Brain protection during neurosurgery: an update from the anaesthetist's perspective

Peri-operative amino acid administration and the metabolic response to surgery

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 has been paid to stimulating 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 enhanced compared with that in the awake state. At awakening from anaesthesia, post-operative hypothermia may be prevented without shivering. The tissues involved and the mechanisms by which nutrients stimulate heat production are still not completely understood. 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 occurs 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 the body's own tissues, needed for wound healing and immunological function. However, other cellular mechanisms may also contribute to this non-shivering thermogenesis.

Perioperative thermoregulation and heat balance

In summary, both regional and general anesthesia markedly impair the normal precise regulation of core body temperature. Consequently, inadvertent perioperative hypothermia is common. Hypothermia develops because the typical operating room environment is cold; however, it is anesthetic-induced impairment of thermoregulatory responses that contributes most. Internal redistribution of body heat is a surprisingly important factor, contributing more to core hypothermia than net heat loss in most patients. There is now convincing evidence that a typical amount of intraoperative hypothermia, say 2 degrees C, predisposes numerous complications and alters patient outcome. Fortunately, effective methods are available for preventing hypothermia.