A 28-year-old man with atypical pneumonia (suspected *Mycoplasma pneumoniae*) is prescribed azithromycin 500 mg on day 1, then 250 mg daily for 4 days. On day 2, he develops palpitations and syncope. ECG shows prolonged QTc interval (520 ms). What is the most appropriate immediate management?

Question

Accepted Answer

D. Discontinue azithromycin immediately and obtain baseline electrolytes (K+, Mg2+, Ca2+); monitor ECG continuously; switch to doxycycline for atypical pneumonia. ## Macrolide-Induced QT Prolongation and Torsades de Pointes

### Risk Recognition
**Key Point:** Azithromycin is a potent inhibitor of cardiac potassium channels (hERG), causing dose-dependent QT prolongation. Syncope with QTc >500 ms signals imminent risk of torsades de pointes.

### Mechanism of QT Prolongation
Macrolides block the rapid component of the delayed rectifier potassium current (I~Kr~), delaying ventricular repolarization. Risk is highest with:
  - Azithromycin > clarithromycin > erythromycin
  - Baseline QTc prolongation
  - Hypokalaemia, hypomagnesaemia, hypocalcaemia
  - Drug interactions (CYP3A4 inhibitors, other QT-prolonging agents)
  - Female sex, age >60 years

### Immediate Action
**High-Yield:** Discontinuation is mandatory when symptomatic QT prolongation (syncope, palpitations) occurs. This is a medical emergency.

### Management Algorithm

```mermaid
flowchart TD
  A[Azithromycin + QTc >500 ms + Syncope]:::urgent --> B[STOP azithromycin immediately]:::action
  B --> C[Check serum K+, Mg2+, Ca2+]:::action
  C --> D[Continuous cardiac monitoring]:::action
  D --> E{Electrolytes normal?}:::decision
  E -->|No| F[Correct deficiencies IV]:::action
  E -->|Yes| G[Supportive care]:::action
  F --> H[Switch to non-macrolide: doxycycline or fluoroquinolone]:::action
  G --> H
  H --> I[ECG follow-up at 24-48 hrs]:::action
```

### Alternative Agents for *Mycoplasma*
| Agent | Advantage | Disadvantage |
|-------|-----------|-------------|
| Doxycycline | No QT risk, good lung penetration | Photosensitivity, esophageal irritation |
| Fluoroquinolone (levofloxacin) | No QT risk, excellent atypical coverage | Less data in young patients |
| Clarithromycin | Macrolide option | Still carries QT risk; NOT preferred here |

**Warning:** Clarithromycin is NOT safer than azithromycin in this context—both prolong QT. Switching to clarithromycin would perpetuate the problem.

**Clinical Pearl:** Syncope in a patient on azithromycin is a medical emergency until torsades de pointes is ruled out. Do not continue or reduce the dose—discontinue immediately.

[cite:Harrison 21e Ch 235; KD Tripathi 8e Ch 49]

Answer

A. Switch to a macrolide with less QT prolongation risk, such as clarithromycin

Answer

B. Continue azithromycin and add a beta-blocker to control arrhythmias

Answer

C. Reduce azithromycin dose by 50% and add intravenous magnesium supplementation

A 28-year-old man with atypical pneumonia (suspected Mycoplasma pneumoniae) is prescribed azithromycin 500 mg on day 1, then 250 mg daily for 4 days. On day 2, he develops palpitations and syncope. ECG shows prolonged QTc interval (520 ms). What is the most appropriate immediate management?

Explanation

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