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Going to High Altitude - Do I need to adjust cpap?
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OMyMyOHellYes Offline

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Machine: Respironics 560 Auto
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Location: Texas

Post: #11
RE: Going to High Altitude - Do I need to adjust cpap?
There should be no adjustment necessary. The pressures the CPAP maintains are relative to the outside ambient pressure, not an absolute pressure.

If the barometric pressure is 29.66 and your machine is set on 12 cm H2O, it will maintain a pressure differential from ambient air pressure of 12 cm. If the barometric pressure is 31.6, then your PAP appliance will maintain a differential of 12 CM H2O.

It is the difference between the outside air pressure and the pressure in your sinuses and throat that matters to keep the airway open, not the pressure setting of machine itself as it is relative, not absolute.

OMM
09-09-2013 05:55 AM
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RonWessels Offline

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Machine: REMstar Auto AFlex DS560TS
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Post: #12
RE: Going to High Altitude - Do I need to adjust cpap?
(09-09-2013 05:55 AM)OMyMyOHellYes Wrote:  There should be no adjustment necessary. The pressures the CPAP maintains are relative to the outside ambient pressure, not an absolute pressure.

That's not necessarily true at all. It very much depends on the pressure transducer technology being used. And simple, absolute-pressure reading transducers are a lot cheaper and easier to engineer into a solution. Differential pressure transducers need separate air lines for the pressure-under-test and reference (ambient) pressure.

In fact, were I to engineer this, I would have two absolute-pressure transducers: one for the delivered pressure and one for ambient measurement. Unfortunately, one of the design criterion would be the sensitivity range of each of the transducers, which may be why there are altitude limitations on the operating specifications of the CPAP units.
09-10-2013 08:00 AM
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me50 Offline

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Machine: resmed S9 VPAP Auto
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Post: #13
RE: Going to High Altitude - Do I need to adjust cpap?
hopefully I can figure out how to "crack" the secret code on my machine to do this if needed! I am a bit freaked out by all the high altitude stories I'm reading on the net and then add the sleep apnea issue to this!
Thanks again,

I hope that you were able to get this solved before you left on your trip. This is why I am so excited to get the Resmed Auto Set (except I will walk away if they try to shove the Escape at me)!
09-10-2013 08:47 AM
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OMyMyOHellYes Offline

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Posts: 913
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Machine: Respironics 560 Auto
Mask Type: Nasal mask
Mask Make & Model: Resmed Mirage FX
Humidifier: Respironics System One (no heated hose)
CPAP Pressure: 08.0-15.0 cm/H2O
CPAP Software: EncoreBasic

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Sex: Male
Location: Texas

Post: #14
RE: Going to High Altitude - Do I need to adjust cpap?
(09-10-2013 08:00 AM)RonWessels Wrote:  
(09-09-2013 05:55 AM)OMyMyOHellYes Wrote:  There should be no adjustment necessary. The pressures the CPAP maintains are relative to the outside ambient pressure, not an absolute pressure.

That's not necessarily true at all. It very much depends on the pressure transducer technology being used. And simple, absolute-pressure reading transducers are a lot cheaper and easier to engineer into a solution. Differential pressure transducers need separate air lines for the pressure-under-test and reference (ambient) pressure.

No, I'm going to disagree. A cpap machine (set at 10 cm of water) is actually giving you about 1,043 cm H2O of ABSOLUTE pressure referenced form a true vacuum state, assuming a barometric pressure of 29.92 in Hg - considering that the ambient pressure is 1,033 cm H20.

Say the barometric pressure swings to 31.07 (highest recorded pressure in here Dallas ~ 1962) then the CPAP machine will be pumping out 1,083 cm H20 ABSOLUTE pressure since the ambient pressure is now 1,073. In the scenario you describe using static absolute pressure setting, the machine would actually have to pull a vacuum of about 30 cm H20 to get to its "preset" of 1043 cm H20 and would suck your airways closed....

CPAP works maintaining a pressure differential against the surrounding air, not a sealed vessel in a transducer. Ambient pressure is pushing on one side of the sensor and the pressurized air is pushing on the other.

