Outflow Tract Abnormalities in Atrioventricular Canal Malformations

Abstract

Atrioventricular canal malformations are commonly regarded as being arrests of development of normal cardiac septation since their morphology strongly resembles the normal heart of Carnegie stages 14 through 18. Similarly, the spectrum of outflow tract abnormalities, which includes truncus arteriosus, transposition of the great arteries, double outlet right ventricle, and tetralogy of Fallot, has been interpreted as a developmental arrest because these malformations resemble the normal embryonic outflow tract in Carnegie stages 14 through 18, respectively. The causes of the developmental arrests in these two classes of malformations are unknown. Observation of an unusual autopsy case with features of both atrioventricular canal defect and tetralogy of Fallot prompted us to review the outflow tract morphology in 38 hearts with an atrioventricular canal malformation and two semilunar valves to determine if there was evidence to support the idea that both conditions may have the same pathogenesis. In all 38 hearts there was fibrous continuity between an anterior mitral leaflet and an aortic valve, which by anatomic definition means that no case had truncus arteriosus, transposition of the great arteries, or double outlet right ventricle. There were three cases with coarctation of the aorta, a lesion which arises when blood flow in the pulmonary trunk exceeds aortic flow during early development. There were four cases with pulmonary to aortic valve caliber ratios similar to those found in tetralogy of Fallot but the aortic-to-pulmonary valve angle relative to the heart base ranged over the full spectrum of angles seen in normal hearts and those with tetralogy of Fallot. Furthermore, this angle showed the same range of values for partial and complete canal defects, and did not correlate with patient age or ratio of pulmonary to aortic valve caliber. We conclude that the pathogenesis of the spectrum of outflow tract developmental arrests is different from that of atrioventricular canal malformations.

Key Words:

• autopsy
• endocardial cushion defects
• fetal development
• congenital heart defects
• tetralogy of Fallot