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Transposition of the Great Arteries (TGA) is a congenital heart defect where the two main arteries leaving the heart are reversed. This means that the aorta arises from the right ventricle, and the pulmonary artery arises from the left ventricle, resulting in systemic and pulmonary circulations running in parallel instead of in series.

In a normal heart, oxygen-poor blood flows from the body into the right atrium, into the right ventricle, and out to the lungs via the pulmonary artery for oxygenation. The oxygen-rich blood then returns to the left atrium, moves into the left ventricle, and is pumped out to the body through the aorta. However, in TGA, the systemic circulation receives deoxygenated blood from the right ventricle, and the pulmonary circulation receives oxygenated blood from the left ventricle, preventing adequate oxygenation of blood delivered to the body.

This parallel arrangement creates a situation where patients require mixing of blood, which might occur via a patent ductus arteriosus or an atrial septal defect, to provide some oxygenated blood to the systemic circulation. Without surgical intervention, TGA is life-threatening, emphasizing the importance of recognizing this condition during assessments