Running out of water during sleep

[JDS74]

Help me understand where my logic goes astray.

Without CPAP I usually inhale about X cu.ft. of the ambient air each cycle. I breath about Y times per minute. The ambient air's humidity is what it is.

When I put on the CPAP the additional pressure in my airways is established almost instantly ("almost", I said). Thereafter, the amount of air entering my body each breathing cycle is about that X cu.ft. and has the same humidity as without CPAP because it is the same ambient air. My rate is about the same, so, for a given period of time, about the same amount of moisture enters my body via the air I'm breathing, with CPAP or without.

Technically, that is correct. However, because of the slight increase in pressure, the total amount of air corrected to non-cpap pressure is just a little bit more.

Some may argue the CPAP is forcing more air into my body. I say it is merely increasing the *pressure* within my airways, not the *quantity*. I'm not taking deeper breaths, I'm not inhaling more frequently. It is the same *quantity* of ambient air, with the same "quantity" of moisture, CPAP or not. The excess air supplied by CPAP is vented out of the mask, before it enters my body.

I've never heard anyone complain about the ambient air he's breathing has too low humidity. Why would he complain the CPAP-supplied air has lower humidity when it is the same air and same quantity of air?

For the most part that is correct.

I know some CPAP users sincerely state it dries out their air passages and creates considerable discomfort unless they really crank up the humidity. From a physiological standpoint, why?

To the extent that the breathing is oral breathing, the humidity in room air may not be nearly enough. The nasal passages provide extra humidification for the air passing through that the mouth cavity does not. So, mouth breathers need more humidity that nasal breathers. Some FFM users and all Oral Mask users fall into this category.

I can breathe through my nose when awake and upright but I cannot while sleeping and lying down. Depending on the angle in a recliner chair its iffy.

[robysue]

Some may argue the CPAP is forcing more air into my body. I say it is merely increasing the *pressure* within my airways, not the *quantity*.

The upper airway and CPAP machine is not a closed system like the innertube of a bicycle tire. It's a semi-closed system with a built in leak. And the way the CPAP maintains the pressure in the semiclosed system is to pump as much air into the system as is being lost through the intentional leak. And that makes things quite a bit different than when you're breathing without a CPAP.

Let's examine what happens to a specific CPAP user---namely me:

Now, you are right, I'm not breathing any deeper when I use the CPAP. But when I don't have the CPAP on, there is not a whole lot of excess air being blown around inside my nose and upper airway unless I'm breathing outside on a (very) breezy day or I'm sitting right in front of a fan. (Both of which my nose and throat dislike, by the way.)

When I put the CPAP mask on, there is a lot of excess air flow in the nose and upper airway. At the pressures I use (4-8cm), the expected leak rate for my mask is roughly 20-25 L/min. In other words, every minute I use the machine, the blower forces another 20-25 L/min into the system to compensate for the air being lost through the exhaust vents. This creates a lot of air flow through the whole system, including the inside of my nose and my upper airway. That extra air flow being created by the CPAP's blower acts on my upper airway in a way that is very similar to wind on a breezy day. And, if I don't use a heated humidifier, the result of breathing with a CPAP all night is the same as what happens when I sit in front of a fan for hours at a time: My nose and throat get very dried out.

I'm not taking deeper breaths, I'm not inhaling more frequently. It is the same *quantity* of ambient air, with the same "quantity" of moisture, CPAP or not. The excess air supplied by CPAP is vented out of the mask, before it enters my body.

Not really: A lot of that air is blowing through your nose (when you're inhaling) and your nose is fighting that airflow when you're exhaling. And if you have a nasal mask and your lips open, then air can swirl through the nasal cavity and into the oral cavity. Under some circumstances it is also possible for air to leak into the sinus cavities, into the esophagus, and even into the eyes through the tear ducts. In short, our upper airway is not air tight. And any small leaks allow the air to swirl around inside the nose and the upper airway.

I've never heard anyone complain about the ambient air he's breathing has too low humidity. Why would he complain the CPAP-supplied air has lower humidity when it is the same air and same quantity of air?

