Just wondering, could you possibly have cranio-cervical instability ?
I’m just considering the mechanics because, I have increasing hypertension when I stand up and the drawing below (sorry it’s very basic!) depicts what happens to the head quite often when people are standing. You are no longer looking straight ahead most of the time and will often be looking down, with your head in ‘flexion’ quite a lot of the time – this in itself, if you do have cranio-cervical instability, can cause a restriction at the cranio-cervical junction, of the cerebro spinal fluid flow.
The way this restriction happens, if you do have cranio-cervical instability, is this:-
The weight of your head (which is very heavy) is affected by gravity and slides, ever so slightly forwards, this closes the gap between the opisthion (back of the opening through the skull) and the back of the dens (uppermost peg-bone of neck) in a pincer-like movement, putting compression on the structures that pass through the skull opening, i.e. brainstem, blood vessels etc.
http://prettyill.com/forums/viewthread/712/P45/#4612
This in itself would cause both high blood pressure and tachycardia, as mechanisms to try to keep the brain oxygenated.
Adding to that, if you’re unfortunate enough to have such additional anatomical differences as a short clivus, or a retroflexed odontoid, this can further add to the compression, when your head is in flexion.
http://prettyill.com/forums/viewthread/712/P15/#4079
Try and do as much as you can in a seated position and when you are upright, try and keep your head ‘squarely’ over your shoulders, to see if this improves things.