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Following myocardial infarction the area of the ventricular wall that has died is replaced by scar tissue. This is never of the same strength as the original tissue. In the ventricle (especially the left) the high pressure that contraction of the myocardium produces can lead to stretching of the scar. The leads to a bulge in the ventricular wall which has no intrinsic contractility.
Idiopathic ventricular aneurysm
There is also a seperate entity of congenital or idiopathic ventricular aneurysm which may be associated with sudden cardiac death in young adults.
Following cardiac rupture, either traumatic (e.g. stab wound to the chest) or post-MI pseudo-aneurysms of the ventricular wall may also be formed. This occurs where the bleeding has been contained within a portion of the pericardium.
A ventricular aneurysm is prone to mural thrombus due to the stasis of blood here. The consequences of this can be very severe as the entire systemic circulation is downstream of the left ventricle. As with other aneurysms the weakened wall in also prone to rupture. Because the heart is contained within the pericardium it is cardiac tamponade that kills.
In appropriately selected patients surgical resection of the aneurysm may be tried. It has a mortality of between 3-8%.