The Physics of CPAP - Questions

[camper]

Point taken.

[palerider]

In particular, they don't explain why inhalation would push the tongue back into the mouth,

Because that's not what happens.

I think I was correct in most of my original post, except in one respect: IPAP positive pressure is very important.

No, it wasn't, and IPAP isn't anywhere near as important as EPAP.

Can it be trained away, by strengthening muscles, by medications,

No.

[Gryphon]

This is going to be interesting - Anyone feel free to weigh in on whatever I write if I'm wrong. I'll take any constructive criticism and file it away, so I can learn from it.
Thanks…. On with the quote and my thoughts.

I wish to understand the underlying Physics of CPAP.
According to various sources, the relaxation that occurs during sleep allows loose structures, such as the tongue or parts of the soft pallette, or something in the nose, to move into and block the airways.
To simply this discussion, I will henceforth assume the tongue is the obstructing element.
1. A truly uniform air pressure everywhere is completely ineffective in moving the tongue, because it is only the difference of the pressure above the tongue relative to the pressure below the tongue that exerts an effective force upon the tongue. Also, you would neither breath in or out if pressure was truly uniform everywhere. Thus, CPAP - Continuous Postiive Air Pressure - is misnamed.
All air movement, and air pressure induced tongue movement, must be driven by non-uniform air pressure. (Tongue movement is also driven by its own elasticity, countered to some extent by frictional forces.)

If your trying to understand the "Physics of CPAP" you wouldn't want to oversimplify the situation and blame all apnea and sleep disordered breathing on the tongue.
A truly uniform pressure everywhere is going to help "Support" everywhere... It doesn't take much to help maintain an open airway - the body's natural mode is to hold the tongue and soft tissues out of the airway at least a little - otherwise we'd all be dead, for some of us - the body doesn't maintain that tension on those tissues as well as it should, but I assume there is still some effort being applied to maintain an open airway, or for most of us OSA would lead to fatal complications much faster than it does now.
Pressure give or take around us in the atmosphere is uniform all the time. It does go up and down a bit with the movement of high- and low-pressure air masses or when we go from a lower point of elevation to a higher one, but we breath in and out every day with in a system that has a uniform pressure all around us every day, but we still breath in and out. The change in pressure as we breath in and out, is created by "US" in our diaphragm etc. By changing the relative volume of our lungs expanding them and contracting them allowing for air to move in and out. High to low, low to high pressures, these are generated by our diaphragm and other supportive tissues with in our bodies. This pressure change occurs with every breath and is independent and not connected to the function of a CPAP machine, just like the pressure of the air we breath with out a CPAP doesn’t change.
CPAP - Continuous Positive Air Pressure - gets its name from the fact that "Relative" to atmospheric pressure - the closed system of a CPAP machine is creating a "Continuously Positive Air Pressure" - environment with in our body's respiratory system, compared to the general atmospheric pressure outside of that system.
All the fundamental mechanics of breathing that go on without CPAP continue to work with CPAP - only with the additional overall presence of a higher pressure inside of the airway relative to outside pressure.
Moving the tongue or other tissues around would / could be done by the diaphragm muscles vs, cpap. Compared to the pressures created by CPAP our diaphragm muscles are capable of much higher-pressure changes then a simple CPAP. “as an example, it was said that Marice Andre (classical trumpet player) could generate enough pressure with his air stream to fill a car tier.” Most XPAP machines you would use at home are unable to create enough pressure on their own to inflate a balloon – so the CPAP is simply there to act as a “Splint or support” of the airway. The CPAP doesn’t play much of a role in the breathing process it’s simply there to support that process.

2. Of course, net atomspheric pressure does have some effect, because it affects the total force that can be generated in either direction. E.g., at high altitudes, the available forces from air pressure changes obtained by expanding and contracting the lungs are less than at sea level. I suppose a CPAP machine could just effectively lower your altitude, making air denser, and increasing induced forces - but I assume that isn't what happens, as I will explain in section 8.

Based on fluid dynamics of air, net atmospheric pressure changes and altitude effects simply increase or lower the overall “Density” of the air we breathe.
So you could argue that a CPAP machine is creating a “lower altitude/denser air” inside of the airway relative to outside the airway. This would increase the density of the air inside our airway, allowing it to push outward with a bit more force. This only works because the density of the air outside our airway is lower then inside when the CPAP machine is working.

