Cardiac resynchronization therapy (CRT) is an established therapy in patients with a left ventricular (LV) ejection fraction (EF) < 35 %, and an electrocardiogram (ECG) with prolonged QRS duration (1). The treatment is implemented by implanting a pacemaker with three pacing leads: One in the right atrium, one in the right ventricle (RV) and one in an epicardial vein through the coronary sinus, thereby establishing atrial-synchronized biventricular pacing to coordinate RV and LV contraction.