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Figure 1: The four main anatomical types of sub-semi-lunar infundibulum or conus arteriosus: subpulmonary, subaortic, bilateral (subaortic and subpulmonary), and absent or very deficient. The upper row of diagrams shows the infundibulum (crosshatched) and great arteries as seen from the front (frontal view). The lower row of diagrams shows the infundibulum (crosshatched), the semilunar valves – the aortic valve, indicated by the coronary arteries, and the pulmonary valve, indicated by the absence of coronary arteries – and the atrioventricular valves – the mitral valve, being a two-leaflet valve, and the tricuspid valve, being a three-leaflet valve – as seen from below (inferior view), similar to a subxiphoid two-dimensional echocardiogram. In all diagrams, a ventricular D-loop is assumed to be present. The subpulmonary conus is normal. Resorption of the subaortic conal-free wall permits aortic–mitral fibrous continuity. The presence of a subpulmonary infundibulum prevents pulmonary valve–atrioventricular valve fibrous continuity. A subpulmonary conus is associated with solitus normally related great arteries (diagrammed here), inversus normally related great arteries, and in tetralogy of Fallot, both with solitus normally related great arteries and with inversus normally related great arteries. A subpulmonary conus can also be associated with double outlet right ventricle with hypoplastic left heart syndrome (e.g., with mitral atresia) and with aortic–tricuspid fibrous continuity. The subaortic conus is characterized by resorption of the subpulmonary conal-free wall, permitting pulmonary–mitral direct fibrous continuity. The presence of a complete muscular subaortic conus prevents aortic–atrioventricular fibrous continuity. The subaortic conus and great arteries shown here are associated with typical D-transposition of the great arteries , that is, transposition of the great arteries (S, D, D). A subaortic conus also occurs with L-transposition of the great arteries, that is, transposition of the great arteries (S, L, L), and with transposition of the great arteries (I, L, L). A bilateral conus, being both subaortic and subpulmonary, prevents semilunar–atrioventricular fibrous continuity. A bilateral conus is associated with typical double-outlet right ventricle, both with D-loop ventricles and with L-loop ventricles. A bilateral conus can also be associated with transposition of the great arteries when there is a muscular subpulmonary outflow tract obstruction (stenosis or atresia). Rarely, it is possible for solitus normally great arteries to be associated with a bilateral conus if the subpulmonary part of the conus is well developed and if the subaortic conal-free wall is present but poorly developed, just 1 or 2 mm in height between the aortic valve above and the mitral valve below; I have seen only one such case in my life, in a patient with the incomplete form of common atrioventricular valve canal with an ostium primum defect at the atrial level, no ventricular septal defect, and a cleft mitral valve. Hence, what matters most morphogenetically is not just the anatomical type of conus that is present but rather how much the sub-semi-lunar conal-free wall is present or has been resorbed. In the rare case that I am referring to, a small amount of the subaortic conal-free wall had not been resorbed, but not enough to disrupt the normal type of aortic valve-to-left ventricular approximation. The bilaterally absent or very deficient conus can be associated with double-outlet left ventricle with aortic–mitral and pulmonary–mitral fibrous continuity, even with an intact ventricular septum. However, double-outlet left ventricle does not always have a bilaterally absent or very deficient conus. AD: Anterior descending (coronary artery), Ant: Anterior (ventral), Inf: Inferior (caudad), Lt: Left, Post: Posterior (dorsal), Rt: Right, Sup: Superior (cephalad) (reproduced with permission from Praagh[6])

Figure 1: The four main anatomical types of sub-semi-lunar infundibulum or conus arteriosus: subpulmonary, subaortic, bilateral (subaortic and subpulmonary), and absent or very deficient. The upper row of diagrams shows the infundibulum (crosshatched) and great arteries as seen from the front (frontal view). The lower row of diagrams shows the infundibulum (crosshatched), the semilunar valves – the aortic valve, indicated by the coronary arteries, and the pulmonary valve, indicated by the absence of coronary arteries – and the atrioventricular valves – the mitral valve, being a two-leaflet valve, and the tricuspid valve, being a three-leaflet valve – as seen from below (inferior view), similar to a subxiphoid two-dimensional echocardiogram. In all diagrams, a ventricular D-loop is assumed to be present. The subpulmonary conus is normal. Resorption of the subaortic conal-free wall permits aortic–mitral fibrous continuity. The presence of a subpulmonary infundibulum prevents pulmonary valve–atrioventricular valve fibrous continuity. A subpulmonary conus is associated with solitus normally related great arteries (diagrammed here), inversus normally related great arteries, and in tetralogy of Fallot, both with solitus normally related great arteries and with inversus normally related great arteries. A subpulmonary conus can also be associated with double outlet right ventricle with hypoplastic left heart syndrome (e.g., with mitral atresia) and with aortic–tricuspid fibrous continuity. The subaortic conus is characterized by resorption of the subpulmonary conal-free wall, permitting pulmonary–mitral direct fibrous continuity. The presence of a complete muscular subaortic conus prevents aortic–atrioventricular fibrous continuity. The subaortic conus and great arteries shown here are associated with typical D-transposition of the great arteries , that is, transposition of the great arteries (S, D, D). A subaortic conus also occurs with L-transposition of the great arteries, that is, transposition of the great arteries (S, L, L), and with transposition of the great arteries (I, L, L). A bilateral conus, being both subaortic and subpulmonary, prevents semilunar–atrioventricular fibrous continuity. A bilateral conus is associated with typical double-outlet right ventricle, both with D-loop ventricles and with L-loop ventricles. A bilateral conus can also be associated with transposition of the great arteries when there is a muscular subpulmonary outflow tract obstruction (stenosis or atresia). Rarely, it is possible for solitus normally great arteries to be associated with a bilateral conus if the subpulmonary part of the conus is well developed and if the subaortic conal-free wall is present but poorly developed, just 1 or 2 mm in height between the aortic valve above and the mitral valve below; I have seen only one such case in my life, in a patient with the incomplete form of common atrioventricular valve canal with an ostium primum defect at the atrial level, no ventricular septal defect, and a cleft mitral valve. Hence, what matters most morphogenetically is not just the anatomical type of conus that is present but rather how much the sub-semi-lunar conal-free wall is present or has been resorbed. In the rare case that I am referring to, a small amount of the subaortic conal-free wall had not been resorbed, but not enough to disrupt the normal type of aortic valve-to-left ventricular approximation. The bilaterally absent or very deficient conus can be associated with double-outlet left ventricle with aortic–mitral and pulmonary–mitral fibrous continuity, even with an intact ventricular septum. However, double-outlet left ventricle does not always have a bilaterally absent or very deficient conus. AD: Anterior descending (coronary artery), Ant: Anterior (ventral), Inf: Inferior (caudad), Lt: Left, Post: Posterior (dorsal), Rt: Right, Sup: Superior (cephalad) (reproduced with permission from Praagh<sup>[6]</sup>)