Cor Triatriatum
A malformation of the heart in which the embryonic common PULMONARY VEIN was not incorporated into the LEFT ATRIUM leaving behind a perforated fibromuscular membrane bisecting the left atrium, a three-atrium heart. The opening between the two left atrium sections determines the degree of obstruction to pulmonary venous return, pulmonary venous and pulmonary arterial hypertension.