Eisenmenger syndrome is most often a complication of having a hole (shunt) between two chambers of your heart. Here, a hole between the heart's main pumping chambers (ventricular septal defect) causes increased blood flow to the pulmonary arteries. Eventually the shunt irreparably damages the walls of your lung's arteries (pulmonary arteries). Eisenmenger syndrome occurs when the pressure in the pulmonary arteries becomes so high that it causes oxygen poor (blue) blood to flow from the right to left ventricle and then to the body, causing cyanosis. The high pressure also causes the walls of your heart's ventricle to thicken (hypertrophy).