good heart doctors in bangalore what is the most common heart defect in down syndrome ? the most common defects are atrioventricular septal defect ventricular septal defect ventricular septal defect , persistent ductus, arteriosus and tetralogy of fallot ...... causes of avsd ......... a complete avsd arises during pregnancy when the common valve fails to separate into the two distinct valves when the septa that split the upper and lower chambers of the heart do not grow all the way to meet in the center of the heart