OMyMy
09-12-2013 06:37 AM
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RonWessels Offline

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Posts: 465
Joined: Jun 2013

Machine: REMstar Auto AFlex DS560TS
Mask Type: Nasal mask
Mask Make & Model: ComfortGel Blue
Humidifier: REMstar Heated Tube DS6T
CPAP Pressure: 11 - 20
CPAP Software: SleepyHead

Other Comments: CMS50F Pulse Oximeter

Sex: Male
Location: Ontario, Canada

Post: #15
RE: Going to High Altitude - Do I need to adjust cpap?
I think we are actually saying the same thing. Let me try again.

To generate a true pressure relative to ambient, you would need a relative pressure transducer with two air inputs: one for the pressure-under-test and one for the ambient pressure. This is expensive and complicated, so what CPAP machines almost certainly do is have two absolute-pressure transducers where one measures the delivery pressure and the other measures ambient pressure. The control algorithm can then subtract the two to determine an approximate value for the pressure relative to ambient.

I say this value is approximate because there could easily be differences in the bias and or scale values for the two pressure transducers. Additionally, you are now subject to the operational requirements of the ambient pressure transducer in terms of what sort of ambient pressures (ie. altitudes) you can reliably operate at.

Now, if the transducers go significantly non-linear at lower pressures (higher altitudes), you may need to artificially adjust the commanded treatment pressure to cancel out the errors and achieve the correct true treatment pressure. Hence the potential for "adjustment" for high altitudes.
09-12-2013 10:51 AM
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archangle Offline
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Machine: ResMed S9 AutoSet
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Post: #16
Going to High Altitude - Do I need to adjust cpap?
The Intellipap Auto automatically adjusts up to 9000 feet according to the manufacturer.

http://www.devilbisshealthcare.com/files/A-DV54%20Rev%20G-FINAL.pdf

Even if the machine blows the "right" pressure, changes in altitude may have effects on your pressure needs.

1) At 11,000 feet, pressure drops by 30% or so. (that's 300 millibar or 300 cmH2O). It's enough different that the sensors or fan action may not work quite the same. I suspect the flow sensor readings may change, too. The pressure won't change by 300 cmH2O (duhh), but it may change a bit. CPAP machines are usually designed to self adjust, but they're usually not specified to go that high.

They'll probably still work, but may go out of spec.

2) The pressure difference is enough that there are some "mechanical" changes in the physics of breathing. Less O2 per liter of air, and some other changes. This might mean you need a different "real" gauge pressure. (Gauge pressure is the pressure in the mask relative to surrounding atmospheric pressure.)

3) There are also physiological/neurological changes in breathing and sleep apnea at altitude. These effects are not that well understood. There are complex control mechanisms in your breathing system. Your CPAP pressure needs may change. In particular, some people develop central apnea at increased altitude. Other changes happen as well.

As I said, the effect of altitude on apnea/CPAP is not really that well understood.

Personally, I wouldn't be too worried, but there's a possibility of some problems.


Theoretical discussions:


Average barometer readings vary around 2.5 millibar (mb) or higher during a day. 1 millibar is approximately 1 cmH2O, so that's 2.5 cmH2O.

I just looked up the National Weather Service records for New York, http://w1.weather.gov/data/obhistory/KLGA.html, and the pressure has varied from 1006 to 1021 in the past few days.

If the CPAP machines pumped a fixed absolute pressure, the effective pressure on your lungs would vary a few cmH2O during the average day, and would have varied by 15 cmH2O in the past few days in New York.

CPAP machines measure or produce "relative pressure," which is what your lungs "see" in terms of how it affects your breathing. Without that, they'd be useless as the weather changes.

Most pressure sensors are inherently differential or "gauge" pressure sensors. There is some sort of element that flexes based on the differential pressure between two chambers. In order to make them read absolute pressure, you just seal off one side to a sealed air chamber. It's actually a little harder to make an absolute pressure sensor than a "gauge" sensor. It may be that the manufacturers sell more absolute gauges, but it's very easy to make a "gauge" sensor that measures relative to ambient pressure. You just don't seal off the "back" side of the sensor.

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09-12-2013 04:47 PM
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