A-fib happens when the electrical activity in the upper heart chambers "short-circuit", so the chambers don't contract properly. Instead, they are just vibrating.
Luckily, not all of those impulses are conducted to the main chambers (that would be equal to cardiac arrest), only a few of them are, but which impulses conduct and not is random, so the effect is a completely irregular heart rhythm. Also, the heart rate can increase inappropriately with exercise and drop inappropriately with rest and sleep, possibly causing symptoms of dizziness and chest pain. This may be regulated with medications.
Also, as the upper chambers aren't contracting properly, blood clots can grow there, which is why the risk of stroke is increased, and blood thinners are used to prevent this.
Risk factors of atrial fibrillation are high blood pressure, stress, smoking, heart diseases such as valve disorders and cardiomyopathy, heart surgery, etc. Treatment is either to control the rhythm with medications, control the heart rate with medications, and invasive or non-invasive surgery (restructure the upper chambers or "burning" away the triggering focus, ablation)
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