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[Treatment] Is AHI misleading?
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PaulaO2 Offline
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Post: #31
RE: Is AHI misleading?
There is no way your machine is 500% off. It could be the sleep center's machine was off or that it was extra sensitive or that during the time you had your device on vs their device, there was a difference. How were the two tests done? They used their equipment (leads, chest strap, oximeter, etc) and you used your machine?

Home machines cannot take everything into consideration. It doesn't know when you are in REM sleep. It doesn't know if you are even asleep. All it knows is the feedback it gets from the pulses it sends to determine if the airway is open or closed. Some detect when you are snoring but even that could be tainted due to other vibrations.

One night one week and one night two weeks later is not a trend. It is two nights two weeks apart. Did you do the exact same things both days? Did you eat the same things, worry about the same things, dream about the same things? This is why we stress giving things time between changes or before making any assumptions or conclusions.

And also remember what AHI is. It's an average based on events and time period. It is just numbers. If, for example, you had REM sleep for just an hour and in that hour you had 29 events, then the REM AHI would be 29. But if you didn't have any events at any other time and you slept for 8 hrs, then your AHI would be 3.6.

The thing to take into higher consideration is how you feel. An nightly AHI of 29 would make you feel like crap on a stick. An AHI of 3.6 would make you feel pretty good.

What are your weekly and monthly averages? How do you feel? Are you getting enough sleep?

You're looking at this picture very narrowly.

PaulaO2
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11-03-2012 12:28 PM
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lv3101 Offline

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Machine: ResMed S9 VPAP ASV 36037A
Mask Type: Nasal mask
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CPAP Pressure: 15/11
CPAP Software: ResScan SleepyHead

Other Comments: severe OSA, diabetes2

Sex: Male
Location: San Francisco Bay Area, California

Post: #32
RE: Is AHI misleading?
@PaulaO2> "There is no way your machine is 500% off"
Are you questioning the accuracy of the sleep lab AHI score? I was surprised, too, this is why I did a second overnight test 2 weeks later in the same lab. The second test confirmed the first test. Years ago I had a split sleep study at Stanford with their BiPAP which showed AHI score in the same range (AHI =~18). All these 3 sleep lab test showed my AHI to be around 18 (while using BiPAP). What worries me is why my BiPAP is consistently showing AHI < 2, while 3 sleep lab tests in 2 different labs showed AHI ~18?

@PaulaO2> "How were the two tests done? They used their equipment (leads, chest strap, oximeter, etc) and you used your machine?"
Yes, they used their equipment (leads, chest strap, oximeter, etc, but not their BiPAP) and I used my machine at a pre-programmed pressure. The sleep lab technician did not change my pressure during these 2 overnight tests. The sleep lab was established with prof. Guilleminault's help (affiliated with the Stanford Sleep Disorders Clinic) and has a setup similar to Stanford.

@PaulaO2> "The thing to take into higher consideration is how you feel. (…) An AHI of 3.6 would make you feel pretty good."
My average AHI calculated by my BiPAP over several months is < 2 (average bed time 7h/day), but I feel EDS and this is why I'm investigating the accuracy of AHI calculated by our machines.
11-03-2012 02:04 PM
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Sleepster Offline
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Post: #33
RE: Is AHI misleading?
(11-03-2012 11:45 AM)lv3101 Wrote:  @Sleepster> "all the machines seem to be accurate"
All machines have a margin of error. Based on my tests described in my fist post in this thread the margin of error of my machine (which is considered market leader) was > 500% (my machine calculated AHI=1.9 while the sleep lab calculated AHI=16.7)

Your machine calculated a AHI of 1.9 under what circumstances? The pressure was set to 15/11 and left there all night? Your machine is going to stay at that set pressure all night.

I can't imagine a sleep lab doing the same thing. Why would they keep you at a fixed pressure of 15/11 for an entire night?

It's not uncommon for a person to have a AHI of 16.7 the first night of therapy, and then a week later have a AHI of 1.9 receiving the same therapy.

Quote:That's what I've been doing with my auto BiPAP (bought in Jan. 2012) for months. It shows average AHI < 2.

Then the machine is showing that you're receiving effective therapy. What is it you hope to gain by making changes in your therapy?

Quote:How can I monitor and adjust my auto BiPAP if the margin of error in the AHI numbers showed by biPAP is above 500%?

You can't, but I don't agree with your premise. I believe your calculation of the margin of error is flawed. When you compare a AHI of 16.7 to a AHI of 1.9 you have to be sure that those data were collected under similar circumstances. To convince me of that you'd have to repeat it in a sleep lab for several nights, and then repeat it at home for several nights. Then you'd have to consider the fact that sleep labs calculate their AHI from data collected only while the patient is sleeping wheras PAP machines calculate the AHI from data collected the entire time the patient is wearing the mask, asleep or awake. Then you'd also have to account for the fact that in the sleep lab the patient is uncomfortable and connected to a bunch of wires as opposed to being at home in comfortable surroundings.

