Low pressures: Suffocation, rebreathing, etc.

[ThisIsMyUserName]

So I use a very low pressure (5 cm), which works great from an apnea perspective: I went from an AHI in the moderate-sever range to less than 1 on average. This is great from the CPAP perspective: no need for APAP, or complicated whatevers. I do have two questions, though: sometimes it feels like I can't get enough air, which I've heard is common at low pressures, but makes no sense to me at all, since at any pressure, the pressure is still well above room pressure. Second, I noticed a disclaimer on my mask info sheet that basically said that low pressures may not be enough to clear the CO2 from my breathing out, which can occassionally lead to, you know, death. I'm not all that worried, but it did get me thinking about whether the pressure is enough to fully clear excess CO2 and, if not, whether that means I'd be breathing low-oxygen air all night. Thoughts or knowledge on this?

[Sheriff Buford]

I know the lowest possible pressure is a 4, and I don't recall too many folks here reporting out that their bottom pressure is a 5.... with very few folks using a 6. Regardless, you are definitely on the bottom-end of the pressure scale. You referred to apap being "APAP, or complicated whatevers", so I'm assuming you are using the cpap mode. To me, in cpap mode, a 5 is way too low. I would bump up my pressure to a 6 or 7 and watch my numbers very closely, especially the central apnea. This may remove the suffocating feeling.

Sheriff

[ThisIsMyUserName]

Is there such a thing as too low? At the titration they started at 5, everything looked good, so they left it; the doc said I could bump it up to six if I want, but not to bother if things are going well. My numbers look great, and I'm definitely getting vastly better sleep, but if a higher pressure would get me better air, I'm happy to try it.

[Julie]

It's too low if a) you can't breathe comfortably, b) your AHIs are high, and c) if you're getting a lot of centrals throughout the night (not just a few when falling asleep or on waking). And 'too high' for centrals is usually something over 15, so I wouldn't worry a whole lot about 5!

In fact though, if your OSA is really 'fixed' by such a low pressure, I wonder if just using a soft cervical collar to prop your head up a bit in front (keeping your airway just that much more open), and side sleeping (with no mask) wouldn't do the trick for you as I know it has for a few people. You'd need to use an overnight oximeter to test things for a few weeks (til the effects of Cpap have worked their down) and/or get a new sleep study after that time, but I'd try it if I were you.

[hueyville]

All my titrations have started at 8 but their may be some reason I dont know for that. Just know even my 1st started at 8 and all zubsequent have also. I always hate that part as cant seem to breath till the machine hits 12. I am a high pressure patient though.

[ThisIsMyUserName]

Interesting! I had always assumed that a low pressure was a good thing; easier to breather out, no need for APAP, etc. On your list of three potential issues: I definitely have the apnea under control; AHI is down at 1.0 or below (and I've checked data on sleepyhead regularly to ensure nothing weird is going on), and haven't seen central apneas. I can breather comfortably once I'm "into it" that is, when I first put on the mask, I sometimes find I need to do a minute or two of mouth breathing, and I feel like I can't get enough air (this still makes no sense to me!); once I'm sleeping, though, I don't wake up or anything. I am now a little worried that I'm not clearing enough CO2, though, so I'll try upping the pressure to 6 and see if it feels any different (my doc told me I might want to do this, anyway). Is there a way to know about whether the air is getting cleared without having to buy an oximeter? My leak numbers look pretty normal and in line with the mask documentation. Other ideas or tests?

[CarpeNoctum]

If you have any trouble breathing out at 6 or 7, kick in the exhalation relief feature. Obviously that's what it's for.
CN

[jaybeem]

Are you using the ramp feature? If so, turn it off.

[Sheriff Buford]

Is there a way to know about whether the air is getting cleared without having to buy an oximeter? My leak numbers look pretty normal and in line with the mask documentation. Other ideas or tests?

