Amiodarone in patients with left bundle branch block: how to assess the QT interval?

Amiodarone is commonly used for the treatment of supraventricular and ventricular arrhythmias. As a class III antiarrhythmic drug, it prolongs phase III of the cardiac action potential leading to QT interval prolongation. Therefore, the QTc interval should be monitored during amiodarone up-titration to prevent proarrhythmia. However, QTc monitoring in bundle branch block requires some modification as outlined in this case report. The normal upper value of QT interval has been set at 450 ms for males and 460 ms for females. Patients with preexisting bundle branch block (BBB) by definition exhibit wider QRS intervals, ranging between 120 and 200 ms. This ‘augmented’ QT interval duration is mainly driven by the prolonged time of ventricular depolarization, rather than the time of ventricular repolarization. This inherent QT interval prolongation in BBB can be corrected with specifically designed electrocardiographic formulas. Nevertheless, accurate QT interval calculation at very low or high heart rates remains challenging.