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[Equipment] Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
Hulk, the anti-asphyxia valve in a full face mask does not prevent airflow back down the tubing on a FFM...the operating pressure above ambient seals it's ports off. It only opens when the pressure in the CPAP tubing is the same as the ambient (surrounding) air...i.e. when the machine is off and you need to draw fresh air with minimal dead air space to minimize rebreathing and CO2 build-up.

I get where you're coming from, but let me try to put your mind at ease.....

Like a PAPR, there is the possibility of pushing exhaled air against the blower's flow and down the tubing; however, the net result is only a minor increase in the equivalent "dead air space" and the volume of exhaled air you re-breathe. Our human body re-breathes a small portion of the same air all the time due to the fact we have no one-way valves and exhaust ports on the airways connecting our mouth/nose and our lungs. This is normal and the body has adapted to it.

With a CPAP, you kind of need to think of it like using a snorkel near the surface of the water....the breathing tube from lungs to fresh air is longer, so you re-breathe some additional air. The difference between a snorkel and CPAP is that air column of air in the tubing (the CPAP equivalent to the snorkel) is constantly being exhausted through the mask vents regardless of whether it is a nasal mask, pillows or full face.

Yes, there is more dead air space volume so you do re-breathes some additional volume of exhaled air when using CPAP. However, unlike a snorkel, the effective dead air space volume is far lower than the amount of volume in the tubing due to the constant air flow from the blower of the CPAP unit itself. In other words, it's not like you're breathing through a 6' hose and, hence, re-breathing 6' of exhaled air....but yes, there will be some minor additional re-breathing of exhaled air for ALL CPAP masks. Again, the effective increase in re-breathing volume would be quite minor (probably less than that of a snorkel, TBH).

I'm not sure why you were having an issue with the pillows mask (unless there was an issue with the exhaust ports), but if working like they should, both the full face F20 and the P10 masks you have used are supposed to function the same way. In terms of your therapy, it sounds like the F20 is doing what you need it to, so enjoy Smile
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
Not sure any further discussion really helps, but following are my rough calculations that suggest to me I have no rebreathing issues associated with my mask/elbow/machine.

my tidal volume is about 500 ml per breath, and it takes me about 4 seconds to breathe in and out once (outflow is about 2 seconds) so I exhale about 250 ml per second.
the engineered-in leakage rate for the mask is about 28 l/min which is roughly 460 ml per second.

it seems logical that most (if not all) of my exhaled air is expelled through the vent holes along with the remaining fresh air supplied from the machine.
flow from me (250 ml/sec) and flow from machine (X ml/sec) totals to 460 ml/sec.  solving for X, the amount the machine adds to the vent flow during exhale is 210 ml/sec.

the special valve (flap) on the elbow before my mask fulfills no purpose while the machine is on.  stated a different way, I have no need for any special ventilation device on the elbow while the machine is on.

Dedicated to QALity sleep.
You'll note I am listed as an Advisory Member. I am honored to be listed as such. See the fine print - Advisory Members as a group provide advice and suggestions to Apnea Board administrators and staff concerning Apnea Board operation and administrative policies. Membership in the Advisory Member group should not be understood as in any way implying medical expertise or qualification for advising Sleep Apnea patients concerning their treatment.
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
Just to clarify for everyone;
I don't think that I have any re-breathing issues with the F20.
There appears to be always plenty of fresh air circulating the mask and whether I am breathing in or out, the circular ring of vents are expelling air.
Each day I become more acclimatised to the mask, hoses, sounds.
For me, the F20 is close to the Fire Fighting Breathing Apparatus that I have been trained on for many years, so consciously and subconsciously, I am able to adapt to it quicker than a nasal mask.
Since switching from the P10 to the F20, there is a steady upward trend of waking up refreshed and alert, even if I have had a terrible night of cushion blow-outs and entanglement in the delivery hose.

TBXx's comment.... "but I doubt anyone is stronger than the turbines in the machines" intrigued me?

I would have thought this to be the case?

To be sure, I thought I would test it by blocking my nose and mouth breathing directly onto the delivery tube. Pressure 6, no EPR.
I was surprised at how easy it was!
But obviously quite dangerous as I was definitely re-breathing my own air. (due to the length of hose and the volume in the CPAP machine)

So, it was only for a half dozen breaths and it is a practice which I would advise against doing unsuperverised.

The question is "why was it so easy?"

Sure, TBMx comment about being awake has merit?

