@Sushi - one more possibility is hypophosphatemia. I first came across this when reading about refeeding syndrome, courtesy of Freddd. Hypophosphatemia is the hallmark of refeeding syndrome, along with hypokalemia and hypomagnesia.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440847/
There is a link between some forms of ME/CFS and hypophosphatemia:
http://www.ncbi.nlm.nih.gov/pubmed/9683977
And I've experienced hypophosphatemia - it can cause severe fatigue. It's also linked to afib and hypertension.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4105385/
http://hyper.ahajournals.org/content/9/1/96.full.pdf
http://www.ncbi.nlm.nih.gov/pubmed/2136290
Several months ago a supplement which initially boosted my energy ended up making me very tired (I wish I could remember which one it was!) Low potassium makes me very tired, but this felt different. After coming across refeeding syndrome, I supplemented with high phosphorus foods (kefir and yogurt), and also small doses of monosodium phosphate from Swanson’s and voila, my energy returned.
Last week I increased my B12 and folate and, after initially feeling better, again developed rather bad fatigue. It wasn’t a crash, wasn’t low potassium and took me a couple of days to remember phosphorus. I had slacked off on the kefir and yogurt. So I restarted on kefir, added some monosodium phosphate and within about a day and half my energy started to come back.
Unfortunately, there does not appear to be very much research on phosphorus. And the literature that does exist generally just says be careful!, that the problem with phosphorus in general is too much, not too little (just as they do with potassium)
But I don’t drink sodas, a large source of phosphorus for many. And people with ME/CFS do have a variety of electrolyte problems.
I think the easiest and safest way to see if hypophosphatemia is a problem for you is to drink a lot of kefir, or have a lot of yogurt – at least 2 decent servings in a day and see if it helps your BP and afib. I would not be surprised if hypophosphatemia was an unrecognized issue for many of us.