Where does the air go?

[jomama]

Okay, so, this is probably a stupid question, but where does the air go when you're not inhaling? So, for example, I just got an Airfit N20, and when the mask is on and the machine is on (at a pressure of 7, say), if I hold my hand in front of the mask I feel air coming out of the vent, which I am sure is normal. Is that "excess air?" What is that air? Is that taking away from the 7 cm of pressure? The machine is supposed to be pushing air through my throat which is supposed to keep the airway open, right? So, where is all that air going? Into my lungs, I'm guessing. Where else? But when I sit/lie there and don't breathe for a number of seconds, I don't feel my lungs filling with air. But I guess that's what's happening? But after 8-10 seconds of air being streamed into my lungs, one would think I'd be able to feel it, like a balloon that's being filled with air, but I don't feel that. Again, I know that this is a stupid question, but for some reason I just can't picture/wrap my head around it....

[Sleeprider]

Keep in mind your CPAP is a flow generator that maintains a fixed pressure. If you hold your breath, the only flow needed is the amount needed to maintain the pressure in the face of the mask's vent. That flow rate is about 25 Liters per minute, and roughly four times your respiratory flow rate. That vent is there to ensure enough air is exchanged when you exhale that you will not re-breathe the exhaled air and its CO2. If not for the vent, there could be no flow at all, and the pressure would be maintained. When you inhale, the CPAP fan actually runs faster to generate more flow and maintain pressure, and when you exhale, the fan runs slower because less flow is needed as you exhale to maintain pressure.

At your settings, the CPAP supplies between 7 and 15 cmH20 of pressure (2.75 to 5.9 inches of water or 0.1 to 0.2 PSI). If you were to blow bubbles into a glass of water through a straw, the most pressure you ever experience is the the amount to hold a bubble at the end of the straw in less than 6 inches of water. So the CPAP cannot inflate even a pool raft because the pressure is too low. It will blow very hard during an open leak, but it only blows at the set pressure, which is enough to stabilize the tissues in your airway, and is actually a very gentle pressure.

[jomama]

Thanks for the reply. Still confused... :/ So, when I am lying there and not inhaling for a few seconds, where is the air going? Into my trachea/windpipe, right? And thus down into my lungs, right? But you're saying it's not enough air/pressure to fill them up?

As a side note, when I take the mast off, the pressure is pretty strong, sounds like a little hurricane, ha ha. But you're saying that when the mask is on, it's not blowing that (much) pressure down my windpipe? When I take the mask off, I'm hearing 25? And it's only sending down 7 (if that's my setting)?

[Sleepster]

Okay, so, this is probably a stupid question, but where does the air go when you're not inhaling?

It doesn't need to go anywhere. You are confusing a pressure with a flow. You can, for example, have full pressure and no flow at all. Pressure is how hard the air is pushing, flow rate is how fast the air is moving.

The engineering marvel of the CPAP machine is that it's able to maintain a constant pressure over a wide range of flow rates.

So, for example, I just got an Airfit N20, and when the mask is on and the machine is on (at a pressure of 7, say), if I hold my hand in front of the mask I feel air coming out of the vent, which I am sure is normal. Is that "excess air?"

That's the intentional leak. You need that to keep from re-breathing your own exhaled air.

But when I sit/lie there and don't breathe for a number of seconds, I don't feel my lungs filling with air.

Right. Because they're not filling. That would require (inspiratory) effort on your part. The CPAP machine just keeps the airway open (a pressure splint) so that your efforts at breathing are not frustrated.

[Sleeprider]

Pressure vs Flow. These are separate, but related things. CPAP is low pressure. If you put your hand over the end of the tube you can completely stop the flow very easily, but when you let the tube flow freely, it will flow a large volume of air. A compressor has very high pressure, and you cannot stop the flow from a compressor's air tube with your hand, but if you release the air pressure and let it flow freely, the compressor will flow a fairly low volume of air once the pressure is released. Similarly, if you have a water hose, you cannot stop the flow with your hand because it is around 70 psi. Your furnace fan blows a tremendous amount of air around your house but it does not pressurize it, and you can stop the flow or divert it with a simple damper.

Your CPAP is not blowing air down your windpipe. You must spontaneously breathe, or no flow happens in or out. It may be slightly easier to inhale with CPAP pressure, but you can easily overcome that pressure. Without your ribs expanding, and diaphram tightening, the CPAP cannot cause you to inhale. There are non-invasive ventilators that can accomplish that magic. The discussion of pressure and flow is actually pretty high level stuff and it's not a dumb question, but I might not be bright enough to explain it. I'm sure another member will step in.

