When the ductus arteriosus stays open, oxygen-rich (red) blood passes from the aorta to the pulmonary artery, mixing with the oxygen-poor (blue) blood already flowing to the lungs. The blood vessels in the lungs have to handle a larger amount of blood than normal. How well the lung vessels are able to adapt to the extra blood flow depends on how big the PDA is and how much blood is able to pass through it from the aorta.
Extra blood causes higher pressure in the blood vessels in the lungs. The larger the volume of blood that goes to the lungs at high pressure, the more the lungs have to cope with this extra blood at high pressure.
Children may have difficulty breathing because of this extra blood flow to the lungs at high pressure. They may remain on the ventilator for a longer period of time if they are premature. The support from the ventilator also may be high, due to this extra blood flow to the lungs.
Rarely, untreated patent ductus arteriosus may lead to long-term lung damage. This is uncommon, however, since most children will have been treated for their PDA before the lungs get damaged.
Often, the patent ductus arteriosus may be "silent," that is, causing no symptoms. This is especially true in older patients (beyond the first few months of life) with small PDAs.