Thermoregulation

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Red flags

• 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.