Mask carbon dioxide levels - toxicity

[JDS74]

The mention of the Mirage Quattro mask as being symptomatic in regard to CO2 rebreathing is interesting in that the dead space in that mask is quite large relative to a "normal" tidal volume. In the user guide, the dead space for the larger cushions is listed as 242 mL. At lower pressures that larger dead space could contribute to a lesser dilution of the exhaled breath. For example, at a pressure of 8 cm H2O, the leak rate is listed as 32 L/min that is a little higher than my Oracle mask that lists 30 L/min. The Quattro user guide mentions meeting the ISO 17510-2 standard for protection against rebreathing.

Just a thought,

[Wulfman...]

Another discussion I was also part of about 7 years ago had to do with certain masks also not sufficiently clearing CO2.

Oh right, back in day:


And before that, back in the frequie days, when the fad was trying to increase FICO2 levels.

I tell ya, if you stand in one spot long enough, what goes around...

Actually, this is the thread I was thinking about when I referenced the Quattro and the possibility of insufficient clearing of CO2.

http : // www . cpaptalk . com/viewtopic . php?f=1&t=38408&p=335958&hilit=+Quattro#p335958

(take out the spaces after copying and posting into the browser)


Den

.

[miltf]

Thanks for all the responses.

Here is the data:

As you can see, the level of CO2 never drops below 2.2% or so, so it doesn't matter what the deadspace volume is, I'm still breathing 2% or higher CO2. I may be a pretty shallow breather and that might affect my results and the pressure is low and that certainly affects the results, but these are the conditions under which I start CPAP (as set by my doctor) and these are the conditions under which I feel like ripping off my mask. Your results may differ.

I appreciate all of the experience expressed here and learning that some people might benefit from higher CO2 to counter central apnia.

As to whether this is safe or not, I can't say. It just feels bad to me and my original point is that some new users might quit from the feeling of not being able to breath. I fixed things for myself by putting a few extra holes into the mask. It feels much better and my AHI is still low, so who can complain.

The parts I used were:
SprintIR sensor from GSS (0-5%)
Generic TTL to USB cable
MegunoLink Pro software for control and data logging
Generic 12V mini-vacuum pump from Ebay
Selectable voltage power supply to control airflow rate (4-12 v)
Generic airlfow meter (0-3 L/min)
Tubing, connectors, and flange nuts

I think I'm over my exploration of CO2 and CPAP. If anyone is interesting in measuring their own mask, take a look at the parts above and make me an offer (by messaging). I can provide all of this (except the software license) plus my control program and tips on use for a reasonable price.

As to my original comments on silicone, yesterday I tried to bake out the silicone volatiles in a convection oven at 450 F for 1.5 hr. I think I got about 90% of the way to clean for me, so maybe I will be back in the normal world of CPAP soon.

All the best,
Milt

[Wulfman...]

If you're going to keep using such a low pressure setting, I just don't see any "improvement" in your future (or at least until you increase it).


Den

.

[miltf]

One more time -- I have an S9 Auto. It starts at 5 cm and then goes up (to about 10) as needed by my breathing. Why the bad vibes?

[chunkyfrog]

That is not what you said earlier.
If you can't be consistent, you have no credibility.

[JDS74]

miltf

I think I see the measurement error in your procedure.
When you exhale, the pump sucks in air from your exhalation and you see a rise in CO2 concentration.
When you inhale, the enhanced CO2 level air still remains in the sampling tubing and is only gradually mixed with the incoming air from the CPAP machine.

You noted that the pump pulling air from the mask is 2 L/min.
Assuming a breathing rate of 15 BPM, the pump pulls in about 150 mL of air with each breath time.
Assuming a large dead space mask such as the Quattro full face, the sample is just a little more than one half of the dead space while the air passing through will be on the order of 1,000 mL.

For your measurement to be accurately reflecting what is going on, the CO2 concentration in the ambient air must be elevated.

That is highly likely to not be true so the conclusion has to be the sampling mechanism must have a built-in bias towards exhaled air only and not a mix of dead air space air (from exhalation) and ambient air.

So, could you do an additional experiment?
Could you try making the same set of measurements for two different setups:
1) CPAP machine turned off and hose disconnected from the machine.
2) CPAP mask disconnected from the hose.

Then compare those sets of measurement with your first studies to see if they both indicate elevated CO2.