3. As near as I can figure out, the tongue acts like a valve. In particular, when you breath out, you have a negative pressure above your tongue relative to the lower airway just below the tongue, so that the relative force moves the air move out, and tends to push the tongue out of the throat, opening the airway. When you breath in , you have a positive pressure above your tongue relative to the lower airway just below the tongue, pulling air in, but also pulling the tongue into the throat, and closing off the throat, and creating a blockage.
Is that correct?

It sounds like your trying to create an overly analytical explanation of snoring. If the tongue is in the way, the changes in air pressure inside our airway can move the tongue around… this occurs with or with out CPAP… The additional pressure inside the airway during CPAP use can help to hold the tongue out of the way, lessening this issue.

4. Normally, these relative positive and negative pressures are created by the expansion and contraction of the lungs - primarally driven by the use of your abdominal muscles. We mostly use abdominal muscles to expand the lungs, and breath in. We can simply relax those muscles and let them contract, causing us to breath out, or we can use extra muscle tension (presumably, from a different set of muscle fibers) to blow extra hard (as is often used for singing and mouth-blown musical instruments). My best guess is that when most people sleep, we use simple muscle relaxation to breath out. I'm not sure how much that matters, but I'm curious if I'm right that sleep-time exhalation is produced solely by abdominal muscle relaxation.
So, is that correct?

I’m not sure if you can simplify the movement of the muscles in the act of breathing that much, simply because we’re asleep.
I’m thinking the same movements and muscles come into play, it would stand to reason that the only significant difference is we’re not deliberately controlling these muscles when we’re sleeping, just like we don’t during day to day activities when we’re not thinking about breathing rather letting the body do what it knows how to do naturally. However, if we stop breathing due to an obstruction, the body knows what to do and will bring about more effort and force to continue to breath. This can cause arousals and wake us up from deeper levels of sleep, disturbing our sleep and causing poor health among other issues. This added effort to breath past an obstruction can cause other issues, like generating such a negative pressure in the abdominal area that stomach acids can be pulled past the muscle responsible for containing them and into our esophagus causing damage. Issues like “Gerd, or heart burn” after sleeping is sometimes a sign of untreated OSA.

5. As I said before, a CPAP machine, can't genuinely create a uniform positive pressure above the tongue, relative to below the tongue in the lower airway, while we are breathing normally, because air pressure would equalize above and below the tongue, and air would not flow past the tongue. (Standard fluid dynamics.)
So it doesn't. It also cannot create uniform pressure at the opening to the mouth or nose, because if it did, air would not flow in or out of the mouth or nose.
In order for you to successfully breath in, you MUST have positive pressure outside the mouth or nose relative to the lower airways.
In order for you to successfuly breath out, you MUST have negative pressure outside the mouth or nose relative to the lower airways.
It's just basic physics.
Is that correct?

That’s not how CPAP works… The CPAP is creating an increase in overall pressure of the entire airway system, relative to atmospheric pressure acting on the outside of the body. This slight increase in the overall pressure inside the airway acts as a splint, to help hold the airway open… That is ALL a CPAP does.

The pressure changes your talking about are not created by the CPAP machine – they are solely created by our breathing muscles like they do all the time even when we are not using a CPAP machine.
CPAP machines don’t fundamentally change the physics of the breathing process or how we breath – they simply provide a SPLINT effect to help maintain an open airway. All air movement in and out of our airway is done by the normal act of breathing in and out like we do when we’re awake.

