The pulmonary veins that bring oxygen-rich (red) blood from the lungs back to the heart aren't connected to the left atrium. Instead, the pulmonary veins drain through abnormal connections to the right atrium.

In the right atrium, oxygen-rich (red) blood from the pulmonary veins mixes with venous (bluish) blood from the body. Part of this mixture passes through the atrial septum (atrial septal defect) into the left atrium. From there it goes into the left ventricle, to the aorta and out to the body. The rest of the poorly oxygenated mixture flows through the right ventricle, into the pulmonary artery and on to the lungs. The blood passing through the aorta to the body doesn't have enough oxygen, which causes the child to look blue (cyanotic).

This defect must be surgically repaired in early infancy. Depending on the exact anatomy, the surgery can be very complicated and difficult. The pulmonary veins are reconnected to the left atrium and the atrial septal defect is closed. When surgical repair is done in early infancy, the long-term outlook is very good. Still, lifelong follow-up is needed to make certain that any remaining problems, such as an obstruction in the pulmonary veins or irregularities in heart rhythm, are treated properly. Lifelong follow-up is important to make certain that a blockage doesn't develop in the pulmonary veins or where they're attached to the left atrium. Heart rhythm irregularities (arrhythmias) also may occur at any time after surgery.