Normally, your heartbeat follows a steady rhythm as your heart contracts and relaxes. When you have AFib, the upper chambers of your heart (atria) beat rapidly and irregularly, sending blood to the lower chambers (ventricles) less efficiently, says Robert Bonow, a professor of cardiology at Northwestern University Feinberg School of Medicine in Chicago. Episodes can last for minutes to hours or longer, and can cause palpitations, lightheadedness, fatigue, and shortness of breath. Over time, AFib tends to become chronic.
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