Transposition of the great arteries occurs during fetal growth when your baby's heart is developing. Why this defect occurs is unknown in most cases.
Normally, the pulmonary artery — which carries blood from your heart to your lungs to receive oxygen — is attached to the lower right chamber (right ventricle).
From your lungs, the oxygen-rich blood goes to your heart's upper left chamber (left atrium), through the mitral valve into the lower left chamber (left ventricle). The aorta is normally attached to the left ventricle. It carries oxygen-rich blood out of your heart back to the rest of your body.
In transposition of the great arteries, the positions of the pulmonary artery and the aorta are switched. The pulmonary artery is connected to the left ventricle, and the aorta is connected to the right ventricle.
Oxygen-poor blood circulates through the right side of the heart and back to the body without passing through the lungs. Oxygen-rich blood circulates through the left side of the heart and directly back into the lungs without being circulated to the rest of the body.
Circulation of oxygen-poor blood through the body causes the skin to have a blue tint (cyanosis). Because of this, transposition of the great arteries is called a cyanotic congenital heart defect.
Although some factors, such as genetics, rubella or other viral illnesses during pregnancy, maternal age over 40, or maternal diabetes, may increase the risk of this condition, in most cases the cause is unknown.
Levo-transposition of the great arteries
In this rare type of transposition of the great arteries — called levo-transposition of the great arteries or sometimes referred to as congenitally corrected transposition — the two ventricles are switched so that the left ventricle is on the heart's right side and receives blood from the right atrium, and the right ventricle is on the heart's left side and receives blood from the left atrium. This type of transposition of the great arteries is sometimes also called congenitally corrected transposition.
However, the blood usually circulates correctly through the heart and body. The right-sided left ventricle attaches to the pulmonary artery, which delivers oxygen-poor blood to the lungs, while the left-sided right ventricle attaches to the aorta, which carries oxygen-rich blood to the body.
Because the circulation is congenitally corrected, some people with this rare condition may not experience symptoms for many years and go undiagnosed until adulthood. However, many people with this condition have other congenital heart defects that may cause symptoms, and these people are usually found to have heart disease as children. Depending on their condition and heart defects, people may have surgery to repair the defects.
The right ventricle may not function well as the main pumping chamber over time, which can lead to heart failure in adults. Adults often also have abnormal function of the valve guarding the right ventricle (tricuspid valve). Abnormal valve and ventricular function can cause heart failure. Treatment such as valve replacement, ventricular assist devices or a heart transplant may eventually be needed.