Abstract
Core body temperature is one of the most tightly controlled physiological parameters. Normal core body temperature ranges from 36.5 to 37.5°C. Peripheral body temperature, involving the skin and subcutaneous tissues of the trunk and limbs, is less well controlled – the difference between core and peripheral temperatures is usually around 2–3°C, but can be as much as 20°C in extreme circumstances.
How is body temperature regulated?
Core body temperature is one of the most tightly controlled physiological parameters. Normal core body temperature ranges from 36.5 to 37.5°C. Peripheral body temperature, involving the skin and subcutaneous tissues of the trunk and limbs, is less well controlled – the difference between core and peripheral temperatures is usually around 2–3°C, but can be as much as 20°C in extreme circumstances.
Body temperature is determined by a balance of heat loss and production:
Heat loss occurs due to radiation, convection, evaporation (sweat and respiration), conduction and loss in urine and faeces.
Heat is produced by basal metabolic processes, exercise, shivering and non-shivering thermogenesis (in neonates and infants).
Temperature is regulated by both feedforward and negative-feedback loop control. Feedforward control is mediated by the central nervous system (CNS) involving interpretation and prediction of the external environment. If it is snowing, you are likely to put on a coat before going outside. This prevents any temperature change from occurring.
Negative-feedback control contains the usual elements for sensing and correcting the internal environment:
Temperature sensors. These are a large family of temperature-gated ion channels known as transient receptor potential channels. Peripheral and core temperature sensors send information to the hypothalamus.
A control centre. The hypothalamus analyses the afferent temperature signals, checks them against a set-point and controls the efferent response.
Effector system.
– Temperature-losing mechanisms involving sweating and skin vasodilatation are controlled by the anterior hypothalamus.
– Temperature-conserving and -generating mechanisms involving vasoconstriction, shivering, non-shivering thermogenesis, behavioural changes (e.g. putting on more clothes or turning up the central heating) are controlled by the posterior hypothalamus.
Whilst basal metabolic rate (BMR) cannot be increased to boost heat production, shivering is extremely effective, producing up to a sixfold increase in heat production.
How does general anaesthesia disturb the normal thermoregulatory mechanisms?
Mild hypothermia (34.0–36.5°C) is common during general anaesthesia – the cause is often multifactorial:
Patients may arrive at theatre cold; for example, following a prolonged wait in a cold preoperative ward whilst wearing a thin hospital gown.
The hypothalamic set-point is lowered during general anaesthesia. Normally, the hypothalamus responds to a core temperature below 36.5°C with vasoconstriction, and shivering commences at 36.0°C. Under general anaesthesia, these threshold temperatures are lowered by 2–3°C.
Behavioural changes are (obviously) lost under general anaesthesia. The patient is no longer able to put on additional layers of clothes.
Muscle paralysis prevents shivering.
Most anaesthetic drugs cause vasodilatation, counteracting the normal vasoconstriction response to hypothermia.
The reduction in core body temperature associated with general anaesthesia normally follows a triphasic pattern (Figure 89.1):
Redistribution phase. A decrease in core temperature of 1.5–2.0°C over 30–45 min. This is a consequence of the administration of a vasodilator (e.g. a volatile anaesthetic) in combination with a reduction in the hypothalamic set-point for vasoconstriction.
Linear phase. A more gradual fall of 1°C over the next 2–3 h due to radiation, convection and evaporation. The degree of evaporation depends on the type of surgery, being highest in open abdominal procedures.
Plateau phase. When peripheral vasoconstriction becomes activated, core temperature reaches a plateau when the rate of heat loss is matched by basal metabolic heat production. However, in patients who have both general anaesthesia and neuraxial blockade, peripheral vasoconstriction is ineffective and core temperature continues to decline.
