exist because of divorce of the apical septum from the subpulmonary infundibulum (Fig 32.7) The muscular defects can be multiple, particularly when they are in the apical part of the muscular septum, or they can coexist with perimembranous or juxtaarterial defects FIG 32.7 How muscular defects, which of necessity are punched into the fabric of the muscular ventricular septum, can be positioned so as to open to the inlet, the apical part, or the outlet of the right ventricle They can also open anteriorly relative to the body of the septomarginal trabeculation Their relationship to the atrioventricular conduction axis, shown in blue, will vary relative to their position within the muscular septum For the perimembranous defects, the situation is more complicated Of necessity, all perimembranous defects, since they represent failure to close the embryonic interventricular septum, must open centrally within the base of the right ventricle (Fig 32.8B) Depending on the extent of deficiency of the environs of the surrounding muscular ventricular septum, however, the defects can extend so as to open primarily to the right ventricular inlet (Fig 32.8A), or to the right ventricular outlet (see Fig 32.8C) They can also be sufficiently large to open to both inlet and outlet and hence be confluent We had previously described perimembranous defects as “excavating” into the inlet, outlet, or apical trabecular components of the septum.15 We now know this to be incorrect Although much of the muscular septum, when viewed from the right side, seems to be positioned to separate the inlet of the right from the inlet of the left ventricle, this is not the case Because of the deeply wedged location of the normal subaortic outflow tract, the majority of the septum as viewed from the right side, which is beneath the septal leaflet of the tricuspid valve, separates the inlet of the right from the outlet of the left ventricle The seemingly septal surface of the right ventricular outflow tract, furthermore, is formed by the freestanding muscular subpulmonary infundibulum The posterior walls of this component separate the cavity of the right ventricle from the aortic root, rather than forming a septum between the cavities of the right and left ventricles It is not possible, therefore, in the normal heart, to recognize the boundaries of a discrete muscular outlet septum (see Chapter 2) Perimembranous defects that open to the outlet of the right ventricle do so because the supraventricular crest is divorced from between the limbs of the septomarginal trabeculation In this situation, it does become possible to recognize a discrete muscular outlet septum, which forms the superior margin of the hole between the ventricles (see Fig 32.8C) In the central perimembranous defect, in contrast, and similarly with the perimembranous defect that opens to the inlet of the right ventricle, the outlet septum maintains its attachment between the limbs of the septomarginal trabeculation The right ventricular edge of the supraventricular crest forms the cranial margin of the defects (see Fig 32.8A–B) When perimembranous defects permit shunting primarily to the inlet of the right ventricle, in addition to the fibrous continuity found between the aortic and tricuspid valves, there is an additional extensive area of continuity in the roof of the defect between the leaflets of the tricuspid and mitral valves This results in loss of the normal offsetting between the valves The septal leaflet of the tricuspid valve in hearts with such defects is frequently divided or deficient If the two components of the abnormal leaflet are at all bound down to the margins, the shunting can occur from left ventricle to right atrium Such shunting is frequently held to be caused by absence of the atrioventricular membranous septum This can occur, but it is very unusual If found, it produces an atrioventricular septal defect; but in the setting of separate right and left atrioventricular junctions (Fig 32.9A) More usually, when there is ventriculoatrial shunting, this is first across a perimembranous inlet defect and only subsequently to the right atrium (Fig 32.9B) The defects that permit ventriculoatrial shunting are described as Gerbode defects.22 Perimembranous defects can also, on occasion, be associated with failure of formation of the freestanding muscular subpulmonary infundibulum In this instance, the defect will be bordered not only by fibrous continuity between the leaflets of the aortic and tricuspid valve but also by fibrous continuity in the roof between the leaflets of the arterial valves Such defects therefore will be both perimembranous and doubly committed and juxtaarterial Of necessity they will open to the outlet of the right ventricle (Fig 32.10) FIG 32.8 Three defects that all have fibrous continuity in their posteroinferior margins between the leaflets of the aortic and tricuspid valves and hence are perimembranous As can be seen, however, the defect shown in B opens centrally at the base of the right ventricle, whereas the defect shown in A opens to the right ventricular inlet, with the defect in C opening to the outlet SMT, Septomarginal trabeculation FIG 32.9 Direct (A) and indirect (B) Gerbode defects The direct defect is due to a deficiency of the atrioventricular component of the membranous septum, whereas the indirect defect is a perimembranous defect opening to