I had borderline low BP most of my adult life, including the early years of ME/CFS. Then somewhere in the past few years my BP started creeping up to high normal. This is about the same time the tachy started giving me real problems. I don't know if this is a progression of the illness thing, a result of clearing up 5 chronic infections, deconditioning from not being to be active when ill... or something else entirely. Struck me as odd, though, that I went from being lowish BP to highish BP over the course of this illness.