I too had the heart flutter thing (as well as most of the other side effects.) I did not go through all the tests though (I do not trust doctors very much) so I decided to get my Mirena removed first to see if it would make a difference and then go from there.
Well I had it removed on 4/17 and I have not had any heart flutters/irregularity at all. Most of my side effects went away quickly although I have had a few relapses but no where near as bad as when it was in. I think you will notice a change as soon as it is out if you have it removed. I have no doubt the Mirena caused this symptom as well as all the other symptoms I had.