Autoimmune Basis for Postural Tachycardia Syndrome

Inclusion Criteria

- 18-50 years old - Postural Tachycardia Syndrome: Heart rate increase >30 bpm from supine within 10 min of standing, in the absence of orthostatic hypotension (>20/10 mmHg fall in blood pressure), with chronic symptoms (> 6 months), and in the absence of other acute cause of orthostatic tachycardia. - Able and willing to provide informed consent - Female premenopausal subjects must utilize adequate birth control and willingness to undergo serum beta-hCG testing - The subject must understand and be able to comply with the study procedures and restrictions.

Exclusion Criteria

- Hypertension (>150 mmHg systolic and >100 mmHg diastolic) based on history or findings at screening. - Orthostatic hypotension (consistent drop in blood pressure >20/10 mmHg with 10 min stand) - Pregnancy - Cardiovascular disease, such as myocardial infarction within 6 months, angina pectoris, significant arrhythmia (sinus tachycardia is not excluded), deep vein thrombosis, pulmonary embolism - History of serious neurologic disease - History or presence of significant immunological or hematological disorders - Clinically significant gastrointestinal impairment that could interfere with dietary compliance or drug absorption - Impaired hepatic function (aspartate amino transaminase and/or alanine amino transaminase >1.5 x upper limit of normal range) - Impaired renal function (serum creatinine >1.5 mg/dL) - Hematocrit <28% - Current or concurrent disease that could affect the absorption, action or disposition of the drug, or clinical or laboratory assessments. - Any underlying or acute disease requiring regular medication that could possibly pose a threat to the subject or make implementation of the protocol or interpretation of the study results difficult - Inability to comply with the protocol Healthy control subjects will be healthy, non-smoking and on no chronic medications at the time of the study. Healthy control subjects will be group-matched to the POTS patients for age and gender. We will attempt to study female patients in the first half of their menstrual cycle to minimize cyclical variability.