connective tissue
From what i understand it is scar tissue within the heart that will limit how well your heart can contract to pump out blood. Unlike your heart muscle tissue, the scar tissue will not contract.A heart attack would be the most likely cause for these scars.
Myocardial infarction (MI) can lead to heart failure in numerous ways. First of all, early on, the heart muscle does not contract well because it is not receiving enough oxygen and other necessary substrates, so stroke volume is decreased, which may lead to congestive heart failure. Later on, if the MI is aborted with thrombolytics or with a cardiac catheterization and thrombectomy, the myocardium may be stunned, or hibernating, because of the lack of oxygen, and may return to normal function over time. If the MI completes, the portion of cardiac muscle that was affected is dead. Depending on how significant a portion that is, losing the muscle alone may result in heart failure. The dead portion of the heart will later turn into a scar, which is noncontractile and also does not allow for the normal stretch, so it may affect preload and contractility in that way as well, decreasing cardiac output and possibly leading to congestive heart failure.