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 associated with a higher than normal rate of this disease include:

• Rubella or other viral illness in the mother during pregnancy
• Poor nutrition in the mother during pregnancy (prenatal nutrition)
• Alcoholism
• Mother's age over 40
• Diabetes

Transposition of the great vessels is a cyanotic heart defect. This means there is too little 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 coming back from the body goes through the lungs to get oxygen and then back to the body. In transposition of the great vessels, the blood that goes through the body lacks oxygen.

Symptoms appear at birth or very soon afterwards. How bad the symptoms are depends on the type and size of the associated 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.