6. Any EXTRA positive pressure outside the mouth or nose (relative to the lower airways) will slow down breathing out, and would at first thought reduce its effectiveness in unblocking the airway.
Any EXTRA positive pressure outside the mouth or nose (relative to the lower airways) will speed up breathing in, and make you breath deeper (an effect I notice while using the machine), but would at first thought force the tongue into the throat harder, and tend to block the airway.
So, to first order, a CPAP machine, viewed as a simple state machine, is counter productive, and tends to block your airway.
HOWEVER (and this a desperate attempt to make the physics somehow work) the tongue has mass and inertia, and perhaps viscosity as well.
So, what I assume actually happens (since CPAP has proved effective for many people), is that it tends to make breathing out slow, giving your tongue time to come more fully out of the throat, and breathing in fast, so it doesn't have time to go into the throat enough to block the airway.
In other words, the net effect is to make breathing in - the problematical phase - occur faster than the cycle time of the tongue-as-a-valve, so the tongue-as-a-valve does not have time to cycle fully closed, and therefore does not block the airway. Breathing out (the good phase) lasts longer, and that portion of the valve cycle does complete, opening the airway.
In other words, CPAP machines work soley because of altered breath timing vis a vis the timing of the tongue-acting-as-a-valve, NOT because of uniform external air pressure.
Is that correct?

That is much different from the published explanations I have seen, in which a net positive outside pressure is supposed to somehow blow your airway open. But I can think of no physically possible way the latter explanation could be correct, unless my assumptions about the tongue are totally wrong.

I think your way over thinking it. The CPAP machine simply acts like a splint… think of buttresses of wood in an old mine shaft, holding the ceiling up so it doesn’t collapse on top of the miners.

7. I'm very bothered by this explanation - tongue size and mass vary a lot. It doesn't seem like a timing-based functionality could work well for everyone, especially since it effectively reduces the forces opening the airway, by slowing down the time over which the outward breath occurs. Likewise, it increases the forces closing the airway, by speed up that time.

There are some tweaks that can come into play for how people handle CPAP and the effects it has on their breathing, Centrals vs, obstructive vs, Mixed. However simply messing with options on a CPAP such as exhalation relief or the changes in pressure from Inhale vs, exhale can help or hurt some people. We’re all different, that’s why dialing in a PAP machine is so important to find the best treatment for each given person.
The overall effect of CPAP use though is to help support the opening of the airway but increasing the air pressure inside the airway vs, outside the airway.

8. Of course you COULD argue that the CPAP machine DOES effectively lower your altitude, and therefore increase the forces available to open the airways while you are breathing out. But I haven't seen them explained that way. And, in that mode of explanation, it would also increase the forces available to close the airways while you are breathing in, so I don't see how that mode of explanation could work.

If you want to think of it that way… I’m guessing you could but you’re still thinking of the forces of air having a fundamental function of pushing tissues in the airway around as part of normal breathing.
You could say that “Inside the airway” when the CPAP is on… the relative Altitude would be lower then the ambient altitude outside of the airway and, that fact is allowing the denser air inside the airway to push gently against the tissues of the airway all around helping to hold them open.
You’re not going to hear the relative altitude explanation from anyone on how CPAP works because aside from Mr. Wizard or Phil Nye the science guy, most people would be confused as heck on hearing it and it wouldn’t be very helpful.
You’re over thinking it.

9. Anyway -- my timing-based explanation of how CPAP works FEELS wrong, and differs greatly from the published literature I have seen, so I find it hard to believe.

It’s your explanation, there could be some merit to it, but I find the overall concept of PAP simply helping to hold the airway open and allow the body to continue the natural function of breathing to be much less complicated.

10. My explanation also does not explain why CPAP machines create strong positive pressure ("clear airways") pulses outside your mouth or nose when they determine there is an airway obstruction. That SHOULD force the tongue further in, unless the obstruction effectively stops the movement of the tongue. It SHOULD NOT, to first order, pull the tongue away from the throat and open the airway.

Once again, making a last ditch effort to make this work, there are only two ways I can think of in which a positive pressure pulse could possibly clear a tongue-blocked airway:

A. It could rely on the inertia of the upper air column. In particular, the machine pushes in a pulse of positive pressure, which bounces off the tongue and back of the throat, leaving the mouth or nose, and creates a negative pressure pulse, which does the desired job.

B. The pulse of pressure could compress the tongue. When the extra pressure is released, the tongue bounces back off of the back of the throat and/or soft pallette and/or throat.

Is either explanation correct?

Both of these effects should be fairly small, because the air column has very limited inertia, and the tongue, being mostly water, cannot compress much. Perhaps they are sometimes enough...

But those explanations doesn't FEEL right.

Maybe the Clear Airways pulse wakes you up a little, and YOU clear the airway???

