Nutritional substrate and perioperative hypothermia

Abstract General anaesthesia causes hypothermia due to decreased metabolic rate and impaired thermoregulation. Many warming devices are in use to prevent heat loss, but little attention is paid to stimulate the body's own heat generation. All nutrients raise energy expenditure, and the highest thermic effect is ascribed to amino acids and proteins, 30–40% in the awake state. Amino acids infused during general anaesthesia exert a thermic effect that is fivefold enhanced compared with in the awake state, and may prevent postoperative hypothermia. It is still incompletely understood in which tissues and by which mechanisms nutrients stimulate heat production. However, these findings support the existence of an inhibitory action normally exerted by central thermosensors, in order to maintain oxidative metabolism within certain limits, to prevent hyperthermia. During anaesthesia, central thermosensors are silenced and hence, amino acid thermogenesis is exaggerated. The amino-acid-induced heat generation during anaesthesia predominantly occurred in extra-splanchnic tissues, most probably in skeletal muscle. It may reflect an increased protein turnover, as both protein breakdown and synthesis are energy consuming processes, known to generate heat. Possibly, amino-acid infusion provides substrates, otherwise mobilized from own tissues, needed for wound healing and immunological function. However, other cellular mechanisms may also contribute to this non-shivering thermogenesis.