Patterns of hypothermia following general anaesthesia

Following the induction of anaesthesia, core hypothermia occurs in three stages;

1. Redistribution
2. Linear phase
3. Plateau phase

This accounts for the largest drop in core temperature of the three stages. Vasodilatation causes redistribution of heat from core to periphery. Body heat content remains unchanged.

Factors affecting redistribution

• Patients initial heat content. More heat will be transferred from the warm core to a cold peripheral compartment and patients with cool peripheries will suffer a greater degree of core hypothermia.

• Neonates and infants have a much larger core compartment than adults, as it extends closer to the body surface.

• Body morphology. Obese patients tend to have warmer peripheries as the adipose tissue acts as a thermal insulator resulting in vasodilatation.

This begins at the start of surgery as the patient is exposed to cold cleaning fluids and cool air flow in the theatre. Heat loss exceeds heat production and most surgery does not extend past the linear phase.

• Radiation contributes the most (40%) and is proportional to environment/core, temperature difference to the power of four.

• Convection contributes up to 30% and is proportional to air velocity

• Conduction contributes up to 5% and is proportional to the difference in surface temperatures

• Evaporation contributes up to 15% and occurs from cleaning fluids, skin, respiratory and wound. A laparotomy can contribute up to 50% of the total.

• Respiratory contributes 10% (8% evaporation of water; 2% heating of air) and is enhanced by the cooling effect of cold anaesthetic gases.

• Cold intravenous fluids.

Once core temperature falls below the thermoregulatory threshold, peripheral vasoconstriction increases and acts to limit the heat loss from the core department. When core heat production = heat loss to the peripheral compartment, core temperature reaches a plateau. Patients with an autonomic neuropathy (diabetics) have impaired sympathetic vasoconstriction and are unable to establish a core plateau in phase 3. Combined general and regional anaesthesia will have a similar effect as the regional anaesthetic (spinal/epidural) will prevent vasoconstriction in the legs; i.e. failure to establish a core plateau.