The outlet ventricular septal defect is a cornerstone of the outflow tract defects and exists on a continuum that is anatomically different from the isolated central perimembranous VSD, according to the results of an observational study of 277 preserved heart specimens with isolated outlet ventricular septal defect without subpulmonary stenosis.

“In all of the specimens studied, the VSD always opened in the outlet of the right ventricle, cradled between the two limbs of the septal band, irrespective of the presence or absence of a fibrous continuity between the aortic and tricuspid valves, and the presence of an outlet septum,” according to the report published in the March issue of the Journal of Thoracic and Cardiovascular Surgery by Dr. Meriem Mostefa-Kara of the Paris Descartes University and her colleagues.

The 277 specimens comprised 19 with isolated ventricular septal defect; 71 with tetralogy of Fallot (TOF); 51 with TOF with pulmonary atresia (PA); 54 with common arterial trunk (CAT); 65 with double-outlet right ventricle (DORV), with subaortic, doubly committed, or subpulmonary ventricular septal defect; and 17 with interrupted aortic arch (IAA) type B ( doi:10.1016/j.jtcvs.2014.11.087 ).

Previous studies have shown that all malalignment defects include a VSD because of the malalignment and the absence of fusion between the outlet septum and the rest of the ventricular septum, and all authors agree that this VSD is cradled between the two limbs of the septal band, according to the researchers.

They found such an outlet VSD in all of the heart specimens studied, Dr. Mostefa-Kara and her colleagues added. In addition, they found that its anatomic variants were distributed differently according to the defect involved. This was especially true when focusing of the posteroinferior rim and particularly on the aortic-tricuspid fibrous continuity. In addition, this continuity occurred with different frequency among the various outflow tract defects studied.

They found the highest rate of continuity in isolated outlet VSD, then decreasing progressively from TOF to TOF-PA, then DORV, becoming “exceedingly rare” in CAT and absent in IAA type B.

The researchers also analyzed 26 hearts with isolated central perimembranous VSD from their anatomic collection and compared these with the outlet VSD hearts. All 26 of these VSDs were located behind the septal leaflet of the tricuspid valve, under the posteroinferior limb of the septal band, and NOT between the two limbs of the septal band as was the case with the outlet VSDs.

This led them to state that there was a “blatant anatomical difference between the these two types of VSDs,” and pointed out the risk of confusion. “The presence of a fibrous continuity at the posteroinferior rim of the VSD is important for the surgeon, because it makes the conduction axis vulnerable during surgery and therefore must be described specifically in the preoperative assessment of the defect,” they warned.

“This anatomic approach places the outlet VSD as a cornerstone of the outflow tract defects, anatomically different from the isolated central perimembranous VSD. This may help us to better understand the anatomy of the VSDs and to clarify their classification and terminology,” Dr. Mostefa-Kara and her colleagues concluded.

The study was sponsored by the French Society of Cardiology. The authors reported having no relevant disclosures.