When human beings get cold, one of the body's first mechanisms to generate heat is to constrict the blood vessels in the arms and legs. This helps to keep blood in the core of the body and prevents heat loss from the skin's surface. Although this helps to conserve heat, prolonged periods of peripheral vasoconstriction reduces the amount of oxygen available to the tissue.
Increased muscle flexion and activity
In the adult human, heat is produced through shivering, but infants have a poor ability to shiver. Rather than shivering, infants increase activity through flexion and movement which generates muscular warmth. Flexion also helps to decrease the body surface area.
Premature and sick infants may be unable to flex in response to cold stimuli the way that mature infants can do to minimize their exposed surface area. The premature infant has poor muscle tone and primarily lies with his extremities extended. The premature infant has a very large surface area to body mass ratio, in other words, a large area is exposed to the environment, and this can result in rapid heat loss.
Brown fat metabolism
This process is initiated in the hypothalamus by way of the sympathetic nervous system. Brown fat is located around the kidneys, adrenal glands, mediastinum, subscapular, and axillary areas and is very limited in premature infants. Storage of brown fat occurs primarily during the third trimester of pregnancy. So remember - the more premature the infant the less brown fat he/she has stored.
Norepinephrine is released at the brown fat site as a result of cold stress. When norepinephrine is released, it causes the release of free fatty acids (FFA), which undergo combustion in the mitochondria of the brown fat cells. This process releases HEAT. One consequence due to the release of FFA is that FFA will compete with bilirubin for albumin binding sites. This increases the risk of hyperbilirubinemia and possibly kernicterus. Kernicterus refers to yellowish staining of bilirubin in nuclear centers of the central nervous system, particularly in the basal ganglia, cerebellum, and hippocampus. The prognosis for the infant with kernicterus is poor if left untreated.
Of the three methods of heat production, brown fat metabolism is the one most utilized by the newborn.