Why is QT interval prolongation clinically significant for patients on certain antiarrhythmics?

• A. It shortens the QT interval.
• B. It increases risk of torsades de pointes.
• C. It has no clinical consequence.
• D. It causes immediate heart attack.

Answer: (B)

QT interval prolongation signals delayed ventricular repolarization, which can create a substrate for early afterdepolarizations. Those extra impulses can trigger torsades de pointes, a dangerous form of ventricular tachycardia that can progress to ventricular fibrillation and sudden death. Many antiarrhythmics prolong the QT by blocking potassium channels (IKr), so patients on these drugs require careful QTc monitoring, electrolyte optimization, and avoidance of additional QT-prolonging medications. Clinically, a substantially prolonged QTc (often cited around or above 500 ms) markedly increases the risk of torsades de pointes, making this a key safety consideration in antiarrhythmic therapy.