Endocardial cushion defects are classified as partial, excessive, or complete according to the degree of development of the atrial septal tissue around the atrioventricular valve and the atrioventricular valve malformation. 1.Partial type Partial endocardial cushion defects mainly include primary orifice atrial septal defect and mitral valve cleft defect and its cause of mitral regurgitation of different degrees. The primary orifice atrial septal defect is crescent-shaped, in the lower part of the atrial septum and above the atrioventricular valve. Most of the mitral regurgitation sites are at the anterior commissurotomy defect. 2. Complete type Complete endocardial cushion defects include primary foramen ovale septal defects and septal inflow tract defects below the atrioventricular valve. A group of atrioventricular valves spans the left and right heart, forming the superior (anterior) and inferior (posterior) bridging valves. The patient forms a “bare zone” on the septal ridge. There are three subtypes of complete endocardial cushion defects: Type A is the most common. Type A is the most common and refers to an anterior bridging valve whose tendon cords are extensively attached to the septal ridge and can be effectively divided into “two valves”, i.e., the left superior valve is entirely in the left ventricle and the right superior valve is entirely in the right ventricle, allowing the surgeon to reconstruct the right and left atrioventricular valves during surgery. It refers to the left anterior bridging valve emitting papillary muscle attached to the right ventricle. Type C refers to the anterior bridging valve suspended over the ventricular septum without tendon attachment. Complete endocardial cushion defects are prone to combined cone stem malformations such as tetralogy of Fallot, right ventricular double outlet, and aortic misalignment, with tetralogy of Fallot being the most common, accounting for approximately 6% of cases. Other combined malformations include patent ductus arteriosus (10%), perpetual left superior vena cava perpetua (3%), and left ventricular outflow tract obstruction due to diffuse subaortic stenosis or residual atrioventricular valve tissue. 3. Excessive type The excessive type of endocardial cushion defect is intermediate between the partial and complete types, with two well-defined groups of atrioventricular valve orifices, the primary atrial septal defect and the ventricular septal defect below the atrioventricular valve. Patients with septal defects are often located in the inflow septum and do not have a distinct “bare zone” on the septal ridge. Patients with endocardial cushion defects often have atrioventricular conduction tissue ectopic due to the absence of the atrial septum, with the atrioventricular node more posteriorly inferior to the normal position and closer to the coronary sinus, and the His bundle often travels along the inferior border of the septal defect, with the bundle branches bifurcating more inferiorly. Therefore, these patients are most likely to have surgically induced conduction system damage.