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Do CPAP-related improvements persist? If so, how long?
Do CPAP-related improvements persist? If so, how long?
Hi all

I'm aware that I already have at least one other thread full of my moaning; I thought this was a sufficiently separate topic to need a new thread, but please let me know if that isn't way things are usually done around here. If you don't want to read more of my ramblings, by all means ignore the "background" bit, and go straight to the "TL;DR" bit.


I've been doing CPAP for only three weeks, and I'm still struggling to understand whether I actually have any kind of OSA or not. My AHI figures with APAP (7cm-16cm) are generally < 1, where my original (home) sleep test reported AHI ~20. On the face of it, this looks like a significant improvement but, because I'm essentially asymptomatic, it's hard to tell. After all, the ResMed CPAP machine and the home sleep test estimate AHI in completely different ways.

Anyway, to try to get more information, last night I set the CPAP machine to a constant 4cm, the lowest it will go. I assumed that this would be sub-therapeutic, and my figures would be worse.

In fact, that really wasn't the case. My AHI did increase a little -- to about 1.3 -- but my average SpO2 actually increased (albeit only by one percentage point). My total apnea time was (and always is) less than two minutes, whatever the pressure. In the sleep test, my average SpO2 was 90%, now it's ~96% with my "usual" pressure, and ~97% with the pressure turned right down (but in all cases I had periods with SpO2 was < 90%).

I'm struggling to understand all this. I had assumed that if I had obstructive sleep apnea -- which is what the sleep test said -- then using a minimal pressure would cause my AHI to increase immediately.


So my questions are:

1. If you have obstructive sleep apnea, and you stop treatment, does the AHI figure revert to its pre-treatment level immediately, or is there some lasting benefit?

2. Could it be that a pressure of 4cm is actually therapeutic? That isn't what I've read, at least for adults. But if your OSA is mild, could 4cm be enough to give AHI < 2?

3. Could it be that a home sleep study, which does not measure actual airflow, could say that I had OSA when the ResMed machine reports that I do not? Which, if either, should I believe?

Thanks in advance.

Best wishes, DS
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RE: Do CPAP-related improvements persist? If so, how long?

  1. No, it doesn't resort back immediately.  If you are getting a new sleep study, they usually ask you to refrain from using your CPAP for about a week prior to the sleep test.

  2. It's possible that 4 cm could offer some help, since this is above the ambient pressure.  At this low pressure, most adults will experience a feeling of suffocation.  Your AHI is not completely tied to your pressure.  Sleeping position, apnea type and severity also play a role.  One night's data does not make a trend.  You need about 7 days data to see any meaningful result.

  3. I don't have an answer about a home sleep study accuracy.  Studies are a one to two day snapshot of a patient's sleeping profile.  I would look at them more as a probable indicator and not the gospel.   Evidently, they must produce some data that is believable, or insurance companies wouldn't allow them for a diagnosis.  However, you ultimately base your therapy on the CPAP's data result.
Crimson Nape
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RE: Do CPAP-related improvements persist? If so, how long?
Depending on the method of sleep study, depends on how accurate it is, and as was said, it's only one night. With my centrals, for example, on plain CPAP, same steady fixed pressure, my AHI would vary from just above 10 to almost 60 events per hour, averaging about 30-35. With the extra oxygen, it's varying from 7 to 35, averaging about 15-20, and the amount of time I spend below 90% varies from 20 minutes to nearly 4 hours (out of 10). And while I definitely feel worse without CPAP straight away, it takes a while to see a clear difference with the change in pressure. 4cm is usually a problem because adults often feel like they're getting too little air - not necessarily because 4cm isn't helpful. Even at 4cm, that's still likely to have some effect.

The only real way to know is to, as suggested, stop therapy for a week and try another sleep study, but if you are seeing such big rises in your oxygen, then even if the only benefit is the increased ventilation at night causing you to blow off more carbon dioxide, so your body has to compensate less during the day for whatever causes your long-term low oxygen levels. My first sleep doctor this time last year told me to absolutely NOT stop using CPAP, even if I wasn't feeling any better physically, because even a little help to reduce the chance of cardiac issues was important. Considering that those have been your only symptom, primarily at night, staying on it long-term is not going to hurt and may well counter the respiratory imbalance.
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RE: Do CPAP-related improvements persist? If so, how long?
@Ratchick, @Crimson Nape: thank you for your detailed responses. I guess I can't conclude much, if anything, from changing pressure for one night.

I'm curious about this however, which you both said:

"It's possible that 4 cm could offer some help, since this is above the ambient pressure.  At this low pressure, most adults will experience a feeling of suffocation."

Why is that? As you say, it's still above atmospheric pressure. For me, 4cm is like not having the mask on.

Best wishes, DS
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RE: Do CPAP-related improvements persist? If so, how long?
I don't honestly know. I guess it's a sense of suffocation with having the mask on and the relatively low flow of air, and the sensation of being unable to breathe as deeply and/or freely. Either that, or it could be down to carbon dioxide build-up potentially, I guess. Not everyone feels that sense of air hunger, but plenty enough do, and for adults, it's usually not therapeutic enough anyway, so the normal recommendation is to go higher for comfort. But everyone's different.

I did see this article: https://www.ems1.com/ems-products/ems-eq...RoZVZGbr3/ which may also play a part, even though what it's describing is a different scenario. Basically, it's that sometimes a person's requirements aren't being met and this causes respiratory distress. I would imagine that's also a potential issue with CPAP too.
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