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Mask Vent Design Question
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PaytonA Offline
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Post: #1
Mask Vent Design Question
I am currently using an F & P Zest Q nasal mask and the design of the vent is beginning to trouble me. This mask has an elbow connected to it that is rotatable in 2 dimensions. The elbow has a series of small holes in it that act as the vent. There is also an attachment that holds a small oval of filter material over the vents to muffle the noise from the vents and limit the flow. This is what makes it the "Q" version of the Zest mask.

Last night I found that putting the mask on with no hose attached and blocking the hose inlet line, it was very difficult to breathe in or out. The literature from F&P indicates that the mask can be used with or without the filter media in place. I tried it without the filter material and ended up with a lot of condensation-some getting into the vent holes-causing a lot of whistling noise which woke me up.

All this messing with the vent made me look at the design more critically. The vent is basically at the end of the incoming line and I am beginning to wonder if it is very efficient at eliminating the exhaled air,since it is trying to do so against the pressure of the incoming air. I am also wondering if it may be having an adverse effect on my oxygen saturation which would not be a good thing.

I am interested in hearing from anyone that has some ideas about this.

Thanks in advance.

PaytonA
(This post was last modified: 01-24-2014 02:20 PM by PaytonA.)
01-24-2014 02:19 PM
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RonWessels Offline

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Post: #2
RE: Mask Vent Design Question
If I understand correctly, you found it very difficult to breathe essentially entirely through the mask vent. This is not surprising, since that vent is not designed to handle the instantaneous flow required for inhalation.

The purpose of the mask vent is to make sure that you do not breathe your own exhaled air. As such, the flow rate out of the vent only needs to be enough to average out to the volume of your exhalation over the period of your breathing. You are correct that some of your exhaled breath probably remains inside the "dead area" of the mask, but if you think about that volume of air compared with the volume of your inhalation, you will realize that this is a trivial amount in comparison. As long as whatever exhaled air got forced back up the hose is eventually vented before you next inhale, everything is working fine. Of course, if you "overload" the system by deeply inhaling/exhaling and then taking a second breath quickly, you will still get some of your previous exhaled air, which is why it will feel particularly warm and stuffy. But people don't breath that deeply and quickly normally.
01-24-2014 03:41 PM
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zonk Offline

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Post: #3
RE: Mask Vent Design Question
(01-24-2014 02:19 PM)PaytonA Wrote:  Last night I found that putting the mask on with no hose attached and blocking the hose inlet line, it was very difficult to breathe in or out.
The purpose of the vents to flush out exhaled air, as fresh pressured air enter the mask flushes out exhaled air
For that reason when power goes out, full face masks have anti-asphyxia valve so you can breathe room air and don,t suffocate but you can open your mouth and breath with a nasal mask
01-24-2014 04:02 PM
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justMongo Offline

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Post: #4
RE: Mask Vent Design Question
As described by Zonk: My FFM mask has two types of vents. A series of holes that are the flush holes to vent exhaled air out of the mask during xPAP operation.
There is a second vent with a flapper valve at the mask elbow. It's purpose it to permit one to breathe when the xPAP is not operating -- such as failure of the xPAP or just in that period between strapping on the mask and pressing the xPAP start button. When xPAP pressure is available the flapper closes that vent.

PaytonA -- perhaps the mask you have is not the best for you...

INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
01-24-2014 05:00 PM
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PaytonA Offline
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Post: #5
RE: Mask Vent Design Question
(01-24-2014 03:41 PM)RonWessels Wrote:  If I understand correctly, you found it very difficult to breathe essentially entirely through the mask vent. This is not surprising, since that vent is not designed to handle the instantaneous flow required for inhalation.

The purpose of the mask vent is to make sure that you do not breathe your own exhaled air. As such, the flow rate out of the vent only needs to be enough to average out to the volume of your exhalation over the period of your breathing. You are correct that some of your exhaled breath probably remains inside the "dead area" of the mask, but if you think about that volume of air compared with the volume of your inhalation, you will realize that this is a trivial amount in comparison. As long as whatever exhaled air got forced back up the hose is eventually vented before you next inhale, everything is working fine. Of course, if you "overload" the system by deeply inhaling/exhaling and then taking a second breath quickly, you will still get some of your previous exhaled air, which is why it will feel particularly warm and stuffy. But people don't breath that deeply and quickly normally.

