A 28-year-old woman presents with acute anaphylaxis following a penicillin injection for pneumonia. She develops severe bronchospasm, hypotension, and angioedema within 5 minutes of drug administration. What is the drug of choice for immediate management of this Type I hypersensitivity reaction?

Explanation

## Management of Acute Anaphylaxis (Type I Hypersensitivity) ### Pathophysiology Type I hypersensitivity is an IgE-mediated, immediate reaction. Mast cell and basophil degranulation releases histamine, tryptase, leukotrienes, and prostaglandins, causing: - Bronchospasm and airway edema - Vasodilation and hypotension - Angioedema ### Drug of Choice: Epinephrine (Adrenaline) **Key Point:** Epinephrine is the ONLY first-line, life-saving drug for acute anaphylaxis. It must be given immediately, intramuscularly (0.3–0.5 mg of 1:1000 solution in adults). ### Mechanism of Epinephrine | Effect | Receptor | Clinical Benefit | |--------|----------|------------------| | Bronchodilation | β~2~-adrenergic | Relieves bronchospasm | | Vasoconstriction | α-adrenergic | Reverses hypotension | | ↓ Mast cell degranulation | β~2~-adrenergic | Prevents further mediator release | | ↑ cAMP in mast cells | β~2~-adrenergic | Stabilizes cell membrane | **High-Yield:** IM epinephrine is superior to IV because it avoids the risk of arrhythmias and provides sustained absorption. Dose may be repeated every 5–15 minutes if symptoms persist. ### Adjunctive Therapy (After Epinephrine) 1. **Antihistamines** (diphenhydramine, cetirizine) — block H~1~ and H~2~ receptors; slower onset, not first-line 2. **Corticosteroids** (methylprednisolone, dexamethasone) — prevent biphasic reactions; take 4–6 hours to work 3. **Supportive care** — IV fluids, oxygen, monitoring **Clinical Pearl:** Biphasic anaphylaxis (recurrence 1–72 hours later) occurs in ~20% of cases; corticosteroids reduce this risk, but they are NOT first-line for acute symptoms. **Warning:** Do NOT delay epinephrine while awaiting IV access or other interventions. IM administration is faster and safer in the acute phase. ### Mnemonic for Anaphylaxis Management **ABCDE:** - **A**irway, **B**reathing, **C**irculation (ABC) - **D**rug: Epinephrine IM immediately - **E**levate legs, establish IV access, monitor [cite:Robbins 10e Ch 6]