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As a relatively new user I'm frequently thinking of questions that may not occur to more seasoned veterans of the CPAP system. I began using the nasal pillows. That only lasted a couple of weeks as i found them to be uncomfortable - which is not to say that a nasal mask (my current choice) is what I think of when I think of "comfort". But it works. I guess I need say nothing else.

So, of the two masks I've experienced, both bleed a small amount of air out as the user cycles from inhale to exhale. I stress the word "small". My thought for the day is - where does all of my exhaled air go? The hose is too long for it to be pushed upstream out the other end. Does it just back flow in the hose until I begin the inevitable inhale process? If so, am I inhaling a mixture of new air, that has been pulled through the device filters, and my previously exhaled air?

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If you wave your hand around in front of your mask, you will find a stream of air coming from the mask vent. If you are using a Dreamwear mask, it will be somewhere over your head. P10 cult members will tell you they can barely feel it, but it's there. All cpap masks are designed to vent air, roughly 20 to 40 liters per minute, depending on pressure. The cpap machine is designed to allow for this air flow, because it is this excess flow that mixes with your exhaled air and pulls it out of the mask. Just for reference, your tidal flow is anywhere from 6 to 10 liters per minute, depending on your size and how fast you breathe.
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My original DME described the exhaled air as being "flushed" out of the mask through the vent. She said it was normal to have the venting and prevented me from rebreathing air enriched in carbon dioxide.

Hope this helps.
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To put it simply, when you breathe out some goes straight out the vent, the rest goes into the hose and is flushed out through the vent before you breath again. 
There might be the tiniest amount of exhaled breath left, but it is tiny and will not do any harm.  When you breath normally, (without CPAP) all your breath is not flushed out anyway if you think about, there is a small amount left that does not get expelled and is left in your lungs and breathing tubes.
Some people have made the mistake of blocking the vent on the mask in the past, lucky they all survived.
I am NOT a doctor.  I try to help, but do not take what I say as medical advice.

I am just a long term user of c.p.a.p. and therefore I have some experience in using these machines and equipment.

However, I am NOT an expert, so advice given should be taken as given in good faith only and NOT medical advice.

Every journey, however large or small starts with the first step.   Coffee
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