I get these episodes too and after all these years I can almost FEEL adrnaline and nor adrenaline getting squirted out of my adrenals. All I can do is wait it out.
A way I've found to use up these "fight or flight" agents is to get right to the gym and knock myself senseless on the elliptical trainer. When I'm done, I feel like a limp disrag but my BP and heartrate are back to normal.
There's always SOMETHING I can trace back too that spurred the whole episode on...usually some aggravation or fear. Neither can stand up long to the rigors of the ELLIPTICAL FROM HELL!