Transposition of the Great Vessels | Practicalhospital.com ...

In this anomaly, the normal relation between the aorta and the pulmonary artery are reversed. The aorta rises anteriorly from the right ventricle, and the pulmonary artery originates posteriorly from the left ventricle. Because this results in two separate circulations, intracardiac or extracardiac shunts (atrial or ventricular septal defects, patent ductus arteriosus) must be present for survival. Shunts through atrial and ventricular septal defects are bidirectional and permit the mixing of oxygenated and unoxygenated blood. In contrast, shunts through the foramen ovale and ductus arteriosus are predominantly right to left and thus of less value. Although the shunts are essential for initial survival, they eventually result in preferential blood flow into the lower-pressure pulmonary circulation. This leads to pulmonary vascular congestion, increased pulmonary venous return, diastolic overloading of the heart, and early congestive failure. Concomitant pulmonic stenosis, which occurs in up to 20 percent of the patients, acts as a protective device in decreasing pulmonary blood flow and consequently diminishing diastolic overloading of the heart. During the first few days of life, the heart is usually normal or nearly normal in size and the pulmonary vascularity is within normal limits. However, pulmonary blood flow soon increases, and venous congestion adds to the radiographic appearance of increased pulmonary vascularity. The enlargement of both ventricles produces a typical oval or egg-shaped configuration of the heart. In patients with associated pulmonic stenosis, th6 pulmonary vasculature is decreased and the heart is more normal in size. A characteristic finding of complete transposition of the great vessels is narrowing of the vascular pedicle on frontal projections. This is due to the superimposition of the abnormally positioned aorta and pulmonary artery combined with the absence of normal thymic tissue because of stress atrophy. On lateral projections, the anterior position of the aorta with respect to the pulmonary artery causes widening of the vascular pedicle. Angiocardiography is required to demonstrate the abnormal positions of the great vessels and to precisely document the type and magnitude of the associated intracardiac and extracardiac shunts.

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