Just a thought experiment.

[JDS74]

That is not what you said earlier.
If you can't be consistent, you have no credibility.

For purposes of the measurement he set it to straight CPAP and pressure of 5 cm H2O.
His treatment levels are different.

Using a variable pressure for this experiment would unnecessarily complicate things.
Another possibly interesting experiment would be to set the pressure to 10 cm H2O and make the same measurements.

[chunkyfrog]

Good point about remaining gas in the sampling tube.
Testing anything always has the possibility of affecting the results.
Five centimeters is rarely adequate for therapy, so any results are affected.
A higher pressure would have cleared the tube better.

[JDS74]

Here is a link to a small study of rebreathing via cpap masks.
Not too recent (2003)
http://www.researchgate.net/profile/Dea ... 738812.pdf

[Todzo]

Thanks for all the responses.

As you can see, the level of CO2 never drops below 2.2% or so, so it doesn't matter what the deadspace volume is, I'm still breathing 2% or higher CO2.

You need to learn to think in terms of what is happening dynamically and in terms of pre and post vent placement running with real time air flow rates.

I believe that if you changed the measurement system sample point physical placement point in the mask your results would vary widely, especially near the nostril.

At the vent the air is flowing out the vent point constantly at the vent rate. Lets say that is about 2/3 of my vent rate of 30 liters per minute. At rest and in bed I would not expect the minute breathing volume to be above six liters per minute for a normal person. If the breathing waveform were a pure sine wave then the maximum expiration rate would be 8.5 liters per minute and all of the expired air would leave the vent port. The inhalation cycle would immediately start a process of dilution of the “breathed” air in the mask and the path to the nose would become very clean very early. Since the air in the mask is not a direct part of the circuit it's effect would depend upon how the air circulates in the mask during the inhalation phase and would be limited.

But the breathing waveform is not a sign wave. At the start of exhalation it may in fact become quite fast and may for a part of a second breach the 20 liters per minute vent rate and re-breathed air could become backed up into the supply tube. But after the beginning of exhalation things tend to taper off rapidly. By the time inhalation starts exhalation is likely to have been near zero for some time. So most likely the fraction of a breath which became backed up is eliminated long before inhalation starts.

I think that if you locate the air sample point of your measuring system very near a nostril you will find that the CO2 levels drop much more and raise a bit more. What happens in the mask away from the actual nasal air flow is likely very different from what happens elsewhere between the mask and face.

I may be a pretty shallow breather

If true your actual re-breathed air amounts should be minimal indeed.

and that might affect my results and the pressure is low and that certainly affects the results, but these are the conditions under which I start CPAP (as set by my doctor) and these are the conditions under which I feel like ripping off my mask. Your results may differ.

When I hear that someone is “ripping off the mask” my first thought is that they have over breathed enough to reduce their blood carbon dioxide levels to the point where it is affecting their chemoreflexes and causing long term facilitated hyperventilation. One very notable thing that I have experienced in that state is a very unnatural air hunger. I know I am at rest and in bed but I am breathing like I am walking up hill – fast. This should not be.

I have found that using eucapnic breathing techniques will evaporate the unnatural breathing hunger in just a few minutes (usually about 40 seconds for me). Some think that the unnatural breathing hunger is caused by changes in the carotid bodies which make them more sensitive in the hypocapnic environment. Others think that the cerebrovascular changes due to the hypocapnic state create such separate states for the central and peripheral chemoreceptors that it throws the system off. Whatever the cause moving toward eucapnia cures it for me every time.

I appreciate all of the experience expressed here and learning that some people might benefit from higher CO2 to counter central apnia.

As to whether this is safe or not, I can't say. It just feels bad to me and my original point is that some new users might quit from the feeling of not being able to breath. I fixed things for myself by putting a few extra holes into the mask. It feels much better and my AHI is still low, so who can complain.

The parts I used were:
SprintIR sensor from GSS (0-5%)
Generic TTL to USB cable
MegunoLink Pro software for control and data logging
Generic 12V mini-vacuum pump from Ebay
Selectable voltage power supply to control airflow rate (4-12 v)
Generic airlfow meter (0-3 L/min)
Tubing, connectors, and flange nuts

I think I'm over my exploration of CO2 and CPAP. If anyone is interesting in measuring their own mask, take a look at the parts above and make me an offer (by messaging). I can provide all of this (except the software license) plus my control program and tips on use for a reasonable price.

