Atrioventricular septal defect (AVSD) is a type of congenital heart defect (a heart defect that is present at birth) in which there is a hole in the wall (septum) between the left and right chambers of the heart, as well as abnormalities with the valves that regulate blood flow between the atria and ventricles. This hole occurs at the point where the upper and lower chambers of the heart meet and can cause a number of complications. 

AVSDs are classified by how serious the defect is. In the partial form of the condition, blood is allowed to travel between the left and right sides of the heart's atria. The tricuspid and mitral valves are also involved in this form of the defect. In the complete form of the condition, blood can travel freely through all four chambers of the heart and involves not only the septa, but also the mitral and tricuspid valves. The AVSD is commonly associated with Down's syndrome.  Nearly half of children with Down's syndrome have congenital heart disease and, of those patients, nearly half have an AVSD.  A large percentage of patients with a complete AVSD also have Down’s syndrome.

Children with an AVSD will usually begin to show signs or symptoms within six to eight weeks of birth, if not immediately. These symptoms are similar to the symptoms of heart failure as the heart struggles to supply the body with oxygenated blood while there is an increased volume of blood circulating through the lungs. The leakage of the tricuspid and mitral valves, especially the mitral, results in the blood being pumped back and forth between the ventricles and atria in a very inefficient fashion. Other complications include an enlarged heart, high blood pressure in the lungs and congested lungs. It may also result in heart failure.

Surgery is always necessary to treat an AVSD. Because of our increased understanding of heart valve anatomy and better imaging techniques, the outcome of surgery is excellent.