My nephew has a "POTS syndrome" for the past 8 months. His heart rate races +40% when he stands up (often fainting), and he experiences a "brain fog." Astronauts experience similar symptoms after returning from long trips and NASA developed a program of extreme exercises for astronauts. 80% of POTS patients are successful after 3 months of the NASA program; he is doing it for 5 months now, without major improvements.
We are trying to reach people familiar with training methods (marathon runners?) to better understand the maximum heart rate levels during such exercises. Maybe by modifying the program he could get better results.
Here is an article that might be helpful to you. It's all advice and things that can be helpful for POTS. Apparently ginger can be helpful, strange. And like robinwilliams (I love that handle lol) hydration is very important as well as not eating a bunch of processed food. Diet is always important. Hope this helps