Summer Research Description: The objective of this study is to assess endothelial function in Postural Tachycardia Syndrome (POTS). POTS is characterized by an excessive increase in heart rate upon assuming upright posture, associated with symptoms suggestive of cerebral hypoperfusion or sympathetic activation. POTS is an important cause of disability in otherwise healthy young adults, disproportionately affecting women of childbearing age. Even though the pathophysiology of POTS is not completely understood, it has been suggested that nitric oxide deficiency, a cause of endothelial dysfunction, is at least partly responsible for this condition. This study plans to assess endothelial function in POTS patients using flow-mediated dilation (FMD) of the brachial artery in response to local ischemia. Measurement of the change in brachial artery diameter pre- and post-ischemia will indicate whether the subject exhibits normal vascular function. The researchers hypothesize that patients with POTS will have vascular endothelial dysfunction compared with control subjects.