I'm sorry your doctors don't understand. Could you present info re this to your doctor like yogini mentions? If not it might be helpful to find doctors that are willing to work with you!
This is from the POTS what helps page: Ritalin increases peripheral vascular resistance via alpha receptor stimulation (Grubb, Kosinski, Mouhaffel & Pothoulakis, 1996). Ritalin is prescribed by some physicians, but can be addictive.