This condition happens when all the Pulmonary Veins coming back from the lungs to the Left Atrium do not in fact empty into the Left Atrium. Instead they go through a small chamber which then empties into a single large vein that goes to the right side of the heart.

The fresh pink blood coming back from the lungs mixes with the blue blood in the Superior Venacava. This mixing of blood then goes to the Right Atrium, through a hole to the Left Atrium, then to the Left Ventricle, and on to the Aorta.

However, most of the blood will end up going through the Tricuspid Valve to the Right Ventricle and on to the Pulmonary Artery. The Right Ventricle and Pulmonary Artery will both become larger because they are handling lots of blood. The lungs will be receiving huge amounts of blood and the baby will be very congested and breathing fast

If an obstruction happens to the major vein going to the right side of the heart the baby will be blue. This condition is usually treated with immediate surgery.