The Physics of CPAP - Questions

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.)

Is that correct?

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.

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?

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 soley by abdominal muscle relaxation.

So, is that correct?

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?

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.

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.

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.

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.

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???

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.

Oy vey.

https://www.youtube.com/watch?v=4GW97Xk06N8
https://www.youtube.com/watch?v=-gie2dhqP2c

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

unless my assumptions about the tongue are totally wrong.

Yeah, pretty much.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

8. Of course you COULD argue that the CPAP machine DOES effectively lower your altitude,

You could argue that the earth is cubical (and probably wouldn't be the first) but that doesn't mean that's not nonsensical.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

9. Anyway -- my timing-based explanation of how CPAP works FEELS wrong, so I find it hard to believe.

Congrats,that's the first sensible thing you've come up with.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

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:

Without reading them, they're both wrong.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

Is either explanation correct?

Nope.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

11. Why not instead create pulses of strong negative pressure outside the mouth to clear obstructed airways?

because you're completely and totally wrong about the whole thing.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

Is that correct?

Nope.

camper wrote: ↑

Sat Jun 08, 2019 10:44 pm

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.

Such a crazy modification must have been thought about by nobody else, ever.
FTFY.

OK, not sure where to start so I will keep it simple.

a) In most instances it is not the tongue slipping back and blocking the movement of air.
b) Yes, CPAPs do work. There is abundant empirical evidence (in fact the proof is in most every sleep study).
c) My sense is that at most xpap pressures, there is some muscle work on exhalation (hence why many report soreness for the first month or so).

Not entirely applicable example: I take a snorkel and replace the tube portion (harder plastic) with a tube made of very thin and pliable plastic (think the material in a sandwich bag). When you breath out of the snorkel, no problem. Your exhalation is enough to hold the plastic "tube" open so that air can pass. But, upon inhalation, the plastic "tube" portion collapses and no air can pass. But if that tube could be held open so it doesn't collapse when you inhale. That could be done with some sort of internal mechanical device or perhaps even with positive air pressure....

Now imagine if I connect that snorkel tube end to a cpap. There is enough air holding the snorkel "tube" open so I can breathe in without it collapsing. You can still breath out but it takes a bit more effort. Now imagine, I instead hook your negative pressure device to the tube end. When I breath in, I am fighting a pulsing vacuum so it is very hard to get any air at all. The vacuum is also trying to collapse the airway. And then there would be trying to sleep with a pulsing suction...

Here is a correct explanation of the pathophysiology:

https://youtu.be/SJ7cDggUJks?t=360

What in the world are you trying to do?
I get the sense that you are not embracing using cpap and trying to find any excuse to not use it.

Is that correct?

I couldn't find anything in all that stuff you said that was correct except where you said you must be wrong in your assumptions.

I think he's just bored and parsing all that was fun!

Julie wrote: ↑

Sun Jun 09, 2019 4:40 am

I think he's just bored and parsing all that was fun!

Perhaps...but it's a good way to find yourself totally ignored and if you scream wolf no one will do anything.

Pugsy wrote: ↑

Sun Jun 09, 2019 4:33 am

What in the world are you trying to do?
I get the sense that you are not embracing using cpap and trying to find any excuse to not use it.

Is that correct?

I couldn't find anything in all that stuff you said that was correct except where you said you must be wrong in your assumptions.

Looking at his past posts I can agree with you

Re: Speculations about CPAP-dependency.
Post by jimbud » Mon Jun 03, 2019 10:39 am

I do believe this camper person is "having you on".
JPB



Hate to remind you guys I told you so, but I told you so.
(Really I don't mind).

He reminds me somehow of this guy with the initials K. S.
JPB

jimbud wrote: ↑

Sun Jun 09, 2019 12:02 pm

Re: Speculations about CPAP-dependency.
Post by jimbud » Mon Jun 03, 2019 10:39 am

I do believe this camper person is "having you on".
JPB



Hate to remind you guys I told you so, but I told you so.
(Really I don't mind).

He reminds me somehow of this guy with the initials K. S.
JPB

The initials "D A" seem appropriate.

jimbud wrote: ↑

Sun Jun 09, 2019 12:02 pm

K. S.

palerider wrote: ↑

Sun Jun 09, 2019 1:45 pm

"D A"

A district attorney in Kansas.

Is that correct?

jnk... wrote: ↑

Sun Jun 09, 2019 3:16 pm

jimbud wrote: ↑

Sun Jun 09, 2019 12:02 pm

K. S.

palerider wrote: ↑

Sun Jun 09, 2019 1:45 pm

"D A"

A district attorney in Kansas.

Is that correct?

Um... sure?

palerider wrote: ↑

Sun Jun 09, 2019 1:45 pm

jimbud wrote: ↑

Sun Jun 09, 2019 12:02 pm

Re: Speculations about CPAP-dependency.
Post by jimbud » Mon Jun 03, 2019 10:39 am

I do believe this camper person is "having you on".
JPB



Hate to remind you guys I told you so, but I told you so.
(Really I don't mind).

He reminds me somehow of this guy with the initials K. S.
JPB

The initials "D A" seem appropriate.

You most likely have applied the appropriate moniker, as usual.
JPB

jimbud wrote: ↑

Sun Jun 09, 2019 5:31 pm

palerider wrote: ↑

Sun Jun 09, 2019 1:45 pm

jimbud wrote: ↑

Sun Jun 09, 2019 12:02 pm

Re: Speculations about CPAP-dependency.
Post by jimbud » Mon Jun 03, 2019 10:39 am

I do believe this camper person is "having you on".
JPB



Hate to remind you guys I told you so, but I told you so.
(Really I don't mind).

He reminds me somehow of this guy with the initials K. S.
JPB

The initials "D A" seem appropriate.

You most likely have applied the appropriate moniker, as usual.
JPB

I wouldn't go THAT far... but sometimes I do get lucky

Julie wrote: ↑

Sun Jun 09, 2019 4:40 am

I think he's just bored and parsing all that was fun!

Fun for who?

We refer to a long post without any paragraph breaks and insufficient punctuation as a 'wall of text'.

I vote that OP's latest be dubbed "Wall of obtuse".