There is no such thing my friend. Either way most cardiology fellowships do accept IMGs, on the other hand what are your chances of being accepted given the increasing competition from AMGs is different story. No fellowship will prefer you over an equally qualified (or even less qualified) AMG. At the time being there is a large presence of IMGs in cardiology fellowships due to the fact that AMGs hate the cardiologist lifestyle. Ultimately as they increase in number and have less options, they will fill fellowships like cardiology too which I expect to happen in 2018-2019.
The point is, to get into a cardiology fellowship, you do everything that you can by applying as widely as you can and go wherever you get an interview.