Atrial fibrillation is an uncoordinated beating of both the left atrium and right atrium of the heart. For the heart to pump blood effectively and efficiently from the atria into the ventricles, both atria must have a normal rhythm.
To start each normal heartbeat, an electrical impulse is sent from the SA node in the right atrium to the rest of the heart, as shown in the illustration. The SA node is the natural pacemaker that coordinates the heart's contractions into a "sinus rhythm".
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