First note that moving air (i.e. wind) is more effective at evaporating moisture than non-moving air. And the stronger the breeze, the more effective the air flow is at evaporating moisture from a damp surface.

And that's what the inside of our nose and upper airway is: A damp surface that has air flowing over it. And if we're breathing in a very breezy environment, there is more airflow over the the mucous membranes lining the nasal passages and the upper airway than when we're breathing in a very still environment. And it turns out that a lot of people do have trouble with their noses or throats drying out on windy days. A lot of people dislike sleeping under a fan (without a CPAP) because they wake up with a dry nose and throat. In other words, some people are just more sensitive to a lot of excess air movement in and around their nose and upper airway than others are.

The folks who don't like windy days or sleeping under a fan are usually the ones who have significant trouble with using a CPAP without a humidifier: Their nose and upper airway react to the additional airflow created by the machine in the same way their nose and upper airway reacts to windy, dry days and/or sleeping under a fan. Using a heated humidifier helps prevent the mucous lining the nose and upper airway from drying out. In other words, moving air is less irritating when it is warm and moist enough to not dry out the mucous membranes lining the nasal passages and the upper airway.

[palerider]

Help me understand where my logic goes astray.

Without CPAP I usually inhale about X cu.ft. of the ambient air each cycle. I breath about Y times per minute. The ambient air's humidity is what it is.

When I put on the CPAP the additional pressure in my airways is established almost instantly ("almost", I said). Thereafter, the amount of air entering my body each breathing cycle is about that X cu.ft. and has the same humidity as without CPAP because it is the same ambient air. My rate is about the same, so, for a given period of time, about the same amount of moisture enters my body via the air I'm breathing, with CPAP or without.

Some may argue the CPAP is forcing more air into my body. I say it is merely increasing the *pressure* within my airways, not the *quantity*. I'm not taking deeper breaths, I'm not inhaling more frequently. It is the same *quantity* of ambient air, with the same "quantity" of moisture, CPAP or not. The excess air supplied by CPAP is vented out of the mask, before it enters my body.

I've never heard anyone complain about the ambient air he's breathing has too low humidity. Why would he complain the CPAP-supplied air has lower humidity when it is the same air and same quantity of air?

I know some CPAP users sincerely state it dries out their air passages and creates considerable discomfort unless they really crank up the humidity. From a physiological standpoint, why?

I can find NO fault in your logic Sir, and have, indeed, wondered the same thing myself, maybe people just *believe* they need more humidity, and so therefore, they do (the mind is a powerful thing and the placebo affect has been documented as being a real, effective thing). There are three people in this house, and none use water in their humidifier, though most of us keep it connected for the slight 'muffling' effect it creates. I *do* add water and turn on the humidifier if I've got some kind of respiratory illness, because the added moisture seems to help loosen up the crap, akin to a nice hot shower, only it goes on all night long.

all that said, I don't encourage anybody not to use humidity, because, if it makes them more comfortable, then... why not?

[palerider]

However, because of the slight increase in pressure, the total amount of air corrected to non-cpap pressure is just a little bit more.

which would mean that the cpap air has *slightly more* moisture in it than before.

[wardmiller]

>> Their nose and upper airway react to the additional airflow created by the machine . . . <<

First, let's assume the person keeps his/her mouth shut. That is not an unreasonable assumption,considering all those who use nasal pillows or nose masks. Then the respiratory system is a *closed* system, with virtually all air entering and exiting the body through the nose. The CPAP increases the *pressure* of the closed system, but does not increase the *volume* of air inhaled in a given period of time. Excess air is vented from the mask, before it enters the body, because the pressure in the closed system is greater than the ambient air pressure around the mask.

If "the additional airflow created by the machine" exists, where does it go? Ignoring the minuscule amount leaking out of the eyes, ears, and swallowed, the air inhaled goes to the lungs. I maintain, awake or asleep, CPAP machine or not, the volume of air inhaled by a person over a period of time is essentially the same. All this ambient air has the same relative humidity.

