The holes within the substance of the septum, and some of those with divorce of the subpulmonary infundibulum from the apical muscular septum, have exclusively muscular rims These channels are therefore described simply as being muscular (see Fig 32.3, left) They can be associated, as will be shown, with malalignment of the septal components Perimembranous Defects The perimembranous defects that open into the base of the right ventricle are positioned directly beneath its inner curvature When the ventricular septum is intact, this area is occupied by the membranous part of the septum, with the supraventricular crest separating the area from the leaflets of the pulmonary valve The membranous septum is crossed by the hinge of the septal leaflet of the tricuspid valve, which divides it into atrioventricular and interventricular components The axis of atrioventricular conduction tissue penetrates through this septum from the apex of the triangle of Koch Having penetrated, in the setting of septal integrity, the conduction bundle runs on the crest of the muscular septum, sandwiched between the fibrous and muscular septal components and emerging in the right ventricle beneath the medial papillary muscle, which takes its origin from the posterocaudal limb of the septomarginal trabeculation (Fig 32.5) Perimembranous defects are found when the ventricular septum is deficient in this area of normal continuity between its membranous and muscular components Such defects, of necessity, will open centrally into the right ventricle Indeed, central defects found at the ventricular base can only be perimembranous Until very recently, we had presumed that their phenotypic feature was fibrous continuity between the leaflets of the aortic and tricuspid valves (see Fig 32.3, right) In most instances, this is the case We are now aware, however, that in a small proportion of cases fulfilling the criteria for the defect to be perimembranous, the fibrous continuity is found only between the leaflets of the mitral and tricuspid valves, with myocardium of the supraventricular crest interposing between the leaflets of the aortic and tricuspid valves The strict definition for the perimembranous defect, therefore, should be the presence of fibrous tissue in the posteroinferior border of the defect producing continuity between the leaflets of the mitral and tricuspid valves, but usually also producing aortic-to-tricuspid fibrous continuity In some instances, the tricuspid-to-mitral fibrous continuity will be through the substance of the central fibrous body FIG 32.5 Landmarks of the normal ventricular septum Its larger part is muscular, but directly beneath the inner curve of the right ventricle there is a fibrous part, known as the membranous septum This is divided by the hinge of the septal leaflet of the tricuspid valve into atrioventricular and interventricular components The atrioventricular conduction axis is directly related to this septum, penetrating the atrioventricular component at the apex of the triangle of Koch, and being sandwiched between the interventricular part and the muscular septum until emerging on the septal surface of the septomarginal trabeculation directly beneath the medial papillary muscle Perimembranous defects were traditionally considered to be “membranous,”21 since they represent failure to close the embryonic interventricular communication by forming the membranous septum The defects, however, are always larger than the normal dimensions of the membranous septum The embryonic interventricular communication almost certainly fails to close because the muscular ventricular septum is deficient around its borders That is why we prefer to describe the defects as being perimembranous.15 Doubly Committed and Juxtaarterial Defects The feature of the third phenotypic type of defect is that they occupy the region that, in the normal heart, is formed by the freestanding component of the muscular subpulmonary infundibulum The phenotypic feature of this third type, which is both doubly committed and juxtaarterial, is fibrous continuity in the roof between the leaflets of the aortic and pulmonary valves (Fig 32.6) Such defects cannot exist in the normal heart, in which the freestanding muscular subpulmonary infundibulum has developed in normal fashion FIG 32.6 Defect that is doubly committed and juxtaarterial It opens to the right ventricle in the area normally occupied by the supraventricular crest (compare with Fig 32.5) Its phenotypic feature is fibrous continuity in its roof between the leaflets of the aortic and pulmonary valves In most instances, as in this example, it possesses a muscular posteroinferior rim SMT, Septomarginal trabeculation Position of Defects Within the categorization of ICD-11, it is the position of the defects relative to the landmarks and components of the right ventricle or, in other words their geography, that serves as the initial feature for description Muscular defects, which are punched within the substance of the muscular septum, can exist so as to open to the inlet, or anywhere in the apical parts of the right ventricle, including those that open anteriorly relative to the body of the septomarginal trabeculation Muscular defects opening to the right ventricular outlet can also