Figure 9
The rule regarding the location of the conduction axis (red dots) in hearts with perimembranous ventricular septal defects holds good for all instances except when there is malalignment between the atrial septum and the apical muscular septum. This is associated with straddling and overriding of the tricuspid valve. As shown in the specimen, the conduction axis, carried on the crest of the muscular septum, can no longer take origin from the regular atrioventricular node, which remains at the apex of the triangle of Koch (yellow lines). Instead, it originates from an anomalous node (red star), which is formed at the site of union between the malaligned muscular septum and the inferior atrioventricular junction.