In transposition of the great arteries, the aorta and pulmonary artery are switched. Normally, the aorta carries blood from the heart’s left ventricle (lower left chamber) to the body, and the pulmonary artery carries blood from the heart’s right ventricle (lower right chamber) to the lungs.
But in the transposition of these two great arteries, the aorta emerges from the right ventricle, sending oxygen-poor blood that has just returned from the body back out to the body without its usual course through the lungs to pick up oxygen. And the pulmonary artery emerges from the left ventricle and sends oxygen-rich blood that has just returned to the heart from the lungs back to the lungs.
Because the blood carried by the aorta to the body does not first go to the lungs to pick up oxygen, it continues to be poorly oxygenated. As a result, the newborn with this heart defect will have intense bluish skin discoloration.
According to the American Heart Association, of the thousands of babies born each year with a cardiovascular defect, 10 to 11 percent have transposition of the great arteries.
Progression and Possible Complications
Life cannot be sustained unless openings between the left and right sides of the heart allow blood to mix so that oxygen in the blood going to the body is increased.
Before birth, there are normally two openings between the heart’s left and right sides – the atrial septal defect (hole between the heart’s two upper chambers) and the ductus arteriosus, a vessel connecting the pulmonary artery and the aorta. These openings typically close soon after birth. When they close, the baby with transposition of the great arteries lacks oxygen and becomes quite blue unless steps are taken to increase blood mixing. Ultimately, without surgical correction, a child with this defect suffers from lack of oxygen and heart failure and does not live long.
As the openings between the heart’s left and right sides narrow, the supply of oxygen delivered to the child’s body decreases. The first steps in treating the defect aim to stabilize the baby’s condition. These steps are designed to keep blood flowing through the openings:
- A medication called prostaglandin E1 (PGE) is given to keep the ductus arteriosus open.
- A balloon-tipped catheter (a long, thin tube) is guided through the child’s arteries to the atrial septal defect (the hole between the left and right atria). Once positioned properly, the balloon is inflated to “tear” and enlarge the atrial septal defect. This enlargement promotes more mixing of blood between the two sides of the heart, thus increasing the amount of oxygen in the blood going out to the body.
These steps help increase the level of oxygen in the blood going out the body, but it never reaches normal levels. After the baby’s condition is stabilized, surgical repair is performed at one to two weeks of age. In this open-heart procedure, a surgeon switches the two great vessels to their normal positions so that the aorta emerges from the left ventricle and the pulmonary artery from the right ventricle. The coronary arteries have to be switched as well so that they are connected to the aorta after surgery. In addition, the openings between the left and right sides of the heart are closed and the patent ductus arteriosus is tied off and closed.
Later, the sites where the arteries are connected to the valves could become narrow. Treatment – either with a balloon catheter and insertion of a metal, mesh cage called a stent or with surgery – may be needed to widen them. Patients are also usually followed for the rest of their lives to monitor for narrowing of the coronary arteries.