Perioperative Hypothermia

Perioperative hypothermia is a core temperature under 36.0 C during or after surgery. Mild is 34.0 to 35.9 C, moderate 32.0 to 33.9 C and severe under 32.0 C.

Epidemiology. It affects 50% to 70% of unwarmed surgical patients. It raises the risk of wound infection (about threefold), blood loss, transfusion, cardiac events and longer stay.

Phases. Redistribution of heat from core to periphery lowers core temperature by 0.5 to 1.5 C in the first hour. A linear phase follows over 2 to 3 hours as heat loss exceeds production, then a plateau after vasoconstriction.

Risk factors. Age over 60 years, low body weight, ASA 3 or more, major or long surgery, large open body cavity, cold fluids, cold theatre (under 21 C), neuraxial block, burns, trauma, hypothyroidism and sedatives.

Effects. Coagulopathy and platelet dysfunction, impaired drug metabolism (longer neuromuscular block and delayed emergence), shivering, increased oxygen use, arrhythmia and pressure injury.

Pathophysiology.

Anaesthesia impairs thermoregulation, causes vasodilation and lowers the vasoconstriction threshold. Heat is lost by radiation, convection, evaporation and conduction.

Cold skin, shivering (absent under anaesthesia and with relaxants), slowed heart rate, hypotension, prolonged block, delayed awakening, J (Osborn) waves on ECG, arrhythmia, and bleeding without a surgical source.

Red flags.

• Core temperature under 32 C.
• Atrial or ventricular arrhythmia.
• Bleeding with prolonged clotting times.
• Cardiac arrest.

Measure core temperature continuously (oesophageal, nasopharyngeal, bladder, pulmonary artery or zero-heat-flux probe) from before induction until recovery. Peripheral and axillary readings underestimate core temperature. Send blood gas, lactate, glucose, calcium, magnesium, coagulation, full blood count and ECG.

Differential diagnosis.

• Measurement error.
• Sepsis or hypothyroidism.
• Adrenal insufficiency.
• Neurological injury.
• Drug effect.

Prevention. Warm patient for at least 20 to 30 minutes before induction, keep theatre at 21 C or more, use forced air warming from induction, warm IV fluids over 500 mL and all blood products, cover exposed areas, and use warmed, humidified gases for long cases.

Treatment.

• Mild: forced air warming at 38 to 43 C, warmed fluids, covering the head, and treating shivering; aim to raise core temperature by 0.5 to 1 C each hour.
• Moderate to severe: add warmed IV fluids (37 to 40 C), warm lavage (bladder, gastric or peritoneal), and avoid rapid peripheral rewarming alone, which can cause afterdrop and hypotension.
• Cardiac instability or arrest: extracorporeal rewarming is preferred. Continue resuscitation until core temperature is 32 C or more, with adrenaline and amiodarone withheld under 30 C and spaced at longer intervals from 30 to 35 C.

Correct coagulopathy with calcium (keep ionised calcium 1.1 mmol/L or more), platelets, fibrinogen and plasma as bleeding dictates, and check glucose. Reduce neuromuscular block doses and confirm recovery with train-of-four.

Monitoring. Core temperature every 15 minutes until 36 C or more, ECG, potassium, glucose and coagulation.

Referral and escalation. Critical care for moderate or severe hypothermia. Cardiothoracic and perfusion teams for extracorporeal rewarming.

Special groups. Neonates and infants lose heat rapidly, so warm theatre to 24 to 26 C and use radiant heaters. Older adults, burns and trauma patients need early warming.