RonWessels,

You understood me correctly but maybe I was not clear enough. My point about it being difficult to breathe with the "muffler" in place was not to say that I should be able to breathe easily but it was very, very difficult. With other masks without the "muffler" it is much easier to breathe through the vent than with this one. Also with the "muffler" on and the machine running at 20 cm H2O it is very difficult to feel the vented air even with the more sensitive areas of my hand. The other thing that this little experiment did was to start me thinking about the placement of the vent.

The following is only to illustrate my logic when thinking about this venting situation.

If one were to hook a pipe to a container full of hydrogen sulfide gas with a small vent in the pipe right before the container and then blow air into the pipe, it would take a long time to remove all of the hydrogen sulfide from the container at the air pressure that we use. Some of the H2S would get entrained by eddy currents and leave through the vent but it would take a while. If one placed the vent on the opposite side of the container from the pipe, it would clear the H2S out faster (more eddy currents and more entrainment).

This all makes me a little unsure. I guess I will use without the muffler for now.

Best Regards,

PaytonA
01-24-2014 10:18 PM
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Sleepster Offline
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Post: #6
RE: Mask Vent Design Question
(01-24-2014 02:19 PM)PaytonA Wrote:  Last night I found that putting the mask on with no hose attached and blocking the hose inlet line, it was very difficult to breathe in or out.

As you indicated in your later post, more difficult than with other masks, and presumably more difficult than it would be without the filter material.

So this establishes that less air flows through the vent when the filter material is present. That is a concern that you might want to research. See just how much the flow is limited. Presumably it's not limiting it below a safe level.

Quote:The vent is basically at the end of the incoming line and I am beginning to wonder if it is very efficient at eliminating the exhaled air, since it is trying to do so against the pressure of the incoming air.

The purpose of the vent is keep the hose cleared of exhaled air.

Sleepster
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INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
01-24-2014 11:49 PM
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archangle Offline
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Post: #7
RE: Mask Vent Design Question
The issue of venting out all the stale air is well understood and designed into the mask as long as you use a proper CPAP mask and don't block the vents. It's pretty much foolproof.

You don't exhale just through the air vents. The CPAP blower keeps a constant pressure at the mask. As you exhale, some of their goes out the vent and some goes back up the hose. Because the mask is at a constant pressure, a fixed amount of air goes out through the vent as you inhale and exhale.

The "leak rate" of the vent is designed to flush out enough air throughout the breathing cycle at the minimum 4 cmH2O pressure that CO2 and O2 levels stay OK. This is true, even if the filter is in place, unless the filter has become clogged or you're using something other than the correct filter.

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01-27-2014 02:34 AM
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PaytonA Offline
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Post: #8
RE: Mask Vent Design Question
(01-27-2014 02:34 AM)archangle Wrote:  The issue of venting out all the stale air is well understood and designed into the mask as long as you use a proper CPAP mask and don't block the vents. It's pretty much foolproof.

You don't exhale just through the air vents. The CPAP blower keeps a constant pressure at the mask. As you exhale, some of their goes out the vent and some goes back up the hose. Because the mask is at a constant pressure, a fixed amount of air goes out through the vent as you inhale and exhale.

The "leak rate" of the vent is designed to flush out enough air throughout the breathing cycle at the minimum 4 cmH2O pressure that CO2 and O2 levels stay OK. This is true, even if the filter is in place, unless the filter has become clogged or you're using something other than the correct filter.

In thinking about this after the inputs that I have received here, I think I see what everyone has been trying to tell me. The dead space in the mask is 90mL. Using the data from the VPAP my breath calculates to around 700mL for each breath. Even if the mask was full of depleted air (from the last exhalation) it is such a minor part of my next breath that it probably does not have much effect.

Thanks for the inputs,

PaytonA
01-28-2014 03:37 PM
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Sleepster Offline
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Post: #9
RE: Mask Vent Design Question
Right. Think of it this way. Without the vent exhaled air would bounce in and out of the hose. Like a snorkeler with a too-long snorkel, you end up breathing in too much of your exhaled CO2. The vent keeps the hose purged, not the mask.

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INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
01-28-2014 07:35 PM
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