My father recently had a mild heart attack in his sleep. He woke up and didn't feel anything weird. He has been to hundreds of doctors and hundreds of hospitals since. There doesn't seem to be a solution, except for maintaining healthy habits.
This worry of your parents is not something I've ever dealt with; I'm lucky that way. My mom and dad are quite independent and take care of themselves well. I've moved back from the States because the distance had started affecting my mom's anxiety severely but knowing that my dad's side of the family have all died from heart attacks, the thought of losing either of my parents is terrifying.
This worry for my parents' health is eating me up from the inside. I'm trying my best to validate my worries and calm the anxiety down, because this is a part of my life. I hope I find an outlet for this but it's something I was going to have to deal with eventually.