Study Overview Brief Summary Detailed Description This is a pilot dose-finding study to test the hypothesis that mirabegron increases systolic blood pressure (BP), prevents syncope/presyncope, and improves the quality of life (QOL), functional capacity, chest pain, and overactive bladder (OAB) symptoms in patients with postural orthostatic tachycardia syndrome (POTS) who have a documented history of hypotension inadequately responsive to conventional treatments. The investigators will perform 24-hour ambulatory blood pressure monitoring (ABPM) and ambulatory skin sympathetic nerve activity (SKNA) recording using a Bittium Faros electrocardiogram (ECG) monitor, assess the number of syncope and presyncope episodes and determine the symptoms using validated questionnaires at baseline. The patients will then be given mirabegron (either 25 mg once daily or 50 mg once daily) for eight weeks. Afterward, the patient will return to the clinic for clinical assessments, complete questionnaires, ABPM, and ambulatory SKNA recording while still on treatment. Mirabegron will be stopped when the data collection is complete. Because mirabegron has a long half-life, the investigators will schedule a video visit with the patient 12 weeks after beginning the treatment and inquire about the patient's symptoms. The investigators will repeat all pertinent questionnaires at that time. https://clinicaltrials.gov/study/NCT06133075
