Phenotyping of Postural Orthostatic Tachycardia Syndrome (POTS)

Inclusion Criteria: POTS Patients * Age ≥ 14 years, able to provide informed consent (assent with parental consent for age \< 18) and comply with procedures * Meets consensus criteria for POTS: (1) sustained increase in heart rate ≥ 30 bpm above supine baseline within 10 min of quiet standing or upright tilt (≥ 40 bpm in individuals 12 to 19 years of age) OR sustained upright HR (heart rate) \>120 bpm, (2) absence of orthostatic hypotension, (3) symptoms with standing that improve with sitting or lying down, (4) resting supine heart rate \< 100 bpm, (5) orthostatic symptoms present for at least 6 months * Stable oral medication regimen for at least 14 days Non-POTS Control Patients * Healthy women, age 18 - 30 years, able to provide informed consent and comply with study procedures * Does NOT meet consensus criteria for postural tachycardia syndrome * No symptoms of orthostatic intolerance or dysautonomia. No history of other major medical disorder * Resting supine heart rate \< 100 bpm Exclusion Criteria: None of the following exclusion criteria: * Use of amphetamine-type stimulants, diuretics, selective norepinephrine reuptake inhibitor, anticholinergic medications (including tricyclic antidepressant medications), fludrocortisone, desmopressin in past 14 days * Use of other autonomic drugs (adrenergic agents, pyridostigmine, droxidopa, triptans, ivabradine) in past 48 hours * Currently receiving IVIG (Intravenous immunoglobulin), subcutaneous IgG (Immunoglobulin G), or any investigational medication (in past year) * Infusion of iv fluids in past 7 days * History or evidence of another condition explaining symptoms or orthostatic tachycardia (e.g. structural heart disease, CSF (Cerebrospinal fluid) hypovolemia, or severe traumatic brain injury)