After a heart attack, fibrosis occurs at the site of cardiac muscle death. this reduces the efficiency of the hearts pumping ability because

Fiboros connective tissue is incapable of contracting

Fibrosis is the scientific name of scar tissue, and scar tissue no matter where in the body is incapable of performing any life sustaining activities. In the heart the muscles primary duty is to contract at specific intervals, so as to pump blood. Because fibrosis is absolutely useless, we can come to the assumption that contraction of the myocardium that has been replaced is not something it can do. This can lead to a deficiency in the pumping of blood throughout the body and the blood will begin to pool causing such issues such as edema (swelling due to fluid buildup). This is called CHF or Congestive heart failure.