My guess – Since I haven’t put a lot of thought into it – is that the little pulses you feel from a modern CPAP now and then is that the PAP is trying to tell what has caused the air movement to stop.
The little pulses of air are not part of the treatment. If the pressure wave is very jagged the pulses of air are acting against a closed airway because the overall volume of air the pulses are pushing against is very small, so those pulses would be felt by the machine as very hard. However, if the pulses of air are felt by the machine as a gentler curve it would stand to reason that those pulses of air are hitting an open airway and are filling the whole volume of the airway and lungs.
Hence obstructive apnea or open airway apnea… “that’s just my guess”

11. Why not instead create pulses of strong negative pressure outside the mouth to clear obstructed airways?
That would effectively pull the tongue back into the mouth out of the throat, driven by the relatively higher pressure in the lower throat.

Is that correct?

If so, why don't CPAP machines do that? They've been around for almost four decades. Such an obvious modification must have been thought of, by many people, many times over.

Ummm First CPAP machines were like getting hooked up the exhaust vent of a vacuum cleaner… no one thought it would be a good idea to reverse that process.
The sort of modification your thinking of would be a system that completely breaths for you. IE: a ventilator. Even with those, most of the time the active process is to fill the lungs and allow the air to leave naturally, not Suck the air out of you if you’re not exhaling normally. (I could be wrong about this)
PAP devices for the most part, are a SPLINT for the airway. They’re not there to breath for you. Only on more advanced machines do more aggressive timing and air pressure changes take place, those more advanced devices our outside of the scope of my experience so I can’t say for sure how they work exactly. There other members of this forum who are way more knowledgeable about how they work.

If your trying to work out the physics of how a CPAP machine works… it’s there to increase the relative atmospheric pressure inside the airway vs the pressure outside the airway. This difference in pressure increase helps to hold open the airway.
Hope this helps, and I didn’t error on any of my details. As I said, feel free to comment or correct any issues you see.

Restwell,
Gryphon

[HoseCrusher]

camper, I think sleep apnea is better understood if you focus on the airway. The tongue is there and it does contribute, but the whole idea is to keep the airway open upon the suction involved when breathing in.

Here is another view on this.

https://www.youtube.com/watch?v=IP3IgvE36GA

One alternative in dealing with the tongue is to cut away at it to make sure it doesn't interfere with the airway.

https://www.youtube.com/watch?v=vdTBRR2nzX8

Unfortunately the tissue may grow back and once again interfere with the airway.

There is an idea that exercising the muscles involved with that section of the airway may be an alternative treatment. Note that wrestling, weight lifting, and sumo wrestling do not strengthen the correct muscles in the neck.

Here is a discussion on that type of alternative treatment.

https://www.youtube.com/watch?v=GbZINDloIrg

[palerider]

(I could be wrong about this)

Nope, spot on.

[camper]

A truly uniform pressure everywhere is going to help "Support" everywhere...

Well, no. Movement of obstructions inside the airway should not be affected by pressure relative to air outside the body, only by relative pressure changes inside that airway.

Oh wait - if you assume it is outside air pressure, acting on the skin of the mouth, neck and throat, that collapse the airway during inspiration, using the floppy pipe analogy, that causes the problem, net internal pressure might work. I guess that is what you are saying, and it does make some sense - but I would bet pressure differentials within the airway matter too.

The problem is, I'm not seeing an explicit physics model anywhere, that explains how much that contributes.

A truly uniform pressure everywhere is going to help "Support" everywhere...Based on fluid dynamics of air, net atmospheric pressure changes and altitude effects simply increase or lower the overall “Density” of the air we breathe.

But when you change lung volume, the resulting change in internal pressure differentials is proportional to net pressure, because
PV=NkT
implies that
Pressure change resulting from a volume change depends substantially on n, which is proportional to initial pressure. [Yes, I initially wrote the wrong derivation, and I'm ignoring delta T - so sue me; think of it this way - if you have almost zero initial pressure, the pressure change developed by a volume change is also almost zero - but if you have a high initial pressure, you get a much higher pressure change from the same volume change.]

That doesn't just affect the relative forces I was thinking of, internal to the airway - it also affects the splint-like support forces you were talking about, created during exhalation. And the loss of the support forces you were talking about, during inhalation. So net pressure should matter - a lot - which means, (oops) I was wrong, and CPAP makes a lot more sense than I thought.