[Gryphon]

Doing the math one way or the other. I think the issue is mute.

If I let my humidifier run out the inside of my mouth can become so dry that if I'm not careful and don't moisten my mouth before moving my jaw around too much the skin can literally crack and bleed. If how ever on the other hand I have my tank filled with water and maintain heated humidity through out the night I find that my mouth is a little dry in the morning but no where near as dry as before.

It's one of those things. I have an understanding of what is happening with the air movement but regardless I don't have to know what is going on to know that there is a significant difference. That can be tested with results that can be consistently repeated.

If you figure that the water tank of a CPAP machine holds several cups of water more or less on average. Also with differences in ambient air humidity and room temperature some machines can run dry by morning even when filled properly.

Lets say you have a cpap user who regularly has their humidifier run dry. If they then go under the same conditions and decide to not put any water in their CPAP... by morning if two cups of water is normally gone from their machine isn't it more or less safe to consider that some of that same moisture might have be stolen from of our soft tissues that were exposed to that same air?

We're talking about natural laws of physics here. More air movement means more evaporation.

[wardmiller]

Doing the math one way or the other. I think the issue is mute.

If I let my humidifier run out the inside of my mouth can become so dry that if I'm not careful and don't moisten my mouth before moving my jaw around too much the skin can literally crack and bleed. If how ever on the other hand I have my tank filled with water and maintain heated humidity through out the night I find that my mouth is a little dry in the morning but no where near as dry as before.
. . . .
We're talking about natural laws of physics here. More air movement means more evaporation.

You just proved my point. It appears you are a mouth breather while using your full-face mask. During the day, do you also breath through your mouth?

[WearyOne]

.
What about a standalone humidifier?

I have a friend whose husband just (and finally!) had a sleep study. It was a split-night study. He's very excited about going on CPAP since he did feel a lot better after being on CPAP during the study. He did have a problem, though, and that was his nose was terribly dry after just a few hours even with the humidifier turned to 4 during the study. (He was using the DreamWear and did very well with it.) The chambers can run dry on some humidifiers if turned all the way up, and they do smell awful when that happens. So I suggested he get a standalone heated humidifier.

This one sure looks like it holds more water than the chambers made for specific xPAP machines:

https://www.cpap.com/productpage/fisher ... ifier.html

[robysue]

A question for those who don't believe that a humidifier is ever needed except for some perverse psychological need of the user.

Can you tell me why my nose and throat drys out if I sit in front of a fan breathing normally through my nose for roughly a 3 or 4 hour period?

Can you tell me why my nose and throat drys out if I'm outside for 4+ hours on a windy day?

As near as I can tell, my nose reacts to CPAP the same way it reacts to fans and windy days: It drys out because there is more air moving through my nostrils than there is on a still day or when I sit around not in front of a fan.

[wardmiller]

As near as I can tell, my nose reacts to CPAP the same way it reacts to fans and windy days: It drys out because there is more air moving through my nostrils than there is on a still day or when I sit around not in front of a fan.

Robysue, the reason I jumped into this thread is because I believe statements like "more air [is] moving through my nostrils" is just not the case. With CPAP, are you inhaling a great deal more air than when not on CPAP -- that is, does your chest expand much more than otherwise? When on CPAP, are you inhaling more frequently per minute than without?

I've never heard anyone answer "yes" to those two questions. It is a fact, when using CPAP, the person breaths about the same *volume* of air per minute as without. It is just the *pressure* within the air passages is increased, not the *amount* of air. The excess air provided by CPAP is vented from the mask, before it enters the body.

If "more air is moving through your nostrils", where is it going? You would have to be inhaling more frequently or taking deeper breaths.

[chunkyfrog]

If the tank were larger, moisture loss due to mouth breathing would be ignored,
and this problem would never be addressed.
If you cannot keep your mouth shut, or cannot wear the appropriate torture device to STOP it,
YOU GET PUNISHED. In any case, the blame is shifted to the patient,
And the mask and machine makers' soft, fat butts are covered.

