Certain ezymes are found in heart muscles and not in the blood stream.
If after a suspected heart attack they are found in the blood stream then they can only have come from a damaged heart.........leaked out into the blood stream....heart attack.
As far as I am aware, when the heart muscle is damaged, an enzyme is released into the bloodstream which can be detected in a blood test. I think there is a period where it is detectable eg 6 - 48 hours after the heart attack but I am sure a google search would be much more informative than me!!