CPAP versus altitude, a warning

I am reverse engineering how the CPAP works. It is a problem in fluid flow dynamics. I am a chemical engineer.

It occurred to me to question whether the CPAP machines would work at different altitudes. Since the great majority of Americans live within 1,000 ft. of sea level for most people this isn't an issue.

I came across this study by the U.S. government:

http://www.ncbi.nlm.nih.gov/pubmed/7497764 I have the text of the page at the end of this posting.

It turned out that 3 out of 4 commercially available machines didn't deliver the required pressure at different altitudes. Remember the pressure you get with your CPAP is relative pressure to the atmosphere. So when they take about the pressure not being correct they mean relative pressure.

If you are living at a higher altitude it may be that you aren't getting your pressure. Unfortunately they don't mention brand names. Also, since air pressure drops exponentially with altitude, differential change dP/dA is highest at lower pressures. That is the error per altitude is highest might be highest at lower altitudes. So if you were titrated at 1,000 feet you might be having a lot of problems at 6,000 feet. But also if you were titrated at 500 feet altitude and are living at 1,500 feet altitude there might be significant error. With P= f(A), and f = 10 exp(-kA) where k is a constant the slope is highest at lower elevation.

If you feel your CPAP isn't working you might consider the elevation of the titration and the elevation of where you live and whether your model compensates for elevation. I live in a mountainous region at an elevation, and it is significantly cooler than the valley. (Temperature drops are adiabatic with pressure, so I have a significant drop in air pressure). Maybe my new machine works so much better because its control mechanisms compensate for altitude. My doctor of course didn't have any thoughts about altitude and the CPAP even though the name of where I live essentially says I live at an elevation.

THE STUDY

Abstract

STUDY RATIONALE AND OBJECTIVE: Sleep-disordered breathing is commonly treated with nasally applied continuous positive airway pressure (CPAP). Typically, pressures are titrated to pneumatically splint the airway to prevent its collapse in response to negative inspiratory pressure. This investigation was prompted by several patient complaints of sleep-related breathing difficulty associated with travel to high altitudes. CPAP devices create pressure with fan-generated airflow; therefore, CPAP performance should behave according to collective fan laws.

MEASUREMENTS AND RESULTS:
In the present study, we examined the effect of simulated altitude change on four commercially available CPAP machines. Machines were tested using anatomic airway mannequins in an altitude chamber. We made three simulated ascents to 12,000 feet with machines set at 5, 10, and 12 cm H2O sea level pressure equivalents. We measured pressure using water manometers at 2,000-foot increments during ascent and descent. Mask pressures varied systematically with changing altitude in three machines. One machine, equipped with a pressure regulation feature, maintained pressure within 1 mm H2O at all pressure and altitude combinations.

CONCLUSIONS:
Altitude significantly alters delivered pressure according to predictions made by the fan laws, unless a unit has pressure-compensating features. Clinicians should consider this factor when CPAP is prescribed for patients who live or travel to places located at significantly higher or lower elevations than the titration site.

Hello Sleepy1235
Well, I have to agree it was an interesting study that was performed back in 1995. Advances in xPAP machines have been great in the years since this study was performed. So, if you have an older machine, or a newer one where the specifications don't say that "Altitude Compensation: Automatic - Maximum height 7500 feet." or something along these lines, and you travel to various heights then this would be a concern for sure.

I even look at the altitude compensation mode and think how accurate is it... I have seen no specifications that say the accuracy - but I tend to believe that a few mm H2O pressure will not cause severe issues.
Paul

If it is an auto machine --- then it is monitoring 'response' to pressure and is less concerned with measured (inaccurate in your analysis) pressure. I recognize that you used the term CPAP and not ACPAP. IMHO

After putting a manometer on various lenghts of tubing, I'm thinking there could be quite a lot to actually maintaining therapeutic pressure.

Though new devices and autopaps are designed to automatically compensate, it might not be a bad idea for a person who travels to, or lives at, higher/lower altitudes to purchase a manometer. Here's a link to our sponsors product made by Tiara.

https://www.cpap.com/productpage/guage- ... ssure.html

By having one of these, you could always check pressure empirically.

'Tis an interesting thing to keep in mind---particularly for the PAPers stuck with bricks from PR. Consider this comparison chart from our host:



Users of PR System One bricks need to manually adjust their machines for changes in altitude.

How many users of PR bricks do you think are aware of this issue?

How many DMEs who sell the bricks tell their customers/victims of this issue?

How many DMEs are unaware of this issue?

Seems that the bricks in the S9 line and the Intellipap line do have Auto Altitude adjustments from what I could determine in a quick search of the machines that CPAP.com is still selling.

But for anybody using machines that are one generation back and who do travel to areas at significantly different altitude, this is something that folks need to be aware of.

I notice my Devilbiss also has automatic altitude adjustment--
--a good thing as that's the one I use when traveling from 1300 feet to 5200 feet.
(I'm tempted to bring the Hybernite next trip)

I actually discussed altitude effects with my sleep doctor, and he mentioned that any of the machines I'd be using automatically compensate. That said, during the approximately 4 months that I was on a trial with a system one 660 bipap, being an avid RV camper I did travel quite often with variations in altitude from a low of approximately 3,000 feet to a little over 10,000 feet on a couple of occasions, with the 10,000 foot elevation being higher than the specs call out for the respironics machine. During the higher elevation trips I didn't notice any difference in AHI readings for OA's, but my centrals tended to run about double. Seems I'm somewhat sensitive to pressure, which is why I can't get good results with a Cpap, or even a bipap for that matter. That said, it's generally the higher pressures that kick up my OA events, so one might think maybe the machine over compensated. Then again, it's hard to guess anything based on just a few nights.

I haven't traveled with my ASV machine yet, but have a couple trips planned over the next couple weeks. I'll be staying at lower elevations though, not more than 7500 feet or so. The ASV machine varies pressure so much over the course of the night I don't imagine the altitude change will make any difference.

I use the Resmed S9 autoset, and I specifically checked the altitude issue before I got a machine as I live in Seattle, but travel to Colorado, Utah, etc and do have some big altitude changes. I had no problems at all.