But it was so easy 6 and if the minimum pressure is 4, (SleepyHead reported a lot of cases where the pressure dropped off the scale. Zero was recorded, but the Manufacture's Specifications say the RESMED S9 will not go below 4) that is 30% easier!

So I question, in simplistic terms, if I can comfortably blow 6 and the CPAP is trying to maintain 6, then does the CPAP pump will roll itself back to near zero? 
Hence I was effectively blowing only against the resistance of the tube, chambers, fan and filter?

So, then I put on the P10, not connected to the CPAP, blocked the end of the hose and blew against it. It was a lot harder than blowing against the CPAP at 6!
Sure the nose can't blow as hard as the mouth, but if it has to only blow at 4, or less?

My Sleep Physician is part of a large sleep research Institute.
My 6 week trial ends Friday this week (21st July 2017).
I will be putting all this to him and the technicians for consideration when I see them on Friday.
I will post the results, even if I am totally wrong about the re-breathing!

Thankyou to everyone who has taken the time to contribute to this discussion.

Looking forward to trialling the AIRTOUCH cushions, when they become available (hope they stop the blow-out issues)

If the Sleep Physician recommends i continue with CPAP, I look forward to getting control of the settings and fine tuning them myself.

Sleep well and may you not have too many dreams of being strangled by a giant squid during a dive (who remembers the voyages of the S.S.R.N. Seaview with Admiral Nelson and Commander Lee Crane?)

Thanks ,
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
that was actually regarding normal operation behavior - meaning: you have a mask attached to it!
I didn't thought of anyone trying to breath down the tube itself - 6 cmH2O is just that: 6 cmH2O. If you would breath into the mask forcefully the pressure would rise - thus the vent-flow would increase accordingly. (As the intentional leakage is highly pressure dependant!) - whatever goes down the tube would and will be washed out before you inhale again.

All that was regarding normal behavior - normal person. If you have like a tidal volume of 600ml or more and you take more than 20 or 30 breaths <-- that is not normal! - there you would for sure rebreath. (I suppose you want that in that very case Wink )

If you have a really huge tidal volume and very strong lungs you might indeed push air down the tube. Depending on your respiratory rate there should be plenty of time for the machine to flush out this "already used" air.
A FFM will have always a greater CO2-Vent-Flow (intentional leakage) than nasal or pillow-masks. I can imagine that pillow-masks can be a problem if you have a huge tidal volume and a (relatively to that) short exhale-phase.

If you really are concerned about that and "feel" that you are rebreathing: up the pressure a bit - more pressure more venting^^
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
For the sake of the small fraction of CPAP users who may be having problems and looking for specific answers its worth being as specific and factual as possible.  

This thread is specifically about the P10 nasal pillows mask venting system and the possibility of CO2 poisoning.  I have been doing a lot of research and found other posts else where describing similar experiences.  Other Nasal Pillows type masks may cause similar effects in unusual circumstances.  

All vents do not vent at the same rate for a given pressure.  Giving venting flow rate stats for one mask proves nothing about another.  The P10 has definitely proven this to me without any doubt.  I have 2 other masks and they are fractionally different from each other as well as the P10.

Venting rate is not directly proportional to pressure, i.e. double pressure does not mean double vent flow rate.  As HULK explained, turbulence (frictional forces) causes a reduction in the expected increase rate of flow.  Vent flow rate increases with pressure but its not a linear relationship.  Imagine a pump with infinite pressure attached to your mask – will you get infinite flow through your vent?  Obviously not.  The vent has a ceiling vent rate that it incrementally reaches as pressure rises.  The vents are so small on our devices that this phenomenon is detectable at human levels.  So small on the P10 that i think its of special concern.

Rebreathing air with elevated CO2 levels might be OK for limited durations but its not OK when you are doing it for extended periods, possibly all night, night after night month after month year after year.  Its of vital importance we remember that duration is a critical element to consider when thinking about CO2 poisoning.  Elevated CO2 levels are toxic to humans.  As the level of CO2 goes up so does the toxicity, ranging from headache, confusion and lethargy up to death.  Death from CO2 poisoning is very real and it’s the small tip of the iceberg of pathological affects.  The earliest and most obvious symptoms are neurological, from temporary to permanent but other organs are also affected.  There’s also the question of O2 deprivation.  CO2 displaces O2 in your blood cells.  O2 transportation is directly affected.   So if we were experiencing CO2 poisoning we’re also experiencing O2 deprivation.

