In total anomalous pulmonary venous return (TAPVR/TAPVD), the pulmonary veins do not return to the left atrium. In the “cardiac” type, the pulmonary veins return to the heart to drain into the right upper chamber (right atrium). The only way for blood to get to the left side of the heart is by crossing through a hole (an atrial septal defect or ASD) between the top two chambers (atria) to reach the left side of the heart. Sometimes the abnormal pathway that drains the pulmonary veins is too small leading to what is called “obstruction”, where the blood cannot flow normally throughout the body and babies can become very sick in these cases.