Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Postural orthostatic tachycardia syndrome ?
    ### Response:
    Postural orthostatic tachycardia syndrome (POTS) is a rare condition that is primarily characterized by orthostatic intolerance (an excessively reduced volume of blood returns to the heart when moving from a lying down to a standing position). Orthostatic Intolerance is generally associated with lightheadedness and/or fainting that is typically relieved by lying down again.
    