I also take metoprolol. My pulse has stayed very consistent for 2 years so that is why I was so surprised when it wasn't.
I've been taking the beta blocker Metoprolol for 5 or 6 years, and it's side effect of slowing my pulse rate is still very effective. It was prescribed after I complained about waking in the middle of the night with an elevated rate. I took it for several years before finding that it was high due to severe sleep apnea. Now on CPAP, I continue to take it today even after my ablation to my control blood pressure.
I also take it to lower heartrate along with other bp Meds. I am also on cardizem and have been for years. It originally helped lower my rate but then lost effectiveness in that area. I was just so surprised today as even exercising its been hard to get it in the 70s. You are right. Aging sure has some effects I am not really liking.
Beema
Sorry to say heart rhythm (and everything else in my experience) gets worse with age (aging). I know that when on a high dose of BB, 100 mg twice a day, it initially made my BB so low that I got dizzy from just standing up...had to get up slow and stand for a minute before walking. After a few weeks the dizziness lessened. Just my way of answering that my experience says, "yes" the human body (mine) adjusts to BB. I suppose they also have less effect on lowering HR, the reason I take it.
My resting HR is in the 70s and standing could be 100. It doesn't take much to get me up to 135 or so, my maximum. Just walking briskly on the level will do it. I ran (10 minute miles) up to the age of 67, but no longer.