Anaes · Applied physiology — thermoregulation and heat balance
Thermoregulation
Also known as Body temperature regulation · Perioperative hypothermia · Heat loss under anaesthesia · Forced-air warming · Shivering · Redistribution hypothermia · Non-shivering thermogenesis · Brown adipose tissue · Malignant hyperthermia
Thermoregulation keeps the human core temperature within a narrow band around 37 degrees C, and anaesthesia dismantles the control system so effectively that inadvertent perioperative hypothermia becomes one of the commonest and most preventable complications in the operating theatre. The framework rests on six exam-critical ideas. First, the hypothalamus is the body's thermostat: it integrates central (preoptic) and peripheral (skin) thermoreceptor input and, through a threshold system, drives heat production (basal metabolism, shivering, and brown-fat non-shivering thermogenesis) against heat loss (radiation about 40 to 60 per cent at rest, convection, conduction, and evaporation). Second, general and neuraxial anaesthesia widen the inter-threshold range by 2 to 4 degrees C, abolishing the vasoconstriction and shivering defences so the patient becomes poikilothermic. Third, the resulting heat loss has three phases: redistribution (core-to-peripheral, a fall of about 1 to 1.5 degrees C in the first hour and not true heat loss), a linear decline of about 0.5 to 1 degree C per hour for 2 to 3 hours, then a plateau when vasoconstriction returns. Fourth, mild hypothermia (below 36 degrees C) is harmful: it increases morbid cardiac events (Frank, relative risk 2.2), triples surgical site infection (Kurz, 19 versus 6 per cent), increases blood loss and transfusion (Schmied), and prolongs drug action (Heier, vecuronium duration doubled). Fifth, malignant hyperthermia is a ryanodine-receptor channelopathy triggered by suxamethonium and volatiles, presenting with unexplained rising end-tidal carbon dioxide and treated with dantrolene. Sixth, prevention by prewarming, forced-air warming, fluid warming, and a warm theatre is the standard of care. Anchored on Sessler's reviews (Lancet 2016, Anesthesiology 2008 and 2013, Journal of Clinical Anesthesia 2024), Cannon on brown adipose tissue, and the four landmark randomised trials of normothermia.
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- General and regional anaesthesia increase the separation of the thermoregulatory thresholds by 2 to 4 degrees C, so the patient mounts NO vasoconstriction or shivering defence until the core temperature has already fallen markedly — the patient is effectively poikilothermic, and the core-to-peripheral gradient drives the first-hour redistribution drop.
- Phase 1 redistribution hypothermia is the steepest early fall (about 1 to 1.5 degrees C in the first hour), is NOT true heat loss, and can only be PREVENTED (not treated) by prewarming the periphery before induction so there is no core-to-peripheral gradient when anaesthesia abolishes vasoconstrictor tone.
- Mild hypothermia below 36 degrees C doubles morbid cardiac events (Frank, relative risk 2.2, VT 7.9 versus 2.4 per cent), triples surgical site infection (Kurz, 19 versus 6 per cent with 2.6 extra hospital days), increases blood loss and transfusion need (Schmied), and roughly doubles vecuronium duration (Heier).
- Shivering increases whole-body oxygen consumption and carbon dioxide production by 200 to 500 per cent — a dangerous load in the patient with cardiac or respiratory disease, and the commonest reason a cold patient in recovery desaturates.
- In malignant hyperthermia the EARLIEST reliable sign is an unexplained rise in end-tidal carbon dioxide despite increased minute ventilation; hyperthermia itself is a LATE sign. Treatment is immediate cessation of triggers and intravenous dantrolene (the ryanodine-receptor antagonist).
- The neonate loses heat rapidly (large surface area to mass ratio) and depends on brown-fat non-shivering thermogenesis; the elderly have blunted vasoconstriction and shivering and a reduced metabolic reserve. Both are high-risk groups.
Meet the patient
A 78-year-old man has a three-hour laparotomy for bowel obstruction in a cool theatre, unwarmed. By recovery his core is 34.8 degrees C, he is shivering violently, his oxygen saturation is 88 percent, and his vecuronium is still working. He will get a wound infection, bleed a little more, and stay another two days — all preventable, all from a degree or two of cold.[1]
Hold the question that runs through the whole topic: why does an anaesthetised patient cool at all, when the thermostat is intact? Because anaesthesia does not abolish thermoregulation — it slides the thresholds down until the defences never engage.[1][3]
References12ShowHide
- [1]Sessler DI. Perioperative thermoregulation and heat balance Lancet, 2016.PMID 26775126
- [2]Sessler DI. The thermoregulation story Anesthesiology, 2013.PMID 23221865
- [3]Sessler DI. Temperature monitoring and perioperative thermoregulation Anesthesiology, 2008.PMID 18648241
- [4]Sessler DI. How three linked clinical observations led to an understanding of perioperative heat balance: A personal reflection on the scientific process J Clin Anesth, 2024.PMID 38733707
- [5]Cannon B, Nedergaard J. Brown adipose tissue: function and physiological significance Physiol Rev, 2004.PMID 14715917
- [6]Frank SM, Fleisher LA, Breslow MJ, et al. Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events. A randomized clinical trial JAMA, 1997.PMID 9087467
- [7]Kurz A, Sessler DI, Lenhardt R. Perioperative normothermia to reduce the incidence of surgical-wound infection and shorten hospitalization. Study of Wound Infection and Temperature Group N Engl J Med, 1996.PMID 8606715
- [8]Schmied H, Kurz A, Sessler DI, et al. Mild hypothermia increases blood loss and transfusion requirements during total hip arthroplasty Lancet, 1996.PMID 8569362
- [9]Heier T, Caldwell JE, Sessler DI, et al. Mild intraoperative hypothermia increases duration of action and spontaneous recovery of vecuronium blockade during nitrous oxide-isoflurane anesthesia in humans Anesthesiology, 1991.PMID 1673591
- [10]Heuer L. [Pre-warming - how can perioperative hypothermia be avoided?] Anasthesiol Intensivmed Notfallmed Schmerzther, 2003.PMID 12975737
- [11]Insler SR, Sessler DI. Perioperative thermoregulation and temperature monitoring Anesthesiol Clin, 2006.PMID 17342966
- [12]Ehlers UE, Ulmer J, Keller M, et al. Comparison of continuous temperature measurement methods in the intensive care unit: standard bladder catheter measurements versus non-invasive transcutaneous sensors J Clin Monit Comput, 2025.PMID 39066870