What do we actually know about orthostatic intolerance?

InitialConditions said:
Cardiac output can be low despite high HR under reduced stroke volume (which is what you'd expect with poor venous return). HR cannot fully compensate.
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Yes, but cardiac output is not the issue, it is cerebral perfusion. If blood pressure is maintained then perfusion pressure for brain is maintained, so low cardiac output is not a problem for the brain. It will only be a problem for tissues with arterial constriction.

So, as you say, if brain perfusion really is to blame for symptoms then something else is going on, which might just as well be going on in people without tachycardia too.
 
ScoutB said:
I think everyone's in agreement that focusing on OI makes more sense than tachycardia?
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Yes, but do we really have good evidence for that OI being related to reduced brain perfusion if we don't have a story for that that adds up? In an acute reaction to typhoid vaccine you feel awful and have to lie down, but there is no reason to think there is a shift in brain perfusion as far as I know.
 
Jonathan Edwards said:
But the association of OI with tachycardia without a BP drop suggests much more that the unpleasantness of standing or sitting up relates to the reflex tachycardia itself rather than any loss of brain perfusion.
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My HR gets 20-30 BPM lower from very low doses of betablockers, but there is no noticeable difference in the amount of «unpleasantness» when sitting or standing.

The unpleasantness also kicks in a while after the HR goes up.

My hypothesis is that the increased HR is a downstream effect of something else going wrong. It could even be a measuring issue where a BP «sensor» is faulty in some way so it thinks the BP is too low and therefore increases the HR to try to compensate.
 
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