The idea being to try and compensate for the lack of ambient O2, due to the low atmospheric pressure. Ambient air is always taken in conjunction and is the bulk of gas processed. Various delivery systems have been used over the years. The ‘Full Flow’ system has come out on top due to its almost faultless simplicity. The drawback is that we breathe in a cycle of three parts; two of these parts are
breathing out. As the name suggests, the O2 is flowing constantly. This means in effect that somewhere in the region of 60-75% of the gas is wasted as it flows into the mask as the user is breathing out (for 2/3 of the breathing cycle) and thus the O2 is pushed out of the mask to atmosphere. The other drawback is that the O2 consumed is from a small diameter tube and can only ever be taken at a set limited rate, regardless of demand. Demand systems have proved unpopular as due to their complexity; they are not as reliable, but they are more efficient!
