Lots of people don't understand the risks. If you don't have the anti-asphyxia valve, and the CPAP shuts off, you will be blowing your exhaled air back up the hose and into the water tank. Then, when you inhale, you'll be breathing back the air you've exhaled. The O2 levels will drop, and CO2 will rise.
In industrial situations, this kind of thing can cause you to lose consciousness and eventually die from lack of O2. Google "confined space training". They tell horror stories about people passing out in confined spaces without ever noticing anything is wrong.
Note that this is different from having your airways blocked off. With a non-functioning CPAP machine, you can still inhale and exhale without much effort because the air can flow through the CPAP machine without that much pressure. The problem is that O2 levels slowly drop and CO2 rises.
In a CPAP user, it's assumed that nasal mask users will reflexively open their mouth before harm is done. FFMs have the anti-asphyxia valve that will open and let stale air out and fresh air in.
Most of us will probably wake up if the CPAP shuts off and we breathe stale air, but it's not 100% certain, especially with the "confined space" low O2 effect. Obviously, the problem is worse if you have certain medical conditions, medications, are drunk, etc. Also, as apneacs, we've somewhat gotten used to sleeping without oxygen to some extent.
Even without a functioning anti-asphyxia valve, there's still SOME fresh air. A little air will leak out the normal vent holes in the mask, but since the pressure is lower, not nearly as much stale air will leak out. There's also going to be some mixing of stale air and fresh air out through the filters on the back of the machine.
You may be worried about the anti-asphyxia valve failing. It's a good idea to check it every so often. Unplug the hose, plug the hose connector with your thumb and be sure you can still breathe fairly easily. You shouldn't have real problems unless the mask and the power fail at the same time.
Some people get power alarms that make noise if the power goes off.
Even without a power failure, or with a backup power supply, the CPAP machine itself can fail, get turned off, or you the hose can come loose from the CPAP machine.
If the power goes off and your anti asphyxia valve does work, or you open your mouth, you're back to having sleep apnea. One night probably won't kill you, but you might have some risk of a heart attack or stroke during that one night because you're not as used to apnea, or you've gotten worse.
That covers the O2/CO2 problems. If you vomit during the night with a FFM, you need to be able to get the mask off quickly. Google "aspiration asphyxiation" if you want some more horror stories. Be sure you can get the FFM off quickly in a panic in the middle of the night.
Some people tape their mouth shut to stop mouth breathing. This is like using a FFM without an anti-asphyxia valve and makes the aspiration risk a lot worse.
There's actually some risk from using a chin strap, but I can open my mouth pretty easily even with a chin strap. My respironics chin strap had a warning about asphyxiation.
Re: vent holes
All home CPAP masks have exhaust vent holes, even nasal masks. These leak air all the time when the pressure is on. Without these holes, there would be no place for your exhaled air to go, and you'd eventually suffocate.
On some FFMs, there are two sets of "vent" holes. One for the "normal" exhaust vent, and one set for the anti-asphyxia valve. They all need to be kept clean.
There are some "hospital use only" masks that do NOT have vent holes. They're meant to be used with a special connector that does the venting of the exhaled air. In theory, you could die if you use one of these with a normal CPAP. You will occasionally see these for sale on "unofficial" sites like craigslist or ebay. Sometimes, someone brings one home from the hospital. Be sure anything you use with CPAP is a real home CPAP mask.
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