Morphologic features of an intact ventricular septum susceptible to subpulmonary obstruction in complete transposition.

Twenty-five autopsy specimens of complete transposition of the great arteries with intact ventricular septum (VS) were categorized as "bulging" (11 cases) or " nonbulging " (14 cases) according to the curvature of the VS. A fibrous ridge was observed on the VS, especially at the site of mitral apposition in 82% of the bulging group. No fibrous ridge was seen in the nonbulging group. An objective index of anteroposteriorness was then designed to measure the distance between the midpoint of the nonfacing aortic cusp line and the left anterior descending coronary artery. In the bulging group, the aorta lay more anterior to the pulmonary trunk, whereas in the hearts with a straight VS, the aorta tended to lie side by side and to the right of the pulmonary trunk. The midmitral line is an imaginary line in the middle of the anterior mitral leaflet. The more the pulmonary valve is wedged toward the right atrioventricular junction, the more the midmitral line will pass through the nonfacing pulmonary cusp rather than right pulmonary cusp or the nonfacing /right commissure. This was so in 100% of the bulging group, but in only 36% of the nonbulging group. These differences between the groups in terms of anteroposterior index and extent of wedging were statistically significant. In conclusion, if the aorta lies more anterior and to the left of pulmonary trunk rather than side by side and to the right, the "wedged" subpulmonary area will be more susceptible to obstruction caused by septal bulging. Cross-sectional echocardiography is the best means of diagnosing these features.