Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) sick sinus syndrome ?
    ### Response:
    Sick sinus syndrome (also known as sinus node dysfunction) is a group of related heart conditions that can affect how the heart beats. "Sick sinus" refers to the sino-atrial (SA) node, which is an area of specialized cells in the heart that functions as a natural pacemaker.
    