The Morphogenesis of Transposition of the Great Vessels

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Abstract
From earlier studies of the embryology of the ventricular skeleton a new explanation of the morphogenesis of transposition of the great vessels is offered. It is simpler than earlier torsion-detorsion theories, and provides criteria for differentiating transposition from other conotruncal anomalies, which previous theories do not. The theory postulates that there is, in effect, a shift in the orientation of the fibroplastic continuum which lines the primitive heart tube, extending from the A-V canal to the truncus arteriosus. Normally this continuum holds the aortic part of the truncus in fibrous continuity with the mitral part of the A-V canal. It is suggested that in transposition it is the pulmonic part of the truncus that is held in continuity with the mitral ring. Such a morphogenetic sequence would be accompanied by predictable abnormalities of other parts of the ventricular architecture. These would include (1) narrowing of the crista supraventricularis, (2) re-orientation of the bulbar part of the right ventricular septal surface, (3) a particular lie of the coronary ostia, and (4) certain deformities of left ventricular musculature. Seventeen hearts with transposition and a number of control hearts and hearts with other types of congenital conotruncal anomalies were studied. In all instances the deformities predicted were found to be present. The abnormality of left ventricular musculature proved to be of special interest. It is probable that the ventricular septal defect which often accompanies transposition is, in many instances, due primarily to an abnormality in the development of left ventricular muscle. This is in contrast with the septal defect seen in tetralogy of Fallot and other conditions without transposition, where the elaboration of bulbar musculature appears to be primarily at fault. Regarding the electrocardiogram in transposition, nearly all cases not complicated by inflow tract deformities showed the S1S2S3 pattern with QRS interval prolongation.