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Operating Altitude Question
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Sleepster Offline
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Post: #11
RE: Operating Altitude Question
A CPAP manufacturer recommends you don't operate the machine at a pressure below 74 kPa, which is equivalent to saying you shouldn't ascend to an altitude over 8500 ft.

When you're in an airplane you go way higher than that so they have to pressurize the cabin. They pressurize the cabin so that it's equivalent to ascending to an altitude of 6000 ft to 8000 ft. An altitude of 8000 ft is equivalent to a pressure of 75 kPa, so you're within the range of pressures recommended by the manufacturer.

At sea level the pressure is about 101 kPa.

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INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
12-24-2012 11:39 AM
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archangle Offline
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Post: #12
RE: Operating Altitude Question
Apparently, there are some changes that happen in your respiratory "feedback" system at higher altitudes separate from CPAP. It can cause more central apnea, periodic breathing, etc., apart from any problems the CPAP machine has.

I'm getting foggy recollections at the moment about the details.

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If it's midnight and a DME tells you it's dark outside, go and check it yourself.
12-24-2012 12:38 PM
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Shastzi Offline

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Post: #13
RE: Operating Altitude Question
Here is some interesting stuff to read about oxygen saturation vs altitude.
If you read through some of the tables you'll see that a subject in excellent health
can only expect a saturation of about 90% when at 10,000 ft altitude.
I live at sea level (close enough) such that I try to stay at 90% or higher using the CPAP.

So if you are at altitude and starting out at only 90% then it can be very easy to drop to much lower levels.
According to a study I read a few weeks ago (about diving medicine) when you drop to 64% you are at
the critical level imminent to loss of consciousness.

This corellates with the charts in the URL below.
ie: when you hit 30,000 feet and 57% O2 saturation you will pass out.

There are a few other interesting gotchas in there too. Check it out. Wink

http://www.dr-amy.com/rich/oxygen/
12-24-2012 09:23 PM
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genes Offline

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Post: #14
RE: Operating Altitude Question
(12-23-2012 10:02 PM)Shastzi Wrote:  I am just guessing at this point though.
Are there any engineers in the house?

Yes!

They are all in the house now busy waiting for Santa and expecting a lot of presents because they have been so cool, personable, and haven't been naughty? this year.

oops!

He just came and all I got was a lump of coal again. I do not know what is going wrong! Oh well I guess I might as well go to bed.

GeneS
12-25-2012 01:35 AM
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vsheline Offline

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Post: #15
RE: Operating Altitude Question
(12-23-2012 09:46 PM)TheDuke Wrote:  My machine also gives an 8500 ft. statement. But I have had occasions to use my machine for a total of 7 or 8 weeks at altitudes of as much as 10,500 ft. and everything seemed to be OK


(12-23-2012 10:02 PM)Shastzi Wrote:  That would be a good place for a pulse oximeter with SO2 Assistant software and data collected for Sleepyhead. You might be running into desaturation events though your BiPAP is running its heart out but not quite able to produce enough pressure at those altitudes. Some O2 to bleed into the system might be a good idea too.
Not medical oxygen though, aviation oxygen (it has a lower dewpoint and wont freeze in the regulator)
I am just guessing at this point though.
Are there any engineers in the house?


(12-24-2012 09:23 PM)Shastzi Wrote:  Here is some interesting stuff to read about oxygen saturation vs altitude.
If you read through some of the tables you'll see that a subject in excellent health 
can only expect a saturation of about 90% when at 10,000 ft altitude.  
I live at sea level (close enough)  such that I try to stay at 90% or higher using the CPAP.

So if you are at altitude and starting out at only 90% then it can be very easy to drop to much lower levels. 
According to a study I read a few weeks ago (about diving medicine) when you drop to 64% you are at 
the critical level imminent to loss of consciousness.

This corellates with the charts in the URL below.   
ie: when you hit 30,000 feet and 57% O2 saturation you will pass out. 

There are a few other interesting gotchas in there too.    Check it out.   Wink

http://www.dr-amy.com/rich/oxygen/


Hi Shastzi,

I don't understand how the medical experts have determined that our CPAP treatment pressure needs to be increased as the altitude increases. Seems counter-intuitive to me as an engineer, because I would have thought the CPAP pressure could be reduced at highter altitudes, by the same amount as the surrounding atmospheric pressure is reduced as the altitude increases.

I would have thought that, in order to "prop open" our airways the differential between CPAP treatment pressure and the surrounding atmospheric pressure needs only to be kept the same. But, instead, experts have determined that we actually need the differential increased, not kept the same.

Although I suppose it is possible the medical professionals are all wrong on this (not being engineers, and this being seemingly an engineering question), still, I think it would be unreasonable for me to assume the experts are all wrong. I think I must be missing something in my understanding. (It happens all the time, actually...) So my guess is medical professionals have run tests on apnea patients at lower atmospheric pressures and have empirically found that the pressure differencial needs to be increased for some reason, not kept the same. (But, of course, I could be wrong in thinking I'm probably not right...)

Anyway, in general I withold myself from offering opinions on this counter-intuitive subject.

But as to your question about the oxygen - sorry, as an engineer I haven't a clue... (Hmmm, by golly, maybe that's it! The medical folks are raising the CPAP pressures in order to counteract the decreasing atmospheric pressure which would have decreased the Oxygen Saturation. Nah, too simple an explanation.)

