Partial anomalous pulmonary venous return

In partial anomalous pulmonary venous return, some of the pulmonary veins incorrectly send blood to the heart's upper right chamber (atrium). This causes some oxygen-rich blood from the lungs to mix with oxygen-poor blood before entering the right atrium. In a normal heart (shown on the left), oxygen-rich blood flows from the pulmonary veins to the upper left chamber (left atrium).