Congenital heart condition in which the connection between the pulmonary artery and the aorta persists. Before birth this artery by-passes the pulmonary artery and leads directly into the aorta. After birth the lungs expand, the connection is no longer needed, and so closes. Inexplicably, this closure does not occur in some children, and because pressure in the aorta is higher than that in the pulmonary artery, pressure in the pulmonary blood vessels rises, causing shortness of breath and pneumonia, and also bringing about a mixture of blood with high and low oxygen content. The heart functions less effectively. Babies with this condition sometimes grow slowly. Treatment is by surgery to close the ductus: it is tied and cut, ideally a few months after birth.