Hi again, HaydenJack, and thanks for your kind words and for this current post.
This is an extremely interesting question - and I think there may be a reasonable and relatively simple explanation.
Last we spoke, you mentioned your bedridden mom's condition included "end stage heart failure, COPD, a host of other stuff." That conversation can be accessed here: https://copd.net/topic/having-a-hard-time-maintaining-o2-saturation/.
It sounds like mom may have a rather weak respiratory effort and the supplemental oxygen is helping her to maintain satisfactory oxygen levels. If, as you say, the cannula seems to work 'better', when there is a mask placed over the cannula - so be it, especially since the physician says "to do what works"!
Now for the 'educated guess' as to 'why?' If mom has a weak respiratory effort - I'm thinking the mask is acting as a 'reservoir' for the oxygen from the cannula. Whatever mom does not inhale during the breathing cycle, builds up in the confines of the mask (which is placed over the cannula). If that is so, then during her next breath (inhalation), she will be drawing in the oxygen, both from the cannula, AND the mask (contents), which has been enriched with oxygen either from the cannula, and/or from the mask's oxygen source gas (if it is powered by oxygen). You did say the nebulizer mask is not plugged in.
From my own perspective, these are the mechanics and physical principles of how this may be working in your mom's situation.
Perhaps, the other clinical moderators / team members will weigh in here with their opinions, too.
What do you think?
Leon (site moderator COPD.net)