Normal healthy outdoor air has CO2 at 400ppm.  Levels as low as 1000ppm, even though its your average classroom level, have been shown to have negative neurological effects in reduced cognitive performance.  We breathe out 50,000ppm!  100,000ppm is lethal within minutes.   Look up ehp.niehs.nih.gov/15-10037/  &  cdc.gov/niosh/idlh/124389.html

If you are in a situation where you are rebreathing continually the total level of CO2 in the air you rebreathe will be accumulating.  

Human CO2 production increases with exertion.

In the case of the P10 nasal pillows mask and CO2 poisoning we have to consider several factors particular to the P10.  

It has a mesh vent with hundreds of tiny micro sized holes.  This is a big departure from other vents I have seen that have a lot less and larger holes.  The frictional forces are proportional to the surface area the air has to pass over.  Logic dictates then that the P10 has greater frictional value than other vent types with fewer but larger holes.  My observations/tests have proven with normal light breathing the P10 can vent sufficient exhaust air.  However, although the mask is continuously venting, its not enough in all circumstances.  Even the manual states ‘some rebreathing may occur’.  What is the value of 'some', at what point, for how long and how often!!??  If you get into a state where you start to breathe heavier it does not vent all exhausted air.  Additionally the micro sized holes are easily restricted or blocked by moisture.  Whether you haven’t dried it properly, have condensation in your tubes, cough, sneeze or just blow out hard you can block up the tiny holes with moisture.  The effect lasts a long time.  Its surprisingly hard to clear moisture from this vent design.  

There are other reasons why you might breathe heavier like sickness; cold, flu, a chest infection.  There are also other mechanical factors that could increase the chances of rebreathing.  A power failure – we get plenty of them.  On a more regular basis you may get blocked vents by rolling into your pillow, your hand, a sheet or cover etc etc… the list goes on.  These are applicable to all vent types so all mask types should be carefully considered.

Where exhaust air is rebreathed a feedback loop can ensue.

If you are breathing air with higher CO2 levels than 400ppm your body automatically makes you breathe faster and deeper to compensate.  The exhale pressure inside the mask will increase as you blow out more volume faster.  The vent flow rate is not proportional so the amount you blow back down the tube increases and so the proportion of air at 50,000 ppm you rebreathe increases.  This new level of CO2 is additional to the amount your body produces and therefore the level of CO2 on your next exhale is higher again, stepping up with every cycle.  

The effort required to breathe harder increases your bodies CO2 output therefore up goes the level of CO2 you are exhausting and so rebreathing…  away it goes again and a negative feedback loop ensues.

Some comments have been made that you have safety features called your mouth and waking up.  With low level CO2 poisoning neither of these may apply, especially when one of the symptoms of CO2 poisoning is sleepiness.  As a nose breather mouth breathing is not automatic for me.  Regardless, my biggest concern is being drip fed low level CO2 night after night, month after month, year after year and the long term neurological effects this may have.  No thanks!  Guessing at whether enough exhaust air is being purged is dangerous.  Do some tests, know for sure.  Upping pressure may be helpful but imprecise for such a serious issue.  

How much rebreathing are you willing to do?  Personally Im not willing to do any.  Why take the gamble if you don’t have to.   I have made a check valve and there is no possibility of rebreathing any more through any mask type in any situation be it low level or power failure.   I can only breathe out through the vent now – this made an obvious change to the normal way it feels and leaves no doubt that I was blowing a lot of air back down the tube.  Breathing normally is pretty much unaffected but breathing out mildly heavier is definitely harder and I can sense the valve closing up.   

With the check valve in, the P10 is pretty much a total fail.  Its uncomfortable breathing out from the start and I woke from it a few hours later and had to change back to the Swift FX.  Yet more proof to me that it is not venting anywhere near enough.  User beware!   

I think we should be demanding more information from the manufacturers.  Vent rates at given pressures and clear warnings as to the risks of CO2 where when and how - if only for the small percentage of users that are at risk.
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
(07-18-2017, 11:43 PM)DaveResmedP10 Wrote: For the sake of the small fraction of CPAP users who may be having problems and looking for specific answers its worth being as specific and factual as possible.  

I have (and like) the P10 mask.

You have written a very thorough statement as to what might be happening in some cases. However, "we have the technology" to make decisions based on actual data.

So, here's my data and analysis:

1) Dreamstation (with assist from SleepyHead) says "leak rate" of 23 liters per minute, or more when it raises pressure.

2) Maybe dreamstation is reporting incorrect number? Probably not. Properly functioning medical devices don't usually make "gross" mistakes. The reported value is almost certainly correct to within 5% or 10%.

