QT Prolongation Drug Combinations

## QT Interval and Torsades de Pointes

The QT interval represents ventricular depolarization and repolarization. Drugs that block the hERG (IKr) potassium channel delay repolarization, prolonging QT. When QTc exceeds 500 ms or increases by more than 60 ms from baseline, the risk of torsades de pointes (TdP) — a polymorphic ventricular tachycardia — rises substantially.

## Drug Categories That Prolong QT

### Known Risk (CredibleMeds Category 1)

- **Antiarrhythmics**: amiodarone, sotalol, dofetilide, quinidine
- **Antibiotics**: azithromycin, erythromycin, fluoroquinolones (levofloxacin, moxifloxacin), TMP-SMX
- **Antipsychotics**: haloperidol (especially IV), ziprasidone, thioridazine
- **Antidepressants**: citalopram, escitalopram (dose-dependent; FDA limit 40 mg/20 mg)
- **Antiemetics**: ondansetron (IV doses above 16 mg), domperidone, droperidol
- **Antifungals**: fluconazole
- **Others**: methadone, vandetanib, arsenic trioxide

### Dangerous Combinations

Combining two QT-prolonging drugs produces additive or synergistic effects:

| Combination | Setting | Risk |
|-------------|---------|------|
| Azithromycin + fluoroquinolone | Pneumonia | Additive QT prolongation |
| Methadone + fluconazole | HIV/addiction | CYP3A4 inhibition + dual QT effect |
| Haloperidol + ondansetron | ICU/postoperative | Additive hERG block |
| Citalopram + azithromycin | Primary care | Both prolong QT independently |
| Amiodarone + fluoroquinolone | Cardiology/ID | Very high cumulative risk |

## Risk Factors for TdP

Drug-induced QT prolongation alone rarely causes TdP. Additional risk factors are almost always present:

- **Hypokalemia** — the single most important modifiable risk factor; depleted by diuretics, diarrhea, vomiting
- **Hypomagnesemia** — often coexists with hypokalemia
- **Female sex** — women have longer baseline QTc and 2-3x higher TdP risk
- **Bradycardia** — pause-dependent TdP; heart rate below 50 increases risk
- **Heart failure** — structural heart disease amplifies arrhythmia vulnerability
- **Pharmacokinetic interactions** — CYP inhibitors increase QT-drug exposure (e.g., ketoconazole + cisapride, the interaction that led to cisapride's withdrawal)

## Clinical Management

1. **Check baseline ECG** before starting known QT-prolonging drugs in at-risk patients
2. **Monitor electrolytes** — maintain K+ above 4.0 mEq/L and Mg2+ above 2.0 mg/dL
3. **Avoid combinations** of two or more QT-prolonging drugs when alternatives exist
4. **Use CredibleMeds** (crediblemeds.org) to check QT risk category of any drug
5. **Repeat ECG** after dose increases or when adding a second QT-prolonging agent
6. **Hold the drug** if QTc exceeds 500 ms or increases more than 60 ms from baseline

## Key Takeaways

- QT prolongation becomes dangerous when QTc exceeds 500 ms or increases 60+ ms from baseline
- Hypokalemia is the most important modifiable risk factor for TdP
- Combining two QT-prolonging drugs creates additive risk
- CYP inhibitors can increase QT-drug levels, compounding the risk
- Women have inherently longer QTc and higher TdP susceptibility
- CredibleMeds.org categorizes drugs by QT risk level