## Fluoroquinolone-Induced Cardiac Toxicity **Key Point:** Moxifloxacin is the fluoroquinolone with the HIGHEST risk of QT prolongation and torsades de pointes among all agents in this class. ### Mechanism of Cardiac Toxicity Fluoroquinolones block cardiac potassium channels (particularly hERG channels), leading to delayed repolarization and QT interval prolongation. This creates a substrate for re-entrant arrhythmias. ### Relative Cardiac Risk Among Fluoroquinolones | Fluoroquinolone | QT Prolongation Risk | Clinical Significance | |---|---|---| | **Moxifloxacin** | **Very High** | **Most cardiotoxic; avoid in QT prolongation** | | Gemifloxacin | High | Second-highest risk | | Levofloxacin | Moderate | Lower risk than moxifloxacin | | Ciprofloxacin | Low | Minimal QT effect | | Norfloxacin | Low | Minimal QT effect | **High-Yield:** Moxifloxacin's enhanced cardiac toxicity is due to its broader spectrum of activity and structural features that increase hERG channel blockade. ### Risk Factors for Torsades de Pointes - Pre-existing QT prolongation - Hypokalaemia - Hypomagnesaemia - Female sex - Bradycardia - Concurrent QT-prolonging drugs (macrolides, antiarrhythmics, antipsychotics) - Heart failure **Clinical Pearl:** Moxifloxacin should be **avoided** in patients with known QT prolongation, electrolyte abnormalities, or concurrent cardiotoxic medications. Baseline ECG is recommended before use in high-risk patients. **Warning:** Do not confuse moxifloxacin's broad spectrum (which makes it attractive for atypical coverage) with cardiac safety — the two are inversely related in this class.
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