I’ve had 4 heart attacks none of them were while exercising. 3 of them were when either sitting down or laying down one was while driving. I was an active bike rider during all of that time, most recent was 2 months ago. I do have coronary artery disease it’s mostly from genetics (high cholesterol). They have occurred at about nine year intervals with no symptoms before the event. The last one was several months after a dye test in the cath. lab that didn’t show a problem that would trigger a repair. For me it’s just a ticking time bomb but I don’t worry about high levels of exercise triggering one.