A patent ductus arteriosus is a functioning vascular connection between the aorta and the pulmonary artery. Through this channel passes a volume of blood, its magnitude determined by the size of the ductus and the pressure differential between the two vessels. In uncomplicated cases the flow is from aorta to pulmonary artery and not vice versa; therefore, the patient is not cyanotic. It has been shown that as much as several liters of oxygenated blood per minute leak from the peripheral to the pulmonary circulation.1 This implies that a volume of blood far larger than normal is flowing through the pulmonary arteries, pulmonary capillaries, and pulmonary veins and through the left auricle, the left ventricle, and the proximal portion of the aorta. The systemic flow may be reduced to below the normal level or may be maintained by a sufficient increase in the output of the left ventricle.
Burwell CS. PATENT DUCTUS ARTERIOSUS AND VASCULAR RINGS. JAMA. 1954;154(2):136-138. doi:10.1001/jama.1954.02940360034009