ABSTRACTThe embryogenesis of tetralogy of Fallot is still much debated. In particular, the dextroposition of the aorta is not considered by all pathologists as a genuine abnormality in this congenital heart disease but rather as a false impression due to an exaggeration of the normal overriding caused by dilatation of the aorta secondary to abnormal hemodynamics. We used two-dimensional echocardiography to examine the spatial position of the aortic root in 22 patients with tetralogy of Fallot (aged 5 days to 24 years, mean 6.4 years) and in 23 normal subjects (aged 1 month to 27 years, mean 7.6 years). Using the parasternal short-axis view, we determined the percent rightward displacement of the aortic root in relation to the plane of the atrial septum, and the relationship between the aortic cusps and the atrial septum. We measured the value of the angle &thetas;, which was defined as the angle between the plane of the atrial septum and the plane of the left coronary-noncoronary commissure and leaflet appositional plane. In the control group, the aortic root was displaced to the right by only 23.6 ± 7.6%; the atrial septum crossed the posterior aspect of the aortic root at the middle (n = 19) or at the right half of the posterior cusp (n = 4), and the angle 0 had a value of 43.3 ± 8.8 degrees. In the 22 patients with tetralogy, the percent rightward shift of the aortic root was augmented to 55.5 ± 9% (p < .001) and the atrial septum was related to the posterior commissure in 14 patients, to the left coronary cusp in two patients, and to the left fourth of the posterior cusp in six patients; the angle 0 had a value of 9.2 11.2 degrees (p < .001). In the two groups, the position of the commissure between the right coronary and left coronary aortic cusps was similar in relation to a line passed forward from the atrial septum. We conclude that the aorta is truly dextroposed in tetralogy of Fallot, with a rightward shift due to a clockwise rotation of the aortic root (looking downstream), and that the axis of this rotation is the anterior aortic commissure. Our echocardiographic findings, which confirm conclusions of previous investigators based on pathologic anatomy, support the embryologic hypothesis of the lack of conal rotation in tetralogy of Fallot and permit a quantitation of the effects of this morphogenetic mechanism.