Heart Condition and Swimming

Hi, I'm a 57 year old male swimmer with atrial fibrillation taking rhythmol twice a day. (I'm in sinus rhythm most of the time.) I also do a 2,800 yard workout four times a week and aspire to NQT for the 50 and 100 freestyle. I can't seem to get straight answers from my cardiologist. He says that it is fine to swim, but deeper questions are beyond him. For example... Is it OK to really press myself in my workouts? (I'm out of breath at the end of some of my sets, but who isn't?) Should I be thinking of long even swims rather than sprint swims? (I do mile and two mile open water swims, but they're not very exciting for me.) I'm fighting for breath sometimes during my sets. Is this just a matter of conditioning, or is there a direct link to the a fib? I have the low side of normal blood pressure. Is there any relationship between blood pressure and whether people are better constituted to be sprinters vs. distance swimmers? Any information from M.D.s and/or others who have heart conditions would be appreciated. See you at nationals!