In Transposition Of The Great Vessels, Life Is Possible Only If There Is Also A Ventricular Septal Defect.

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In Transposition of the great vessels, the two circulations run in parallel because the aorta arises from the right ventricle and the pulmonary artery from the left ventricle. For life, you need mixing between the two streams so that oxygenated blood can reach the systemic circulation. A ventricular septal defect provides a direct path for blood to mix between the ventricles: oxygenated blood from the left ventricle can enter the right ventricle via the defect, and then part of that mixed blood is ejected into the aorta. This mixing raises systemic oxygen content enough to sustain life, which is why this defect is classically associated with survival in TGA. Other forms of mixing—like a patent foramen ovale or a ductus arteriosus—can also help, but the ventricular septal defect is the most dependable and commonly discussed mechanism for maintaining life in this condition. Pulmonary stenosis does not provide the necessary mixing mechanism in the same direct way and isn’t the factor that ensures survival.