Episode
Podcast 994: Biphasic Anaphylaxis
- Podcast
- Emergency Medical Minute
- Published
- Feb 16, 2026
- Duration seconds
- 194
- Processing state
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Summary
Contributor: Aaron Lessen, MD Educational Pearls: What is anaphylaxis and what are its treatments? Anaphylaxis is a broad term for potentially life threatening allergic reactions that can progress to cardiovascular collapse (anaphylactic shock). It is triggered by IgE and antigen cross-linking on mast cells to induce degranulation and the release of histamines, which can cause diffuse vasodilation and respiratory involvement with end-organ hypoperfusion. First line treatment is the immediate administration of epinephrine at 0.01 mg/kg (max dose for pediatrics is 0.3 mg and for adults is 0.5 mg) as well as removal of the offending agent causing the reaction. Additional pharmacologic treatments such as anti-histamines and steroids should be considered but not used instead of epinephrine when anaphylactic shock is evident as the sole therapy. What is biphasic anaphylaxis and what is its occurrence? Biphasic anaphylaxis is the return of anaphylactic symptoms after the initial anaphylactic event. Previous studies have reported an incidence ranging from 1-20% of patients having an initial anaphylactic reaction having biphasic anaphylaxis, at a range of time from 1-72 hours. The mechanism of biphasic anaphylaxis is not completely known, but can be contributed to by initial interventions wearing off (and why patients will be monitored for 2-4 hours after initial symptoms and treatment), or delayed immune mediators beginning to take effect. Recent studies show that the rate of biphasic anaphylaxis may be closer to 16% occurrence with a median time of occurrence being around 10 hours. What is the key take away and patient education on biphasic anaphylaxis? After patients have been observed for the initial 2-4 hours in the emergency room, they are generally safe to go home. Patie…