Severe blockage of the arteries that supply blood to heart muscle is known as a heart attack (the medical term is “myocardial infarction”). Heart muscle is weakened immediately after a heart attack but gradually regains its strength. Occasionally, the heart muscle progressively weakens, leading to chronic failure of the heart to work as an efficient pump. Although several techniques and drugs are used to improve blood flow to heart muscle rapidly after a heart attack (known as reperfusion therapy), the factors that influence development or improvement of heart muscle weakness are not known.