Inadvertent Left Ventricular Pacing Through an Unrecognized Sinus Venosus Atrial Septal Defect

Inadvertent left ventricular (LV) lead placement is rare but exposes patients to systemic thromboembolism.

An 89-year-old man received a dual-chamber pacemaker for atrial fibrillation with paroxysmal complete heart block. Severe right atrial dilatation prevented right ventricular lead placement, and the operator converted to a sheath-delivered "left bundle branch area" approach. Seven months later, imaging unexpectedly showed the lead coursing from the superior vena cava through an unrecognized superior sinus venosus defect into the left atrium, across the mitral valve, into the LV. Transesophageal echocardiography confirmed a 30 × 20 mm superior sinus venosus defect with partial anomalous pulmonary venous return.

A paced right bundle branch block pattern is expected during left bundle branch area pacing, and activation-time criteria may be falsely reassuring when the lead paces the LV endocardium directly. Atrial septal defect screening is warranted whenever right-sided dilatation is unexplained.