Transposition of the great arteries refers to an abnormality in the developmental separation of the great vessels, which results in the aorta emanating from the venous ventricle and the pulmonary artery coming off the systemic ventricle (ventriculoarterial discordance). Two varieties are most commonly seen in adults. The first type is dextrotransposition of the great arteries (d-TGA), with “dextro” initially meant to describe the location of the aorta in respect to the pulmonary artery. In this condition, the right ventricle gives rise to the aorta and the left ventricle gives rise to the pulmonary artery, but both atria are appropriately connected to their respective ventricles (AV concordance). This condition is not compatible with life unless there is a naturally occurring shunt (ASD, VSD, or PDA) or surgically created shunt. Often, these patients have undergone repair during childhood with a Senning or Mustard procedure, in which blood is baffled from the venae cavae to the left atrium and from the pulmonary veins to the right atrium . The primary long-term concern in these patients is that the right ventricle is ill prepared to serve as the systemic ventricle. It can weaken and fail over time (usually when the patient enters the third or fourth decade), and these patients also develop significant systemic AV regurgitation, with the tricuspid valve in the mitral position.
The other type of TGA is the congenitally (naturally) corrected lesion, levotransposition of the great arteries (l-TGA). In this case, the ventricles are also inverted (both AV and ventriculoarterial discordance are present). This variation results in a circulation in which blood flows from vena cava to right atrium to left ventricle to pulmonary artery to pulmonary veins to left atrium to right ventricle to aorta. Again, the problem remains a right ventricle pumping into the systemic circulation. This condition is also associated with about a one in three lifetime prevalence of complete heart block.