atrioventricular valve (B) Note the location of the incomplete left ventricle (LV) in (B) It is located posteroinferiorly relative to the dominant ventricle and, as shown, is usually left-sided Since it is located posteroinferiorly, it is not seen in the heart shown in (A) Straddling Atrioventricular Valves Patients with straddling of one or other atrioventricular valve, or rarely both atrioventricular valves, form a spectrum between univentricular and biventricular atrioventricular connections This is because the straddling of the atrioventricular valvar tension apparatus is almost always associated with overriding of the atrioventricular junction The extent of the overriding junction supported by the dominant as opposed to the incomplete ventricle will determine whether the atrioventricular connections are effectively double inlet or biventricular The incomplete ventricle will itself be appreciably larger when the connections are biventricular, but the mere presence of straddling of the tension apparatus can be sufficient to tilt the strategies for surgical repair toward the Fontan option despite the presence of a ventricular cavity and mass that may be normal in size Either the right or left atrioventricular valve can straddle or override, and depending on the ventricular topology, this can produce spectra extending from either concordant or discordant connections to double-inlet left ventricle or double-inlet right ventricle (Figs 69.19 and 69.20) It is straddling of the tricuspid valve that leads to double-inlet left ventricle, whereas straddling of the mitral valve is associated with a spectrum leading to double-inlet right ventricle Whether the atrioventricular connections are concordant or discordant in either situation depends on the combinations of atrial arrangement and ventricular topology In all instances, however, the tricuspid valve straddles and overrides the inlet part of the ventricular septum This has important consequences for the disposition of the atrioventricular conduction axis, even if the atrioventricular connections are potentially concordant This is because overriding of the junction supporting the morphologically tricuspid valve produces malalignment between the atrial and apical ventricular septa Because the apical ventricular septum carries the atrioventricular conduction axis on its crest, this means that the atrioventricular bundle is unable to make contact with the regular atrioventricular node at the apex of the triangle of Koch Instead, the axis takes its origin from an anomalous atrioventricular node formed at the point where the malaligned ventricular septum meets the atrioventricular junction (Fig 69.21A) When the mitral valve straddles and overrides, it does so through an anterosuperior septal defect, typically in the setting of either transposition or double-outlet right ventricle with subpulmonary interventricular communication In this setting, when the atrioventricular connections are potentially concordant, the conduction axis maintains its origin from the regular atrioventricular node When straddling of either the mitral or tricuspid valve is found in the setting of discordant atrioventricular connections, however, the conduction axis should be anticipated to arise from an anterolateral atrioventricular node However, there can be a sling of conduction tissue when it is the mitral valve that is straddling in this setting FIG 69.19 Spectra found in the setting of straddling of the morphologically tricuspid valve The upper panels show how, when the tricuspid valve is right-sided, the atrioventricular (AV) connections with minimal straddling are concordant, whereas with excessive straddling and