Ventricular septal defect in tetralogy of Fallot

A study of 67 surgically unaltered specimens of tetralogy of Fallot at The Johns Hopkins Hospital revealed a group of 28 hearts in which the ventricular septal defect was surrounded entirely by muscular tissue. Since part of this muscle lay in the space between the tricuspid and aortic valve annuli, where it occupied a position taken by the parietal band in the normal heart, it is postulated that this muscle represents an inferior division of the embryonic infundibular septum (parietal band) in these specimens. The superior division of the parietal band forms the anterior margin of the ventricular septal defect and the superior border of the stenotic infundibulum. A similar but more rudimentary inferior portion of the infundibular septum may have been present in another 16 of the 67 specimens, as represented by a fibrous or fibromuscular band separating the aortic and tricuspid valve annuli.