TETRALOGY OF FALLOT, TRUNCUS ARTERIOSUS, AND ABNORMAL MYOCARDIAL ARCHITECTURE

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Rat fetuses exposed to bis-diamine develop various cardiovascular anomalies. The relation between anomalies of the heart and the aortic arch arteries were quantitatively analyzed in 65 treated rats at a late fetal stage under a dissecting microscope. There were 26 rats (40%) with a ventricular septal defect in the perimembranous portion, of which 14 rats (22%) had tetralogy of Fallot, four (6%) had truncus arteriosus, and eight had relatively small septal defect with no other major anomalies. Of the four rats with truncus, two had a vestigial vasculature on the truncus root and three had a rudimentary infundibulum. In 44 rats (68%) the middle latitudinal muscle bundle of the ventricular septum was not continuous with the left ventricular free wall but rather with the right ventricular free wall. There were, isolated or in association, a double or right aortic arch in six rats (9%), aberrant subclavian arteries in nine (14%), right ductus arteriosus in 12 (18%), and agenetic ductus in four (6%). The cross-sectional area of the ductus, as corrected by that of the aortic isthmus, was abnormally small in 47 rats (72%). The rats with a septal defect and/or an abnormal myocardial architecture usually had a small ductus; it was very small or absent in the rats with tetralogy. It is speculated that the defect may be the anomalous and reduced development of the 6th arch arteries, which, imposing pressure load on the fetal right ventricle, may have led to the persistence of ventricular septal defect as a vent and/or to the formation of myocardial architecture favorable for the generation of pressure in the right ventricle.