Causes

Eisenmenger syndrome usually develops due to a hole between the chambers of your heart. To understand how Eisenmenger syndrome affects your heart and lungs, it's helpful to know how your heart works.

How the heart works

Your heart is divided into four chambers, two on the right and two on the left. The right side moves blood into vessels that lead to your lungs. In your lungs, oxygen enriches your blood, which then circulates to your heart's left side. The left side of your heart pumps blood into a large vessel called the aorta, which circulates blood to the rest of your body.

Valves control the flow of blood into and out of the chambers of your heart. These valves open to allow blood to move to the next chamber or to one of the arteries, and then close to keep blood from flowing backward.

How Eisenmenger syndrome develops

Eisenmenger syndrome is typically due to an unrepaired hole (shunt) between the main blood vessels or chambers of your heart. This shunt is a heart defect you're born with (congenital). Heart defects that can cause Eisenmenger syndrome include:

• Atrioventricular canal defect. In this heart defect, there's a large hole in the center of the heart where the walls between the upper chambers (atria) and lower chambers (ventricles) meet. Some of the valves in your heart also may not function properly.
• Atrial septal defect. An atrial septal defect is a shunt in the wall of tissue that divides the right and left sides of the upper chambers of your heart (atria).
• Patent ductus arteriosus. This heart defect is an opening between the pulmonary artery that carries oxygen-poor blood to the lungs and the artery that carries oxygen-rich blood to the rest of your body (aorta).
• Ventricular septal defect. This shunt in the wall of tissue that divides the right and left sides of your heart's main pumping chambers (ventricles) is the most common cause of Eisenmenger syndrome.

In any of these defects, blood is flowing in a way it normally doesn't, which increases the pressure in your pulmonary artery. Over time, this increased pressure damages the smaller blood vessels in your lungs. The damaged blood vessel walls make it difficult to pump blood to the lungs.

Eisenmenger syndrome causes increased blood pressure in the side of the heart that has low oxygen-containing blood (blue blood). This allows the low oxygen-containing blood to cross the hole (shunt) in the heart or blood vessels, which lets oxygen-rich and oxygen-poor blood mix. This lowers the oxygen level in your blood and causes a bluish tint to your skin (cyanosis). This also leads to an increase in your red blood cell count to try to make up for the lack of oxygen.