I used it with DD.... sort of. I used it wrong and didn't actually get the physical effect of the gas - as I found out after the birth, when my placenta had to be extracted with forceps and the MW kept telling me "suck more gas", and I was all "Ohhhh, this is what they meant!". :p Psychologically, though, I did feel more in control while I was holding the tube, knowing I could suck on it if I wanted... it kinda gave me something to do, and also something to resist ("I can get through this next contraction without it").
I'm planning to do Hypnobabies this time, waterbirthing and so on, and obviously I'd prefer to go all-natural; but our birthing centre offers nitrous, and I won't feel bad about using it if I need to. It doesn't cross the placental barrier; it's user-controlled, not controlled by a nurse or anything; it wears off very fast (I think it exits the body in something like 7 seconds - or is it 7 breaths?); and as far as medical pain relief goes, I don't think it has anywhere NEAR the implications of an epi or even pethadine or whatever.
That said, yes, it does alter your mental state - when I was using it properly I definitely had that "Wow, this is agony, but I don't care" thing going on, which was a rather unusual sensation! (Given the alternative, though, I was happy with it...) I can't imagine using it for several hours at a go or anything, which is why I'm planning to use it only if necessary and as a last resort. Your "security blanket" definition is a good one!
And yeah, learn from me - if you do use it, keep taking breaths of the gas. Don't stop as soon as you "feel funny". You're supposed to, that's the point. :p (I mean, obviously, stop if you feel sick or whatever - but I was so out of it with labour-land and lack of sleep that I kept thinking "Ooh, that feels a bit odd" and stopping. Silly me.)
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