I finally had a cardiologist appointment that was helpful, and swapped out the beta blockers for fludrocortisone. Technically, he's calling it orthostatic hypotension even though I've yet to catch a low blood pressure, and my heart rate jumps when I stand.
I experience visual blackouts on standing too fast and nausea and fatigue from prolonged sitting, which was very much indicative of "less blood in brain."
The mechanism the cardiologist described is: I stand up and blood can't make it where it needs to go. My vision goes black because the eye is very sensitive to blood pressure and won't work right if it's low. The brain responds to this by sending signals to the heart through Beta 1 receptors to increase heart rate, which helps get some blood back and restores BP. This is apparently cardiology 101 so my Doc joined me in expressing frustration with that this pattern isn't well understood by most medical professionals, despite how common it is.
He calls this OH because the root problem is a lack of blood pressure, but I have seen that same mechanism be "hypovolemic POTS" instead. The most prominent example would be our friend Dr. Nathan Keiser who mostly uses the word POTS to refer to this mechanism.
So my question is: even though my diagnosis is technically OH, mind if I stick around?