How high does a heart rate have to go to become a heart attack?

Cardiac arrest and heart attack are two different things. Heart attack, or myocardial infarction, is when blood supply to the heart is interrupted, which causes heart cells to die. Cardiac arrest is when the heart stops beating. What you're talking about, a rapid heartbeat, is called tachycardia. If it persists long enough, it can cause a myocardial infarction due to low blood pressure. You need to just see your doctor about all this. You won't find a definitive answer here.