The cause of total anomalous pulmonary venous return (TAPVR) is unknown.
In normal circulation, blood is sent from the right ventricle to pick up oxygen in the lungs. It then returns through the pulmonary (lung) veins to the left side of the heart, which sends blood out through the aorta and around the body.
In TAPVR, oxygen-rich blood returns from the lungs to the right atrium or to a vein flowing into the right atrium, instead of the left side of heart. In other words, blood simply circles to and from the lungs and never gets out to the body.
For the infant to live, an atrial septal defect (ASD) or patent foramen ovale (passage between the left and right atria) must exist to allow oxygenated blood to flow to the left side of the heart and the rest of the body.
How severe this condition is depends on whether the pulmonary veins are blocked or obstructed as they drain. Obstructed TAPVR causes symptoms early in life and can be deadly very quickly if it is not found and corrected with surgery.