Cor triatriatum is an extremely rare congenital heart defect. Normally, the human heart has four chambers of which the upper two are the atria. These two are separated from each other by a partition (septum) called the atrial septum. The other two chambers, known as ventricles, are also separated by a septum.
In cor triatriatum there is a small extra chamber (XC) above the left atrium of the heart. The pulmonary veins, returning blood from the lungs, drain into this extra "third atrium." The passage of blood from the lungs into the heart (left atrium and ventricle) is slowed by this extra chamber.
Cor triatriatum may eventually lead to features of congestive heart failure and obstruction over time.