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Pressure Question
I have been doing some reading and have not found an answer to my question.
I see most of you that are on an auto machine set a range. Let's say 9-13. I can see and have experienced the reasoning behind having a bottom number. Until the machine adjusts up to the area of your top number it makes breathing hard and is not fun. Why limit the top number? In other words, why not have a range of 9-20? If your number is 12 and you set the top to 13 in theory the machine will always be in the range you need, but why not have the top at 20? The way I understand it, the machine will not go higher and run 19 if all you need is 12. What is the advantage to limiting the range? What happens if my number is 12 and the range is 9-13 but a position or some other reason causes me to need 16 briefly and the machine doesn't go there because of the limit?
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This may not be the best reason but it is one reason that caused me to tighten my limits.
I have experienced leaks that temporarily disrupt the machine's best operation and cause it to increase pressure above what I am used to, or need, which wakes me up. Where as if I decrease the upper limit to just a little above where it wants to operate, these short leaks just occur without incident.
if you can't decide then you don't have enough data.
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I try to keep a tight range because my APAP responds to Flow Limations, RERAs and snores by raising the pressure. Sometimes it raises it way too much and according to sleepyhead, there are no apneas occuring when it decides to raise pressure. This increase in pressure, if too high disturbes my sleep, so I have to "tame the beast."

MobileBassit is correct. If you have a lot of leaks, and if too many, the machine sometimes raises pressure which can wake you up. One reason why you have to control leaks.
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There are several reasons one might want to limit upper number the most important being that some people begin to experience more Central Apneas due to higher pressures. Other reasons might be increased problems with aerophagia or stomach bloating from higher pressure forcing air into stomach; increased leaks around mask.

In my case the Resmed is overly sensitive to Flow Limitations for my particular circumstances and wants to keep increasing pressure due to Its sensing these FLs. It will want to go way up past my prescribed pressure and if I let it go to these higher pressures I begin to experience all the problems listed above, but most notably the increase in Central Apneas/Clear Airway Apneas.

The first versions of Auto machines could actually experience a form of run away pressure increases so it was more important in the earliest versions to set an upper limit, but the newer generations of Auto machines don't suffer from those issues at this point.

If you are an individual that does not have an issue leaving the top wide open, then no problem, go ahead and leave the top at 20 cm, but for some folks it does create problems. It's funny that both my Resmed machines (S9 and Airsense Autos) want to keep increasing pressure, but my other Auto machines (F&P Auto Icon, Z1 Auto, and Transcend Auto) don't react the same way, they all top out either right at my prescribed pressure or just slightly above, so my conclusion is this happens due to the fact that each manufacturer uses their own proprietary Auto Algorithm - and from studying Sleepyhead graphs it looks like the Resmed Algorithm is much more sensitive to Flow Limitations in terms of increasing pressure.
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the first time I adjusted my top limit down from 20 is the morning after the bad night where it climbed to 15. Boom, I will not subject myself to that anymore. Luckily, my pressures don't slam against my current top, but if they did, I would have to choose between lowest AHI and best fit comfort and sleep quality. I think of this as obtaining the lowest practical AHI.

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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