The purpose of this clinical study is to further assess the clinical effect of high dose midodrine hydrochloride (ProAmatine®), an approved treatment for orthostatic hypotension. During the course of the study, participants will receive either ProAmatine® or a placebo. Assessments will be made using questionnaires that measure symptom and activity levels. Blood pressure in the lying down, sitting and standing positions will be measured. Patients will also complete standing time assessments. They will be asked to remain standing without moving until they feel sufficiently lightheaded, or dizzy, or feel faint so that they would feel more comfortable sitting down.