overriding there is double-inlet left ventricle (LV) with a right-sided incomplete right ventricle In contrast, as shown in the lower panels, the spectrum is between discordant AV connections and double-inlet LV with left-sided incomplete right ventricle when the tricuspid valve is left sided FIG 69.20 Comparable spectra to those illustrated for straddling of the tricuspid valve in Fig 69.19 In this instance, the mitral valve is straddling, leading to spectra between concordant atrioventricular (AV) connections and double-inlet right ventricle (RV) with the left-sided incomplete left ventricle (upper panels) and discordant connections and a double inlet with the right-sided incomplete RV (lower panels) FIG 69.21 (A) A morphologically tricuspid valve straddles and overrides (arrows) the inferior and inlet part of the apical ventricular septum (star) Because of the malalignment between the atrial septum and the apical muscular ventricular septum, the conduction axis (red line) takes origin from an anomalous atrioventricular node located in the inferior atrioventricular junction (cross-hatched oval) (B) The mitral valve morphologically straddles through an anterosuperior septal defect that opens to the outlet of the right ventricle In this instance, as shown with concordant atrioventricular connections, the atrioventricular conduction axis maintains its origin from the regular atrioventricular node Solitary atrioventricular valves can straddle and override when one of the atrioventricular connections is absent The solitary valve can be right or left sided and can exist in the setting of either right- or left-sided ventricular topology (Fig 69.22) These are the combinations that produce the uniatrial but biventricular atrioventricular connections Because of the absence of one of the atrioventricular connections, such patients will be suitable only for treatment by conversion to the Fontan circulation The arrangements are often described in terms of double-outlet atrium, but all patients with straddling atrioventricular valves have a double outlet from one or the other atrium Such “double outlets” can also be found in the setting of atrioventricular septal defect with a common atrioventricular junction when there is gross malalignment of the atrial septum versus the ventricular septum Further complications can be produced when the solitary straddling atrioventricular valve has dual orifices, with the connecting tongue of valvar tissue attached to the crest of the ventricular septum This seemingly produces an intact ventricular septum, but ventricular septation is of necessity deficient when there is malalignment, as found with straddling atrioventricular valves