2. We will apply external abdominal compression (up to 40mm Hg) to evaluate the ability of this counterpressure method in combination with a pressor agent to improve orthostatic tolerance. For this purpose, we will use an abdominal binder with an inflatable cuff placed underneath, which will be inflated to exert pressure at the level of the umbilicus while patients are standing. Patients may be randomly assigned to 5mm Hg (sham treatment) and/or 20-40mm Hg external abdominal compression with an abdominal binder applied at baseline of some of the therapeutic trials described above. The principal investigator will decide which medication will be used in each trial.