The ductus arteriosus is a blood vessel that connects the pulmonary artery and the aorta. In the fetus, it enables blood to bypass the lungs. The fetus does not breathe air, and thus blood does not need to pass through the lungs to be oxygenated. After birth, blood does need to be oxygenated in the lungs, and normally the ductus arteriosus closes quickly, usually within days up to 2 weeks.

In patent ductus arteriosus, this connection does not close, allowing some oxygenated blood, intended for the body, to return to the lungs. As a result, the blood vessels in the lungs may be overloaded and the body may not receive enough oxygenated blood.