A 28-year-old woman 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 blood cultures are pending. She is started on azithromycin 500 mg on day 1, then 250 mg daily for 4 days. On day 3 of therapy, she develops palpitations and syncope. ECG shows QTc interval of 520 ms (baseline 380 ms). Which of the following is the most appropriate immediate action?

Explanation

## Clinical Scenario Analysis This patient has developed **QT prolongation** (QTc 520 ms, baseline 380 ms = 140 ms prolongation) secondary to azithromycin therapy. The combination of palpitations, syncope, and ECG changes suggests risk of **torsades de pointes**. ## Macrolide-Induced QT Prolongation **Key Point:** Azithromycin and other macrolides (erythromycin > clarithromycin) block cardiac potassium channels (hERG/IKr), prolonging the QT interval and increasing arrhythmia risk. **High-Yield:** Macrolide-induced QT prolongation is: - Dose-dependent - More common with IV erythromycin and clarithromycin than azithromycin - Potentiated by hypokalaemia, hypomagnesaemia, female sex, and concurrent QT-prolonging drugs - Can occur even at standard doses in susceptible patients ## Management of Macrolide-Induced QT Prolongation | Finding | Action | |---------|--------| | QTc < 500 ms, asymptomatic | Monitor; continue if benefit outweighs risk | | QTc 500–550 ms, asymptomatic | Consider alternative antibiotic | | QTc > 550 ms OR symptomatic (palpitations, syncope) | **DISCONTINUE immediately** | | Syncope + QT prolongation | High risk of torsades de pointes — stop drug | **Clinical Pearl:** Once QT prolongation becomes symptomatic (syncope, palpitations), the drug must be stopped immediately. Continuing it or reducing the dose carries unacceptable arrhythmia risk. ## Why Fluoroquinolone? For community-acquired pneumonia (CAP) with atypical features (interstitial infiltrates, need for macrolide coverage), a **respiratory fluoroquinolone** (levofloxacin, moxifloxacin) is an appropriate alternative that provides: - Atypical coverage (Mycoplasma, Chlamydia, Legionella) - Gram-positive and gram-negative coverage - No significant QT prolongation at standard doses **Warning:** Do NOT continue the offending drug, reduce the dose, or add electrolyte supplementation when symptomatic QT prolongation is present — these measures do not eliminate arrhythmia risk. ## Mnemonic: **STOP QT** - **S** — Syncope or severe symptoms → Stop immediately - **T** — Torsades risk is high - **O** — Ongoing QT > 550 ms - **P** — Prolongation is symptomatic - **Q** — QT-prolonging drug must be withdrawn - **T** — Try alternative antibiotic class