On a standard ECG, the QTc interval corrects QT for heart rate (Bazett's formula: QTc = QT/√RR). In a patient taking sotalol, QTc is 510 ms (normal <440 ms in men). The electrophysiological risk of this prolonged QTc is:

• A Ventricular fibrillation due to accelerated phase 0 depolarization
• B Complete heart block due to AV nodal conduction slowing
• C Torsades de pointes — early afterdepolarizations during prolonged repolarization can trigger polymorphic VT ✓
• D Atrial fibrillation due to inhomogeneous atrial repolarization

Explanation

QT prolongation indicates prolonged ventricular repolarization (predominantly phase 3 of the action potential — delayed IKr closure). During a prolonged action potential plateau, oscillatory depolarizing currents (early afterdepolarizations, EADs) can occur and trigger ectopic beats — leading to torsades de pointes (polymorphic VT with a twisting QRS axis around the isoelectric baseline). Sotalol blocks IKr (class III antiarrhythmic) as well as beta-receptors. Hypokalaemia and hypomagnesaemia worsen this risk by further impairing repolarization.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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