**5. Pathogensis**

The pathogensis reflects two different types of rupture. The first, a simple rupture, is a direct through-and-through defect that is typically located anteriorly (when associated with a LAD territory infarct). Conversely, complex ruptures are believed to result from tracking of blood as it dissects thru the septum with left ventricular entry sites remote from right ventricular exit sites – these tracks then enlarge over time due to the pressure gradient between the left and right ventricle. Multiple defects are found in 5-11% of cases and emphasized the need for a complex pre-operative and intra-operative assessment of all pathways to insure a complete repair (Edwards, 1984). Incomplete closure of residual or secondary defects can account for post-operative recurrences. Transmural infarcts can be quite extensive with defects developing to several centimeters in diameter and can often involve extensive areas of the left ventricular free wall. For complex defects, as blood dissects through the necrotic myocardium there can be further expansion and damage with loss of cellular integrity. With local cellular destruction there is fragmentation with degeneration of myocytes with enzymatic digestion and destruction. In patients who survive the acute presentation, up to 66% develop chronic ventricular aneurysms and a third will have significant functional mitral regurgitation from the secondary effects on the ventricular free wall.

The pathologic consequences and outcomes of surgery of anterior and posterior defects appear to be different in ways beyond what can be explained by the degree of shunting. Earlier autopsy studies have shown that anterior PI-VSDs were associated with 33% of the LV and only 10% of the RV being infarcted, while posterior defects were associated with only 20% of the LV and 33% of the RV being infarcted (Cummings, 1977). Particularly considering the acute pressure/volume overload and associated RV failure, it becomes understandable why posterior based defects have a worse prognosis.
