Yes, Metoprolol is a beta blocker and used to lower blood pressure and heart rate. I take 50 mg extended release to slow down tachycardia due to Afib. So my GUESS, is yes beta blocker is lowering HR. Too Low? I'd think, again guess, anything under 50, unless just waking up from a restful sleep, is too low. But I am not trying to argue with your medical advice, I am myself "just" a patient, a long time sufferer from Afib. This has been going on for 20 years or more and while it was under control for a few years I am now in permanent Afib. Fyi, I underwent open heart surgery in 2007, and still hear, God bless my surgeon. That was specific to valve leakage, mitral. Optimism, besides being at advanced senior age, I am still getting around so heart problems can be lived with. Hope your doctor can give you more professional advice and ease of the mind.