Effects of anaesthesia on thermoregulation

Hypothermia is a common and serious complication during anaesthesia and surgery. It mainly results from anaesthetic-induced inhibition of thermoregulatory control and exposure to cold operating room environment. Perioperative hypothermia develops in three distinct phases: (1) anaesthetic-induced vasodilation during induction of anaesthesia results in core-to-peripheral redistribution of body heat and decreases core temperature 1–1.5°C during the first hour of general anaesthesia; (2) subsequently core temperature decreases linearly as heat loss to the environment exceeds metabolic heat production; (3) after 3–5 h of anaesthesia, core temperature often stops decreasing. This core temperature plateau results from reactivation of thermoregulatory vasoconstriction which decreases cutaneous heat loss and constrains metabolic heat to the core thermal compartment. Perioperative hypothermia is associated with numerous complications such as myocardial ischaemia, increased risk of wound infection and coagulopathy. On the other hand temperatures only 1–3°C below normal provide substantial protection against cerebral ischaemia and hypoxaemia in numerous animal species. Consequently, most anaesthesiologists believe mild hypothermia is indicated during operations likely to cause cerebral ischaemia such as carotid endarterectomy and neurosurgery or cardiac procedures. Thermal perturbations, therefore, deserve the same risk/benefit analysis as other medical interventions. Fortunately, effective methods of cooling and warming surgical patients are now available.