Normal conotruncal alignment
Septation of the heart is an immensely complex and elegant process. The apical aspect of the ventricular septum (muscular septum) accrues as the ventricles balloon out from the primitive heart tube. The primary ring tissue between the expanding ventricles is a primitive heart tube and grows very little; thus, as the chambers expand apically, the septum develops between the expanding ventricles. Ridges fuse together to septate the primitive outflow into the pulmonary artery and aorta in a rotational fashion, creating the normal contortion of the pulmonary trunk (green) around the aorta (orange). These ridges also contribute to the formation of the semilunar valves. Proximal to valve formation, the fusing ridges form the infundibular septum (pink). This structure joins the superior endocardial cushion (yellow) to form the membranous septum. The infundibular septum closes the connection between the RV the LV outflow, which forms from the primitive outflow foramen. Once this happens, there are no remaining connections between the right and left ventricles. Defects in this process cause the family of conotruncal defects, which include truncus arteriosus (defect in arterial septation), transposition of the great arteries (defect in arterial rotation), tetralogy of Fallot (anterior deviation of arterial septation and of conal septal alignment), and posterior malalignment defects (e.g. CoA/VSD or IAA/VSD). A doubly committed sub arterial VSD is due to failure of formation of the infundibular septum.
Anterior malalignment defect. Anterior deviation of arterial septation creates small pulmonary arteries, RVOT obstruction (red), and ‘pulls’ the aorta anterior and rightwards over the RV. This results in TOF and DORV anatomies, which may also include defects with even more disturbance of outflow rotation (e.g. DORV with subpulmonary VSD).
Posterior malalignment defects. Posterior deviation of arterial septation creates a small aorta, CoA, and crowds the LVOT (red) between infundibular septum and the anterior MV leaflet, resulting in VSD with CoA or IAA.