This communication between the pulmonary artery and thoracic aorta is normally completely closed within two, or at most three, weeks after birth. It may, however, remain patent when from any cause the circulation is abnormally directed through it. This may occur by reason of obstruction of the pulmonary artery or aorta, or from stenosis of the mitral orifice. It has also been observed without any other malformation in cases of obstruction to the pulmonary circulation from insufficient inflation of the lungs (atelectasis).