Do you have anxiety otherwise or just this one thing? This is going to become a more and more common obsession, as devices such as smart watches are becoming more and more prevalent and people are becoming obsessed by measuring things that change naturally during the day. For those who have a demonstrated medical problem, these devices can be useful, but for the rest of us it's just an obsession. If you can't stop this or suffer other anxiety problems, you might want to consider therapy. Something you can try is meditation to see if you can calm yourself. Regular exercise can help as well.
Oh my gosh, health anxiety is awful. Lots of people suffer it, you are NOT alone. And focus on the heart as the primary source of concern is also very common -- I read 70 percent of those with health anxiety worry about their heart. The others are more worried about cancer. Sigh. Most have neither.
The first thing to do is to get a full physical. That is important and everyone should do this but it also will help you feel more confident in how you next go about things. If your physical is all good, no heart issues as I suspect you'll find, then you can turn to how to treat the worry and anxiety.
You can possibly start with natural and life style things if your anxiety is not debilitating. This would include proper diet, rest and includes exercise (good for calming anxiety AND preventing heart disease!), then add meditation, yoga, deep breathing, spirituality, etc. You can consider adding counseling with a professional such as psychologist or counselor/therapist and learn triggers, coping skills, etc. And then there is the medical route. Your doctor can point you in the direction of a therapist and talk to you about medication if you can't combat this yourself. Short bouts of fear and anxiety are something that happens to everyone but if this is all the time or very frequent and consuming your mind very often, seek some help for it. It's treatable.