Does mask SIZE make a difference (regarding pressure)?

[Broomstick]

Leak rate was the same with both masks.

Are you sure?  Keep in mind that larger masks would need a larger "Intended" leak to purge the exhaled Oxygen.

to the OP, it is not at all uncommon to need to tweak settings when changing the delivery system, not always, but not uncommon.  I believe that a change in the volume of the system is the largest factor.

Fred

Interesting. I did wake up with the "too much air" feeling that I haven't felt in a while. I think the body has to adjust to these things and switching back and forth may not be the best idea. I'm not sure what size is best since I seem to fall in between sizes. 

I think this is the most likely explanation for what happened.

[Gideon]

oops, correction exhaled CO2 Carbon DiOxide, not Oxygen.

[Fats Drywaller]

Why would the volume of exhaled carbon dioxide change with a larger mask? You seem to be saying that the larger air volume in a larger FF mask causes deeper breathing, with the patient taking in more air, absorbing more oxygen, and exhaling more carbon dioxide with each breath. Is that really what's happening?

[Gideon]

You have more volume to purge to dump your exhaled Carbon Dioxide.  Keeping it in the breathing circuit would cause you to inhale more than a standard amount and disrupt the Oxygen / Carbon Dioxide balance in your blood which in turn impacts your bodies signaling to breathe.

[Fats Drywaller]

OK, I realized that a possibly useful way of thinking about it is as a container of a given size with an inlet tube and an outlet tube. The outlet pressure (to the patient's airway) is a function of the inlet pressure and the volume of the container.

I assume that for practical purposes, with a mask, it'd be a stairstep function, with only a slight increase in container (mask) volume not requiring an increase in inlet pressure. But past a certain point, an increase in inlet pressure is needed, and then again past another certain point, and so on. (Why don't the CPAP manufacturers tell us about that? Are the FF mask volumes all assumed to be close enough that no pressure increase should be required?)

That doesn't take the vent into account, though.

[jaswilliams]

Surely it is a closed loop there will bea resistance to flow due to the tube, but tbe mask resistance is minimal. Pressure is calculated at the mask so it could be a fairly simple flow down a pipe of known dimensions calculation

[Gideon]

BUT it is not a closed loop.  UNLESS you close the vent, which is NOT recommended.

[Walla Walla]

OK, I realized that a possibly useful way of thinking about it is as a container of a given size with an inlet tube and an outlet tube.  The outlet pressure (to the patient's airway) is a function of the inlet pressure and the volume of the container.

I assume that for practical purposes, with a mask, it'd be a stairstep function, with only a slight increase in container (mask) volume not requiring an increase in inlet pressure.  But past a certain point, an increase in inlet pressure is needed, and then again past another certain point, and so on.  (Why don't the CPAP manufacturers tell us about that?  Are the FF mask volumes all assumed to be close enough that no pressure increase should be required?)

That doesn't take the vent into account, though.

I think the mask settings on the machine are about putting you in the ball park for correct event reporting and leak rates. It was never intended to assure the correct pressure for any given mask. too many variables to be able to do that.