Anaphylaxis under anaesthesia

What it is

Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction. Most cases are IgE-mediated: a previously sensitised patient meets the allergen, and mast cells and basophils release histamine, tryptase, leukotrienes and prostaglandins; other cases activate mast cells directly without IgE. Under anaesthesia, the usual culprits are drugs and materials given close together around induction: neuromuscular blocking agents, antibiotics, chlorhexidine, latex and dyes, among others. Recognition is harder than on the ward because the patient cannot report symptoms, the skin is covered by drapes and many anaesthetic drugs also lower blood pressure. The commonest presentations are sudden hypotension and tachycardia, bronchospasm with high airway pressures and a falling end-tidal CO2, and cardiac arrest; skin flushing, urticaria and angioedema may be absent or appear late. Adrenaline is the first-line drug and should be given early, with high-flow oxygen, rapid intravenous fluids and removal of the trigger; antihistamines and corticosteroids are only second-line. Timed blood tryptase samples support the diagnosis, and every patient must be referred for allergy testing so that the culprit is identified and avoided in the future.

How to recognise it

• Sudden, marked hypotension and tachycardia within minutes of a drug, colloid or exposure (bradycardia can occur in severe cases).
• Bronchospasm: high airway pressures, wheeze or a silent chest, upsloping or falling end-tidal CO2, desaturation.
• Flushing, urticaria or angioedema of the face, lips or tongue; look under the drapes.
• Poor response to the usual vasopressors, or cardiac arrest with pulseless electrical activity.
• Timing points to the trigger: reactions to intravenous drugs come quickly; to latex or chlorhexidine, more slowly.

Why it happens

• In IgE-mediated reactions, the allergen cross-links IgE on mast cells and basophils, which degranulate.
• Histamine and other mediators cause vasodilation and leaky capillaries, so fluid leaves the circulation and blood pressure falls.
• The same mediators contract bronchial smooth muscle and swell the airway mucosa.
• Some drugs activate mast cells directly without IgE; the treatment is the same.

What to do at once

1. Call for help, declare anaphylaxis and note the time — a crisis role for each person speeds treatment.
2. Stop or remove every possible trigger: drug, colloid, chlorhexidine, latex — continued exposure keeps the reaction going.
3. Give pure oxygen and secure the airway, intubating if swelling is developing — angioedema can close the airway quickly.
4. Give adrenaline early, intramuscular or titrated intravenous boluses, and repeat — it reverses vasodilation, oedema and bronchospasm.
5. Raise the legs and give rapid intravenous crystalloid — large volumes leak out of the circulation and must be replaced.
6. Reduce or stop anaesthetic agents that lower blood pressure, keeping the patient asleep — they worsen the hypotension.
7. If there is no pulse, start CPR and follow advanced life support — anaphylaxis is a reversible cause of arrest.
8. If refractory, start an adrenaline or noradrenaline infusion, then vasopressin; glucagon for beta-blocked patients — they work by other receptors.
9. Treat persistent bronchospasm with an inhaled beta-2 agonist; add antihistamine and steroid only once stable — they are second-line.
10. Take the first tryptase sample as soon as resuscitation is under way — the rise is short-lived and confirms mast cell activation.

Clues to anaphylaxis under anaesthesia and the usual pitfall

Costly mistakes

• Delaying adrenaline while giving antihistamines or steroids.
• Treating the hypotension as a simple drug effect with repeated small vasopressor doses.
• Missing a reaction because the skin was hidden under the drapes.
• Forgetting tryptase samples, or not referring the patient for allergy testing so the trigger is used again.
• Not recording every drug, fluid and material given in the minutes before the reaction.

Next step

• Admit to intensive care or a monitored bed; watch for a biphasic reaction hours later, and for airway swelling before extubation.
• Decide with the surgeon and a senior whether to finish, shorten or postpone the operation.
• Repeat tryptase a few hours after the reaction and take a baseline sample a day or more later.
• Refer to an allergy service for skin or blood testing of every agent given; tell the patient and give a written record of the reaction.
• Record the event, report it to the drug safety system, and debrief the team.

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Elsewhere in DurgiPedia

• Type I hypersensitivity (immediate, IgE)
• Mast cell and basophil
• Medicines, allergies and habits: tobacco, alcohol and other substances
• Shock physiology

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: Anaphylaxis (StatPearls (NCBI Bookshelf), 2023).

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