Eisenmenger (I-sun-meng-uhr) syndrome is a complication of a heart defect that you're born with (congenital). A heart defect that causes a hole (shunt) to develop between two chambers of your heart is the most common cause of Eisenmenger syndrome. This hole causes blood to circulate abnormally in your heart and lungs. Increased blood flow returns to your lungs instead of going to the rest of your body. The blood vessels in your lung arteries become stiff and narrow, increasing the pressure in your lung's arteries. This permanently damages the blood vessels in your lungs.
Eisenmenger syndrome occurs when the increased pressure of the blood flow in the lung becomes so great that the direction of blood flow through the shunt reverses. Oxygen-poor (blue) blood from the right side of the heart flows into the left ventricle and is pumped to your body, making it so that you don't receive enough oxygen to all your organs and tissues. Eisenmenger syndrome is a life-threatening condition requiring careful medical monitoring. Medications can improve symptoms and prognosis.