Midodrine is a vaso-constrictor and yes it's fast acting, but short lasting. It is helpful for the POTS by raising BP which helps the tachy rates. I take it along with Florinef (another common med for POTS). However, you said your BP doesn't drop upon standing...just that your HR increases. Is your HR high otherwise? What does it run on average not during events?

Mayo is best if you want work up all at once. It will be multiple tests a day. They are efficient for sure. What I don't like about them (and many others who say the same) is that they do not look outside the box. They have criteria for everything and if you don't fit 'clearly' within the criteria, then you don't have a diagnosis. This is especially true for autoimmune conditions...and even PN. This can make it difficult for patients like you with lots of negative tests but clearly something going on. I can guarantee you that they would never say 'you were just having a good day' with the gastroparesis. If the test is negative...it is negative.

You should get online and review each of their websites for info on comprehensive work-ups. Make some calls and consider insurance options. There are lots of good hospitals...but I only have experience with JH, Mayo and a couple in the Dallas area when I lived there.