An atrial septal defect (ASD) is a congenital heart condition characterized by a hole in the wall (septum) that separates the two upper chambers of the heart, known as the atria. This defect allows oxygen-rich blood from the left atrium to mix with oxygen-poor blood in the right atrium, which can lead to increased blood flow to the lungs and potential complications over time. ASDs can vary in size and may not cause noticeable symptoms in some individuals. However, if left untreated, they can lead to heart enlargement, arrhythmias, and pulmonary hypertension. Diagnosis typically involves echocardiography or other imaging techniques, and treatment options may include monitoring, medication, or surgical repair.