Anaphylaxis — Recognition & Triggers
A patient receives IV antibiotics and within minutes develops lip swelling and wheezing — but the blood pressure is still normal. Is this anaphylaxis? The answer determines whether you have seconds or minutes to act.
Core Concept
Anaphylaxis is a severe, rapid-onset systemic allergic reaction mediated by massive IgE-triggered histamine and mediator release from mast cells and basophils. It is a form of distributive shock caused by widespread vasodilation and capillary permeability, not volume loss or pump failure. Recognition hinges on identifying involvement of two or more body systems within minutes to hours of exposure. The classic triad: skin (urticaria, flushing, angioedema), respiratory (laryngeal edema, stridor, bronchospasm, wheezing), and cardiovascular (hypotension, tachycardia, weak pulse). Skin signs are present in up to 90% of cases but can be absent — anaphylaxis without urticaria is the presentation students miss. GI involvement (cramping, vomiting, diarrhea) counts as a system. Onset is typically within 5–30 minutes of IV exposure, up to 2 hours for oral ingestion. A biphasic reaction can recur 1–72 hours after the initial episode without re-exposure, which is why observation periods of at least 4–6 hours are standard.
Watch Out For
Don't confuse anaphylaxis (multisystem, requires epinephrine) with a localized allergic reaction (isolated hives, no respiratory or cardiovascular compromise). Students mistake vasovagal syncope post-injection (bradycardia, pallor, diaphoresis) for anaphylaxis — anaphylaxis produces tachycardia, not bradycardia. Angioedema of the lips and tongue signals airway threat even before wheezing begins; waiting for stridor means the window is closing.
Clinical Pearl
Two systems, think anaphylaxis. Hives plus wheeze? Hives plus vomiting? Wheeze plus hypotension? Stop the trigger and call for epinephrine — don't wait for all three.
Test Your Knowledge
3 quick questions — see how well you understood Anaphylaxis — Recognition & Triggers