For so many things proven to be wrong in ME/CFS and Long Covid bodies its astounding the lack of diagnosis and treatment. Just another biomarker to add to the list (a historical one and quite old at that) of things doctors wont use.
I had 2 impedance cardiographies done around 2005-2007 I think and both were significantly abnormal, along the lines predicted by Peckerman. But I believe the devices and testing are very hard if not impossible to find now. But as @BrightCandle said, another biomarker that has gone ignored. It wasn't ignored by my doctor at the time - he was remarkably prescient and aware - but unfortunately died several years ago.