Chris, I'm so sorry to hear what has happened. I agree about finding a good cardiologist and trying to get a holter monitor. It's shameful that this has not already happened if you have a history of arrhythmia.
Also, depending on the type of tachycardia/arrhythmia, maybe the Valsalva maneuver would help? This has sometimes, but not always, been able to interrupt my own episodes --
http://medical-dictionary.thefreedictionary.com/Valsalva's maneuver
As with anything, ask your doctor, be careful, read all the warnings about who should not try this maneuver, etc, etc. It's not something to take lightly.
I have had some kind of SVT (supraventricular tachycardia), completely unrelated to ME/CFS (that started at age 29), since I was 11. Never captured on a holter monitor because it is so random but probably
AV nodal reentrant tachycardia based on my symptoms, age of onset, etc. Mine usually goes away in 5-6 minutes but some are longer, the longest episode was about 25 minutes. That may seem short but when your heart rate is 190-200 BPM then every minute seems like an hour.
As others have posted, it's not a panic attack. I feel completely drained afterwards (like I've run many miles) but no fear either during or after. When I was onlly 11 years old then of course I was scared the first few times. But after 40 years, and knowing that in my case it's just a short circuit in my heart and it will "reboot" to a normal rhythm on its own, the SVT is just another annoying symptoms. And my SVT (which sounds so scary to the average person) has never interfered with exercise, school, or work, unlike ME/CFS/NMH symptoms which made me disabled. Then add the fact that I have a reasonably understanding and competent cardiologist and my SVT is one of my smaller problems.
HOWEVER -- SVT is very different when it starts to happen suddenly, three days in a row, on top of a bunch of already very bad ME/CFS symptoms and an ER staff that doesn't seem very helpful. My heart goes out to you, Chris, that your situation is so much worse than mine. I hope my sharing the details of my own case is not taken as a comparison in any way. I wanted to explain my own type of SVT in case it helps folks figure out whether the Valsalva maneuver would help them. I do know that there are different kinds of SVT but have no idea which kinds responds to this maneuver.
Also, I'm not sure whether you could do it when you're feeling that weak (wasn't able to watch the video right now but I did read your blog) . Still, maybe others reading this thread who have some form of SVT might find the information useful?
Sending you lots of hugs, Chris!