A 28-year-old man with atypical pneumonia (suspected *Mycoplasma pneumoniae*) is started on azithromycin 500 mg once daily. On day 5, he develops palpitations, dizziness, and syncope. ECG shows a prolonged QTc interval of 520 ms (baseline 400 ms). Serum potassium is 3.2 mEq/L (normal 3.5–5.0). What is the most appropriate immediate next step?

Explanation

## Clinical Presentation: Macrolide-Induced QT Prolongation The patient has developed **symptomatic QT prolongation** (syncope, palpitations) with a marked increase in QTc interval (520 ms from baseline 400 ms) and concurrent hypokalemia. This is a serious, potentially life-threatening adverse effect of macrolide antibiotics. ## Pathophysiology of Macrolide Cardiotoxicity **Key Point:** Macrolides (particularly azithromycin and erythromycin) block cardiac potassium channels (hERG channels), delaying repolarization and prolonging the QT interval. This risk is **amplified by hypokalemia**, which further impairs repolarization and increases the risk of **torsades de pointes**. ## Risk Stratification | Feature | Significance | |---------|-------------| | QTc > 500 ms | Significant prolongation; risk of arrhythmia | | Symptomatic QT prolongation (syncope, palpitations) | **Indicates imminent risk of torsades de pointes** | | Hypokalemia (K⁺ < 3.5) | Synergistic risk factor; impairs repolarization | | Azithromycin use | Known QT-prolonging agent | **Warning:** Symptomatic QT prolongation with syncope is a **medical emergency**. Continuing the offending agent risks sudden cardiac death from torsades de pointes. ## Management Algorithm ```mermaid flowchart TD A[Macrolide-induced QT prolongation]:::outcome --> B{Symptomatic?}:::decision B -->|Yes: syncope/palpitations| C[DISCONTINUE macrolide immediately]:::urgent B -->|No: asymptomatic| D[Risk assessment] C --> E[Switch to alternative antibiotic]:::action E --> F[Correct hypokalemia]:::action F --> G[Repeat ECG after K+ normalization]:::action D --> H{QTc > 500 ms?}:::decision H -->|Yes| C H -->|No| I[Continue with monitoring]:::action ``` ## Correct Management ### Step 1: Discontinue Azithromycin - Symptomatic QT prolongation is an absolute contraindication to continued use - Risk of sudden cardiac death outweighs any benefit from continuing the macrolide ### Step 2: Switch to Alternative Antibiotic - **Doxycycline 100 mg twice daily** is first-line for *Mycoplasma pneumoniae* - No QT-prolonging effects; excellent intracellular penetration - Oral bioavailability is adequate for atypical pneumonia ### Step 3: Correct Hypokalemia - Oral potassium supplementation (20–40 mEq daily in divided doses) is appropriate - IV potassium is reserved for severe hypokalemia (< 2.5 mEq/L) or inability to take oral medication - Target K⁺ > 4.0 mEq/L to restore repolarization reserve ### Step 4: Repeat ECG - Obtain ECG 24–48 hours after discontinuation and K⁺ correction - QTc should normalize as the drug is eliminated and electrolytes are corrected **High-Yield:** Azithromycin is the **most arrhythmogenic macrolide** in clinical use. In patients with QT prolongation, syncope, or risk factors (female sex, bradycardia, electrolyte abnormalities, heart failure), it should be avoided or used with extreme caution. ## Clinical Pearl Macrolide-induced torsades de pointes can occur even in young, otherwise healthy patients. The combination of QT prolongation + hypokalemia + symptoms is a medical emergency requiring immediate drug discontinuation and electrolyte correction.