Morphological characteristics of perimembranous ventricular septal defects and their surgical significance.

The recognition of perimembranous ventricular septal defects and their subcategorization into inlet, outlet and trabecular types is important surgically because of the information it gives concerning the relationship with the atrioventricular conduction bundle. Inlet and outlet defects are well defined. The trabecular defect, in contrast, is less well defined. To obtain a better understanding of the perimembranous trabecular defect we studied 30 hearts, each having a ventricular septal defect known to be perimembranous. Of these 30 hearts, 16 were classified as inlet and 10 as outlet types. The "in between" trabecular defect in each of the remaining four hearts was slit-like, extending from the membranous septum into the area of the medial papillary muscle complex. Each was associated with a complex arrangement of the adjacent tricuspid valve leaflet tissues. In three hearts, leaflets and chords were attached to both the upper and lower rim of the defects. In the other specimen a shelf-like anomalous leaflet attachment was present along the lower rim. The atrioventricular conduction tissues were studied in two of the hearts, showing a close relation between the conduction bundle and the rim of the defect and the anomalous tricuspid valve leaflet attachments. In one case a most unusual disposition was encountered. A fan-like right bundle originated from the main axis at the same level as the origin of the left bundle branches. The anomalous fan penetrated into the connective tissue core of the valve leaflet. The bundle was then traced anteriorly to its usual intramyocardial position. The observed features of perimembranous trabecular ventricular septal defects warrant further caution should they require surgical repair.