Atrial Septal Defect Ostium Secundum

A 26-year-old primigravida presents with effort dyspnea and palpitations (NYHA class II) detected during antenatal booking. On examination, she has a hyperdynamic right ventricular impulse, a soft systolic ejection murmur at the left upper sternal border, and a mid-diastolic flow murmur at the left lower sternal border. The hallmark finding is wide, fixed splitting of the second heart sound that does not vary with respiration. Transthoracic echocardiogram with bubble study demonstrates a 16 mm defect at the **fossa ovalis** (marked **A** in the diagram) with left-to-right shunting and a pulmonary-to-systemic flow ratio (Qp:Qs) of 2.1. Which of the following best explains the fixed splitting of the second heart sound in this patient?