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.
11-03-2012 02:10 PM
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SuperSleeper Offline

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Post: #34
RE: Is AHI misleading?
When comparing these AHI numbers, you must have nearly exactly equal circumstances to draw solid conclusions from anything.

Was the sleep lab's AHI during treatment (under CPAP pressure) or simply without CPAP during the diagnosis phase to show what your true AHI was? If using CPAP, what was the pressure set at by the lab? Did the lab adjust the pressure throughout the night (titration) to get to the optimum straight CPAP pressure or where exactly did their AHI number come from? Was that using the Auto in the lab?

Saying that your CPAP machine is showing a 500% error in AHI is meaningless unless and until we know what the parameters of the sleep study were and that those parameters used at home with your machine were exactly equal (same machine, same settings (auto, bpap, whatever) and same delivered pressures, etc. as the lab settings.

Also, AHI as measured in a lab is always going to be different than that which is measured in your home. The environment and surroundings in your home are much more comfortable, familiar and conducive to sleep, usually. Very few people sleep well in a sleep lab with wires attached, uncomfortable bed, noises outside the room and the knowledge that someone is watching you via video camera all night long.

SuperSleeper
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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.

11-03-2012 05:57 PM
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lv3101 Offline

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Other Comments: severe OSA, diabetes2

Sex: Male
Location: San Francisco Bay Area, California

Post: #35
RE: Is AHI misleading?
Sleepster, PaulaO2 and SuperSleeper - thank you for your valuable comments and questions. Please find my responses (and new questions...) below.

@Sleepster> "Your machine calculated an AHI of 1.9 under what circumstances? The pressure was set to 15/11 and left there all night?"
Yes - in the fist sleep study.

@Sleepster> "I can't imagine a sleep lab doing the same thing. Why would they keep you at a fixed pressure of 15/11 for an entire night?"
The fixed pressure 15/11 is my prescribed pressure I've been using at home. I wanted to compare the lab AHI score with my BiPAP AHI score under the prescribed pressure.


@Sleepster> "Then the machine is showing that you're receiving effective therapy."
My auto BiPAP is showing AHI < 2 and misleading us to think its therapy is more effective than it really is - but other lab equipment running in parallel to my BiPAP is showing a much higher AHI ~ 18. AHI = 18 means somewhat effective therapy as my diagnostic AHI is 85.

@SuperSleeper> "When comparing these AHI numbers, you must have nearly exactly equal circumstances to draw solid conclusions from anything."
The circumstances were equal as I was sleeping in the lab with my machine counting AHI and lab equipment counting AHI in parallel.

@SuperSleeper> "Was the sleep lab's AHI during treatment (under CPAP pressure) or simply without CPAP during the diagnosis phase to show what your true AHI was?
Sleep labs AHI was calculated during treatment (under my BiPAP's pressure). My diagnostic AHI was calculated a few years earlier at 85.

@SuperSleeper> "If using CPAP, what was the pressure set at by the lab?"
Fixed pressure 15/11 (which is my prescribed pressure).

@SuperSleeper> "Did the lab adjust the pressure throughout the night (titration) to get to the optimum straight CPAP pressure?"
No the lab did not adjust the pressure during this test.
Years earlier I had a titration study which concluded with a prescribed optimum pressure 15/11.

@SuperSleeper> "or where exactly did their AHI number come from?
Their AHI came from lab equipment (leads, chest strap, oximeter, etc, but not from their BiPAP)

@SuperSleeper> "unless and until we know what the parameters of the sleep study were and that those parameters used at home with your machine were exactly equal (same machine, same settings (auto, bpap, whatever) and same delivered pressures, etc. as the lab settings."
At home and in the lab: I used the same BiPAP machine, same BiPAP settings, same delivered pressures.


How did you calculate the margin of error of your machines? Is there any standard xPAP test protocol which would validate xPAP performance? In another thread on this forum I noticed that ResMed is frequently changing the firmware version in their machines, for example this year 2012 we've seen so far firmware versions 903, 904 and 905. Does anybody know which improvements were made and which errors were fixed in which firmware version?
11-04-2012 02:13 AM
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SuperSleeper Offline

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Post: #36
RE: Is AHI misleading?
Hi again lv3101,

Quote:How did you calculate the margin of error of your machines? Is there any standard xPAP test protocol which would validate xPAP performance?

I don't think there's any standard at all. Other than trying out a completely different data-reporting CPAP machine to see if there's a huge discrepancy. Thinking-about

With regard to the firmware version changes, those changes could be for any reason, not simply an adjustment in the AHI calculations, although it's possible that's what they've corrected. They keep the reasons for the version changes close to their vest, that's for sure, so we really have no clue whether a new firmware version is actually something that vastly improves our machines or not.

