A 70-year-old woman without any history of pulmonary or cardiac disease developed breathlessness with severe arterial hypoxemia. Cardiac ultrasound examination and spirometry were normal. Cardiac catheterization and intravenous microbubble injection demonstrated an interatrial right-to-left shunt. The defect was closed surgically, and the patient recovered immediately. On surgery, the aorta was elongated, inducing an angulation of the interatrial septum and leading to the atrial septal defect just above the inferior vena cava.