Mondo Description A localized bulging or dilatation in the muscle wall of a heart (myocardium), usually in the left ventricle. Blood-filled aneurysms are dangerous because they may burst. Fibrous aneurysms interfere with the heart function through the loss of contractility. True aneurysm is bound by the vessel wall or cardiac wall. False aneurysms are hematoma caused by myocardial rupture.