Atrial fibrillation (AFib) is a heart rhythm disorder. In a typical heart, a group of cells called the sinus node sends electrical signals that start each heartbeat. The signals go across the upper heart chambers to the AV node, where they usually slow down. Then the signals go to the lower heart chambers, causing them to squeeze and pump out blood. But in AFib, the signals in the upper chambers are chaotic. The AV node can't stop all of the chaotic signals from entering the lower chambers. This causes a fast and irregular heartbeat.
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