My understanding is the heart muscle primarily creates it's own electric impulse.
Not the brain.
When that pulse originates from several areas...out of synch...the vandals have taken the handle and the pump don't work.
The lower big Ventricles/muscles pull blood returning from the extremities and organs and then the top Atrium sends it out.
Atrial fib problems are not as concerning as ventricle fib.
But the longer in a-fib, the more difficult it is to reset:
-One person had 18 cardioversions. Yes 18. Doc was stuck on stupid. A shame and pox upon them. Patient is finally improved now after changing facilities.
-One person had 3-4 cardioversions, followed by ablation which lasted half a year, followed by another combined with Warfarin/Coumadin. That did the trick. Like magic. YMMV.
I bought several generic cuff machines but the results were inconsistent.
This brand does HR/BP/plus fibrillation warnings:
https://omronhealthcare.com/blood-pressure-monitors