[mesenteria]

Thanks for the reply. Still confused... :/ So, when I am lying there and not inhaling for a few seconds, where is the air going? Into my trachea/windpipe, right? And thus down into my lungs, right? But you're saying it's not enough air/pressure to fill them up?

As a side note, when I take the mast off, the pressure is pretty strong, sounds like a little hurricane, ha ha. But you're saying that when the mask is on, it's not blowing that (much) pressure down my windpipe? When I take the mask off, I'm hearing 25? And it's only sending down 7 (if that's my setting)?

Think flow.  Not pressure.  I know, it's a head-shaker, but that's what we're interested in.  If you hold your breath, and put your hand close to the mask or headgear vent, you feel air.  That's because the fan always runs, sometimes faster, sometimes slower, depending on what the flow meters think is happening down at your end of the tube.  You haven't also covered the vent while you're holding your breath, so...………………………………………………..you feel air because...……………………………………………...the fan is still working.  What doesn't go into your lungs as you breathe must exit someplace, and that is past, around, under your skin surfaces inside the mask and out the handiest escape route.  The vent.

The iron machine operated via a vacuum at first, but nowadays, if ever used at all, they force air into your lungs and then back off the pressure to let your chest walls force your lungs to expel the air.  Your PAP therapy works on positive pressure creating flow as well, but a fraction of the 1 bar pressure the newer iron lungs use.  So, while you mechanically try to breathe past whatever is obstructing or closing off your airway, the PAP device adds a little 'help', variably adjustable, and ideally just enough to keep you going.  And asleep.

[PoolQ]

to answer what you asked, you need to break down what is going on into 3 steps: you are not inhaling or exhaling, you are inhaling, you are exhaling.

Basic things that need to be accepted:
The pressure in your airway is the same as in the mask and the same as in the hose all the way back to the "fan"
Yes the flow rate is important to the treatment, but not so much with what you are asking

First set, from above, what is going on when you are NOT inhaling or exhaling:
The CPAP machine has a target pressure setting, you at this point are a fixed volume. so, you have a volume, you have an exit port (the vents)- all those little holes or not depending on your mask usage. so the "fan" builds up the pressure to maintain the mask/airway at the pressure specified. This pressure should splint you airway open.

2nd from above, you are inhaling. In this case you are increasing the size of the hose/mask/airway so the fan must speed up to fill the new volume and bring it up to target pressure

3rd you are exhaling. You are the one increasing the pressure, so the fan decreases speed as you are now controlling the pressure. Most of your air is going out the vents

These are the only things that are going on and what is happing and where the air is going

[DaveL]

Hope this helps...
this is my take. If I'm wrong, please correct me!!

cpap is just a fan that blows.....constant pressure
bpap is a two pressure fan; I think! I've never had one. One pressure blowing while you inhale; one pressure while you exchale.
apap is a variable pressure fan. I've never had one. It sits there waiting, and coasts along at a set lower pressure until you need higher pressure. It responds when an apnea event starts, and this is what I wish I had.

My ResMed is a cpap that reports pressure, leakage and a bunch of things on an SD card I have. I can download it using OSCAR. I have a lot to learn, so there's a lot of information there that I don't understand yet.

When I started a long time ago I was concerned about the air that was vented. It's important that it be allowed to vent. Another thing that concerned me was the noise from that vent, and my machine. I'm lucky. My DME is good and helps me make good decisions. The people here are really good! They report really well on equipment, and help me from choosing bad masks and bad machines....

[jomama]

Okay, so this is interesting. Before, I thought that the "splint" was maintained by a constant flow of air moving down through your throat/windpipe, but what you're saying is that that is not happening?...that it's just pressure that is what's keeping the airway open. But wouldn't that mean that that pressurized air (sorry, not sure how to say it), doesn't stop at the bottom of the throat, but goes all the way down the windpipe into the lungs?...

Look at this video:

https://youtu.be/6Up8Ciyyfzg?t=69

"CPAP delivers a flow of air to the upper airway" it says. And it shows air flowing down into the throat/airway. My question is, where does that air (that is flowing into the nose/mouth) go once it passes the throat/airway? Does it just dissipate? Or is a certain amount of pressure created all the way down through the airway, and including the lungs?

Man...this is even more confusing than I thought it was, haha...

[sheepless]

thanks for asking these questions jomama. it's basic to cpap but it's not a stupid question. after more than 2.5 years of hanging around here I still struggle to understand and remember how this works and what the data mean.