To be honest, I don't remember exactly what initially convinced me that you have hypovolemia, but it was something in your description of your symptoms, and test results, back when we first discussed POTS.
When a doctor recommends that you drink plenty of fluids before having blood drawn, or when she/he recommends a saline IV infusion, that is because she/he thinks that you have hypovolemia, otherwise, you would not need the additional fluids. If drinking more water, or having a saline solution infused by IV, helps to significantly increase your blood pressure, then that is pretty strong evidence that you have hypovolemia, because if you did not have it, the extra fluids would not provide much help.
I've forgotten, I may have already asked this earlier, when we first discussed POTS. Have you been tested for adrenal insufficiency? IOW, has your doctor checked to make sure that your adrenals are producing plenty of cortisol. If your body is low on cortisol, one of the effects is low blood pressure.
Anemia can cause hypovolemia. Are you sure you're not anemic?
Also, what about the link between Chronic Fatigue Syndrome, (CFS), and POTS. Somewhere, I saw a report where approximately two-thirds of patients with postural tachycardia syndrome suffered from chronic fatigue. I remember you mentioning that chronic fatigue is a serious problem for you. The reason I bring this up is because of the fact that you mentioned that you are taking a beta blocker.
The beta blocker is prescribed to limit the maximum heart rate. However, in your case, I believe that the reason why the heart rate increases in the first place, is because it is trying to compensate for the insufficient circulation, due to the hypovolemia, not because of a neural disorder. As the blood pressure goes down, it is necessary for the heart to pump faster, in order to maintain sufficient blood in circulation.
On the other hand, if the tachycardia is due to Neurally-Mediated Hypotension, (NMH), then a beta blocker would probably be a good idea. The big question is whether it's a good idea, in the case of hypovolemia. The following quote is from the POTS information site that I believe you often refer to:
I don't believe that NMH is your primary problem - I think it is hypovolemia. Even if I am wrong, though, and it should turn out to be NMH, the odds are very high that you have CFS, and that suggests that taking a beta blocker may not be a good idea, in either case. Still, without the beta blocker, the chest pain will probably be worse, during a tachycardia episode. The problem is that a higher heart rate does not necessarily move any more blood, if the blood supply is short to begin with. From that viewpoint, the beta blocker is probably a good idea. And, if it helps to keep your BP from falling as low as it would without any meds, then it is obviously beneficial.Beta blockers are used by many doctors to treat POTS and neurally mediated hypotension (NMH). I have heard of more bad drug reactions to beta blockers than any other drugs used for the treatment of POTS and NMH, especially for those patients who have NMH secondary to Chronic Fatigue Syndrome. In these patients beta blockers can actually cause POTS. Some patients have found beta blockers to be helpful, however, especially those patients who develop POTS because of an overly sensitive beta adrenergic system. For these sensitive patients only very low doses of beta blockers are usually required.
Yes, hypovolemia can definitely cause hypotension. Here's a quote from Wikipedia:
http://en.wikipedia.org/wiki/Orthostatic_hypotensionHypovolemia
Orthostatic hypotension may be caused by hypovolemia (a decreased amount of blood in the body), resulting from bleeding, the excessive use of diuretics, vasodilators, or other types of drugs, dehydration, or prolonged bed rest. It also occurs in people with anemia.
The combination of CFS, and hypovolemia/hypotension could definitely make it almost impossible for you to do anything that requires much physical exertion, (such as exercise).
Remember though, when you read anything that I write, I am not a doctor, I'm just a researcher, trying to learn as much as I can about what ails us.
Love,
Tex
P S You're right about most doctors thinking that one has to have massive bleeding in order to have hypovolemia. They can be very narrow minded at times. As Polly says, "Doctors are trained to look for horses, not zebras".
(I will include myself). 
