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Transposition of the great vessels (TGV) is a congenital heart defect that involves the incorrect placement of primary blood vessels of the heart. In cases where the pulmonary artery and aorta are switched, the condition may be referred to as transposition of the great arteries (TGA). In other cases, the blood vessels involved may be the pulmonary artery and veins — superior and inferior vena cavae and the aorta. Congenital heart defects, such as TGV, are known as cyanotic defects, due to the fact that they result in too little oxygen circulating in the blood.
There are two main categories of the defect, known as levo- and dextro-transposition of the great vessels. Dextro-transposition of the great vessels involves literal swapping of two or more primary blood vessels. In cases where both arteries and ventricles are swapped, the term levo-transposition of the great vessels may be used. The catch-all term TGV generally means any case where there is an incorrect spatial alignment of the primary blood vessels of the heart, regardless of whether two vessels are actually swapped with each other.
In addition to the two main categories of TGV, there are further categorizations of simple and complex transposition of the great vessels. TGV is often accompanied by other associated heart defects, and in these cases it may be known as complex transposition of the great vessels. In situations where it is manifested alone, it may be known as simple transposition of the great vessels.
Like other congenital defects, TGV is a condition that affects the developing fetus and can usually be diagnosed prior to, or at the time of, birth. A fetal echocardiogram can often detect TGV in the womb, so that preparations can be made ahead of time to properly care for the infant. If it goes undiagnosed before birth, TGV may be diagnosed via a chest x-ray, checking the oxygen level in the blood, or in a number of other ways. Chest x-rays may be effective due to the characteristic shape of the heart in many cases of TGV, while a low oxygen level is associated with the defect due to the blood not being properly oxygenated before being returned to the body from the heart. If it is detected early, many hospitals are capable of performing a surgery known as an arterial switch to correct the defect.
There are several possible complications that can potentially lead to a fetus developing some form of TGV. Some of these include the mother contracting certain viruses during pregnancy, like rubella or German measles, or developing gestational diabetes. Additionally, the risk of TGV may be increased in cases where the mother has pre-existing diabetes or suffers from poor prenatal nutrition.