Go ahead and bump up to a 6 or 7 and see how you feel. Sometimes we chase numbers without noticing how we feel during the day. Give it a month. Your mask vent is to remove CO2. You can go on line and read the documentation, but most cpap machines masks will properly remove the CO2 with most masks. I would worry about obstructing the venting.... you can get in trouble by trying to block or reduce the vent rate. Some folks have tossed the idea around about trying to reduce the vent rates with tape because the vent was too noisy or it was blowin' on them. Now THAT can get you in trouble real fast!!

Let me know how it goes.

Sheriff

[ThisIsMyUserName]

I bumped up to six last night. No big change in comfort level, though I think I felt a little more "airy", so I'll leave it as is for a while and see how it goes. I do havea mask with a diffuser now (the Eson), so it's less obvious how much air is venting out. I am a little tired today, but I can definitely blame that on monday Night Football.

[Todzo]

So I use a very low pressure (5 cm), which works great from an apnea perspective: I went from an AHI in the moderate-sever range to less than 1 on average. This is great from the CPAP perspective: no need for APAP, or complicated whatevers. I do have two questions, though: sometimes it feels like I can't get enough air, which I've heard is common at low pressures, but makes no sense to me at all, since at any pressure, the pressure is still well above room pressure. Second, I noticed a disclaimer on my mask info sheet that basically said that low pressures may not be enough to clear the CO2 from my breathing out, which can occassionally lead to, you know, death. I'm not all that worried, but it did get me thinking about whether the pressure is enough to fully clear excess CO2 and, if not, whether that means I'd be breathing low-oxygen air all night. Thoughts or knowledge on this?

If you can look up the vent rate for your mask and note the pressure used to determine that rate - then calculate the vent rate at your current pressure. What is the result?

[Todzo]

At night the amount of air I use averages some 6.5 liters per minute. However, a portion of my exhale will often reach over thirty liters per minute. If my vent rate is say 20 liters per minute then for a very short time air is being pushed back down the pipe. But then as I continue to exhale at a lower rate the air goes out the vent before inhale begins.

CO2 when increased in the blood tends to open up circulation. While part of me looks at the potential for a very low vent rate to not clear as much CO2 in the process of breathing with the mask and that the body would respond by increasing respiration which would make for a higher amount of not clearing CO2 at the same time the increased circulation tends to calm things down when in comes to breathing control so it would seem that a reasonable equilibrium might be found.

With EERS[1] the last bit of air exhaled becomes the first bit of air inhaled and the amount is consistent breath to breath. This tends to prevent hypocapnia during times of high air use (over breathing). But I suppose if the actual vent rate is too low it is possible that you could end up breathing a whole lot more resulting in a very large portion of the air being rebreathed. If that were the case simply making a bigger vent port would easily solve the problem.

[1]: Gilmartin G, McGeehan B, Vigneault K, Daly RW, Manento M, Weiss JW, Thomas RJ.
Treatment of positive airway pressure treatment-associated respiratory instability with enhanced expiratory rebreathing space (EERS).
Source: J Clin Sleep Med. 2010 Dec 15;6(6):529-38. Division of Pulmonary, Critical Care and Sleep Medicine, Beth Israel Deaconess Medical Center, Boston, MA, USA.
Link: http://www.ncbi.nlm.nih.gov/pubmed/21206741

[Todzo]

Of course doctors do not modify anything. I think they do make a vent port section however and the pressure you are using is about half of mine which is not even high by any standard. So perhaps the doc can add a vent port section and get the rates up to where they will work?

[Todzo]

Ya know this suggest a potential vent system that could help many who intermittently over breath, which is suspect is all of us. Two vents. A small one near the entrance of the mask and a large one say some 150ml to 250ml or so down stream. So when the person is breathing quietly as one would expect at rest in bed there would be no rebreathed air. But if the persons breathing rose toward points where one could predict that hypocapnia would be a result the amount of rebreathed air would increase – to a point.

I think this could very much increase ventilatory stability while using CPAP.

I think that ventilatory instability is the greatest cause of people giving up on CPAP.

[CarpeNoctum]

Looks like the neighborhood has gone to hell...time to move out.
CN