Autonomic Determinants of Postural Tachycardia Syndrome

Eligibility Details: Inclusion Criteria: - female subjects, age 18-55 years, - criteria for postural tachycardia syndrome (POTS): 1. a heart rate increase of ≥30 beats/min within 10 minutes of upright posture; 2. lack of orthostatic hypotension (blood pressure fall ≥ 20/10 mmHg within 10 minutes of standing); and 3. chronic symptoms during upright posture over at least 6 months, in the absence of any other acute cause. - in the follicular phase of the menstrual cycle (day 5-13 of a 28-day cycle) - POTS with primary central sympathetic activation (psPOTS) as defined as having resting muscle sympathetic nerve activity (MSNA) greater than or equal to 25 bursts/min - able and willing to provide informed consent. Exclusion Criteria: - pregnancy, - smoker, - BMI>30 kg/m2, - deconditioned status (if available VO2max<80% of predicted) - unable to withdraw from medications known to affect autonomic function, blood pressure or blood volume - systemic illnesses known to produce autonomic neuropathy, including but not limited to diabetes mellitus, amyloidosis, monoclonal gammopathies, and autoimmune neuropathies. - Arteriosclerotic disease of carotid artery. History of neck surgery. - conditions associated with inflammatory processes, such as coronary artery disease, hypertension, smoking, hypercholesterolemia (or on statin therapy), rheumatoid arthritis, diabetes - treatment with oral corticosteroids, current infections (e.g., urinary tract infection), or use of non-steroidal anti-inflammatory drugs - other factors which in the investigator's opinion would prevent the subject from completing the protocol including clinically significant abnormalities in clinical, mental or laboratory testing.