The aorta normally originates from the left ventricle and is the main artery that carries oxygen-rich blood to the body. The pulmonary artery is the main artery that carries deoxygenated blood from the right ventricle to the lungs. In a case of transposition of the great arteries, these two main arteries are inverted. In other words, the aorta originates from the right ventricle and the pulmonary artery originates from the left ventricle.
Therefore, the pulmonary artery carries oxygenated blood from the left ventricle back to the lungs and the aorta carries deoxygenated blood from the right ventricle to the body causing the baby to appear cyanotic, or blue. Without a way for the oxygenated blood of the left ventricle to reach the aorta and, in turn, the rest of the body, the child will not survive. Often, an associated ASD or VSD is present allowing blood to flow between the right and left sides of the heart.
In other cases, a PDA is present allowing blood flow between the pulmonary artery and aorta. Early surgical correction is essential. The procedure, known as the Arterial Switch operation, involves transposing the two great arteries as well as the coronary arteries (the small blood vessels that supply oxygenated blood to the heart muscle itself).
View an animated heart with Transposition of the Great Arteries