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Anaphylaxis during anaesthesia
Under anaesthesia anaphylaxis usually appears as sudden hypotension or bronchospasm without a visible rash, after an antibiotic, a relaxant or chlorhexidine — and the treatment is small IV doses of adrenaline and fluids.
Why it matters in the exam
The exam describes sudden collapse after induction and asks what the diagnosis is, what the first treatment is and at what dose, and when tryptase is taken. It also asks about the common causes, what to do in a patient on beta-blockers who does not respond to adrenaline (glucagon), and why antihistamines and steroids are not first-line treatment.
In the UK national audit NAP6 the common causes were antibiotics (mainly teicoplanin and cefazolin), relaxants, chlorhexidine and blue dye (patent blue); the common first sign was hypotension. The rash is often hidden by drapes or appears only once the blood pressure recovers. A low EtCO2 and high airway pressure point to bronchospasm and a fall in cardiac output.
It is important to tell anaphylaxis from other causes of collapse after induction: hypotension from the anaesthetic drugs, bronchospasm in an asthmatic, an oesophageal or endobronchial tube, pneumothorax, embolism and bleeding. An important clue is the timing in relation to an antibiotic or a relaxant, and a lack of response to the usual treatment of hypotension. After the event the patient is told what happened and it is documented so that he does not receive the same agent at the next anaesthetic. In a severe case calcium, blood gases and cardiac function on echo are also checked, and the patient is moved to close observation in intensive care.
Key points
- Under anaesthesia: hypotension or bronchospasm, sometimes without a rash.
- Common causes: antibiotics, relaxants, chlorhexidine, blue dye.
- IV adrenaline 50 mcg, fluids 20 mL/kg.
- Beta-blockers and refractoriness → glucagon.
- Tryptase early, at 1–2 hours and baseline; referral to an allergist.
Summary table
| Step | What is done | Details |
|---|---|---|
| Recognition | Hypotension, bronchospasm, tachycardia, rash or oedema | After an antibiotic, a relaxant or chlorhexidine |
| Immediate | Stop the suspect, call for help, 100% oxygen, raise the legs | Stop volatile agents if there is collapse |
| Adrenaline | IV 50 mcg in an adult, repeat by response; IM 0.5 mg if no IV access | After several doses — an infusion |
| Fluids and refractoriness | 20 mL/kg crystalloid; on beta-blockers — glucagon; an additional vasopressor | CPR if systolic pressure is below 50 mmHg |
| Afterwards | Tryptase early, at 1–2 hours and a baseline after 24 hours; refer to an allergist | Record all the drugs that were given |
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Sources
- Royal College of Anaesthetists 6th National Audit Project (NAP6): Perioperative anaphylaxis, 2018 — Causes, presentation and treatment
- Resuscitation Council UK: Emergency treatment of anaphylaxis, 2021 — Adrenaline and refractory anaphylaxis
- Miller's Anesthesia, 9th edition (2020) — Anesthesia and Immune Function: Perioperative Anaphylaxis
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.