This defect allows blood to mix between the pulmonary artery and the aorta. Before birth there's an open passageway (the ductus arteriosus) between these two blood vessels. Normally this closes within a few hours of birth. When this doesn't happen, however, some blood that should flow through the aorta and on to nourish the body returns to the lungs. A ductus that doesn't close is quite common in premature infants but less common in full-term babies.

If the ductus arteriosus is large, a child may tire quickly, grow slowly, catch pneumonia easily and breathe rapidly. In some children symptoms may not occur until after the first weeks or months of life. If the ductus arteriosus is small, the child seems well. If surgery is needed, the surgeon can close the ductus arteriosus by tying it, without opening the heart. If there's no other defect, this restores the circulation to normal. A new technique has been developed allowing the cardiologist to close the PDA at cardiac catheterization using an implantable coil or plug. However, not all infants and children with PDAs are suitable for this new technique. If the baby is very premature, it is sometimes possible to close a PDA using a medicine called Indomethacin.