Anaphylaxis under anesthesia
Why examiners ask this
Anaphylaxis is a judgment test disguised as a protocol test. The diagnosis is clinical and the presentation under anesthesia is incomplete, so the examiner wants to see you act on hypotension and bronchospasm before the rash appears, give epinephrine without delay, and then investigate properly so the next anesthetic is safe. It appears as an additional topic set outside the operating room, as an intraoperative curveball after antibiotics or rocuronium, and as a cardiac arrest.
The 60-second answer
Sudden hypotension with bronchospasm or a rising airway pressure shortly after a drug is anaphylaxis until proven otherwise, and I treat it as such because delay is what kills. I stop the likely trigger, call for help, give 100% oxygen, and give epinephrine IV in 50-microgram boluses for an adult, repeated and escalated to effect, with an infusion if I need repeated boluses; if the systolic falls below 50 mmHg or the patient arrests, I start CPR.1 I open the fluids wide, 500–1000 mL boluses in an adult,1 because the patient has lost intravascular volume into the tissues, and I lie them flat with the legs raised. I secure the airway early if the airway is involved, because edema makes it harder with time. Once stable I add second-line drugs: an antihistamine, a steroid, and a bronchodilator if wheeze persists. I send a tryptase once the patient is stable, ideally 1–4 hours after onset, and a baseline at least 24 hours later,1 document the timeline, and refer for allergy testing.
Decision points and the "why"
- Epinephrine first, IV, titrated — it treats all three problems: vasodilation, bronchospasm and mediator release. The dose is proportionate to the severity; the error is being timid with a collapsing patient or giving a full milligram to someone merely hypotensive.
- Stop the trigger and the surgery — in NAP6 antibiotics were the leading culprit (teicoplanin disproportionately), followed by neuromuscular blockers, chlorhexidine and Patent Blue dye;2 I remove chlorhexidine-coated lines and latex from the field if those are plausible.
- Volume — crystalloid 500–1000 mL in an adult or 20 mL/kg in a child, repeated;1 up to half the circulating volume can shift into the interstitium.
- Vasopressin and glucagon — for epinephrine-refractory shock I add a second vasopressor, norepinephrine or vasopressin 1–2 units; if the patient takes a beta-blocker, glucagon 1 mg IV bypasses the receptor.1
- Airway — early intubation if there is any stridor, tongue or lip swelling; I use a smaller tube and have a surgical airway plan.
- Second-line drugs are second-line — hydrocortisone and diphenhydramine do nothing for the acute hemodynamics; I give them after epinephrine and fluids and say so; steroids are for refractory reactions,1 and neither drug prevents a biphasic reaction.3
- Investigation — tryptase confirms mast-cell degranulation but a normal level does not exclude the diagnosis; I document every drug and the timing, give the patient a written account, and refer to an allergist for skin testing 4–6 weeks later.
The complication they will inject
The patient arrests, usually with pulseless electrical activity, often preceded by bradycardia.2 I start CPR and ACLS with the modifications anaphylaxis demands: epinephrine 1 mg IV repeated as in the algorithm, aggressive isotonic volume, ECPR if available, and prolonged resuscitation,4 because these patients are young and the cause is reversible. I ask for vasopressin and consider glucagon if the patient is beta-blocked. If the patient has a difficult airway from edema, I prioritize oxygenation with whatever device works and move to a surgical airway without delay. After return of circulation I keep the epinephrine infusion running, because biphasic reactions occur in up to 20% of patients, from 1 to 78 hours later,3 and I admit the patient to the ICU. The examiner may then ask whether the surgery proceeds: an elective case does not; a life-saving one continues once the patient is stable, with the suspected drug avoided.
Pitfalls that cost points
- Giving steroids and antihistamines first because they are familiar.
- Quoting the intramuscular dose when you have an IV and the patient is monitored.
- Waiting for a rash or confirming the diagnosis before treating.
- Saying "I would send a tryptase" without the timing and without saying what you would do with the result.
- Forgetting the differential: vasovagal or spinal hypotension, bronchospasm from light anesthesia, tension pneumothorax, pulmonary embolism, mesenteric traction syndrome, and transfusion reaction if blood is running.
Rapid-why
Q: Why is the IV epinephrine dose so much smaller than the ACLS dose? A: A conscious or anesthetized patient with a pulse needs titration to effect; 1 mg into a perfusing heart causes hypertensive crisis, arrhythmia and ischemia.
Q: Why does the reaction present as hypotension before anything else under general anesthesia? A: The cutaneous signs are hidden under drapes and the patient cannot report symptoms, so the first visible sign is the cardiovascular one; in NAP6 every patient became hypotensive, and bronchospasm was the presenting feature in 18%.2
Q: Which neuromuscular blocker is most often implicated? A: In NAP6 succinylcholine carried about twice the risk of the other relaxants, which were similar to one another;2 cross-reactivity through the quaternary ammonium epitope is common, which is why allergy testing matters for future anesthetics.
Q: Can the patient be re-exposed to the drug if testing is negative? A: Only after a formal allergy evaluation; for antibiotics and latex, avoidance is simpler than challenge.
Numbers to know
| Item | Value |
|---|---|
| Epinephrine IV bolus (adult) | 50 mcg, repeated and escalated to effect1 |
| Epinephrine, refractory or collapse | Infusion about 0.1 mcg/kg/min; CPR and 1 mg if SBP < 50 or arrest |
| Epinephrine, no IV access | 10 mcg/kg IM, maximum 0.5 mg, lateral thigh1 |
| Crystalloid bolus | 500–1000 mL adult; 20 mL/kg child, repeated1 |
| Vasopressin | 1–2 units IV bolus, then infusion |
| Glucagon (beta-blocked) | 1 mg IV1 |
| Tryptase | Once stable, 1–4 h after onset, and ≥ 24 h later1 |
Links
- Intraoperative bronchospasm, Intraoperative hypotension: differential and management, Vasopressors and inotropes: choosing and titrating, ACLS adapted to the operating room, Transfusion reactions, TRALI and TACO (coming)
- Atlas: Monitor atlas: other perioperative and peripartum events (coming)
- Association of Anaesthetists / NAP6 report; World Allergy Organization anaphylaxis guidance
Sources
- 1.Perioperative anaphylaxis treatment algorithm · Resuscitation Council UK / Association of Anaesthetists, 2024 · resus.org.ukAdult IV adrenaline 50 mcg; IM 10 mcg/kg max 500 mcg; fluids; glucagon 1 mg; tryptase timing.
- 2.Anaesthesia, surgery, and life-threatening allergic reactions: epidemiology and clinical features of perioperative anaphylaxis in the 6th National Audit Project (NAP6) · Royal College of Anaesthetists NAP6 (Br J Anaesth), 2018 · eprints.whiterose.ac.uk
- 3.Clinical Guideline Highlights for the Hospitalist: Anaphylaxis Management in Adults and Children · Journal of Hospital Medicine (summary of AAAAI/ACAAI 2020 practice parameter), 2021 · mdedge.comPrimary practice parameter (J Allergy Clin Immunol 2020) was not reachable; summary used for biphasic incidence and timing.
- 4.Part 10: Adult and Pediatric Special Circumstances of Resuscitation (2025 AHA Guidelines) · AHA, 2025 · cpr.heart.org
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.