A 34-year-old woman with a history of atrial fibrillation presents with community-acquired pneumonia caused by Streptococcus pneumoniae. She is started on azithromycin 500 mg daily. On day 3 of therapy, she develops palpitations and syncope. ECG shows prolonged QT interval (QTc 520 ms) and polymorphic ventricular tachycardia. Which of the following is the most appropriate immediate management?

Explanation

## Clinical Scenario Analysis This patient has developed **drug-induced torsades de pointes** secondary to azithromycin-induced QT prolongation in the setting of pre-existing atrial fibrillation (a risk factor for arrhythmia). ## Mechanism of Azithromycin-Induced QT Prolongation **Key Point:** Macrolides, particularly azithromycin, block cardiac potassium channels (hERG/IKr), leading to delayed repolarization and QT prolongation. This is dose-dependent and concentration-dependent. **High-Yield:** Azithromycin carries a **black-box warning** for QT prolongation and torsades de pointes, especially in patients with: - Pre-existing QT prolongation - Electrolyte abnormalities (hypokalemia, hypomagnesemia) - Female sex - Older age - Concurrent QT-prolonging drugs - Cardiac arrhythmias (like this patient's AFib) ## Immediate Management of Torsades de Pointes 1. **Discontinue the offending agent** — azithromycin must be stopped immediately 2. **Correct electrolyte abnormalities** — administer IV magnesium sulfate (1–2 g bolus) regardless of serum level 3. **Supportive care** — continuous cardiac monitoring, oxygen, IV access 4. **Switch to alternative antibiotic** — use fluoroquinolone (levofloxacin) or beta-lactam (amoxicillin-clavulanate) for pneumonia ## Why Other Options Are Wrong | Option | Problem | |--------|----------| | Continue azithromycin + beta-blocker | Beta-blockers do NOT reverse QT prolongation or prevent torsades; the offending drug must be removed | | Reduce dose by 50% | Dose reduction is insufficient once torsades has occurred; the drug must be discontinued entirely | | Switch to another macrolide | All macrolides carry similar QT risk; switching within the class does not eliminate the danger | **Clinical Pearl:** Magnesium sulfate is the first-line antiarrhythmic for torsades de pointes, even in normomagnesia, because it stabilizes the cardiac action potential and suppresses early afterdepolarizations. ## High-Yield Macrolide Cardiotoxicity Ranking **Mnemonic: AZI-RISK** — **A**zithromycin > **Z**ithromax (highest risk), **I**ncreased QT, **R**isk in AFib, **I**V formulation worse, **S**witch early, **K**eep magnesium ready - **Azithromycin** — highest QT risk (black-box warning) - **Clarithromycin** — moderate QT risk - **Erythromycin** — lower QT risk (but GI side effects) [cite:KD Tripathi 8e Ch 48]