Prolongation of the QT interval can lead to a life-threatening ventricular arrhythmia known as torsades de pointes, which can result in sudden cardiac death. There are a number of widely used drugs that are known to cause QT prolongation.
The QT interval
The QT interval varies with heart rate, so correction formulas are used to calculate a heart rate–adjusted value known as the QTc interval. This corrected value is typically provided on the ECG printout.
A normal QTc interval is <440 ms. A QTc between 440–500 ms is considered borderline prolonged, although definitions vary slightly in the literature. In such cases, it may be appropriate to reduce the dose of QT-prolonging medications or switch to a non–QT-prolonging alternative.
A QTc >500 ms is considered clinically significant, as it is associated with a higher risk of arrhythmias. Any QT-prolonging drugs should be reviewed and managed urgently.
Drugs causing prolongation of the QT interval
Some of the more commonly encountered drugs that are known to prolong the QT interval are shown below:
| Antimicrobials | Erythromycin Clarithromycin Moxifloxacin Fluconazole Ketoconazole |
| Antiarrhythmics | Dronedarone Sotalol Quinidine Amiodarone Flecainide |
| Antipsychotics | Risperidone Fluphenazine Haloperidol Pimozide Chlorpromazine Quetiapine Clozapine |
| Antidepressants | Citalopram/escitalopram Amitriptyline Clomipramine Dosulepin Doxepin Imipramine Lofepramine |
| Antiemetics | Domperidone Droperidol Ondansetron/Granisetron |
| Others | Methadone Protein kinase inhibitors, e.g. sunitinib Some antimalarials Some antiretrovirals Telaprevir Boceprevir |
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