It is still not clear to me how anyone can tell whether the tachycardia in POTS is a response to a blood distribution change that causes symptoms or a response to the symptoms, which may be caused by something else. POTS is also a very confusing term because it is often used as if it was itself a cause, that might cause other things like brain fog.
If you mean that the tachycardia of POTS may be an anxiety/stress response
to the symptom of OI, whatever the cause, I'd argue against that for the simple reason that, for me at least, the tachycardia would
precede OI.
It didn't happen to me immediately after onset, but, by three years into the illness, I noticed that I was getting weird symptoms if I stood still for very long. I was not bed-bound, nor do I suspect was I much more deconditioned than a sedentary office worker.
Never-the-less, my pulse was now going up very quickly upon standing. It would easily go from 55 to 110+.
Only after about 10 minutes on my feet, with my pulse already accelerated, would I become aware that my body was in someway "signaling" me to get off my feet. The urgency of this signaling increased the longer I remained standing. It's hard to convey just how "imperative" this warning could become. It was basically commanding, "Sit down
or I will put you down!" I assumed I would probably black out if I remained standing. Inevitably, I would sit down or lean back against a wall (which was less effective than sitting).
As others have mentioned, I found some relief if my legs were in motion. I assumed this was probably because moving my leg muscles helped my circulation in overcoming some kind "blood pooling" in my legs, but it's just a guess.
I wouldn't be surprised if the initial tachycardia was a response to the brain not getting enough blood upon standing. The subsequent OI "warning" to sit down might be a response to prolonged tachycardia, as though the body expects some brief increase in pulse rate upon standing but eventually decides "this is ridiculous!"