What is an atrial septal defect (ASD)? An ASD is a hole in the wall between the heart’s upper chambers. One of the most common heart defects in infants, ASDs vary in size and in the severity of symptoms they may cause. In some children, an ASD may close on its own without any treatment. If the ASD is still present by age 3, it likely will require surgery. Learn more. http://bit.ly/2KZB48E | The Society of Thoracic Surgeons (STS)