3) That 23 liters of air is going somewhere.

4) It could be leaking before it gets to my pillows. In the dreamstation itself or in the hoses to the P10 mask. Or in the P10 mask ahead of the pillows and vents. I haven't detected such a leak.

5) It could be leaking right at my nostrils. I don't think so. I'd notice something like that.

6) I could be exhaling it through my mouth. No, I'd notice that too. I have no problems keeping my mouth closed while sleeping.

7) There's only one place left for those 23 liters per minute. They're flowing out of the vents in the P10 mask. Where else could the air be going?

8) I claim (and this is the only possibly dubious statement here) that this rate of air flow is more than sufficient to rapidly clear any and all of my exhaled CO2.

9) Is there anything wrong with #1 thru #8 above?
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
The first thing that comes to mind is that the vent is not the only place for exhaled breath to go. It can force its way back down the tubing. Second, many things occur while you are sleeping that you do not notice, like leaking from your mouth or your lacrimal duct (tear duct). Some leaks occur without your being able to sense them.

Interesting but I am afraid not definitive. I think that different people have different sensitivities to CO2 concentration.

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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
As data 1-8 is interesting, appreciated and I agree... people are obviously having good experiences with the P10.  But I didnt add my 3 cents for those people.  I did it for the few (like me) who were wondering what is going on and maybe foolishly persisting, not realising the risks - thanks again HULK.   
"9) Is there anything wrong with #1 thru #8 above?"  No but as an argument supporting the P10 I don’t find personal data useful.  Apart from the fact that it basically says 'my P10 works just fine so all P10s must work just fine', everybody’s setup and conditions are different.
I use sleepyhead and the Leak Rate is mountainous ranging from 0 to spikes of 25.  The average over the last 30 days is 3.7 lpm.  My settings are CPAP fixed at 6 no EPR.  Point being your setup vs mine is very different.  It may be that the different experiences at different settings point to the cause so well worth posting the data.  Im going to test the P10 with your settings and see if it even stays on my face : ).  

I re-post excerpts from the P10 user manual.  “The mask contains passive venting to protect against rebreathing. As a result of manufacturing variations, the vent flow rate may vary.”  So clearly there are differences between P10s.  And  “As with all masks, some rebreathing may occur at low CPAP pressures.”
 I use low pressures.  Clearly there is reason for me to use caution.  For me and others there is a real issue with it.  The reports are all very similar - issues with rebreathing.  The experiences are purely subjective but real enough.
Any which way the risks are too high not to talk about them which is all I wanted to do.
As far as comfort and reliability go the P10 is by far the most well designed mask I have used yet and the sole reason why I have tried to persist with it and didn’t just send it back.  The vent is the only issue I have with it.  

As a separate issue regarding the technology, how much can we rely on the data.  How precise is the reporting.  We are relying on it to making some pretty important decisions.  Do the units have a flow sensor or just making calculations based on current draw of the motor.   Its one thing to use data to inform choices and improve sleep experience its totally another to use the data as precise risk assessment.
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
(07-19-2017, 08:23 PM)DaveResmedP10 Wrote: I use sleepyhead and the Leak Rate is mountainous ranging from 0 to spikes of 25.  The average over the last 30 days is 3.7 lpm.

A big problem is that in the terminology of my Respironics Dreamstation "leak rate" actually means total airflow. It's a misleading term. I use that term because it's what SleepyHead uses.

It's especially misleading because in your Resmed equipment IIRC it really does mean a "leak" of air.

Quote: My settings are CPAP fixed at 6 no EPR. Point being your setup vs mine is very different. It may be that the different experiences at different settings point to the cause so well worth posting the data.

Your fixed setting of 6 is quite low. I don't feel comfortable under my minimum of 7. My machine adjusts pressure during the night and sometimes goes as high as 12. Based on comments I've read here and elsewhere I'm not surprised if you're having problems with the P10 with such a low fixed setting.

Quote: Im going to test the P10 with your settings and see if it even stays on my face : ).

Smiley or not, don't go making changes based on what I say. I'm a random guy who is still figuring things out. Better to take advice from those on this forum who have been helping people for years.
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RE: Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)
Errrm no, I didnt mean I was going to use your prescription... just a short test, see what leak rate etc I got.

I have an issue with bloating on higher settings.  Love to be able to get rid of that.  Aerophagia may be the technical term but Im not swallowing the air, its being pumped into me...  

It would be nice to know exactly what the base vent rate is.  Is there a method anyone?  That way we would actually be able to compare real numbers.
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