I have no experience with Nadolol, but I read it is a non-selective
beta blocker that is administered to lower HR and BP. So, if you un-medicated HR is low, say 60 bpm or less, then it seems reasonable that with the medication you could go as low as 59 bpm or so.
Have you checked you BP, it may be lower too.
The beta blocker may be administered to lower the work you heart is doing to aid in the healing process, not sure on this point, just seems like a possibility on why is was prescribed.