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Causes
By Mayo Clinic staff
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.
A normal heart has four chambers. 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 right 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 congenital cyanotic heart defect. Transposition of the great arteries is the most common cyanotic heart defect.
One or more connections needed
In order to survive, infants born with transposition of the great arteries must have a conduit — an additional heart defect such as a hole in the wall between the chambers of the heart (ventricular septal defect or atrial septal defect) or a surgically created channel — that allows some mixing of oxygen-rich blood with the oxygen-poor blood. Some babies will need medications to keep these connections open or a procedure to create or enlarge an atrial septal defect to increase the amount of oxygen delivered to the body.
Although some factors, such as rubella or other viral illnesses during pregnancy, maternal age over 40 or diabetes, may increase the risk of this condition, in most cases the cause is unknown.
