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?

Question

Accepted Answer

D. Epinephrine (adrenaline). ## 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]

Answer

A. Methylprednisolone

Answer

B. Diphenhydramine

Answer

C. Sodium bicarbonate

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

Drug of Choice: Epinephrine (Adrenaline)

Mechanism of Epinephrine

Adjunctive Therapy (After Epinephrine)

Mnemonic for Anaphylaxis Management

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Effect	Receptor	Clinical Benefit
Bronchodilation	β₂-adrenergic	Relieves bronchospasm
Vasoconstriction	α-adrenergic	Reverses hypotension
↓ Mast cell degranulation	β₂-adrenergic	Prevents further mediator release
↑ cAMP in mast cells	β₂-adrenergic	Stabilizes cell membrane