This is actually an acknowledged neurosis and is treated by specialists. Here is the definition;
"A form of anxiety neurosis concerning the state of the heart, as a result of palpitation, chest pain, or other symptoms not due to heart disease. Also called cardioneurosis."
This is much more common than some realize. There is a considerable amount of information available on the Internet, but it is best to discuss this with a therapist. In most cases, the Internet has much to do with the level of anxiety one feels as most people are not really able to understand all the information available as laymen. Normally, what is read on the Internet is "worse case" scenarios. Look at this site for instance, it is a great resource but normally the people that post here are experiencing issues, those that are perfectly healthy don't normally post to say how great they are feeling. This leads to an illusion that there are more people with heart disease than those without which fuels the neurosis.
My advice would be to speak to a therapist, they will help you understand what is normal and what is not and get to the bottom of you fears.
Good luck!
Jon
I'm not sure if there's a textbook definition but what I understand is you may have it if you've had all kinds of tests done on your heart, been given a clean bill of health but you still obsess about having a heart problem.