A 38-year-old woman from Mumbai presents with a 5-day history of productive cough, fever (38.5°C), and dyspnea. Chest X-ray shows bilateral interstitial infiltrates. Sputum culture and standard bacterial cultures are negative. Serology is positive for Mycoplasma pneumoniae IgM. She is started on azithromycin 500 mg once daily. On day 3 of therapy, she develops palpitations and dizziness. ECG shows a prolonged QTc interval (520 ms; baseline 410 ms). Which of the following is the most appropriate immediate management?

Explanation

## Clinical Scenario Analysis This patient has macrolide-induced QT prolongation, a well-recognized and potentially life-threatening adverse effect of azithromycin. ### Mechanism of QT Prolongation **Key Point:** Macrolides, particularly azithromycin, block cardiac potassium channels (hERG channels), delaying repolarization and prolonging the QTc interval. This risk is dose-dependent and increases with: - Hypokalemia or hypomagnesemia - Concurrent QT-prolonging drugs - Female sex - Older age - Renal or hepatic impairment ### Risk of Torsades de Pointes A QTc >500 ms or an increase >60 ms from baseline significantly elevates the risk of torsades de pointes, a potentially fatal polymorphic ventricular arrhythmia. ### Correct Management Strategy **High-Yield:** When QT prolongation occurs during macrolide therapy: 1. **Discontinue the macrolide immediately** — this is non-negotiable 2. **Switch to an alternative antibiotic** — fluoroquinolones (levofloxacin, moxifloxacin) are effective for Mycoplasma and atypical pneumonia 3. **Check serum electrolytes** (K^+^, Mg^2+^, Ca^2+^) and correct any deficiencies 4. **Perform continuous ECG monitoring** until QTc normalizes 5. **Avoid other QT-prolonging agents** (antiarrhythmics, antipsychotics, antiemetics) ### Why Fluoroquinolone? Fluoroquinolones have excellent coverage of Mycoplasma pneumoniae and are not associated with significant QT prolongation at therapeutic doses. Levofloxacin is the preferred choice in this scenario. ### Comparison of Macrolides and QT Risk | Macrolide | QT Risk | Clinical Use | |-----------|---------|---------------| | Azithromycin | **High** | Avoid if baseline QTc >450 ms or risk factors present | | Clarithromycin | **Moderate-High** | Similar risk to azithromycin | | Erythromycin | **Moderate** | Older agent; less commonly used | | Roxithromycin | **Low** | Preferred in some regions with QT risk | **Clinical Pearl:** Azithromycin is increasingly restricted in many countries (including India) for respiratory infections due to QT risk and antibiotic resistance concerns. Fluoroquinolones are now first-line for atypical pneumonia in many guidelines. **Warning:** Continuing or reducing the macrolide dose while the patient has symptomatic QT prolongation is dangerous — the risk of arrhythmia remains unacceptably high. Beta-blockers and verapamil do not address the underlying electrophysiologic abnormality and may mask symptoms.