[palerider]

If you figure that the water tank of a CPAP machine holds several cups of water more or less on average.

1.33 cups for respironics, 1.6 cups for resmed.

Also with differences in ambient air humidity and room temperature some machines can run dry by morning even when filled properly.

by morning if two cups of water is normally gone from their machine isn't it more or less safe to consider that some of that same moisture might have be stolen from of our soft tissues that were exposed to that same air?

no, not at all. why would it? the *vast majority* of the air flow that the cpap generates goes straight out the vent. look at your minute vent charts vs the vent rate.

We're talking about natural laws of physics here. More air movement means more evaporation.

but, that's just it, all the air movement in my p10 is happening outside my nose where it's coming out the hose, swirling around and going out the vent.. if there were a lot of air jostling around just inside my nostrils, then it'd tickle the hair of my nose much more than it does.. (which is something that a full face mask does do to me) and as far as getting up through my narrow nasal passages INTO my nose, much less my throat, or mouth... no way, not happening. the only air movement there, as wardmiller says, is normal respiration.

[palerider]

This one sure looks like it holds more water than the chambers made for specific xPAP machines:

https://www.cpap.com/productpage/fisher ... ifier.html

the F&P standalone holds 400ml, the 380, the respironics hold 325. (just fyi, comparing specs)

[palerider]

A question for those who don't believe that a humidifier is ever needed except for some perverse psychological need of the user.

Can you tell me why my nose and throat drys out if I sit in front of a fan breathing normally through my nose for roughly a 3 or 4 hour period?

Can you tell me why my nose and throat drys out if I'm outside for 4+ hours on a windy day?

As near as I can tell, my nose reacts to CPAP the same way it reacts to fans and windy days: It drys out because there is more air moving through my nostrils than there is on a still day or when I sit around not in front of a fan.

if I could tell you that, I might could tell you why some people need no extra humidity, some need a little, and some need a lot.

maybe it has to do with the internal varied geometry of the nose, turbinate size, etc?

I don't know WHY some people need more than others, just that it's not *logical* given easily observable data.... which, likely means it's internal unknown physiological differences in respiratory structures that contribute to the differences.

[Gryphon]

Following up to the question asked me about mouth breathing.

My sinuses are sometimes shot. I grew up most of my life dealing with allergies and so some times I don't breath through my nose very well. I have to use a full face mask. There are some times were after my cpap has been on for a wile the swelling in my sinuses will lessen and I can breath ok through my nose but after almost 40 years of breathing a certain way even though I've had my cpap now for over 8 years, I find I still breath through my mouth. I've tried taping. Though chin straps just sound a bit masochistic to me "my own opinion" nothing against people who find they work or need them. I might need one too but maybe we'll try that later...

One thing to consider as far as "Added air flow" Think of it this way. Sit in front of a small desk fan. Open your mouth but lock your tong back so the only air you breath in and out is going through your nose... Now keep your mouth open but continue to only breath in and out through your nose. The fan has not added any additional air to the volume of air that you are breathing. However after a few min you'll notice that the soft tissues of your mouth will become very dry as the added air movement swirling around in front of your face is causing a higher rate of evaporation.

My pressure level is about 14 and if I fill my CPAP water tank to max I'm usually ok depending on the time of year. I also keep my humidity level set to 6 "Max" on the S9.

On a side note - If any one wants a stand alone heated humidifier I recommend the Fisher & Paykel highly. It can literately drown you out if you so wish. If you crank it up to max and put your cpap on it's like breathing the air in a steaming shower stall. Make sure your CPAP hose is insulated or your room temp is a little warm because if you crank the F&P and you have a cold room you will get rain out like no body's business.

When I first started with CPAP I used a REM Star Pro and I would fill the water tank on that and run it at 4 and then would daisy chain in a F&P set to 3 and run them both. My DME at the time "back when I had a good one" thought that was a bit much but if it worked for me he wasn't going to tell me no.