Well... they did the tilt table test. And my HR went up to 125. BP stayed the same. Then they gave me some nitroglycerin. And my heart rate went up to 168, BP stayed the same. Then they brought me back down and the doc came in... he looked at the results... thought it over... asked a bunch of questions... and decided that I do have tachycardia that is caused by being upright and aggravated by heat... but POTS is not my problem. He thinks it would not cause the kind of 'crashes' that I have been feeling. He thinks there is an underlying problem that is causing the tachycardia that merits further investigation.
In the meantime I need to drink lots of water, eat lots of salty food, and try a low dose of Pindolol for a couple of weeks to see if it helps. When have you ever heard a cardiologist give the following advice: "Put salt on everything. Go out and buy a couple of bags of your favorite chips and pretzels and eat them every day." :-) Isn't that everyone's dream prescription?!