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Eisenmenger syndrome progresses over time as a result of the effects of high blood pressure in the lungs. This high blood pressure, or pulmonary hypertension, occurs because of congenital heart defects that cause blood flow from the left side of the heart to the right side of the heart (left-to-right shunt). Congenital heart defects of this type include:
Patent ductus arteriosus (PDA). A connection between the aorta and the pulmonary artery which allows oxygen-rich (red) blood that should go to the body to recirculate through the lungs.
Atrial septal defect (ASD). An opening in the atrial septum, or dividing wall between the two upper chambers of the heart known as the right and left atria.
Ventricular septal defect (VSD). An opening in the ventricular septum, or dividing wall between the two lower chambers of the heart known as the right and left ventricles.
Atrioventricular canal defect (AV canal). A complex heart problem that involves several abnormalities of structures inside the heart, including an ASD, VSD, and improperly formed mitral and/or tricuspid valves.
Because the pressures within the left side of the heart are normally greater than those within the right side of the heart, an opening between the left and right side of the heart will cause blood to flow from the left side of the heart into the right side. This left-to-right shunting of blood within the heart causes increased blood flow in the blood vessels of the lungs. The increased blood flow in the lungs' blood vessels causes increased pressure in these vessels (pulmonary hypertension).
If the pulmonary hypertension continues without treatment, the pressure in the right side of the heart may increase to the point that the right side pressure is greater than the left. When this occurs, blood will flow from the right side of the heart to the left (right-to-left shunt), which means that oxygen-poor blood is mixed with the oxygen-rich blood pumped out to the body from the left ventricle.
A reversal of the shunt resulting in a right-to-left shunt causes insufficient oxygen in the blood. In addition, high pressure in the lungs causes progressive changes in the pulmonary blood vessels, which results in irreversible damage to the lining of these blood vessels. Fibrosis (the growth of scar tissue due to infection, inflammation, injury, or even healing), and thrombus (clotting) may also occur. The changes inside the lung blood vessels may be referred to as pulmonary vascular obstructive disease or secondary pulmonary arterial hypertension (PAH).