Partial anomalous pulmonary venous return, sometimes called partial anomalous pulmonary venous connection, is a heart defect present at birth (congenital) in which some of the pulmonary veins carrying blood from the lungs to the heart flow into other blood vessels or into the heart's upper right chamber (right atrium), instead of correctly entering the heart's upper left chamber (left atrium). This causes some oxygen-rich blood from the lungs to mix with oxygen-poor blood before entering the right atrium.
