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Altitude and AHI
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aehjr Offline

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Post: #1
Altitude and AHI
I've noticed an interesting pattern to my AHI numbers. I live at 7800' elevation and occasionally travel to sea level for work. Each time I am at sea level, my AHI numbers get low (<1.0) and stay there. Last week, I had two nights with an AHI of 0.0. Each time I return home, my AHI goes up to the 5-6 range the first night, drops to about 3 the next night and then slowly drops back down to its normal range (0.8-2.0) and stays there.

Anyone else see this pattern of AHI with altitude changes?
05-01-2013 03:05 PM
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Shastzi Offline

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Post: #2
RE: Altitude and AHI
Are you checking the altitude compensation on your S9?

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05-01-2013 04:14 PM
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aehjr Offline

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Post: #3
RE: Altitude and AHI
(05-01-2013 04:14 PM)Shastzi Wrote:  Are you checking the altitude compensation on your S9?

I didn't think there was an altitude compensation. I thought it was automatic on the S9 AutoSet.

Thanks, I'll check that.
05-01-2013 07:31 PM
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Shastzi Offline

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Machine: ResMed S9 Autoset
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Post: #4
RE: Altitude and AHI
ah, ok. It is supposed to be auto compensated up to 8,500 ft. Above that and you need to dig into some engineering texts.

Hope that helps. Smile

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05-01-2013 08:06 PM
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PaulaO2 Offline
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Post: #5
RE: Altitude and AHI
The machine may be autocompensated but your body is not. That's a big change for any person, sleep apnea or not.

When you return and your AHI rises, do you also hit the upper limits of your settings? Bumping the maximum up for just a few nights may not be worth the trouble, though.

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05-01-2013 08:34 PM
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aehjr Offline

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Machine: ResMed S9 AutoSet
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Post: #6
RE: Altitude and AHI
My pressure graphs look about the same as most other nights, whether at sea level or at home. Average pressure is around 8 cm with two or three excursions up near the max of 12.
05-01-2013 09:26 PM
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aehjr Offline

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Post: #7
RE: Altitude and AHI
(05-01-2013 08:06 PM)Shastzi Wrote:  ah, ok. It is supposed to be auto compensated up to 8,500 ft. Above that and you need to dig into some engineering texts.

Hope that helps. Smile

I'm at 7800 ft so the machine should be able to compensate appropriately. That being said, I have not been able to find any info as to what is the proper compensation for altitudes above 8500 ft in spite of hours spent trying to find info on the internet. I routinely sleep at 9600 ft during ski season; hence the reason for my attempts to find info.
05-01-2013 09:29 PM
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JJJ Offline

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Post: #8
RE: Altitude and AHI
(05-01-2013 03:05 PM)aehjr Wrote:  I've noticed an interesting pattern to my AHI numbers. I live at 7800' elevation and occasionally travel to sea level for work. Each time I am at sea level, my AHI numbers get low (<1.0) and stay there. Last week, I had two nights with an AHI of 0.0. Each time I return home, my AHI goes up to the 5-6 range the first night, drops to about 3 the next night and then slowly drops back down to its normal range (0.8-2.0) and stays there.
Anyone else see this pattern of AHI with altitude changes?

I live close to sea level and rarely go to high altitudes, so I have no experience with your problem.

However, the physics is pretty straightforward. At sea level atmospheric pressure is about 14.7 pounds per square inch. The higher you go the lower the atmospheric pressure. Now, the purpose of the CPAP machine is to increase air pressure at your face. If it is starting at a lower pressure than it was designed for it will fail to increase the pressure to the amount you have set it for. It seems to me that the solution is simple: Just increase the settings on the machine when you are at high altitude. Exactly how much to increase the settings should be a simple calculation once you figure out the atmospheric pressure at your altitude.

Having said all that there is one thing I do not understand. According to what I read the S9 Autoset can adjust for altitude. The only way it could do so is if it had an air pressure sensor so it would know what altitude it was operating at. Does it? I'm curious how it knows how much to compensate. Once you know the methodology you can fake it by increasing the settings.
05-01-2013 09:56 PM
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vsheline Offline

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Post: #9
RE: Altitude and AHI
Hi aehjr,

I suppose you have verified that the higher AHI when you return to high altitude is not caused by the max pressure setting ever being too low, but maybe it would help to raise the lower pressure, even if for just a few nights after returning home.

Some forum members report better sleep quality (not merely lower AHI) after they increased the bottom number of their pressure range.

I think the increase in CPAP pressure is approximately 0.6 cmH2O for each 1500 ft increase in elevation above sea level, which is not much more than 1% of the atmospheric change in absolute pressure between the two elevations.

When you go to higher elevations perhaps you can try manually adjusting your pressure range (both top and bottom numbers) by 0.6 cmH2O per 1500 ft.

(05-01-2013 09:56 PM)JJJ Wrote:  However, the physics is pretty straightforward. At sea level atmospheric pressure is about 14.7 pounds per square inch. The higher you go the lower the atmospheric pressure. Now, the purpose of the CPAP machine is to increase air pressure at your face. If it is starting at a lower pressure than it was designed for it will fail to increase the pressure to the amount you have set it for. It seems to me that the solution is simple: Just increase the settings on the machine when you are at high altitude. Exactly how much to increase the settings should be a simple calculation once you figure out the atmospheric pressure at your altitude.

Hi JJJ,

The physics involved were explored in an earlier thread:
http://www.apneaboard.com/forums/Thread-...t=altitude

My guess is we need slightly higher pressure at higher altitudes because the airflow fluid dynamics change slightly, because of the change in air density as the altitude increases.

Take care,
--- Vaughn

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. The Advisory Member group provides advice and suggestions to Apnea Board administrators and staff on matters concerning Apnea Board operation and administrative policies - not on matters concerning treatment for Sleep Apnea. I think it is now too late to change the name of the group but I think Voting Member group would perhaps have been a more descriptive name for the group.
05-02-2013 12:52 AM
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craigrrr Offline

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Post: #10
RE: Altitude and AHI
I used to live in the high desert of Arizona 7200' elevation, I had to move to the low desert it was just getting to hard to breath up there. now I am at 1200' and it is great I also have Afib and it was a lot worse at higher elevations. might be time to move to a lower elevation.
05-02-2013 10:06 AM
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