4. Hemodynamic symptoms scoring: Hemodynamic symptoms will be measured by using the Vanderbilt POTS Symptom Score. The patients will be asked to rate the severity of 9 symptoms on a 0-10 scale (with 0 reflecting an absence of symptoms). The sum of the scores at each time point will be used as a measure of symptom burden. The 9 symptoms are: mental clouding, blurred vision, shortness of breath, rapid heartbeat, tremulousness, chest discomfort, headache, lightheadedness, and nausea. This symptom score has been previously used by our center, and the symptoms were chosen as they reflect common complaints of patients with POTS.