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, larger defects can lead to heart murmurs, fatigue, and shortness of breath. Treatment options include monitoring, medication, or surgical procedures to close the defect and improve heart function.