The clinical manifestations of atrioventricular canal malformations vary according to the type of lesion, the size of the left-to-right shunt, the severity of atrioventricular regurgitation and the elevated pulmonary vascular resistance. The majority of patients have no clinical symptoms in early life, but they develop palpitations after exertion, shortness of breath, reduced exercise tolerance, respiratory infections and right heart failure. The natural course of the disease is similar to that of the larger atrioventricular septal defect of the second orifice, for example, without other congenital cardiovascular malformations. Nearly 10-15% of cases present with documented pulmonary vascular obstructive disease due to pulmonary hypertension in the mid-20s to mid-40s. Complete atrioventricular canal malformations are less common, and there is insufficient information on the natural course of the disease. It is estimated that approximately 80% of those treated for life without depression surgery die at age 2 years from pulmonary vascular obstructive lesions and congestive heart failure. Therefore, early life treatment with depression surgery should be sought in all cases of AV canal malformations. However, if the pulmonary circulatory resistance exceeds 70% of the body circulatory resistance, liver disease is contraindicated for depression surgery life treatment. Some types of AV malformations with mitral valve insufficiency of moderate or greater magnitude have high left-to-right shunt flow and elevated pulmonary artery pressure. At least 20% of cases present with significant mass symptoms of acid reflux during the infantile pushing period and die before the age of 10 years.