In total anomalous pulmonary venous return the pulmonary veins do not return to the left atrium. In the supracardiac (meaning “above the heart”) type, the pulmonary veins drain to vessels located above the heart with the oxygenated blood returning to the right side of the heart. 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.