Here's my non-professionals take on it.

I'm currently on Toprol & Benicar, which I believe do about the same
thing to BP & HR.

At my re-hab, I was working pretty hard on a recumbant machine, and was
told by the staff to slow down, as I had a few PVC's.  My HR at the time
was only about 108 to 110,  slower than what I *thought* I needed to
achieve for a proper workout.
I was told that with my med's, a 110 HR indicated an exertion level
comparable to a HR of 140 without med's.

I now wear a HR monitor during all my workouts. I set the watch to start
beeping at me at HR below 85 for my cool down, and at rate of 112 to
tell me my "safe" maximum.

The very low HR you reference above make me think Propranolol must be
substantially more powerful than my meds. My resting rate is about 60, I
go up to 75-80 while walking around the house, and I need to be really
exerting myself & sweating like crazy to get as high as 110.

David,
happily alive in my 4th month post-attack