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Transposition of the great arteries (TGA) is a type of congenital heart defect in which the two great arteries (the pulmonary artery and the aorta) are in reversed (transposed) locations. This reverses circulation in the heart. Normally, oxygen-rich blood travels from the left ventricle through the aorta to nourish the cells and tissues, while oxygen-poor blood travels from the right ventricle through the pulmonary artery on its way to the lungs for fresh oxygen.
By contrast, TGA results in two kinds of parallel circulation. Oxygen-poor blood from the body is pumped back out to the body through the aorta, which is connected to the right ventricle. 
Meanwhile, oxygen-rich blood is pumped back to the lungs through the pulmonary artery, which is connected to the left ventricle. Early survival depends on the presence of other heart defects that allow some mixing of oxygen-rich and oxygen poor blood.
Even with other defects, however, the baby will likely not be getting sufficient oxygen from the blood. As a result, the infant will often be born with cyanosis, a bluish tone to the skin, lips, nails and other parts of the body (blue baby).
In an effort to compensate for the lack of oxygen-rich blood reaching the body, the heart will overwork itself. Untreated, this added strain could eventually lead to heart failure, a serious condition in which the heart is not pumping efficiently enough to meet the body’s demand for oxygen.
The other defects that may occur with TGA include:
 Atrial septal defect. A hole in the septum between the heart's two upper chambers (atria).
- Ventricular septal defect. A hole in the muscular wall (septum) between the heart’s two lower chambers (ventricles). About 20 percent of children born with TGA will also have a ventricular septal defect.
- Patent ductus arteriosus. A heart defect that occurs when a blood vessel called the ductus arteriosus fails to close after birth, as it normally should. The ductus arteriosus is an open channel in every fetus that allows blood to bypass the lungs, which are not used until the baby takes the first breath after birth. Shortly after the baby’s first breath, the ductus arteriosus should close permanently. If it does not, it is known as a patent ductus arteriosus (PDA). Among babies with TGA, a drug called prostaglandin E1 will often be administered to ensure that the ductus arteriosus stays open instead of closing within a few days of birth.
The presence of one of these defects actually enables a TGA baby to survive the first few days of life. Without some mixing of oxygen-rich and oxygen-poor blood, the tissues of the body and organs would rapidly die. TGA accounts for 5 to 7 percent of all congenital heart defects, affecting approximately 40 out of every 100,000 babies. If left untreated, only about 10 percent of TGA babies will live for more than a year. | 
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