[Equipment] Rebreathing causing Hypercapnia (CO2 – Carbon Dioxide Poisoning)

[PaytonA]

Hi Hulk,

The statement that you posed,"in a system where there is no non-return valve, to avoid expiration going back down the tube and being re-breathed, the pressure provided by the CPAP machine must be always higher than the back pressure from the mask vent." does not seem to hold true for CPAP systems. The CPAP (using CPAP to encompass all types of CPAP machines) are carefully engineered to maintain their target pressure at all times. Without the use of EPR as you exhale the machine will reduce pressure to maintain the target pressure even with the additional pressure of your exhale. It would seem to me that amount of exhale that goes down the tube would depend on the parameters of the vent.

SCUBA gear has a check valve because it does not have a continuous flow of supplied air through the breathing system. CPAP depends on a continuous flow of fresh air through the mask to reduce the CO2 level of any retained air. The sleeping exhale peaks rapidly to a high flow and then tails off slowly until the inhale begins (your breathing while asleep is different than while awake) and you can not demonstrate this to yourself without recording the flows and looking at a measured flow chart when you are awake hence our love for the Sleepyhead software. My point here is that there is a reasonable time for the continuous flow of air through the mask to wash out the CO2 before your next inhale. I have been told that the reason Resmed CPAPs will not go below a pressure of 4 cm/H2O is due to the probability of CO2 build up.

My comments here are based on experience with full face masks and nasal masks. I have little experience with nasal pillows masks like the P10.

With respect to your second question,"The point I am to focus on is the question of whether the settings on the CPAP machine are critical to avoid Hypercapnia due to rebreathing or does the design of the Mask and it's integration with the CPAP machine prevent the user suffering the symptoms of Hypercapnia?" The design of the mask and the CPAP should prevent Hypercapnia. I do think, however, that in some cases some particular people may get borderline with respect to CO2 levels.

Increased pressure can increase blood oxygenation but it is a good idea to look at your data in detail like with Sleepyhead to ensure that one is not causing pressure induced central apnea which would cause your AHI to increase. It might be permanent at the increased pressure or it might just take a little time for the mind/body to adjust but that could be determined with the software. I feel that you could also reduce residual CO2 by using a mask with more venting (my opinion).

Data from Sleepyhead could also help you to convince your MD that there is a problem that needs attention and I do think that your problem needs a doctor's attention. If you can't convince your present doc it may be time to switch assuming that is possible.

Best Regards,

PaytonA

P.S. EPR: EPR settings give you the amount of pressure relief at exhale that the setting indicates. For instance an EPR setting of 3 will reduce the pressure 3 cm/H2O during exhalation and so forth.

[SnoringInOregon]

Hi HULK,

I think your SCUBA background is making you overthink this, but it's certainly possible that your mask isn't working properly.

I also use the P10 mask. Sleepyhead software tells me there is approximately a 23 liter per minute "Leak Rate" when my pressure is 7 cm. In this context that means a total air flow out of the machine, thru the mask mesh, and out into the room.

You will agree that your body doesn't need anywhere near that amount of air while sleeping. Any air that doesn't first flow into your lungs goes directly out the mask mesh (unless you're opening your mouth).

Yes, you will certainly rebreathe a little bit of exhaled air. But not much. The machine is continuously pumping more than a liter of air thru the mask every 3 seconds. That's more than enough to rapidly flush out any exhaled CO2.

You should immediately install Sleepyhead and read out the data from your machine. What is your leak rate?

[ajack]

that works with your machine because dreamstation shows total leak. Resmed only shows excess leak. so I wonder if a leak rate would work with s9?

[HULK]

Thanks for the tips, tricks and advice.
Extremely helpful during this very difficult time.
Have downloaded and installed SleepyHead.
Thanks to the guys who have obviously put considerable time and effort into it.
There sure is a lot of Data and I won't presume understand it at this early stage!

However a couple of questions:

1) on some days there is flow-data, on others there is not?

2) In the 1st week of using the CPAP (auto-mode, EPR On) there are lots of cycles whereby the Pressure starts off from a peak of somewhere between 9 and 11, so with the EPR set at 3 the EPAP will run 3 lower i.e. say 6 to 8. then there is a gradual reduction in pressure to about 6 and the EPAP drops to zero. No mask leakage. Then there is a spike back up with a huge mask leak?
Refer attached snip.
Is this normal?

[dwd1249]

On the pressure chart the red line is the pressure when you breathe in and the green line shows a drop of 3 when you breathe out.  So when you are 6 on the red you are 3 on the green and the chart is bottom at 4. Hope you followed that.
 The leak doesn't look too bad to me but the more here will be able to give you more info.

[HULK]

In the first 2 weeks of usage (auto, EPR 3) there was over 40 cases of the EPAP pressure dropping to zero!

Then in the next week, with fixed pressure of 10, EPR ON - 3, the pressure graph just shows a flat line of 10 and lots of leaks!

[TBMx]

it does not drop to zero! - the minimum pressure is 4. if you are on 6 on inhale with an EPR of 3 your EPAP - or exhale pressure will be 4 - it will be always 4 unless your inhale pressure finally goes above 7. (if you "see" 0 in the charts - it's a display-bug)

as far as it went for your "rebreathing concerns" - if you could inlcude the left sidebar (use F10 to make a screenshot or make the calendar disappear and maybe also disable the cake-diagram in the options) and maybe show the respiration time curves - it would be easier to clarify that "issue".

[HULK]

In chewing though all this data i cannot reconcile the basic observation "I noted that there was venting out of the P10 Mast when breathing in but either very little or none, when breathing out!"

Perhaps I should try a different mask for the next week of the trial? (starting this Thursday 6th July 2017)

Any suggestions?

[OpalRose]

In the first 2 weeks of usage (auto, EPR 3) there was over 40 cases of the EPAP pressure dropping to zero!

If EPR is set to 3, the EPAP pressure will drop 3cm.  The machine's pressure cannot drop to 0.  

Example: If your minimum pressure is set to start at 6 and you have EPR at 3, then the EPAP pressure can only drop 2cm, because the lowest it can go is 4.   Hope that makes sense.


Then in the next week, with fixed pressure of 10, EPR ON - 3, the pressure graph just shows a flat line of 10 and lots of leaks!

With a fixed pressure of 10 and EPR of 3, you will see a fixed top line of 10 and a bottom line showing EPAP of 7cm.

[HULK]

How does this look?