I read with interest the input from Jim McIrvin and agree with all of his
observations on the effects of 02 deprivation.  I share his understanding of
the insidious nature of this killer based on my own years of experience in
many Air Force altitude chamber training flights and one exceptionally close
call I experienced with a loss of 02 in an early U-2 flight.  Time for a
short war story.  

At 70,000 feet, the cabin pressure in the U-2 (early A models. . .I know,
never fly the A model of anything) is 29,000 feet.  While in these
conditions, my breathing hose to my pressure suit helmet became disconnected
from a nearly inaccessible hose cluster on the lower right side of the suit,
due to a faulty locking device.  My chamber training alerted me immediately
to the onset of oxygen loss and I knew I had only seconds at that altitude to
check all aircraft and suit systems in an attempt to locate the problem.   It
is impossible to bring the plane down from that altitude in a meaningful time
frame due to Mach limit so that option was a non-starter.  The ability to
twist around and make a visual search is very limited due to the bulky
pressure suit and tiny cockpit.  I was down to "gun barrel" vision when I
spotted the disconnected hose by looking in one of the cockpit mirrors, which
had been installed to aid in looking into the aft portions of the cockpit
consoles.  I barely had the consciousness left to reach back and reconnect
the system.  Recovery was immediate, so being young and bullet proof, I
continued on my 8 hour mission.  My flight surgeon gave me hell for that
stupid decision, but I thought it was the cool thing to do at the time.  My
point is that if some of  you pressurized folks are flying at around 25,000
ft without using your 02 system, or having it IMMEDIATELY available, you
don't have a hell of a lot of time to execute Plan B if you experience a
total decompression.  Your ability to make and execute appropriate decisions
degrades quickly, and you wont even realize it.  My case wasn't coupled with
a decompression (thank God), but the 02 deprivation effect is the same.   Pat
Halloran