Anatomic obstruction of the right ventricular outflow tract in transposition of the great arteries.

In 126 hearts with transposition of the great arteries (TGA), morphologic features of the right ventricular (RV) outflow tract were analyzed. Distinct anatomic outflow tract obstruction was found in 15 of the 55 hearts with a ventricular septal defect (VSD) (27%): 12 had a perimembranous defect (predominantly extending into the infundibulum), 2 had an inlet, and 1 had an infundibular defect. In 12 hearts, outflow tract obstruction was caused by anterior displacement of infundibular septum and ventriculoinfundibular fold; in 3 hearts, it was caused by bulging of the trabecula septomarginalis or ventriculoinfundibular fold, or both. Of 71 hearts with an intact ventricular septum, only 2 (3%) had RV outflow tract obstruction: 1 had a trabecula in front of the aortic ostium and in the other an aberrant hypertrophic muscle bundle crossed the RV cavity, obstructing the entrance to the infundibulum. Associated malformations included aortic arch malformations (6 cases) and tricuspid valve anomalies (6 cases). Thus, anatomic RV outflow tract obstruction may constitute a complicating factor in TGA, notably in cases with VSD. A systemic RV blood pressure may trigger hypertrophy, resulting in progression of the obstruction; establishing normal RV pressure by a "switch" procedure might prevent this progression.