Eisenmenger syndrome is a condition that affects blood flow from the heart to the lungs in some babies who have structural problems of the heart.

Eisenmenger syndrome is caused by a defect in the heart. Most often, babies with this condition are born with a hole between the two pumping chambers -- the left and right ventricles -- of the heart (ventricular septal defect). The hole allows blood that has already picked up oxygen from the lungs to flow back into the lungs, instead of going out to the rest of the body.

Other heart defects that can lead to Eisenmenger syndrome include:

• Atrioventricular canal defect
• Atrial septal defect
• Cyanotic heart disease
• Patent ductus arteriosus
• Truncus arteriosus

Over time, increased blood flow can damage the small blood vessels in the lungs. This causes high blood pressure in the lungs. As a result, the blood backs up and does not go to the lungs to pick up oxygen. Instead, the blood goes from the right side to the left side of the heart, allowing oxygen-poor blood to travel to the rest of the body.

Eisenmenger syndrome usually develops before a child reaches puberty. However, it also can develop in young adulthood.