ANESTHESIA BLOCKS NONSHIVERING THERMOGENESIS IN THE NEONATAL RABBIT

Nonshivering thermogenesis (NST) is a normal physiological response of the neonate to cold exposure, characterized by increased blood flow to metabolically active brown fat stores. It is standard practice during neonatal surgery to warm the ambient environment in order to avoid consuming vital energy stores. While NST has been well-studied in the neonate, the response during anesthesia and paralysis has not been fully characterized. Rabbit pups (aged 1 to 7 days) were randomized into several groups. The experimental groups consisted of animals mechanically ventilated and administered either metocurine, pancuronium, curare, fentanyl, nitrous oxide (N2O), or halothane. The controls were spontaneously breathing animals. Oxygen consumption (V̇o2), an index of metabolic activity, was measured at thermoneutrality (39°C) and after cold exposure (25°C). Control and metocurine animals had a significant increase in V̇o2 in response to cold exposure. The increase in V̇o2was not noted in animals that received curare, pancuronium, fentanyl, N2O, or halothane. To test the effect of anesthetic withdrawal during cold exposure on V̇o2, additional series of animals were studied. One group received continuous halothane throughout the period of cold exposure; the other had cessation of the halothane during cold exposure. Both groups were rewarmed subsequently. The animals that had withdrawal of halothane during cold exposure had a marked and significant increase in V̇o2 compared with the control group (continuous halothane). V̇o2 returned to near-baseline levels upon rewarming. The authors conclude that many commonly used anesthetic and paralyzing agents inhibit the thermogenic response to cold exposure. However, cessation of anesthesia (halothane) in a cold environment results in a marked increase in metabolic activity. © 1994.