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Group of congenital heart defects

Transposition of the great vessels (TGV) is a group of congenital heart defects involving an abnormal spatial arrangement of any of the great vessels: superior and/or inferior venae cavae, pulmonary artery, pulmonary veins, and aorta. Congenital heart diseases involving only the primary arteries (pulmonary artery and aorta) belong to a sub group called transposition of the great arteries (TGA), which is considered the most common congenital heart lesion that presents in neonates.

Transposed vessels can present with atriovenous, ventriculoarterial and/or arteriovenous discordance. The effects may range from a slight change in blood pressure to an interruption in circulation depending on the nature and degree of the misplacement, and on which specific vessels are involved. Although "transposed" literally means "swapped", many types of TGV involve vessels that are in abnormal positions, while not actually being swapped with each other. The terms TGV and TGA are most commonly used in reference to dextro TGA – in which the two main arteries are in swapped positions; however, both terms are also commonly used, though to a slightly lesser extent, in reference to levo TGA – in which both the arteries and the ventricles are swapped; while other defects in this category are almost never referred to by either of these terms.

Dextro Transposition of the great arteries (also known as dextro TGA) is a cyanotic heart defect in which the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. This switch causes deoxygenated blood from the right heart to be pumped immediately through the aorta and circulated throughout the body and the heart itself, bypassing the lungs altogether. In this same condition, the left heart continuously pumps oxygenated blood back into the lungs through the pulmonary artery, instead of out into the body's circulation as it normally would. In effect, two separate "parallel" circulatory systems are created. It is called a cyanotic congenital heart defect (CHD) because the newborn infant turns blue (cyanotic) from the lack of oxygen.

Levo Transposition of the great arteries (also known as Levo TGA, congenitally corrected TGA, double discordance, or ventricular inversion) is a rare, acyanotic heart defect in which the primary arteries are transposed, with the aorta anterior and to the left of the pulmonary artery, and the morphological left and right ventricles with their corresponding atrioventricular valves are also transposed. In other words, the right ventricle is on the left side of the heart and the left ventricle is on the right side of the heart. The systemic and the pulmonary circulation are connected in this condition. Complications can arise from the pressure change due to the fact that the right ventricle, which is adapted for pumping blood into the low pressure pulmonary circulation, is being tasked with pumping blood at a much higher pressure against the high resistance of the systemic circulation, since it is now in the position of where the left ventricle is typically located.

In many cases, TGV is accompanied by other heart defects, the most common type being intracardiac shunts such as atrial septal defect including patent foramen ovale, ventricular septal defect, and patent ductus arteriosus. Stenosis, or other defects, of valves and/or vessels may also be present. When no other heart defects are present it is called 'simple' TGV; when other defects are present it is called 'complex' TGV.