BTW, I hadn't considered the effects of vacuuming digestive fluids into the upper airway - so, I admit, I guess negative pressure makes little sense.

[camper]

Here is a discussion on that type of alternative treatment.

https://www.youtube.com/watch?v=GbZINDloIrg

He implies that the only musical instrument that might help with sleep apnea is the Digeridoo, because of circular breathing, which forces you to puff out the cheeks while you inhale through the rest of the airway. In particular

the muscles you use are exactly the muscles that affect the area of sleep apnea

Actually, AFAIK, circular breathing technique is a part of moderately advanced playing of EVERY mouth-blown musical instrument - I learned it a little in high school.

Alas, I was never very good at it - I could keep the sound going, but the fresh air never fully reached my lungs, so I would eventually run out of oxygen, and had to stop playing to take a normal breath. But others can keep it going longer. It takes more practice than I put into it. And I don't play any more, so any apnea reduction benefit it might possibly have given me is long gone.

But just imagine - if all the sleep disordered patients whose snoring or arousals keeps their sleep partners awake, were to sing (or play another musical instrument) with circular breathing, for an hour or two every night, and it could prevent sleep apnea - they could STILL keep their sleep partners awake, by singing.

[chunkyfrog]

Circular breathing or not--unconsciousness causes various tissues in the airway
to relax which may collapse, blocking the airway.
Nobody to my knowledge can play the didge in their sleep.
And thank goodness for that!

[jimbud]

I eventually run out of oxygen, and stop writing to take a normal breath.

Please do, every chance you get.
JPB

[Gryphon]

Circular breathing or not--unconsciousness causes various tissues in the airway
to relax which may collapse, blocking the airway.
Nobody to my knowledge can play the didge in their sleep.
And thank goodness for that!

I think what they were showing was that the circular breathing techniques that are fundamental to the didgeridoo would help strengthen muscles in the airway... and like a person with 6 pack abs vs someone with dunlap's disorder [their tummy "done lapped over their belt"] there would theoretically be less loose flappy tissue surrounding an area that was constantly being worked out...

But in reality most people are not going to take the time or have the dedication needed to play and practice enough to see any benefits, whether real or not.

Besides, we know cpap works and is way less difficult then mastering a musical instrument.

Restwell,

Gryphon

[jnk...]

Anyone feel free to weigh in on whatever I write if I'm wrong. I'll take any constructive criticism and file it away, so I can learn from it. . . . movement of the muscles in the act of breathing . . . asleep. I’m thinking the same movements and muscles come into play, it would stand to reason that the only significant difference is we’re not deliberately controlling these muscles when we’re sleeping, just like we don’t during day to day activities when we’re not thinking about breathing rather letting the body do what it knows how to do naturally.

Only because you asked, and I wouldn't exactly call it "wrong," but respiratory-muscle behavior, particularly for those muscles used for breathing other than the diaphragm, does change a bit during sleep, as I understand it.

Diaphragmatic function is largely preserved, which is essential for the maintenance of adequate ventilation during sleep. However, accessory muscle function is reduced, particularly during REM sleep . . . -- https://www.ncbi.nlm.nih.gov/pubmed/12814041

Normally I wouldn't mention it, since that side point has no real impact on the logic or overall thrust of your well-thought-out statements.

[zonker]

Besides, we know cpap works and is way less difficult then mastering a musical instrument.

Restwell,

Gryphon

i'm playing my cpap as a musical instrument right now.

[jnk...]

Besides, we know cpap works and is way less difficult then mastering a musical instrument.

Restwell,

Gryphon

i'm playing my cpap as a musical instrument right now.

I prefer the the telepapper over the stratopapper, myself. But either one can play good music if set up well.

[jimbud]

Your well-thought-out statements.

Huh? Where did you see an example of that?

Nice to see you posting. Always a pleasure.
JPB

[zonker]

Besides, we know cpap works and is way less difficult then mastering a musical instrument.

Restwell,

Gryphon

i'm playing my cpap as a musical instrument right now.

I prefer the the telepapper over the stratopapper, myself. But either one can play good music if set up well.

can i use the wah-wah pedal with either one?