Full face masks, from a viewpoint of an engineering student

[tl424]

Hi everyone,

This is not a question related to treatment per se, but just a curious question from an engineer who uses a full face cpap mask. May any expert chime in :p

Here it goes:
So most of us knows how a cpap machine works, it pumps pressurized air down your airway and effectively acts as a splint to hold your tongue and palate from collapsing. From a physics standpoint, this is due to the force created by the pressure differential between the room air pressure (which in this case is in your mouth) and the cpap air. This is definitely the case for nasal mask and nasal pillows.

However it gets interesting when you use a full face mask... It is often said that full face masks are for mouth breathers as any mouthleak would not result in a loss of therapy pressure. But now imagine a situation where a full-face mask user opens his/her mouth while using the cpap. The pressure equalizes, and there is no longer a pressure differential creating force between the mouth and the airway. Theoretically this would just result in a situation where the user is just breathing without the cpap, and the tongue/soft palate would as easily collapse as if there was no cpap.

But obviously we know that fullface masks work, as evidenced by many users in this forum! So does anyone know where in my reasoning I have gone wrong?

Cheers

[grayghost4]

I think it is difference in pressure between the cpap pressure in your airway and the ambient pressure outside of the body .... but I think you are partially correct .. a nasal mask seem to work at a lower pressure than the full-face mask. I have no empirical data to back up these statements ... just my opinion

[Julie]

Thank you for asking that... I want to see more replies from people who know them.

[Sir NoddinOff]

The way I look at is this: Your example is good observation on compensatory fluid dynamics (often directly comparable to air pressure dynamics). If one's mouth opens then a 'tributary' temporarily releases the pressure to a side loop - so what's the end result and where does that release of pressure go? Back into the FFM mask naturally. I don't see why that is so hard to understand... granted there may be a small delay factor when this occurs, however I suspect that is part of the designed compensation of the air circuit loop for all full face masks - presumably taken care of by the machine's software.

[tl424]

I think it is difference in pressure between the cpap pressure in your airway and the ambient pressure outside of the body .... but I think you are partially correct .. a nasal mask seem to work at a lower pressure than the full-face mask. I have no empirical data to back up these statements ... just my opinion

Hmm I never really thought of it that way, very interesting! However this would likely mean that the force generated to prop the airway up is less than that of a nasal mask (for the same pressure). What's is even more interesting is that since the divergence of air can be looked at as (roughly) a 3 dimensional sphere, your airway could actually be pushed backwards! (Although you would have to have a really weird anatomy for that to happen).

[K34RT]

I concur with grayghost, that the reason that therapy works is that there is a pressure differential between your airways and the external pressure. So in theory whether you breathe through your mouth or nose, is of no concern as it is in the same pressure environment. Funny enough I'm also a biomedical engineering student!

[tl424]

The way I look at is this: Your example is good observation on compensatory fluid dynamics (often directly comparable to air pressure dynamics). If one's mouth opens then a 'tributary' temporarily releases the pressure to a side loop - so what's the end result and where does that release of pressure go? Back into the FFM mask naturally. I don't see why that is so hard to understand... granted there may be a small delay factor when this occurs, however I suspect that is part of the designed compensation of the air circuit loop for all full face masks - presumably taken care of by the machine's software.

*edit*
I understand what you are saying. However, you seem to be explaining how a full face mask equalizes the pressure within itself when you open your mouth, but not how the force between the airway to prop it up is created.

[tl424]

I concur with grayghost, that the reason that therapy works is that there is a pressure differential between your airways and the external pressure. So in theory whether you breathe through your mouth or nose, is of no concern as it is in the same pressure environment. Funny enough I'm also a biomedical engineering student!

Thats cool! Good to know that Im not the only one using cpap in college!

[Goofproof]

First your tongue and palate don't collapse, your airway closes off. Think when you inhale the suction in the airway allows the airway to suck closed, by putting air under pressure it, stops suction from closing the pipe. with a nasal mask, that pressure can vent out of the mouth, preventing the pressure from being effective. A FF. Mask, blocks the nasal air from leaking out of the mouth, making the pressure effective, so the airway can remain open.

The tongue and palate, stay the same size, but sometimes they can shift location because of gravity, and cause the obstruction, tissues in the airway can also be the problem. Jim

[palerider]

facts removed.

[palerider]

I think it is difference in pressure between the cpap pressure in your airway and the ambient pressure outside of the body .... but I think you are partially correct .. a nasal mask seem to work at a lower pressure than the full-face mask. I have no empirical data to back up these statements ... just my opinion

nasal, ffm's and pillow masks all operate at the same pressure, however, the force of that pressure increases as the surface area that it pushes against increases. so the larger the surface a mask covers, the more force it generates, and the harder it is to hold it in place.

for example, if you have something that presses on one square inch with the force of one pound per square inch, at and put that on a scale, it'll read one pound, if you put two of them, so, two square inches... still one PSI, you'll now read 2 pounds pressing on the scale. four square inches, four pounds...still only one PSI.

I've been watching a series on youtube about a family that bought a nuclear missile silo and are showing some of the fun they're having getting in there, the 'front door' is rated at 1000psi, and the guy comments many times that 6-7 psi from a tornado is enough to destroy a house.... just for comparison.

I've got a chamber vacuum sealer in the kitchen, it's lid is about 12"x11". it weighs a few ounces, but as soon as the pump starts, it's impossible to open, because that 12x11 size works out to 132 square inches. as the pressure drops inside, the weight of the atmosphere starts building up, even a 1 psi drop means that there's 132 pounds of air pressure pushing down on that lid, and that happens very fast once the pump starts. by the end of the cycle, (around 30 seconds, the pressure inside is low enough that water boils at room temperature, and there's over 1900 pounds of pressure pressing on that plastic lid.

so, same pressure, just more force that has to be exerted by the straps to hold it on your face.

[tl424]

it pumps pressurized air down your airway

if it pumped air down your airway, you'd eventually explode. assuming that you don't have gills.

pressure differential between the room air pressure (which in this case is in your mouth) and the cpap air.

no, between the pressure in your throat and the room air.

But now imagine a situation where a full-face mask user opens his/her mouth while using the cpap. The pressure equalizes, and there is no longer a pressure differential creating force between the mouth and the airway.

that's because your assumptions are wrong, it works by increasing pressure in the airway, which includes the nose/mouth/throat as applicable) above ambient.

Theoretically this would just result in a situation where the user is just breathing without the cpap, and the tongue/soft palate would as easily collapse as if there was no cpap.

only if you operate on a flawed theory.

No need to be an asshole, I'm asking a fairly innocent question, not hurting anybody..

[palerider]

.

[Goofproof]

No need to be an asshole, I'm asking a fairly innocent question, not hurting anybody..

well, since you're of such delicate sensibilities that a straightforward factual reply hurts your little feelers, I'll refrain from reading anything else you write, or ask for help with, lest I wound you again.

ta, and good luck with your treatment.[/quote}

Could the rest of us get a little of the same treatment? If so, thanks in advance. Jim

[Gryphon]

I'd say regardless of mask type it's going to pressurize your whole respiratory system and anything that ties in to it. its' why at higher pressures I'll see odd things happen like liquid backing up through my tear ducts.

I think it's one of the main reason's you have "Total face masks" for very hi pressure users who were having problems with leaks in the tear ducts. If you bring the eyes into the same pressure environment then the pressure is equal and you don't have air leaking through the tear ducts. I could be wrong though.