Atrial septal defect (ASD) is a relatively common congenital heart disease in clinical practice. From a medical perspective, the severity of ASD depends on whether the patient is an adult or a child. For adults with ASD, generally speaking, defects larger than 20 millimeters are considered large atrial septal defects, which are more severe. If the defect size is between 10 and 20 millimeters, it is often classified as a moderate atrial septal defect. Defects smaller than 4 millimeters are considered small atrial septal defects. Generally, small and moderate atrial septal defects are not severe. For children, an atrial septal defect larger than 10 millimeters is considered a large atrial septal defect, which is more severe. Sizes between 5 and 10 millimeters are classified as moderate atrial septal defects. Defects smaller than 5 millimeters are considered small atrial septal defects, which generally are not severe.