Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the symptoms of Atrial Fibrillation ?
    ### Response:
    Atrial fibrillation (AF) usually causes the heart's lower chambers, the ventricles, to contract faster than normal.
                
When this happens, the ventricles can't completely fill with blood. Thus, they may not be able to pump enough blood to the lungs and body.
    