Medical Definition of atrial fibrillation
: very rapid uncoordinated contractions of the atria of the heart resulting in a lack of synchronism between heartbeat and pulse beat If you're experiencing chest pain, palpitations, lightheadedness or shortness of breath, you could have atrial fibrillation (AF), a condition in which the heart's upper chambers (atria) "quiver."— Abigail L. Cuffey, Woman's Day —abbreviation AF
— called also A-fib, auricular fibrillation
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