An atrial septal defect is a congenital heart defect (present at birth) in which the wall that separates the upper heart chambers (atria) does not close completely, leaving a hole. In this condition, too much blood flows from the left side of the heart (left atrium) to the right side of the heart (right atrium). This excess blood flow makes the heart work harder. Most atrial septal defects are discovered during childhood when a murmur (an extra heart sound) is heard during a physical examination. Small holes may close on their own by the time a child is 2 years old. In some people the defect does not cause symptoms until later in life.
 
Signs and symptoms

• Heart murmur, heart rhythm abnormalities (arrhythmias)
• Shortness of breath, fatigue
• Swelling of the legs
• Frequent lung infections
• Stroke
• Bluish skin color.

Over time, if a large atrial septal defect is not repaired, the extra blood flow to the right side of the heart can damage the heart and lungs, causing heart failure (weakened heart) and arrhythmias (irregular heartbeat).