High Sodium Diet and External Abdominal Compression in POTS

The study will involve a crossover design in which each subject will be assessed (as below) while on a very low-sodium (10 milliequivalent/day) diet compared with a very high-sodium diet (300 milliequivalent/day). These acute dietary interventions will be part of the parent study ("Dietary Salt in Postural Tachycardia Syndrome" IRB#111261) for 4-5 days at the time of the study. Dietary success will be assessed using a 24h urine for sodium and creatinine as a part of the parent study. Study Day: * Studies will be performed at the end of the low and high-sodium diet phases. * Blood pressure recording will not begin until at least 2 hours after the last meal (to avoid any confounding hypotension from the last meal). * Subject will be asked to void prior to data collection. * The subject will be seated in a chair, with their feet comfortably on the floor. * The Dinamap electrocardiographic and blood pressure (brachial cuff) recorder will be attached to the patient and set up for measurements every 10 minutes throughout the study with digital download into the Autonomic Dysfunction Center BP database. * After a 20-minute seated baseline, the subject will stand for up to10 minutes. Heart rate and blood pressure (HR/BP) will be measured at 1, 3, 5, and 10 minutes of standing. At the end of the baseline stand period, subjects will be asked to rate their symptoms using the provided orthostatic intolerance Symptoms questionnaire. * The subject will then be seated and the abdominal binder will be applied without compression. * The Dinamap electrocardiographic and blood pressure (brachial cuff) recorder will be attached to the patient and set up for measurements every 10 minutes throughout the study with digital download into the Autonomic Dysfunction Center BP database. * At 1 and 2 hours post intervention, the subject will be asked to stand for up to 10 minutes while applying external automated abdominal binder compression (up to 40 mmHg). HR/BP will be measured at 1, 3, 5, and 10 minutes of standing. o If nursing staffing shortages make q1h standing difficult, then the subject should be asked to stand at least at 2 hours post intervention. * Study termination for that day.