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Perioperative hypothermia and thermoregulation
In the first hour of anaesthesia core temperature falls mainly because heat moves from the core to the periphery, and later because of heat loss to the environment; early forced-air warming and warmed fluids prevent hypothermia and the bleeding, infection and cardiac events that come with it.
Why it matters in the exam
The exam asks about the three phases of temperature fall under anaesthesia (redistribution, linear fall, plateau), the main route of heat loss (radiation), the complications of hypothermia, the most effective warming method (forced warm air), and the treatment of postoperative shivering (a small dose of pethidine). Questions also cover neonates and brown fat.
Hypothermia is defined as a core temperature below 36 degrees. General and regional anaesthesia dilate the vessels and abolish the vasoconstriction that protects the core, so heat "flows" from the core to the hands and feet — a fall of about 1–1.5 degrees in the first hour. Then the patient loses heat by radiation, convection, conduction and evaporation faster than it is produced. Finally, when the peripheral vessels constrict again, temperature levels off.
Even mild hypothermia increases bleeding (impaired platelets and clotting enzymes), wound infection, cardiac events and discomfort, prolongs the action of muscle relaxants and slows waking. Shivering raises oxygen consumption and is dangerous for a cardiac patient. Under NICE, temperature is measured before surgery, the patient is warmed if below 36 degrees, and active warming is used for surgery longer than 30 minutes. In neonates the large surface area and heat production from brown fat (without shivering) make cold a fast danger.
Key points
- Hypothermia = core temperature below 36 degrees.
- Phases: redistribution, linear fall, plateau.
- Radiation is the main route of heat loss.
- Complications: bleeding, infection, cardiac events, prolonged paralysis.
- Prevention: prewarming and forced warm air; shivering — pethidine 25 mg.
Summary table
| Issue | Main point | Exam point |
|---|---|---|
| Phases | Redistribution (first hour), linear fall, plateau | The first fall is not heat loss to the environment |
| Heat loss | Radiation (the main route), convection, conduction, evaporation | Radiation is the largest route |
| Complications | Bleeding, wound infection, cardiac events, prolonged paralysis, shivering | Even mild hypothermia harms |
| Prevention | Prewarming, forced warm air, warmed fluids in large volumes, a warm room | Forced warm air — the most effective |
| Shivering | Warming; pethidine 25 mg; clonidine or dexmedetomidine | Raises oxygen consumption |
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Sources
- NICE guideline CG65: Hypothermia — prevention and management in adults having surgery, 2008, updated 2016 — Warming and monitoring
- Miller's Anesthesia, 9th edition (2020) — Temperature Regulation and Monitoring
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.