the problem with the pulse oximeter is that if you have it on your finger, and then become hypoxic, you may not regcognize there is a problem and ignore the number and continue on...    it certainly helps and provides information    bottom line is if you become hypoxic with an po2 saturation of less than 85 you will have impaired decision making regardless of what type of mask is on your face.  dont use a caunula only above fl180       if you are going above 18,000 use a mask and an oximeter.  if above 25,000 you need positive pressure mask and realize you are taking a calculated risk if you lose oxygen  as you may not recognize the loss of flow, and if you do you have a small window to get down.  Please do the chamber in Oklahoma City as offered on this list..    it's free and an eye opener

Matt Miriani
AME