Transposition of the Great Arteries

Transposition of the great arteries (TGA) is a congenital cardiac defect caused by an embryological discordance between the aorta and pulmonary trunk or by both atrioventricular and ventriculoarterial discordance—a condition often termed "double discordance."

Dextro-TGA (d-TGA) occurs during cardiac development when the conotruncal septum, which normally spirals toward the aortic sac, forms linearly. This anomaly interferes with the normal division of the truncus arteriosus into the pulmonary and aortic channels, which form the pulmonary arteries and aorta. As a result, the aorta arises from the right ventricle, and the pulmonary trunk emerges from the left ventricle. As the most common form of TGA, d-TGA is characterized by the right ventricle being positioned to the right of the left ventricle, with the aorta arising anterior and rightward to the pulmonary artery, creating 2 parallel circulatory circuits (see Image. Dextro-Transposition of the Great Arteries).

In the systemic circuit of d-TGA, deoxygenated blood returns to the right atrium, flows through the tricuspid valve, and is then pumped back into systemic circulation by the contraction of the right ventricle as it enters the abnormally formed aorta. The pulmonary circuit, on the other hand, involves oxygenated blood from the pulmonary veins draining into the left atrium, passing through the mitral valve, and then being pushed back into the lungs via the contraction of the left ventricle and the pulmonary arteries. Patients with d-TGA typically present with cyanosis within the first 30 days of life. Completely parallel circulatory circuits are incompatible with life and require a patent ductus arteriosus (PDA) or an atrial (ASD) or ventricular septal defect (VSD) to allow the mixing of oxygenated and deoxygenated blood (see Image. Dextro-Transposition of the Great Arteries and Parallel Circulation).

Congenitally corrected TGA (ccTGA) is a complex congenital heart defect (CHD) characterized by misalignment of the ventricles and transposition of the great arteries, resulting in ventricular discordance where the ventricles are not properly aligned with their corresponding atria. This condition is highly variable and can present with a range of anatomical features and clinical manifestations. CcTGA may be associated with other cardiac anomalies, including mirror-image atrial arrangement, atrial isomerism, dextrocardia, VSDs, an Ebstein-like tricuspid valve, pulmonary stenosis, and atresia (see Image. Congenitally Corrected Transposition of the Great Arteries).

Symptomatic infants often have additional cardiac defects that require early intervention. In contrast, patients without significant intracardiac anomalies may remain asymptomatic during childhood and maintain good health well into adulthood. However, over time, the right ventricle and tricuspid valve, which support systemic circulation, can gradually deteriorate, leading to dysfunction and eventual failure.