Medications That Prolong the QT Interval

Drug-induced QT prolongation is an important clinical concern as it can lead to severe ventricular arrhythmias, especially Torsades de Pointes (TdP). This guide presents the main drug classes associated with this effect.

Why Does This Matter?

QT prolongation represents a delay in ventricular repolarization. When excessive (usually QTc > 500ms), it significantly increases the risk of:
- Torsades de Pointes - polymorphic ventricular tachycardia
- Syncope - sudden loss of consciousness
- Sudden cardiac death

Drug Classes

1. Antiarrhythmics

Antiarrhythmics are the best-known class for prolonging QT, especially Class III:
| Drug | Risk | Notes |
|------|------|-------|
| Amiodarone | High | Significantly prolongs QT, but TdP is relatively rare |
| Sotalol | High | Dose-dependent risk |
| Dofetilide | High | Requires monitoring in hospital setting |
| Ibutilide | High | Use only in monitored setting |
| Dronedarone | Moderate | Less prolongation than amiodarone |

2. Antibiotics

Several commonly used antibiotics can prolong QT:
| Drug | Class | Risk |
|------|-------|------|
| Azithromycin | Macrolide | Moderate |
| Erythromycin | Macrolide | High (especially IV) |
| Clarithromycin | Macrolide | Moderate |
| Levofloxacin | Fluoroquinolone | Moderate |
| Moxifloxacin | Fluoroquinolone | High |
| Ciprofloxacin | Fluoroquinolone | Low-Moderate |

3. Antipsychotics

Virtually all antipsychotics can prolong QT:
| Drug | Risk | Notes |
|------|------|-------|
| Haloperidol | High | Especially IV at high doses |
| Droperidol | High | Black box warning |
| Ziprasidone | Moderate-High | Highest prolongation among atypicals |
| Quetiapina | Moderate | Dose-dependent |
| Risperidone | Low-Moderate | Relatively safer |

4. Antidepressants

Some antidepressants require special attention:
| Drug | Class | Risk |
|------|-------|------|
| Citalopram | SSRI | Moderate (dose-dependent, max 40mg) |
| Escitalopram | SSRI | Moderate (max 20mg) |
| Amitriptyline | Tricyclic | Moderate-High |
| Imipramine | Tricyclic | Moderate |

5. Antiemetics

| Drug | Risk | Notes |
|------|------|-------|
| Ondansetron | Moderate | Avoid high IV doses |
| Domperidone | Moderate | Restricted in some countries |
| Metoclopramide | Low | Relatively safe |

6. Other Medications

| Drug | Class | Risk |
|------|-------|------|
| Methadone | Opioid | High (dose-dependent) |
| Hydroxychloroquine | Antimalarial | Moderate |
| Tacrolimus | Immunosuppressant | Moderate |
| Sumatriptan | Triptan | Low |

Additional Risk Factors

TdP risk increases with predisposing factors:
1. Prolonged baseline QTc (>450ms men, >470ms women)
2. Electrolyte disturbances:
- Hypokalemia (low K+)
- Hypomagnesemia (low Mg2+)
- Hypocalcemia (low Ca2+)
3. Bradycardia (<50 bpm)
4. Multiple QT-prolonging drugs
5. Heart failure
6. Congenital long QT syndrome
7. Female sex (naturally longer QT)
8. Advanced age
9. Renal or hepatic impairment (altered metabolism)

Clinical Recommendations

Before Prescribing

1. Assess baseline ECG - measure QTc before starting high-risk medications
2. Review history - personal/family long QT, unexplained syncope
3. Check electrolytes - especially K+, Mg2+, Ca2+
4. Review other medications - avoid risky combinations

During Treatment

1. Monitor ECG - especially in the first 48-72h
2. Maintain normal electrolytes - K+ > 4.0 mEq/L, Mg2+ > 2.0 mg/dL
3. Avoid combinations - do not combine multiple QT prolongers
4. Reassess if QTc > 500ms - consider discontinuing medication

Useful Resources

- CredibleMeds - updated medication list
- QTc Calculator - monitor QTc during treatment

Where can I check which medications prolong QT?
CredibleMed is the international reference maintained by AZCERT (Arizona Center for Education and Research on Therapeutics).

TdP Risk Categories:
Known Risk: Substantial evidence of QT prolongation and Torsades de Pointes risk
Possible Risk: Can cause QT prolongation, but lacks evidence of TdP risk
Conditional Risk: Associated with TdP under specific conditions (overdose, drug interactions)
Avoid if LQTS: Avoid in patients with congenital long QT syndrome
Tip: Always consult CredibleMed before prescribing medications in patients with borderline or prolonged QTc.

Conclusion

Knowledge of QT-prolonging medications is essential for safe clinical practice. Individualized risk assessment, considering predisposing factors and drug interactions, allows safe use of these drugs when necessary.
Use QTc Calculator to monitor your patients during treatment with QT-prolonging medications.
This content is informational and does not replace medical evaluation. Always consult package inserts and updated guidelines.

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