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1) AHI by altitude? 2) Elevated AHI but decent SpO2
A couple of questions. I have been using my ResMed S9 VPAP in S setting with an AirFit P10 nasal pillow for about a month, with no machine prior to that. My AHI has consistently been under 5.0. But SleepyHead and the machine are also telling me I'm experiencing large leaks, often well over 24%. This also occurs with a Swift fx and a ZestQ. So I have an appointment with a respiratory tech to check it out. The SleepyHead leak graph shapes suggest I may be unknowingly mouth breathing and that a chin strap may be a fix. That's the prelude - I see the tech next week. But here are two new problems which have just emerged.

This past weekend our family went up to Wintergreen Resort up along the Blue Ridge. The altitude change wasn't enormous - we were sleeping in a home somewhere between 3500 and 4000 feet ASL. But Friday night and Saturday night both yielded AHI's of about 32! This after routinely seeing AHI's under 5. As with other readings, the large leak number was significant. I've read that elevation can trigger central apneas - but at 3500 feet??

Question 2. I'm back home now - around 500 feet ASL. I used the machine last night, but my AHI had not returned to NL - I ended up with an AHI of 17, in the moderate range. This is on a 5.5 hour recording, with the usual leaks issue. But here's what was strange. I also used my CMS 50H recording pulse oximeter in parallel with the Bilevel. The CMS has been very reliable and accurate. The SpO2 reading was as good as I've seen for me during sleep. Basal SpO2 was 95.2, Average low was 92.2 and minimum SpO2 was 91 - and it didn't drop to that level often. The adjusted SpO2 index per hour was 4.8. The graph, with a few dips, is relatively flat! Before starting the BiPAP a month ago, the pulse oximeter showed excursions below 88%, including some drops to 84% - and a lot of variation in pulse and SpO2. So if it weren't for the AHI of17, I'd be really delighted with the pulse ox data. What's going on here? Thanks for any help and input.
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I recently got a copy of the clinician's manual for my CPAP machine. It mentions an altitude setting. I have a Phillips Respironics machine. The altitude setting is supposed to be set to 1 for elevations under 2500 ft., 2 for elevations 2500-5000 ft., and 3 for elevations of 5001-7500 ft.

I don't know if your ResMed machine has a similar setting, or how much the altitude would affect the pressure supplied by the machine.

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What was the make-up of your AHI at higher elevation? What was the make-up when you got 17 back closer to sea level? What I am asking is how much of your AHI was OAs, CAs, and Hypopneas?

I go from my normal home at less than 100' above sea level to a residence that is almost 6,000 feet above sea level. I make no adjustments. Sometimes my AHI goes up, and sometimes down, and sometimes about the same. I personally feel that for me, any changes are due to external factors other than elevation. That is probably just me.

I think that I am using the same machine as you.

Best Regards,


Admin Note:
PaytonA passed away in September 2017
Click HERE to read his Memorial Thread

~ Rest in Peace ~
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Hi glaxelson,
WELCOME! to the forum.!
Hang in there for more suggestions and answers to your questions.
Much success to you with your CPAP therapy.
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Thanks for the responses! Green - the resmed manual says the ResMed S9 adjusts automatically for altitude. Looking at the numbers, I'm not convinced, but I'll have the RT check the machine next week when I go in for a leak consult and other questions.

Payton - does look like we are using the same machine at a bilevel and S setting. Your questions are excellent. So here are some numbers.

Night before we went to mountains, at home.
6.5 hours AHI 0.77 LL 48% CA 0.15 UA 0.15 OA 0.00 Hyp 0.46

Night one at about 3700-4000 feet. Not all that high!
6.5 hours AHI 33.58 LL 23% CA 12.82 UA 2.9 OA 13.6 Hyp 4.27

Night two in the mountains - same altitude
7.0 hours AHI 32.84 LL 33% CA 12 UA 14 OA 2 Hyp 5

First night home
5.5 hours AHI 17 LL 62% CA 2.2 UA 11 OA 0 Hyp 4 : Note. Also recorded pulse ox in parallel with 95% average SpO2, 91% min, Av pulse 53

Second night home
4.25 hours AHI 0.71 LL 50% CA 0 UA 0 OA 0 Hyp 0.71

For comparison - past month average - all at home except weekend in mountains
AHI 6.88 (mean skewed a bit by two mountain readings - almost always below 5.0
OA 0.79
Hyp 2.10
CA 1.76
Av Large Leak 25% (but also skewed down by weekend)
90% leak rate 60%
Average time about S9 leak threshold 54%

So it seems it's hard to trust data until the leak problem is solved - probably chin strap or, ugh, full face. But the jump from under 5 to 33 is puzzling and concerning. It may be altitude induced central events. Or, if the machine didn't compensate and pressure changed, it could be BiPAP induced central events. Then the first night home is so odd. On the way down again - but at 17. Yet my pulse ox shows nothing below 91% SpO2 and a mean of 95. AHI says there's a problem. Pulse ox says I'm fine. BTW, low pulse rate is secondary to atenolol.

Trish - Thanks for the welcome!
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I agree about the leak data and the amount of trust that it engenders in the rest of the data. It does seem to me that life changes - even small ones - can bring on an temporary increase in CAs. I have never had anywhere as large an increase as it seems like you *might* be having but my CAs have gone up with some changes that have happened in my life recently.

If it were my data I would concentrate on trying to improve the leak rate situation and not worry about any altitude change difference for now. Believe me, I have been where you are with the leaks and occasionally I manage to back into that large leak area but not near as bad as it was before I went with my Mirage Quattro and Simplus FFMs.

Best Regards,


Admin Note:
PaytonA passed away in September 2017
Click HERE to read his Memorial Thread

~ Rest in Peace ~
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