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Atrial fibrillation - Symptoms and causes - Mayo Clinic

Atrial fibrillation is an irregular and often rapid heart rate that occurs when the two upper chambers of your heart experience chaotic electrical signals. The result is a fast and irregular heart rhythm. The heart rate in atrial fibrillation may range from 100 to 175 beats a minute. The normal range for a heart rate is 60 to 100 beats a minute.