AKT · Acute care/Resuscitation & shock

Anaphylaxis

IgE-mediated mast-cell degranulation (type I hypersensitivity)

Overview

A life-threatening generalised hypersensitivity reaction with rapid-onset Airway/Breathing/Circulation compromise and usually skin changes. The single most important action is prompt IM adrenaline — delay kills.

Recognise

Sudden onset after a trigger (food, drug, sting, latex)
Airway: swelling, hoarseness, stridor · Breathing: wheeze, dyspnoea, hypoxia · Circulation: hypotension, tachycardia, collapse
Skin/mucosa: urticaria, angioedema, flushing (absent in ~10–20%)

Red flags

Any Airway/Breathing/Circulation compromise = anaphylaxis → IM adrenaline now; biphasic reaction (recurrence hours later)

Differentials & how to tell them apart

Severe asthmawheeze/dyspnoea but no urticaria/angioedema or clear trigger; no hypotension

Vasovagal syncopebradycardia, pallor, quick recovery lying flat, no urticaria/airway

Septic shockfever/infection source, slower onset, no acute trigger

Panic attackno objective airway/skin/BP signs

Investigations

Clinical diagnosis — do not delay treatment. Mast-cell tryptase (timed samples: at presentation, ~1–2h, and >24h/baseline) confirms retrospectively.

Management

IM adrenaline 500 micrograms (1:1000) to the anterolateral thigh, repeated at 5 min

1Remove trigger, call for help, ABCDE. IM adrenaline 500 mcg (1:1000) anterolateral thigh; high-flow O2; lie flat with legs raised; IV crystalloid bolus.Gate: No improvement after 2 IM doses (refractory) → IV adrenaline infusion by a specialist + critical care
2Non-sedating antihistamine and steroids are NO LONGER routine first-line; do not let them delay adrenaline.
3Observe for biphasic reaction; prescribe 2 adrenaline auto-injectors + allergy clinic referral on discharge.

Adrenaline IM — 500 micrograms (0.5 mL of 1:1000) IM anterolateral thigh; repeat after 5 min if no improvement

High-flow oxygen + IV fluid bolus — crystalloid bolus for hypotension

Refractory: IV adrenaline infusion — specialist/critical care only

Key points

Adrenaline is IM (not IV) in the periarrest/conscious patient, repeated every 5 min as needed. Steroids/antihistamines are adjuncts, NOT the treatment (steroids no longer routine). Observe for biphasic reactions: 2h if good response/treated promptly, 6h if 2 doses needed, 12h if severe/asthma/late presentation. Discharge with 2 adrenaline auto-injectors + training + allergy referral; serum mast-cell tryptase confirms.

Monitor & prognosis

Continuous ABC, SpO2, BP/ECG; serial tryptase; observation period before discharge.

Excellent with prompt adrenaline; fatal if treatment is delayed.

Source: Resuscitation Council UK (Anaphylaxis 2021)