atrioventricular valve In these variants, the ventricular septum no longer meets the atrioventricular junction at the crux, with important consequences for the disposition of the atrioventricular conduction axis (see later).24 A similar spectrum obviously exists when the right ventricle is dominant, with the extreme end of this spectrum being double-inlet right ventricle with a common atrioventricular valve The concept of chamber dominance can also be extended to include the atria When one of the atria is dominant, the common atrioventricular junction is more or less equally shared by the ventricles but is mostly connected to the dominant atrium The only exit for the other atrium is across the atrial component of the atrioventricular septal defect This arrangement is often termed double-outlet atrium, albeit this description is applicable to hearts having absence of one atrioventricular connection and straddling of the other atrioventricular valve (see Chapter 49) Associated Malformations If not ruled out by its anatomy, any lesion must be anticipated to exist in hearts having an atrioventricular septal defect with a common atrioventricular junction We have already mentioned some of the more frequent malformations, notably obstructions within the left ventricular outflow tract, and those affecting the left atrioventricular valve Additional deficiencies of the atrial septum are important and are sometimes described in terms of a common atrium Common atrioventricular valves can also be found in hearts with abnormal segmental connections, such as double-inlet ventricle and discordant or ambiguous atrioventricular connections In these settings, the patients frequently also exhibit abnormal ventriculoarterial connections Double-inlet ventricle, however, is usually not classified as an atrioventricular septal defect Discordant ventriculoarterial connections, for example, are the rule in association with either double-inlet left ventricle or discordant atrioventricular connections A double outlet from the right ventricle is frequently found, particularly when there is isomerism of the atrial appendages (see Chapter 27) Of the other associated lesions, tetralogy of Fallot or pulmonary stenosis is particularly important, occurring in up to one-tenth of patients with atrioventricular septal defect and a common atrioventricular junction Presence of a second muscular ventricular septal defect is also significant In those hearts with obstruction of the left ventricular outflow tract, right ventricular dominance, along with coarctation or interruption of the aortic arch, should be anticipated Atrioventricular Conduction Tissues In most instances the atrial and ventricular septal structures are appropriately aligned in the setting of atrioventricular septal defect with a common atrioventricular junction The arrangement of the atrioventricular conduction axis is different from normal but comparable in all the phenotypic variants with septal alignment.25–27 The difference from the normal arrangement reflects the lack of the atrioventricular septal structures and the concomitant lack of a normal central fibrous body Because of the deficient atrioventricular septation, the inferior edge of the margin of the atrial septum usually makes contact with the ventricular septum only at the crux It is at the crux, therefore, that the atrioventricular conduction axis usually penetrates from the atrial tissues to reach the crest of the muscular ventricular septum In consequence of this arrangement, the entire nodal area is displaced posteriorly and inferiorly Although a well-formed triangle can be seen at this location, this nodal triangle is not the same as the normal triangle of Koch (Fig 31.12) FIG 31.12 Location of the atrioventricular conduction axis in the setting of aligned atrial and ventricular septal structures (A) and rightward malalignment of the muscular ventricular septum (B) The best guide to the location of the atrioventricular node is the point at which the muscular ventricular septum joins the inferior atrioventricular junction Septal alignment permits recognition of an inferior nodal triangle, which has the coronary sinus at its base The inferior triangle, however, is no longer the same as the triangle of Koch The area of union between the posteroinferior extremity of the ventricular septum and the atrioventricular junction provides the most reliable guide to the position of the conduction axis This landmark is to be found even when the coronary sinus is unroofed or opens to the left atrium It is also present when there is malalignment between the atrial and ventricular septal structures, but in this latter setting the atrioventricular node will be positioned away from the cardiac crux (see Fig 31.12B) Having taken origin from the atrioventricular node, either in the nodal triangle or along the inferior atrioventricular junction, the elongated nonbranching bundle runs either on the crest of the muscular ventricular septum or to its left side, being covered by the inferior bridging leaflet The bundle branches are found more posteriorly than in the normal heart Only the right bundle branch extends along the crest in the bare area found in the presence of a common orifice In hearts with separate valvar orifices for the right and left ventricles, this part of the axis is covered over by the connecting tongue of leaflet tissue This feature is of major surgical importance, since it permits sutures to be secured on the left side of the fibrous raphe or within the bridging leaflets themselves without courting damage to the underlying conduction tissues The left ventricular outflow tract, by virtue of its unwedged location, is unrelated to the conduction axis This feature eliminates the risk of surgical damage compared with the normal heart should removal be attempted of the obstructing lesions Related Lesions Knowledge of the anatomic hallmarks of atrioventricular septal defect with a common atrioventricular junction permits recognition of other lesions with similar features that do not fit into the group as defined because they do not possess a common atrioventricular junction The most obvious candidate in this regard is the heart with a normally located subaortic outlet but a deficient atrioventricular component of the membranous septum Such lesions, known as direct Gerbode defects, permit shunting from the left ventricle to the right atrium This is possible only because there are separate right and left atrioventricular junctions (see Chapter 32) Patients with these defects have a normally wedged subaortic outflow tract and are very rare.28 Closely related are the hearts with separate right and left atrioventricular junctions and a normally wedged aorta but with direct shunting across a perimembranous inlet ventricular septal defect into the right atrium because of anomalous attachment of the