determining its hemodynamic consequences Size can be described according to taste, either using subjective adjectives such as large, medium, or small or by relating the different dimensions of the plane of space measured as the defect to the diameter of the aortic root In this regard, defects less than one-third the aortic valve annulus are considered small, whereas those greater than one-half the aortic valve annulus are considered large.23 The shape of the defect may also vary Muscular defects are typically round or oval when seen from the right ventricle, although they can also be crescentic or cashew-like Defects that are bordered by atrioventricular valves in continuity with arterial valves, or arterial valves in isolation, are typically half-moon shaped The shape of the defect may appear different when it is encroached upon by prolapsing leaflets of the aortic valve When it is not round, the defect may appear larger on one plane than on another as seen using cross-sectional and angiographic imaging The size of the defect plays a role in determining which channels are most likely to become smaller or to close spontaneously Because perimembranous defects are closely related to the septal leaflet of the tricuspid valve, there is always the possibility that they may be closed by plastering down of the leaflet across the defect, with small ones being those most likely to close Defects between the outlet components, particularly when complicated by malalignment, are unlikely to close by this mechanism Neither are extensive inlet defects A more frequent and closely related mechanism of closure is aneurysmal enlargement of fibrous tissue in the environs of the defect Although often described as aneurysms of the membranous septum, it is unusual for the remnant of a membranous septum itself to be involved In most cases the grape-like lesions are sculpted from the underside of the leaflets of the tricuspid valve Aneurysmal prolapse of the right coronary leaflet of the aortic valve can be found in the setting of both the doubly committed and the perimembranous defects, particularly those latter defects that extend to the right ventricular outlet, when the outlet septum is markedly deficient Sometimes the noncoronary aortic leaflet can also prolapse through the defect Rarely, such valvar prolapse may produce partial or complete plugging of the defect and result in spontaneous closure It can also provide a false sense of security that the defect is small when, in reality, it is a large defect The prolapse can result in aortic regurgitation In all of these possible mechanisms of closure or diminution in size, contraction and fibrosis around the edges of the defect are additional contributory factors Relationship to the Conduction System The phenotypic variation is key to determining the location of the atrioventricular conduction axis In the normal heart the penetrating component of the atrioventricular conduction axis passes through the central fibrous body and branches on the crest of the muscular ventricular septum (see Fig 32.7) and the central fibrous body forms the posterocaudal margin of perimembranous defects The conduction axis, therefore, will always penetrate posterocaudally As in the normal heart, the atrial landmark to the site of penetration is the apex of the triangle of Koch When a perimembranous defect extends to open to the right ventricular inlet, the triangle itself is displaced inferoposteriorly, but its apex still serves as the guide to the site of penetration of the conducting bundle (Fig 32.13A) Having penetrated through the central fibrous body, the axis is related to the posteroinferior rim of perimembranous defects It is much closer to the septal crest when a defect opens to the inlet, becoming more remote as the defect extends to open between the outlets (Fig 32.13B) Should the posteroinferior rim be muscular, as is the case in most doubly committed defects (Fig 32.14) and in muscular outlet defects, the muscular margin will protect the conduction axis FIG 32.13 How the atrioventricular conduction axis maintains its location in the posteroinferior quadrants of perimembranous defects irrespective of whether they extend so as to open to the inlet (A) or the outlet (B) of the right ventricle The atrioventricular node and the triangle of Koch are displaced inferiorly when the defect extends so as to open to the ventricular inlet FIG 32.14 How the presence of a muscular posteroinferior rim to an outlet ventricular septal defect protects the atrioventricular conduction axis This situation is shown in the setting of a doubly committed juxtaarterial defect