The nonshivering thermogenic response of the newborn to cold exposure and the potentially detrimental effects of the resultant hypermetabolic state have been documented by numerous animal and clinical studies. It is standard practice to protect the newborn infant from cold stress by controlling environmental temperature and preventing heat loss. Although these measures are well accepted, little attention has been paid to the effect of surgical factors, such as operative trauma, anesthetic agents, and muscle relaxants on nonshivering thermogenesis. The purpose of this investigation is to develop an experimental model to study the effect of surgical factors on the neonatal thermogenic response. Newborn rabbits were chosen as the experimental subject because these animals rely solely on nonshivering thermogenesis. One-hundredeleven animals were studied. In the newborn rabbit, there was an inverse relationship between age and the percentage of brown fat. Body weight doubled in the first week of life, while the quantity of brown fat fell from a maximum of 2.9% of body weight in the first day of life to 1.56% at 7 days of age. It was possible to maintain the newborn rabbit in a warm environment for periods of up to 2 hours without changes in blood flow to the brown fat or oxygen consumption. The thermogenic response to cold did not appear to be altered in terms of oxygen consumption when the animals were lightly anesthetized with pentobarbital or paralyzed with metocurine and locally anesthetized. The rise in oxygen consumption was similar in awake, intact animals and those that received pentobarbital or metocurine and xylocaine. There was a similar increase in blood flow to the brown fat on cold exposure in the animals receiving pentobarbital and those receiving metocurine plus local anesthesia. After 37 minutes of cold exposure, body temperature of newborn rabbits fell from 38.2°C to 32.7°C, but inspite of these "hypothermic" levels, oxygen consumption and blood flow to the brown fat was high, increasing 94% and 282%, respectively. This suggests that the metabolic response to cold persists for a prolonged period of time even in the face of low body temperature and continued cold exposure.
- nonshivering thermogenesis
ASJC Scopus subject areas
- Pediatrics, Perinatology, and Child Health