With your set up I think you could do some good Capnography and so measure your actual PetCo2 and infer your PaCO2. I would love to have that capability.

As to my original comments on silicone, yesterday I tried to bake out the silicone volatiles in a convection oven at 450 F for 1.5 hr. I think I got about 90% of the way to clean for me, so maybe I will be back in the normal world of CPAP soon.

All the best,
Milt

It would be ironic if the volatiles used in CPAP machines also caused OSA.

Some additional search terms for you for Google Scholar, PubMed, or other favorite medical information search engines:

cerebrovascular CO2 reactivity

sleep apnea ventilatory instability

sleep apnea CO2 retention

Well that should get you started.

[TangledHose]

It is interesting to see how if issues which deal with air hunger and carbon dioxide maintenance come up here the shills, trolls, and their sock puppets also appear. Sometimes you can tell them because they tend to use very gross avatars.

Is this any better for you Toad.....er, I mean Todzo??

[Sludge]

As you can see, the level of CO2 never drops below 2.2% or so, so it doesn't matter what the deadspace volume is, I'm still breathing 2% or higher CO2. I may be a pretty shallow breather and that might affect my results and the pressure is low and that certainly affects the results, but these are the conditions under which I start CPAP (as set by my doctor) and these are the conditions under which I feel like ripping off my mask. Your results may differ.

Actually, your results differ.

The normal ETCO2 waveform, and one that is measuring hyperventilation, look like this:



ETCO2 has to be near 5%, as well as the waveform having several distinguishing characteristics (more on that later) so what you're measuring there is some localized something or other that is less than completely participatory.

I fixed things for myself by putting a few extra holes into the mask. It feels much better and my AHI is still low, so who can complain.

Yeah, well, increase leak rate enough and I can see AHI = 0.

Especially if the entire night is spent in LL.

[Todzo]

Thanks for all the responses.

As you can see, the level of CO2 never drops below 2.2% or so, so it doesn't matter what the deadspace volume is, I'm still breathing 2% or higher CO2.

You need to learn to think in terms of what is happening dynamically and in terms of pre and post vent placement running with real time air flow rates.

I believe that if you changed the measurement system sample point physical placement point in the mask your results would vary widely, especially near the nostril.

At the vent the air is flowing out the vent point constantly at the vent rate. Lets say that is about 2/3 of my vent rate of 30 liters per minute. At rest and in bed I would not expect the minute breathing volume to be above six liters per minute for a normal person. If the breathing waveform were a pure sine wave then the maximum expiration rate would be 8.5 liters per minute and all of the expired air would leave the vent port. The inhalation cycle would immediately start a process of dilution of the “breathed” air in the mask and the path to the nose would become very clean very early. Since the air in the mask is not a direct part of the circuit it's effect would depend upon how the air circulates in the mask during the inhalation phase and would be limited.

But the breathing waveform is not a sign wave. At the start of exhalation it may in fact become quite fast and may for a part of a second breach the 20 liters per minute vent rate and re-breathed air could become backed up into the supply tube. But after the beginning of exhalation things tend to taper off rapidly. By the time inhalation starts exhalation is likely to have been near zero for some time. So most likely the fraction of a breath which became backed up is eliminated long before inhalation starts.

I think that if you locate the air sample point of your measuring system very near a nostril you will find that the CO2 levels drop much more and raise a bit more. What happens in the mask away from the actual nasal air flow is likely very different from what happens elsewhere between the mask and face.

I may be a pretty shallow breather

If true your actual re-breathed air amounts should be minimal indeed.

and that might affect my results and the pressure is low and that certainly affects the results, but these are the conditions under which I start CPAP (as set by my doctor) and these are the conditions under which I feel like ripping off my mask. Your results may differ.

When I hear that someone is “ripping off the mask” my first thought is that they have over breathed enough to reduce their blood carbon dioxide levels to the point where it is affecting their chemoreflexes and causing long term facilitated hyperventilation. One very notable thing that I have experienced in that state is a very unnatural air hunger. I know I am at rest and in bed but I am breathing like I am walking up hill – fast. This should not be.

