A heart attack is a plumbing problem in the blood supply to the heart. This plumbing problem is usually from a blood clot or vessel spasm. During the early onset of the problem it only irritates the tissues called ischemia. Ischemia is a lack of blood and oxygen to the cells in the heart however it does not cause permanent damage. If Ischemia is unrecognized and left untreated it leads to infarction. Infarction is permanent cell death of your heart muscle and cannot be reversed.
The amount of ischemia or infarction that the heart attack causes is dependent on the size of the vessels with the plumbing problem.