For example, after my bypass surgery, my heart health increased dramatically, I was on a much stronger beta blocker (Coreg)
and it was discontinued
and I was given Metopropolol. I still had problems like you describe. During a walk I noticed a very high heart rate, about 135 bpm which continued for hours. After about 8 hours I went to hospital, and it was determined that my pacemaker needed to be reprogrammed, my heart had become healthier than my pacemaker could adapt to.