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30th August 2024
QTc prolongation
The QT interval is a measurement on an electrocardiogram that represents the time it takes for the heart’s ventricles to depolarise and then repolarise. A corrected QT interval (QTc) is often used to account for variations in heart rate. QTc is calculated using formulas like Bazett’s formula to provide a standardised measurement.
Prolongation of the QT interval on the ECG reflects delayed ventricular repolarisation. Normal QTc reference ranges are < 440ms for men and <460 for women. A potentially dangerous condition called Torsades de Pointes, which can lead to sudden cardiac arrest. Many factors can cause QTc prolongation, as summarised in the table below.
| Category | Causes |
|---|---|
| Medications | Antiarrhythmics (e.g., sotalol), antibiotics (e.g., erythromycin), antipsychotics (e.g., haloperidol), antidepressants (e.g., citalopram) |
| Electrolyte Imbalances | Hypokalaemia, hypomagnesemia, hypocalcaemia |
| Medical Conditions | Heart failure, bradycardia, myocardial infarction |
| Genetic Factors | Congenital long QT syndrome (LQTS) |
| Others | Starvation, severe diarrhoea, certain toxins |
References
- Acquired long QT syndrome: Definitions, pathophysiology, and causes. UpToDate. Available from: https://www.uptodate.com/contents/acquired-long-qt-syndrome-definitions-pathophysiology-and-causes. Accessed 26 Jun 2024
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