## Macrolide Cardiac Toxicity **Key Point:** Erythromycin is the macrolide with the HIGHEST risk of QT prolongation and torsades de pointes among all macrolides. ### Mechanism of QT Prolongation Macrolides block cardiac potassium channels (hERG channels), delaying repolarization and prolonging the QT interval. This creates a substrate for re-entrant arrhythmias, particularly torsades de pointes. ### Relative Cardiac Risk Among Macrolides | Macrolide | QT Risk | Mechanism | Clinical Use Impact | |-----------|---------|-----------|---------------------| | **Erythromycin** | **Highest** | Strong hERG channel blockade | Avoid in QT prolongation, elderly | | **Azithromycin** | Moderate | Moderate hERG blockade | Preferred over erythromycin | | **Roxithromycin** | Low | Minimal channel interaction | Safer alternative | | **Spiramycin** | Low | Minimal cardiac effects | Used in toxoplasmosis | **High-Yield:** Erythromycin > Azithromycin > Roxithromycin in terms of QT prolongation risk. Erythromycin is now reserved for specific indications (e.g., gastroparesis) and avoided in patients with baseline QT prolongation, hypokalaemia, or concurrent QT-prolonging drugs. ### Risk Factors for Torsades de Pointes with Macrolides - Female sex - Hypokalaemia or hypomagnesaemia - Bradycardia - Concurrent QT-prolonging agents (fluoroquinolones, antiarrhythmics, antipsychotics) - Hepatic or renal impairment - Advanced age **Clinical Pearl:** In modern practice, azithromycin is preferred over erythromycin for most infections due to better cardiac safety profile, despite both being macrolides. [cite:KD Tripathi 8e Ch 47]
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