In atrial septal defect, the aoa is

The chest x-ray film is often, but not always, abnormal in patients with significant ASD. Cardiomegaly may be present from right hea dilation and occasionally from left hea dilation if significant mitral regurgitation is present in the patient with an ostium primum ASD. Right hea dilation is better appreciated in lateral films. The central pulmonary aeries are characteristically enlarged, with pulmonary plethora indicating increased pulmonary flow. A small aoic knuckle is characteristic, which reflects a chronically low systemic cardiac output state because increased pulmonary flow in these patients occurs at the expense of reduced systemic flow.(Ref: "Atrial Septal Defects in the Adult - Recent Progress and Overview". By Webb & Gatzoulis. Circulation. 2006; 114: 1645-1653.)