FIG 31.8 Arrangement of the leaflets of the common atrioventricular valve as seen when the common junction is viewed from the ventricular apex The location of the muscular ventricular septum is shown by the dashed black lines, with the yellow arrow showing the zone of apposition between the two leaflets that bridge the ventricular septum These five leaflets are seen to their best advantage when the common junction is guarded by a common valvar orifice (see Fig 31.7, right) When a tongue of valvar tissue joins together the two leaflets of the common valve that bridge the ventricular septum, the effect is to produce separate orifices within the common junction for the inlets to the right and left ventricles (see Fig 31.7, left) This tongue is usually attached directly to the musculature along the length of the crest of the ventricular septum The essence of the so-called ostium primum defect, therefore, is that the fused bridging leaflets are depressed into the ventricular cavity and also fused to the crest of the scooped-out ventricular septum (see Fig 31.6, left) This produces a double-orifice common atrioventricular valve When considering the basic arrangement, three of the leaflets are confined to one or other of the ventricles, with one being exclusive to the left ventricle and two to the right ventricle It is the left mural leaflet, which is much less extensive than the mural leaflet of the normal mitral valve, that is tethered between the superior and inferior papillary muscles of the left ventricle The anterosuperior leaflet and the right mural leaflet are confined to the right ventricle The bridging leaflets have no counterparts in the normal heart The space between them is their zone of apposition (see Fig 31.7) Although often still called a “cleft” for brevity, there is no question that the space is their zone of apposition Reflecting the zones of apposition, the left valve closes in trifoliate fashion (Fig 31.9) FIG 31.9 Intraoperative image showing the trifoliate configuration (dashed lines) of the closed left atrioventricular valve of an atrioventricular septal defect with common atrioventricular valve Such trifoliate closure is markedly different from the pattern of closure of the mitral valve, the two leaflets of which come together along a solitary zone of apposition (see Fig 31.3, left) Today the surgeon seeking to correct the lesion will usually close the space between the left ventricular components of the bridging leaflets This surgical maneuver, however, never produces an arrangement of leaflets for the newly constructed left atrioventricular valve that replicates the arrangement seen in the normal mitral valve The arrangement of the left ventricular papillary muscles reflects the location of the leaflets The muscles are paired, as in the normal heart But instead of being located in anteroinferior and posterosuperior positions, they are in more-or-less direct superoinferior positions On occasion they may be even more abnormally arranged, producing the so-called parachute arrangement.18 In reality, the abnormal valve looks more like a funnel, representing the spatial inverse of a parachute.19 The arrangement of the right ventricular muscles is comparable with that of the normal heart, although the position of the medial papillary muscle is variable This variability has significance, since it reflects the extent to which the superior leaflet bridges into the right ventricle, underscoring the variability seen in the Rastelli classification (see later) As already emphasized, another difference characterizing the morphology of atrioventricular septal defects with a common atrioventricular junction is the dimensions of the ventricular mass In the normal heart, the inlet and outlet dimensions of the left ventricle are approximately the same (see Fig 31.4A) In atrioventricular septal defects with a common junction, the dimension of the outlet is considerably greater than that of the inlet (see Fig 31.4B) It is of little consequence morphologically whether this is because the inlet is shorter than normal or because the outlet is longer Probably it is a combination of the two The disproportion is within the same range, be there a common valvar orifice or separate valvar orifices at the inlets to the right and left ventricles Indeed, once the curtain of valvar leaflets is removed from the atrioventricular junctions in any individual heart, it is not possible to judge simply from examination of the ventricular mass whether there had initially been a common valvar orifice or separate right and left atrioventricular orifices The extent of deficiency of the midpoint of the septum, which we describe as the degree of “scooping,” is usually greater in those hearts having a common atrioventricular valvar orifice.20 Categories of Atrioventricular Septal Defect All the lesions discussed in this chapter are unified by presence of a common atrioventricular junction Despite the commonality of the junctional arrangements, as already discussed, it is still usual for clinicians to recognize two major categories These are considered to be “partial” and “complete,” despite their anatomic comparability Two anatomic features underscore this clinical stratification The first is the arrangement of the individual leaflets within the overall curtain of valvar tissue guarding the common atrioventricular junction The second is the relationship between the bridging leaflets of the common valve and the atrial and ventricular septal structures Problems are produced when these two features are combined so as to identify the presumed complete and partial variants, since the two aspects of anatomy are mutually independent