Eric:

Sorry, I didn't mean post to the whole group using technical terms.

Dead-space is the volume of the conducting airways through which we
breath. In order to ventilate the alveoli where gas exchange with the
blood takes place, it's neccessary to breath more than this volume so
that gases are exchanged, not just rebreathed. A shallow breath, as
you correctly surmise, may not clear the dead-space. End-tidal is
fancy talk for the end-expiratory portion of a resting breath.

My EDS system did indeed come with a mask. Since I'm not flying above
18,000 feet, I don't use it.

My point on demand systems using masks is that they supply a full
breath of 100% O2, not just a pulse. The one I've used didn't supply
oxygen under pressure. I'm not, BTW, suggesting that such systems are
needed below 18,000 feet.

Raphael Warshaw
Claremont, CA

Eric Greenwell wrote in message ...
Raphael Warshaw wrote: