Atrioventricular septal defect causes

Causes
Atrioventricular septal defects (AVSDs) occur during gestational development of a fetus. The endocardial cushions are two areas of thickening that eventually develop into the wall (septum) that separates the four chambers of the heart. They also form the mitral and triscuspid valves, the valves that separate the atria from theventricles.

The lack of separation between the two sides of the heart causes several problems:
 * Increased blood pressure in the lungs. In persons with this condition, blood flows through the abnormal openings from the left to the right side of the heart, then to the lungs. The increased blood flow into the lungs leads to a rise in blood pressure in the lungs.
 * Lung irritation and inflammation. Increased blood flow into the lungs causes irritation and swelling.
 * Heart failure. Because the heart has to pump more blood to the lungs, it has to work much harder than normal. The heart may enlarge and weaken.
 * Cyanosis. As the blood pressure increases in the lungs, blood flow starts to move from the right side of the heart to the left. The oxygen-poor blood mixes with the oxygen-rich blood, and blood with less oxygen than usual is pumped out to the body.

AVSDs are strongly associated with Down syndrome. Several gene changes are also connected to ECD. However, the exact cause of ECD is unknown.

AVSDs may be associated with other congenital heart defects such as:
 * Double outlet right ventricle
 * Single ventricle
 * Transposition of the great vessels
 * Tetralogy of Fallot