I have found that using eucapnic breathing techniques will evaporate the unnatural breathing hunger in just a few minutes (usually about 40 seconds for me). Some think that the unnatural breathing hunger is caused by changes in the carotid bodies which make them more sensitive in the hypocapnic environment. Others think that the cerebrovascular changes due to the hypocapnic state create such separate states for the central and peripheral chemoreceptors that it throws the system off. Whatever the cause moving toward eucapnia cures it for me every time.

I appreciate all of the experience expressed here and learning that some people might benefit from higher CO2 to counter central apnia.

As to whether this is safe or not, I can't say. It just feels bad to me and my original point is that some new users might quit from the feeling of not being able to breath. I fixed things for myself by putting a few extra holes into the mask. It feels much better and my AHI is still low, so who can complain.

The parts I used were:
SprintIR sensor from GSS (0-5%)
Generic TTL to USB cable
MegunoLink Pro software for control and data logging
Generic 12V mini-vacuum pump from Ebay
Selectable voltage power supply to control airflow rate (4-12 v)
Generic airlfow meter (0-3 L/min)
Tubing, connectors, and flange nuts

I think I'm over my exploration of CO2 and CPAP. If anyone is interesting in measuring their own mask, take a look at the parts above and make me an offer (by messaging). I can provide all of this (except the software license) plus my control program and tips on use for a reasonable price.

With your set up I think you could do some good Capnography and so measure your actual PetCo2 and infer your PaCO2. I would love to have that capability.

As to my original comments on silicone, yesterday I tried to bake out the silicone volatiles in a convection oven at 450 F for 1.5 hr. I think I got about 90% of the way to clean for me, so maybe I will be back in the normal world of CPAP soon.

All the best,
Milt

It would be ironic if the volatiles used in CPAP machines also caused OSA.

Some additional search terms for you for Google Scholar, PubMed, or other favorite medical information search engines:

cerebrovascular CO2 reactivity

sleep apnea ventilatory instability

sleep apnea CO2 retention

Well that should get you started.

With your set up I think you could do some good Capnography and so measure your actual PetCo2 and infer your PaCO2. All you should need to do is to move the sensor point very near the nostrils. Please post the results if you do this (with some SleepyHead graphs as well if possible).

Thanks!

[miltf]

miltf

I think I see the measurement error in your procedure.
When you exhale, the pump sucks in air from your exhalation and you see a rise in CO2 concentration.
When you inhale, the enhanced CO2 level air still remains in the sampling tubing and is only gradually mixed with the incoming air from the CPAP machine.

.

I believe you are talking about hysteresis, the tendency of the measurement to lag behind a change in conditions, and yes, this is a good thing to be concerned about. I measured the hysteresis in the system by switching the measured air from 5% to 0% quickly and there is a lag in response of about 1% CO2/sec. This is not enough of a lag to account for the results. In other words, the CO2 does not go to zero in the mask but may go to about 1.7% at its lowest value for a very short time.

Several people have commented that a vent rate of 20-30 L/min should dilute that CO2 to a much lower level than what I am seeing and that therefore I must be wrong. I would agree with this if indeed the mask dead space was well mixed with fresh incoming air and was then vented. Unforturnately, this is not happening in many masks. For example, both the Profile Lite and the Swift FX have vents on the elbow connector, so the air being vented is only that which enters the tube. In contrast the Quattro FX has a vent over the nose, so the air being vented has to first enter the mask dead space and mix with the exhaled air. Both papers that are cited in earlier posts point out the benefit of a vent located in the mask rather than the elbow or tubing, but neither of these has resulted in a change of practice by manufacturers.

Current masks must meet an ISO requirement to prevent rebreathing but this does not have to be demonstrated directly. Instead, new masks must only show that they have an adequate vent rate vs pressure to be accepted as equivalent to already approved masks. That is why manufacturers always publish the vent rate vs pressure curve. There is no requirement for the vent to be located where it can vent expired air (though perhaps there should).

A couple of people have suggested that higher pressures will not have this effect and I have been asked to test that. I am sure that this problem will be lessened at higher pressures. My point is that at the beginning of sleep with an autoset machine, the pressure is often low. It is under these conditions that with some masks the CO2 may be high and may lead to discomfort. I am not claiming anything else.

Bottom line, I have now switched to using a Quattro FX primarily because it has the vent over the nose and provides good venting. If anyone has any further questions for me on this topic, please feel free to send me a private message. I will be happy to respond.