Effects of Hypothermia:
Increased morbidity; hypoxia. myocardial ischaemia, arrhythmias and cerebral ischaemia
Decreased drug metabolism and prolonged duration of action
Coagulopathy; the clotting cascade is enzymatic and platelet function is temperature dependent.
Increased incidence of wound breakdown and infection
During anaesthesia, hypothermia may be defined as a core body temperature less than 36°C. This can cause physiological derangement in the operating theatre and in recovery, and may increase perioperative morbidity.
The effects of hypothermia are proportional to the change in temperature. Metabolic rate is reduced by up to 10% for every 1°C fall in body temperature. There is a reduction in cardiac output and an increase in haemoglobin oxygen affinity. This leads to a decrease in tissue oxygen delivery. Significant hypothermia is associated with metabolic acidosis, oliguria, altered platelet and clotting function and reduced hepatic blood flow with slower drug metabolism. The MAC of inhalational agents is reduced and muscle relaxants have a prolonged effect. Postoperative shivering increases oxygen consumption and myocardial work. There may also be an increased incidence of wound breakdown and infection.
- Abolished behavioural responses
- Increased heat loss through:
- Radiation (accounts for over 50% of heat loss). This is exacerbated when the ambient temperature falls below 24°C
- Evaporation from body surfaces (cleaning fluids)
- Evaporation from open body cavities
- Cooling effect of cold anaesthetic gases and intravenous fluids
- Widened interthreshold range. Heat production falls as anaesthetic agents alter hypothalamic function. The lowering of the hypothermic threshold is related to MAC. The interthreshold range widens so that thermoregulatory responses are not triggered until core temperature has deviated much further than normal from 37°C.
- Reduced metabolic heat production (15-40%), particularly by:
- Reduced muscle activity
- Decreased brain metabolism
Induction of general anaesthesia typically results in a 1°C fall in core temperature within the first 30 minutes. Changes in core temperature are due to redistribution at this early stage. As a result, the core compartment cools and expands, while the peripheral compartment warms and contracts.
i] Barash PG (ed). ASA Refresher Courses in Anesthesiology. 1993; 21: Ch. 7.