Definition of Transposition of the great vessels:

Transposition of the great vessels is a congenital heart defect in which the two major vessels that carry blood away from the heart -- the aorta and the pulmonary artery -- are switched (transposed).

Causes, incidence, and risk factors:

The cause of most congenital heart defects is unknown.

Factors in the mother that may increase the risk of this condition include:

• Age over 40
• Alcoholism
• Diabetes
• Poor nutrition during pregnancy (prenatal nutrition)
• Rubella or other viral illness during pregnancy

Transposition of the great vessels is a cyanotic heart defect. This means there is decreased oxygen in the blood that is pumped from the heart to the rest of the body. Low blood oxygen leads to cyanosis (a bluish-purple color to the skin) and shortness of breath.

In normal hearts, blood that returns from the body goes through the right side of the heart and pulmonary artery to the lungs to get oxygen. The blood then comes back to the left side of the heart and travels out the aorta to the body.

In transposition of the great vessels, the blood goes to the lungs, picks up oxygen, and then goes right back to the lungs without ever going to the body. Blood from the body returns to the heart and goes back to the body without ever picking up oxygen in the lungs.

Symptoms appear at birth or very soon afterward. How bad the symptoms are depends on the type and size of heart defects (such as atrial septal defect or patent ductus arteriosus) and how much oxygen moves through the body's general blood flow.

The condition is the second most common cyanotic heart defect.