Perioperative hypothermia and shivering

What it is

Perioperative hypothermia is a core temperature below 36 °C around the time of surgery, and it is the commonest thermal problem in anaesthesia. Both general and neuraxial anaesthesia impair thermoregulation: they lower the threshold at which the body vasoconstricts and shivers and raise the threshold for sweating, so the range of temperatures that triggers no response widens from a few tenths of a degree to several degrees. Cooling then follows a typical pattern. In the first hour, vasodilation lets warm blood from the core flow to the cooler arms and legs, so core temperature falls quickly by redistribution even though little heat has been lost; then heat loss to a cold theatre, open body cavities, cold fluids and skin preparation exceeds the reduced heat production and temperature keeps falling; finally a plateau is reached when vasoconstriction returns. Mild hypothermia is not harmless: it increases surgical site infection, impairs platelet and clotting-enzyme function so bleeding and transfusion increase, adds cardiac stress, prolongs the action of anaesthetic drugs and neuromuscular blockers, delays recovery and makes patients uncomfortable. Shivering on waking raises oxygen consumption and is distressing. Prevention is the rule: measure core temperature, prewarm, warm actively during the operation and in recovery, and treat shivering by rewarming and, if needed, drugs.

Key points

• Monitor core temperature in operations expected to last more than about half an hour and in all patients at risk.
• Reliable core sites: distal oesophagus, nasopharynx, pulmonary artery and bladder; tympanic and oral readings are easy but less exact.
• Higher risk: older people, babies and small children, low body weight, burns, long or open cavity surgery, combined general and neuraxial anaesthesia, cold theatres.
• Neuraxial anaesthesia also blocks vasoconstriction and shivering below the block, and the patient may not feel cold.
• Hypothermia is part of the lethal triad of trauma with acidosis and coagulopathy: in bleeding patients, warming is part of resuscitation.

How it works

• The hypothalamus compares core temperature with set thresholds and responds with behaviour, vasoconstriction, shivering or sweating.
• Anaesthetic drugs widen the gap between the cold-response and warm-response thresholds, so the patient drifts with the room.
• Heat is lost mainly by radiation and convection from exposed skin, plus evaporation from open wounds and conduction to cold fluids.
• Cold slows enzyme reactions: clotting is weaker, drug metabolism slows and neuromuscular block and sedation last longer.
• Vasoconstriction during rewarming reduces tissue oxygen delivery, which is one reason wound infection rises.

Step by step

1. Identify risk before surgery — age, size, type and length of surgery decide how much warming to plan.
2. Prewarm and keep the patient covered — this reduces the early fall from redistribution.
3. Measure core temperature from induction — you cannot treat a fall you do not measure.
4. Warm actively with forced air and warmed fluids — passive blankets alone rarely keep up with heat loss.
5. Check temperature on arrival in recovery and rewarm below 36 °C — cold patients bleed more and wake slowly.
6. Treat shivering with warming, oxygen and, if needed, drugs — shivering raises oxygen demand in patients with little reserve.

Accidental hypothermia stages (core temperature)

In practice

• Prewarm with a forced-air blanket before induction when possible; it reduces redistribution cooling.
• During surgery cover exposed areas, warm the theatre at the start, use forced-air warming and warm intravenous and irrigation fluids.
• In recovery, measure temperature on arrival and keep warming until the patient is above 36 °C before discharge from the unit.
• Shivering can also come from pain, fever, transfusion reactions or sepsis; think of these when it does not settle with warming.
• Rising temperature with rising CO2 and heart rate is not rewarming: think of malignant hyperthermia.

Next step

• Record core temperature at the start, during and at the end of surgery, and on arrival in recovery.
• Keep actively warming a patient below 36 °C and do not discharge them from recovery until they are warm.
• Call a senior if temperature keeps falling despite warming, if there is unexplained bleeding, or if hyperthermia with rising CO2 suggests malignant hyperthermia.
• Review neuromuscular block reversal and sedation in a cold patient: drug effects last longer.
• Hand over temperature, warming used and any shivering treatment to the ward team.

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Elsewhere in DurgiPedia

• Body temperature regulation
• Hypothermia and cold injury
• Body temperature and fever patterns
• Measuring body temperature: sites, devices and technique

Connections

← All of anaesthesia

Educational content; it does not replace supervised training, your institution's protocols or the anaesthetist's or intensivist's decision. It contains no doses and no ventilator or infusion settings.

Sources: Hypothermia (StatPearls (NCBI Bookshelf), 2026).

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