I'm on 25 mg of Atenolol daily for a heart attack in 2000. Right after the heart attack, I was on 100 mg. There were times I couldn't get my heart rate above 100 bpm. Fortunately my cardiologist has reduced the amount over the years. I also wear a heart rate monitor set to beep at 160 bpm. Of course my problem is different than yours.

The 25 mg of Atenolol still negatively affects my heart rate while riding. I've been known to cheat, like taking half a dose, when doing something like a century ride. I also do not take it if riding in very hot weather. Blood flow helps cool the body and reducing the bpm's doesn't help with heat. Again I wear a heart rate monitor and slow down when it gets near 160 bpm.