Hi. I was a member a few years ago, but have had to re-register to post. My anxiety has been caused by having very frequent palpitations (pvc's or 'missed' beats), often hundreds if not thousands a day. Although told that they are benign they are so frequent and frightening that I struggle to come to terms with them being harmless.
Anyway, I often look at the All Experts Cardiology forum where two retired cardiologists answer heart questions (Dr T and Dr David Richardson).
I have found their answers very reassuring. They always advise that anyone with palpitations should have them checked by a doctor (eh ekg and/or echocardiogram), but in the absence of any structural defect or heart disease, they always seem to say that pvc's are harmless, no matter how many a day (even say 50% of beats), no matter how strongly you feel them.
I would suggest that anyone worrying about their heart gets it checked out, then has a look at the forum. I even printed out a list of their answers to read through when I feel very anxious.
http://en.allexperts.com/q/Heart-Cardiology-964/
My other advice is to get Claire Weekes 'Self Help for Your Nerves' and Richard Carlson 'Stop Thinking Start Living' - two really great books that have helped me through some very tough times with health anxiety.