You can check this by having someone check your BP laying down, then sit up, check it and repeat standing up.If you have an automatic BP cuff you could do it yourself.

I deduce that you are still being evaluated for a primarily cardiovascular cause of the fainting, as you are on a heart monitor right now, but I imagine that your doctor will still not rule out possible POTS as at least a contributing additional cause, short of your being able to confirm this diagnosis via tilt-table testing. Therefore I wonder how your doctor would feel about your undertaking B1 supplementation as something that could support both heart health AND possible dysautonomia/POTS? Traditional MDs are often not comfortable suggesting even famously nontoxic supplements unless they can perceive of these as providing a pharmaceutical-style benefit, but my own internist is an example of an MD who welcomes any harmless supplement, so long as it is not used to substitute for proper testing and pharmaceutical prescriptions. I am thinking that you might get the best insights into how B1 is being used in POTS/dysautonomia patients from this: https://www.standinguptopots.org/nutritionalimbalances#:~:text=Vitamin%20B1%20(thiamin)%3A%20Thiamin,and%20dramatic%20improvement%20in%20POTS. And if you want to see some specific protocols for B1 supplementation when itś used to address a wide range of other conditions, possibly check into the online materials of Eric Overton (EONutrition), a UK naturopath who holds a certificate in the practice of Functional Medicine -- a certificate unavailable in the U.S. that is nonetheless apparently available to non-MDs in the UK who work in this area.