Fig. 4

The image in the left hand panel shows a defect opening centrally at the base of the right ventricle, with the supraventricular crest normally inserted between the limbs of the septomarginal trabeculation. The right hand panel shows the left ventricular aspect of the defect. It has been transilluminated to show the atrioventricular component of the membranous septum, which produces fibrous continuity between the leaflets of the tricuspid and mitral valves. There is also fibrous continuity between the leaflets of the aortic and mitral valves (yellow dotted line). Fibrous continuity is lacking, however, between the leaflets of the aortic and tricuspid valves (red dotted line). The defect remains perimembranous because of the tricuspid-to-mitral valvar continuity via the substance of the atrioventricular component of the membranous septum