The Florinef (as Still Hoping said) is very different than prednisone. Florinef will help you retain fluids better.

The Midodrine is a vaso-constrictor and works by compressing the blood vessels to keep the blood from pooling in your feet/legs. It works very quickly (within 15-20 minutes) but is short lasting (about 3-4 hours, then starts to wear off), so has to be taken 3 times a day (only during daytime hours). I take it at 6 am, noon and 5 PM. Nothing at night, as you don't want it to increase BP while you are laying down...and possibly cause hypertension at night.

Typically you start with one medication, then add the other if you still need help in reducing symptoms. You are obviously way beyond sodium helping you, and need other intervention. So talk to your doctor about which one to start and go for it. Your symptoms of lightheadedness/dizziness, etc are from not getting enough blood to your brain. This is not good to deprive your brain of necessary oxygen rich blood.