Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Atrial Fibrillation ?
    ### Response:
    Atrial fibrillation (A-tre-al fi-bri-LA-shun), or AF, is the most common type of arrhythmia (ah-RITH-me-ah). An arrhythmia is a problem with the rate or rhythm of the heartbeat. During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm.
    