My middle kid (female) had a heart rate of 140 at rest for awhile and then it went back to normal rate, mysteriously. I believe she had POTS from the tilt table test though the nurse turned around too quickly and didn't see the bp drop. Twenty years later she still has moments of near fainting in heat, because it dilates blood vessels and lowers blood pressure, I assume. She just recently went to a cardiologist for the first time in decades due to lupus.

My oldest kid (male) had fast heart rate after exercise and saw a pediatric cardiologist. I honestly forget why. They were very concerned and we called in his heart rate from the school nurses' office every day for awhile, after exercise. That too mysteriously righted itself.

I would find a really good pediatric cardiologist. Your daughter has had EKG, echo, stress test and svt and POTS are ruled out. What issue was found with the stress test?

I would ask what is causing the low blood pressure since that can make the heart rate go up, as I understand it. @gloaming knows more! I assume she is hydrated and you said she is taking salt.

I really hope you can find a doctor who listens. We found it could be hard to find doctors to take problems seriously with young females. The doctor should definitely test thyroid, cortisol, autoimmune markers, anything that could be contributing and whatever was found on the stress test must be relevant.