The effects of hypoxia are not static.  Everything changes;  we age, drink, smoke, forget to exercise, etc, etc

Bill,

 

Thanks, I was going to compose a similar message, but your explanation is right on.  Even your sitting position may make the difference between deep and shallow breathing. 

 

Do not leave the O2 decision to judgement - you may not have the capability to make a thoughtful decision.  Use hard rules and practice them so that O2 use is automatic - and, I mean this is true for those of us that don't fly in the flight levels, too.

 

I recently purchased the Mountain High digital O2 delivery system because makes it easy for me to put on the miniscule cannula and activate the system when high altitude (over 8000 ft) flight is comtemplated.  Using the D5 setting, no O2 is delivered until I climb above 5000 feet. 

 

My gear up landing (June '02) was directly traceable to mild hypoxia (swiss cheese reasoning ability) as demonstrated by reviewing the sloppy notes taken on the last 10,500 foot leg of a long trip (d-alt 13,000).  Note that I had an emergency small cylinder O2 system in the seat pocket under my knee, a digital pulse oximeter hanging from my neck and the regular O2 system deployed on the seat next to me - alas, all requiring reasoning to utilize them.  Being older and overweight, stuffed into my little airplane, didn't help the breathing function either.  Flying, apparently, doesn't require a higher brain function as the approach and sled-like landing were beautifully executed.  BTW, I can't make myself younger, but I can make myself thinner, having lost 30 pounds and I'm not through yet.