One More Mask Setting Technical Question

[AndyB]

This subject has already been extensively discussed previously, but I want to be certain I understanding correctly how different mask settings work, so please forgive the repetition.

As I understand it, on ResMed machines a mask setting of "Pillows" means the mask is expected to be more resistant to pressure from the machine than the resistance that would occur when the mask has a "Nasal" or "Full Face" setting.  Put another way, a "Pillows" setting will cause the machine to compensate for this expected increased resistance by providing somewhat higher pressure at the machine than it would if the setting was "Nasal" (medium restrictive) or "Full Face" (least restrictive).  The objective of this compensation is, as I understand it, to deliver at the mask the desired therapeutic pressures.

Can any of you experts confirm this, or correct me if I'm wrong?

Many thanks,
AndyB

[Ratchick]

I'm not sure of the exact measurements but yes, it's going to be down to different pressure expectations of the masks, or the area it has to fill with air. A FFM has most, pillows have least, so the flow has to be bigger for a FFM and less for pillows. So maybe the opposite from what you said? But I'm not sure. The fact that the motor has to work harder if there's a leak suggests that it's probably working harder with a FFM than a nasal or pillows.

[AndyB]

I'm not sure of the exact measurements but yes, it's going to be down to different pressure expectations of the masks, or the area it has to fill with air. A FFM has most, pillows have least, so the flow has to be bigger for a FFM and less for pillows. So maybe the opposite from what you said? But I'm not sure. The fact that the motor has to work harder if there's a leak suggests that it's probably working harder with a FFM than a nasal or pillows.

I'm pretty sure I have it right, but it's a confusing issue, as our opposing views show.

PaytonA (RIP) and several others discussed this 5 years ago.  The majority view then seemed to be consistent with my description, and my current review of the "pressure flow curves" and "resistance" specs for different masks seems to support the view that small masks like pillows masks offer more resistance to pressure from the machine than large masks like full face masks, with nasal masks somewhere in between.

But hey, I'm no expert, which is why I asked . . .

[Ratchick]

I mean, no, I can see why that's true, because the outflow for a nasal mask (and pillows) are much narrower than from an FFM, for obvious reasons. So that's absolutely going to change the flow of the air like pinching a hosepipe does with water. So it could easily be either way. I genuinely don't know so I'm curious to see what others say.

[AndyB]

I mean, no, I can see why that's true, because the outflow for a nasal mask (and pillows) are much narrower than from an FFM, for obvious reasons. So that's absolutely going to change the flow of the air like pinching a hosepipe does with water.  So it could easily be either way. I genuinely don't know so I'm curious to see what others say.

Nobody else seems interested in this question.

Anyone?

[Crimson Nape]

After experimenting with the different mask settings while using the same mask, it seems to boil down to the free airspace between the different masks and machine response sensitivity. An FFM has a larger volumetric area as opposed to a pillow mask, thus allowing for more volume of air for the same compression pressure.

For an analogy, think of a spray can versus an air compressor tank. If we want to raise the air pressure of both vessels by 1 cm in pressure, it is going to take a lot more air for the air compressor tank than the spray can.

I hope this helps.

- Red

[AndyB]

After experimenting with the different mask settings while using the same mask, it seems to boil down to the free airspace between the different masks and machine response sensitivity.  An FFM has a larger volumetric area as opposed to a pillow mask, thus allowing for more volume of air for the same compression pressure.  

For an analogy, think of a spray can versus an air compressor tank.  If we want to raise the air pressure of both vessels by 1 cm in pressure, it is going to take a lot more air for the air compressor tank than the spray can.

I hope this helps.

- Red

Red,

So telling the machine that it's delivering pressure to a FFM means the machine will push out air at a slightly higher pressure than if it was told it's delivering pressure to a pillow mask?

Sorry to beat a dead horse, but that seems in conflict with the fact that Resmed (for example) states that "resistance" for its pillow masks is higher than resitance for its FFMs.  Or am I wrong about that also (a distinct possibility)?

Anyway, I greatly appreciate your tremendous contributions to this wonderful forum, and your long-standing commitment to helping others with apnea issues!

AndyB

[Crimson Nape]

No, the resistance is the difference between the IPAP and EPAP pressures and how sensitive it is to these changes. While it is in milliseconds, the pressure change going from IPAP to EPAP is going to be faster with a pillow mask as opposed to an FFM. This is because the FFM has a larger volumetric area for the air to compress.

[pholynyk]

I think Red has the correct idea. The full face mask has a larger volume, so it takes more air flow to change the pressure from EPAP to IPAP. To get more air flow the flow generator (which is a centrifugal pump) has to create more pressure by spinning faster. The amount of extra flow, both in volume and duration, is dependent on the volume of the mask to be filled, hence the need to know the mask type.

As for ResMed saying it is based on something they call resistance, I think it a bit of a misnomer. The volume of air in the mask is compressible, so it acts as a sort of spring. The large mask is 'springier', the nasal mask less so, it is harder, or one might say more resistant.

Obviously, I don't have the means to make the measurements that would confirm my suppositions, so I'm open to other explanations that take into account the viscosity and compressability of air and the characteristics of centrifugal pumps.