The "great arteries" in this anomaly refer to the aorta and the pulmonary artery, the two major arteries carrying blood away from the heart.
In cases of transposition of the great arteries, these vessels arise from the wrong ventricle. They are "transposed" from their normal position so that the aorta arises from the right ventricle and the pulmonary artery from the left ventricle.
Other heart defects may occur along with transposition of the great arteries. About 25 percent of children with transposition will also have a ventricular septal defect (VSD) . In nearly a third, the branching pattern of the coronary arteries as they leave the transposed aorta is unusual. Infants may also have narrowing below the pulmonary valve that blocks blood flow from the left ventricle to the lungs.