Depends, your chances are pretty good if you want to stay at your home institution. That also is impacted by how many people in your PGY year want to stay. In my limited experience, once you have gotten into and completed a general Cardiology training program, if you are competent and desirable to the program they are willing to keep you on for interventional training.
Programs typically take their own. So, your chances are dependent upon how many programs there are that won't fill internally. This is one situation where having somebody to bat for you will really help. Hopefully some of the cardiologists at your institution that know you know other people and can call around to see if there are spots open for you.