From: SURNAME@panix.removethispart.com   
      
   In the previous article, Mark Shaw   wrote:   
   > Interesting, and contrary to my crossthread guess. How do you keep   
   > the mask from leaking O2 - well, room air - into it? Or will such   
   > leakage simply not be enough to matter?   
   >   
   > Positive pressure differential? I think that that would make sense.   
      
   Well, the masks are supposed to deliver O2, of course. And the idea is   
   to increase the O2 "FI" (fraction inspired) of that gas. You're not   
   *totally* excluding room air -- the high end of FI is 90% (not   
   counting the 21% in the room air). I don't know that the mechanism is   
   "positive pressure," because in the hundreds, maybe thousands, of   
   times I've put a NRB on a patient, I never noticed that it was pushing   
   air (so to speak) out of the sides of th mask. There's a little bag   
   that may or may not inflate to some degree. There's an exhaust valve.   
      
   The seal is never going to be perfect -- it's a lot stronger with a   
   BVM (bag-valve mask, which you use to force air into the patient's   
   lungs by squeezing a bag while someone else presses it against the   
   patient's face). Still, I never really thought where that 10%-40% of   
   room air is actually coming from, exactly. If you wore a NRB without   
   an O2 line hooked up to it, and you kept it in place, I guess you   
   *would* suffocate.   
   --   
   jd   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)