Just seen this thread. I had a heart attack some 5 years ago after a lifetime of heavy smoking and being a professional couch potato. I sustained some permanent heart damage..... Am on medication for the rest of my life...... Had a stent (inserted through the wrist not the groin) fitted and have subsequently had 3 skin cancer operations, pneumonia + cataract and prostate operations.
During this time I have taken up cycling in a big way and, according to my Garmin 705, have now covered well over 16,000 miles and climbed over 820,000 feet (I live in Wales).
I always use a heart rate monitor and find it very useful as a means of seeing how hard I am working. I too am on beta-blockers but I get up to 150bpm regularly (I am 64) and my heart rate seems to be pretty normal except for being, I assume, controlled to some extent.
My only issue seems to be that I have reached a fitness plateau and can't seem to get beyond doing 60/70 mile rides. I am a little frustrated by this as every 100k ride with over 4000ft is a continuing big challenge that leaves me knackered.