Quote from offlabel
Right, and healthy normal people don't need nasal cannulae. The point is that the amount of O2 that can be delivered via NC plateaus at about 6 lpm and if hypoxia that will respond to more 02 is the problem, moving up to a device that can deliver more is indicated. Having a general awareness of ballpark fiO2 when treating hypoxia is fairly important when teasing out fixing a problem. Interpreting an ABG without having an approximate fi02 in mind doesn't have the contextual value of having one.
No, it was easier to study the FiO2 in healthy subjects who can control their breathing.
A person can be breathing with very large slow tidal volumes or very fast small volumes and vary the FiO2 significantly. And, if you put a baby on 1 liter of O2 straight from the wall that infant will be getting greater than an FiO2 of 0.90 but some nurses still document 0.24 for FiO2. Can you see where this is a problem and even dangerous? You can easily give misinformation about the true clinical picture which leads to under or over treatment. For the adult on too little oxygen they must increase work of breathing.
Until there is an understanding of minute volume along with rate and depth, those cannula estimations are only good for a textbook test. When documenting an ABG, the RR rate and liter flow are the main concern. Only if you are using a Venturi type device (not necessarily a "Venturi mask") or other high flow system or a closed system like a vent or BiPAP would you document actual FiO2.
BTW, the literature representIng the research for high flow NC, oxymask and even regular NC for smaller adults as well as children discredit those numbers you stated earlier. Not everyone is a 75 kg male breathing a tidal volumes of 500 ml 12 x per minute.