What Causes Rapid Increase In Heart Rate Post Mitral Valve Surgery?

I had mitral valve replacement surgery almost two years ago along with the implanting of a pacemaker. My heart rate just after surgery was 100ish. It is now in the high 70s which is about 10 beats faster than prior to surgery. My pacemanker is set to a maximum of 160. I play active sports but find that I get tired (exhausted) far more quickly than prior to surgery. I understand that this would be expected just after surgery but it s been almost two years. Why am I still getting so tired?