the inlet of the right ventricle in association with a deficiency of the septal leaflet of the tricuspid valve FIG 32.10 Defect that opens to the outlet of the right ventricle, which is bordered by fibrous continuity both between the leaflets of the aortic and tricuspid valves and between the leaflets of the two arterial valves Hence it is both perimembranous and doubly committed SMT, Septomarginal trabeculation Valvar Override and Septal Malalignment The third important feature of any channel between the ventricles, and one that always requires description if present, is valvar override and its associated feature of malalignment of the septal components Malalignment usually involves the outlet septum, as described for the perimembranous defect opening to the outlet of the right ventricle (see Fig 32.8C) In the situation illustrated in Fig 32.8C, the outlet septum is a right ventricular structure, as is also the case in tetralogy of Fallot (see Chapter 36) The outlet septum can also be malaligned in posterior and caudal direction and will become a left ventricular structure, thus obstructing the outflow tract of the left ventricle (Fig 32.11) Such caudal deviation can also be found when the borders of the defect, as viewed from the right ventricle, are exclusively muscular The outlet septum can similarly be deviated in either cephalad or caudal direction when it is fibrous rather than muscular In the latter settings, the defect will of necessity be doubly committed and juxtaarterial It can again have a muscular posteroinferior rim, as is usually the case for doubly committed defects (see Fig 32.6), or it may extend to become perimembranous (see Fig 32.10) FIG 32.11 Autopsied heart that has been sectioned to replicate the longaxis parasternal echocardiographic plane It shows deviation of the muscular outlet septum in the setting of a muscular outlet defect obstructing the left ventricular outflow tract More rarely there can be malalignment between the muscular ventricular septum and the atrial septum This is the phenotypic feature of hearts with override of the orifice of the tricuspid valve and straddling of its tendinous cords (Fig 32.12) The defects of necessity open to the inlet of the right ventricle Some have described these defects as being of “atrioventricular canal” type, since the ventricular septum extends along the full width of the overriding tricuspid valvar orifice In the allegedly “traditional” approach, however, it was suggested that the perimembranous inlet defect was of the “AV canal” type; hence it was called the “type III” defect.21 Both of these variants were said to be of the “atrioventricular canal” type; however, they should be distinguished from atrioventricular septal defects with common atrioventricular junction, since they have separate atrioventricular junctions, with a mitral valve guarding the leftsided junction It is the malalignment between the atrial and ventricular septal structures that is the distinguishing feature of the lesion associated with a straddling tricuspid valve, over and above the fibrous continuity between the leaflets of the aortic and tricuspid valves (see Fig 32.12, left), since the septal malalignment underscores the anomalous location of the atrioventricular conduction axis (see Fig 32.12, right) FIG 32.12 Features of the perimembranous defect opening to the inlet of the right ventricle in association with atrioventricular septal malalignment The location of the atrioventricular node has been superimposed on the image at left, as shown by the white star It is formed at the point where the malaligned ventricular septum meets the inferior part of the right atrioventricular junction The node is no longer in its anticipated location at the apex of the triangle of Koch (red star) The image at right is a diagrammatic representation of the anomalous location of the conduction axis Size and Shape of the Defect The size of any channel between the ventricles is obviously important in