This got very long but I'm not sure what to cut out. I have not idea whether it will help anyone but I do hope someone gets something out of it!
... I was curious if you ever discovered what caused the tachycardia in your sleep?
As far as I can tell the tachycardia I described in this thread (I describe a second type of tachycardia below) was related to my problems with autonomic dysfunction and NMH. The problem seemed to be related to pushing too hard during the day. I was gradually ramping up my activities (not exercise, just errands and household duties) without realizing it. At the same time, I was gradually reducing my resting time without realizing it.
So, I got serious about reducing my activities and resting more (which is VERY HARD because I hate doing nothing). I also started mixing up a homemade electrolyte mixture (something close to Pedialyte in sodium/potassium levels) more often. I drink that instead of dinner maybe 3-4 times a week. Unlike so many folks on this forum I sleep better when I don't eat dinner (no blood sugar problems).
It may also be that I had (have?) some sort of co-infections that got re-activated as
@CBS suggested (have not re-read the thread but I think that was mentioned?). But whatever the reason for my problem it finally did subside and has not come back.
By the way, none of my doctors were helpful, sadly. And the sleep specialist that I visited had not heard of Orthostatic Intolerance. What's worse, she did not even glance at the patient handout from Johns Hopkins.
I did an at-home sleep screening test (with a device like this --
http://www.resmed.com/epn/products/apnealink/apnealink.html?nc=patients ). The screening test showed no apnea and excellent blood oxygen levels, which was good to know. There was some other problem (hyponea?) that looked borderline. But I was not going to bother with a full sleep test with a doctor who did not even know about autonomic problems (and who did not want to learn!). I think it's pretty well known that autonomic dysfunction can interfere with sleep. And if this doctor's whole focus is apnea and CPAP machines, which would not help me, then why go for a full sleep test with her?
So, when people talk about getting detailed tests for co-infections I just shake my head in awe. I'm overwhelmed by the idea of trying to find a doctor who would test for such problems. I feel lucky to have a supportive primary care doctor and a cardiologist. I'm sure they were a huge factor in my successful claim for social security disability. From what I read on these forums finding such doctors is quite rare.
I doubt that I could find a doctor *locally* to do these extra tests for co-infections. And I'm not up to any serious investment of time/money/energy/travel to go to an ME/CFS specialist. I guess I just don't believe the trade-off would be worth it. Plus, there's always the risk of running into doctors who would do more harm than good.
Okay, now I should try to describe what I think are my two different kinds of tachycardia.
One kind is due to Orthostatic Intolerance (OI) -- NMH (Neurally Mediated Hypotension) and POTS. My symptoms come on due to various NMH triggers like posture (standing still is the worst, but sitting still and upright at a desk is also bad), getting overheated, and after eating (splanchnic pooling, blood pools in the abdomen). The tachycardia is just one of the many pre-syncope symptoms that I get. It does not come alone. I also get nausea, dizziness, brain fog, fasciculations (muscle twitches), sometimes muscle pains, and others that I can't remember right now. This, along with PEM from the ME/CFS/whatever, is disabling. I am not in bed all day but I am mostly at home. I am on disability, don't exercise, spend a lot of time resting, can't do a lot of household things, etc.
This tachycardia from OI is always part of a larger set of symptoms. What helps me is to rest in the recliner with my feet up (or lie down completely), get cooler, drink electrolytes, etc. And the drugs/supplements already mentioned (midodrine, florinef, potassium RX, salt tablets, magnesium).
I also have another kind of tachycardia that first started when I was about 11 years old. It sounds more frightening than the OI/NMH/POTS to most folks when I describe it. But is
not disabling at all. For example, I would never drive with a really bad episode of dropping blood pressure/nausea/dizziness/"brain fog". But I have driven with this other type of tachycardia.
What happens is that my heart changes abruptly, in a single beat, from a normal rate to 185-195 bpm. (even higher when I was a kid) And when it stops it also changes back to normal in a single heartbeat. It is not caused by exercise or exertion. It may be somewhat related to being tired (e.g., international travel and jet lag makes it happen more).
It was frightening when it first started happening when I was a kid. But it always went away on its own (no need for drugs or any other treatment) after only a few minutes. I went to the doctor and he said that as long as it went away in less than 5 minutes then there was nothing to worry about. I had a completely normal life, went to college, got a job, exercised (hiking, weight lifting, running, aerobics classes), travel, and all that. This weird tachycardia continued to happen at random times. And it still happens to this day. It even wakes me up from sleep at times. But it is a completely different feeling than the problem I described in this thread.
This other type of tachycardia was--and still is--no big deal for me. It is not painful. It does not make me dizzy or nauseated. The only problem is that it can be tiring when it goes on for more than a couple of minutes (the longest has been 45 minutes). But when it's only a few minutes or less (sometimes just seconds) then it's just a nuisance, like hiccups. It freaks out my husband (he's the only one who's ever been around to feel my heart rate when it happens) way more than it bothers me.
When I first saw the cardiologist for my problems with ME/CFS/NMH (sorry to those who don't like this combo of terms but I never know what to call my illness) I mentioned this other type of tachycardia as part of my medical history. Because it has never been caught on an EKG (even though I've had two holter monitors since 1990) no one knows for sure what it is.
However, based on my symptoms, my age at initial onset, my otherwise seemingly healthy heart, etc., my cardiologist believes it is most likely
AV Nodal Reentrant Tachycardia . This is a type of short circuit in the heart. There is ablation surgery available if it ever gets too bad. But for now I'm not even taking beta blockers for it. I've tried using the Valsalva maneuver (recommended by doctors) and sometimes that makes it stop, but not always. Usually I just lie down until it goes away.
I'll end by saying that I've been sick since 1990. I think having been sick for so much longer means that I don't know whether what's going on for me is going to be even remotely relevant to you. I was much sicker early on, esp. during 1991 (difficult just to walk across the room). Many of my symptoms have changed over the years. I'm not even sure I could list all the different changes after all this time. For example, I had lots of night sweats in the beginning and now these are quite rare (once every year or two?).
Okay, that's more than enough rambling from me!

PS. I just re-read my first post on this thread and realized that I repeated a lot of what I said then about my suspected AV Nodal Reentrant Tachycardia. Oops! Sorry, I guess this did not have to be as long as it was.