warmer then will produce excessive amounts of heat in an attempt to raise the infant's core temperature, resulting in overheating.

• Factors that result in insufficient heat production by the radiant warmer:

  • Covering the infant and probe with a bubble blanket

  • placing the probe over the infant's liver

  • setting the skin set point 0.5°C below the neutral thermal point (the temperature at which the least amount of calories is required for thermoregulation)

  • having the temperature more than 10°C below the infant's skin temperature

AAP Prep
updated 11/9/2005