Well, few of us on here are doctors or otherwise qualified to answer your question. However, my limited understanding is that 1) a lowered heart rate tends to reduce the incidence of AF getting started, 2) the bisoprolol in your system limits the extreme high rates that can occur when AF does start. e.g. my first attack reached 280bpm on the paramedics ECG due I suppose to me thinking I was dying and being very scared. Since then, and taking the bisoprolol, I've not to my knowledge exceeded 200 bpm (I have a Kardia).
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