I had the same issue of sudden racing heart/palpitations after having Mirena in for a year and a half. I had it removed, and while the symptoms got MUCH better, I continued to have them for over a year post-removal. My doctor put me on a small dose of a beta-blocker (atenolol) and that helped a lot. I had to quit drinking caffeine altogether. Even 2 years out, if I have some, I get palps again.