An atrial septal defect of 3-6 mm in diameter is a small defect, 6-12 mm is a medium defect, and greater than 12 mm is a large defect. The primary orifice defect is a partial endocardial cushion defect, so it is often combined with mitral or tricuspid valve malformation or dysplasia. Secondary orifice defects are often simple, so they can be classified as central, superior, inferior, or mixed defects depending on the location of the defect. However, the size of the defect can determine the amount of fractional flow, which can affect cardiac function. If there is a persistent increase in pulmonary blood flow, it can cause pulmonary stasis, resulting in increased right ventricular load, which can develop into pulmonary hypertension over time. Therefore, patients with atrial septal defect are recommended to undergo interventional or surgical treatment as soon as possible.