A form of dysautonomia.  Often abbreviated as POTS, it is an abnormal increase in heart rate upon standing.   POTS is diagnosed by a tilt table test, or prolonged standing test, and a test is considered positive for POTS when an adult experience an increase in heart rate of 30 beats per minute within ten minutes of standing (or 40 bpm for an adolescent), or if the heart rate reaches 120 or above.  A diagnosis also requires that a patient experiences symptoms such as lightheadedness, syncope (fainting), shakiness, fatigue, anxiety, nausea, vomiting, sweating and fatigue.