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.
(This post was last modified: 12-25-2012 05:03 AM by vsheline.)
12-25-2012 04:04 AM
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Shastzi Offline

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Post: #16
RE: Operating Altitude Question
I think you are right vsheline. pressure differential to keep your airway inflated.
The only thing I can relate to there is from aviation. Some engines have an aneroid (mechanical barometer) connected to a waste gate controller. The controller regulates the turbocharger output so that intake manafold pressure is maintained up to 25,000 feet.
So it would be the same thing with people. Keeping the human intake manifold pressurized to sealevel plus CPAP set pressure.
Wink
How many machines are "altitude compensated" though? o.O
12-25-2012 07:11 AM
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Sleepster Offline
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Post: #17
RE: Operating Altitude Question
(12-25-2012 04:04 AM)vsheline Wrote:  I don't understand how the medical experts have determined that our CPAP treatment pressure needs to be increased as the altitude increases. Seems counter-intuitive to me as an engineer, because I would have thought the CPAP pressure could be reduced at highter altitudes, by the same amount as the surrounding atmospheric pressure is reduced as the altitude increases.

The pressure measured by CPAP machines, and prescribed by physicians, is the difference in pressure between the inside of your lungs and the outside of your body. This difference in pressure is called the gauge pressure.

As you increase your altitude atmospheric pressure decreases. The CPAP pressure decreases by the same amount if you keep the gauge pressure the same.

Quote:I would have thought that, in order to "prop open" our airways the differential between CPAP treatment pressure and the surrounding atmospheric pressure needs only to be kept the same.

You are correct. That is what CPAP machines do.

Quote:But, instead, experts have determined that we actually need the differential increased, not kept the same.

Probably by only a small amount to compensate for the reduced oxygen at lower pressures.

Sleepster
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INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
12-25-2012 11:23 AM
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Shastzi Offline

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Post: #18
RE: Operating Altitude Question
Hmm. If we check the charts, we see that the *percentage* of oxygen (18-20%) stays the same all the way up to 60,000 feet. (this is so high you need a space suit and almost no civilian aircraft venture there)
In an airplane for instance,
even though the plane is cruising at 37,000 feet. the cabin is at a more manageable 7,000-8,000 feet.
By maintaining some air pressure inside plane, the oxygen is able to keep up a good transfer/exchange rate across the blood/air interface in the lungs.
Cabin pressurization works in commercial aircraft for this reason.
See again: http://www.dr-amy.com/rich/oxygen/

So the big IF here is how well can the CPAP "keep the pressure maintained"?

What does this mean if you are parked at 10,000 ft on a mountain?
The drop in pressure from sealevel to 10k ft is 237mm of mercury (Hg) (SL=760mm, 10k=523mm)
or 3.222 meters of water. (322 cm of water or 4.5 psi) Blink

I would think the CPAP might be running full blast in some cases just to hold on for dear life, which may be why the manufacturor reccomends 8,500ft or less.
Using the CPAP would, of course be better than NOT using it.

Until someone takes one of these CPAP machines up Pike's Peak and logs a bunch of data
we might not know for sure. Better to err on the side of safety.

Just between you, me and the fence post I would like to know how well some of the CPAP's compensate for places like Mexico City and Denver Colorado...
Thinking-about
12-25-2012 11:57 AM
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vsheline Offline

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Post: #19
RE: Operating Altitude Question
(12-25-2012 11:23 AM)Sleepster Wrote:  
(12-25-2012 04:04 AM)vsheline Wrote:  I would have thought that, in order to "prop open" our airways the differential between CPAP treatment pressure and the surrounding atmospheric pressure needs only to be kept the same.

You are correct. That is what CPAP machines do.


vsheline Wrote:But, instead, experts have determined that we actually need the differential increased, not kept the same.

Probably by only a small amount to compensate for the reduced oxygen at lower pressures.

Hi Sleepster,

I think you are right that it is only by a small amount. (Based on measurements I took today, I think the increase in CPAP pressure is not much more than 1% of the total change in atmospheric pressure.)


(12-25-2012 11:57 AM)Shastzi Wrote:  Hmm. If we check the charts, we see that the *percentage* of oxygen (18-20%) stays the same all the way up to 60,000 feet. (this is so high you need a space suit and almost no civilian aircraft venture there)
...

So the big IF here is how well can the CPAP "keep the pressure maintained"?

What does this mean if you are parked at 10,000 ft on a mountain?
The drop in pressure from sealevel to 10k ft is 237mm of mercury (Hg) (SL=760mm, 10k=523mm)
or 3.222 meters of water. (322 cm of water or 4.5 psi) Blink
...

Until someone takes one of these CPAP machines up Pike's Peak and logs a bunch of data we might not know for sure.

Hi Shastzi,

We might be in luck. A visit to Pikes Peak might not be neccessary. My old Fisher-Paykel SleepStyle 604 supports manual entry of the elevation. I suppose the new models with automatic elevation compensation will adjust their pressures by about the same amount that the SleepStyle 604 does when it is manually set for differennt elevations.

So, my measurements today on the model 604 (using economical manometer SP Medical LLC model LB1075-E, bought from Supplier #2 on our Suppliers List) lead me to believe that the CPAP pressure is increased by approximately 0.6 cmH2O for each 1500 ft change in elevation.

This would be a change in CPAP pressure of approximately 4 cmH2O for a 10,000 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.

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.
(This post was last modified: 12-25-2012 04:30 PM by vsheline.)
12-25-2012 03:54 PM
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Shastzi Offline

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Post: #20
RE: Operating Altitude Question
Oh good! I am not in any shape to climb Pikes Peak anytime soon.
Big Grin
12-25-2012 04:36 PM
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