Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Sudden Cardiac Arrest ?
    ### Response:
    Ventricular fibrillation (v-fib) causes most sudden cardiac arrests (SCAs). V-fib is a type of arrhythmia.
                
During v-fib, the ventricles (the heart's lower chambers) don't beat normally. Instead, they quiver very rapidly and irregularly. When this happens, the heart pumps little or no blood to the body.
    