Thus far, there's no way for end-users to upgrade the firmware version for any CPAP machine that I know of, other than sending it into an authorized repair facility or to simply purchase a new machine. They don't make the firmware easily accessible or upgradable, because that would make it that much easier for other competitors to reverse-engineer their proprietary algorithms and code that makes their machines unique, thus eroding any competitive advantage they have in the CPAP marketplace.

Regarding your CPAP vs. Lab AHI issues... just something to think about, but it's possible that there was some type of error with the lab equipment or the way in which they conducted the test - not likely, but possible. Dont-know

If you have major concerns about this, I would see if you could borrow a completely different Bi-level machine (different brand) from someone (perhaps from a local DME who has a used data-capable bi-level sitting around) and set it as much as possible to the same settings as your existing machine, and see what it reports as your AHI. Not sure this is possible, but it might provide useful insight. Tell them that if it reports the AHI correctly, you may be ordering a new machine from them.

To be honest with you, this discrepancy just sounds way out of the ordinary. Yes, CPAP manufacturers might have a vested interest in under-reporting AHI to make their machines look more effective; these machines are controlled by government regulation, so if there were such an obvious problem with the veracity of the reported AHI, I would think that the manufacturers would suffer the wrath of the regulators for falsely reporting data on their machines.

We have had folks here (with two CPAPs made by two different manufacturers) use both machines with the same settings, and the while their reported AHI levels do have a small difference, usually it's more like 1-2 points for AHI, and that's about it.

The machine's reported AHI is not that important anyway as far as pure accuracy is concerned. Sure, it should be calibrated to as close as possible to real AHI, however the data we all get from our CPAP machines should be used for trend analysis only. So, the 1-2 point AHI variance between different machines should not matter for our purposes as patients - we simply need it to accurately record the differences in AHI from previous nights.

The truth is that there is always going to be some differences in AHI, even between labs using the exact same equipment. It's not like measuring a weight or distance, where there is only one variable and pre-agreed upon, unchangeable fixed numbers. AHI measurements are much more subjective, considering there is no standard on what is considered an obstructive or central event, exactly, or how severe that event must be before being recorded as an actual event with regard to the nightly AHI reporting.

But the AHI differences you report seem vastly different from normally acceptable variance in calibration. If you truly think your machine is messed up with regard to how accurately it's reporting AHI, I'd seriously consider getting another machine, since you may not be getting the most effective treatment. But I don't think this issue is common - you just may have a defective machine.

Coffee

SuperSleeper
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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.

11-04-2012 09:02 AM
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Sleepster Offline
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Post: #37
RE: Is AHI misleading?
(11-04-2012 02:13 AM)lv3101 Wrote:  @Sleepster> "Your machine calculated an AHI of 1.9 under what circumstances? The pressure was set to 15/11 and left there all night?"
Yes - in the fist sleep study.

Do you have the sleep study report? This would be very unusual for a sleep lab to do this. I can't imagine a sleep lab spending all that time and money just to measure the effectiveness of a pressure setting.

Typically the first sleep study is a collection of data while there's no PAP therapy.

The second sleep study is a titration where PAP therapy is used to find the best possible pressure for a patient. In this case the pressure is adjusted by the technician and not left at a specific setting all night.

In fact, in a titration where a AHI of 23.1 was measured at a pressure of 15/11 the tech would raise the pressure until it was high enough to reduce the AHI to something lower.

As I said, this is the typical situation. Your situation may not be typical, in which case there would have to be some atypical factors such as a sleep-disordered breathing condition that you haven't told us about.

Calculation of the AHI just ain't that complicated. A flow meter determines when your breathing falls below a threshold for 10 seconds, and that gets counted as an event. Total number of events divided by total number of hours equals AHI.

I still don't understand what your goal is. Do you simply want a machine that reports a more reliable AHI? You ain't gonna find one. Pay for another sleep study if you want a more accurate AHI than what you're getting from your machine.

Quote:What worries me is why my BiPAP is consistently showing AHI < 2, while 3 sleep lab tests in 2 different labs showed AHI ~18?

If that's indeed the case, and all other things are equal, your AHI is lower because you're more comfortable when you're sleeping at home. The only other thing I can think of is that the machine measures your AHI all the time, not just when you're asleep. You'd have to be spending lots of time with you PAP machine on while you're awake, like most of the time!

In those sleep lab measurements of your AHI, was your AHI composed mostly of OA's, CA's, or hypopneas?

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.
(This post was last modified: 11-04-2012 12:17 PM by Sleepster.)
11-04-2012 12:13 PM
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Dawei Offline

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Post: #38
RE: Is AHI misleading?
I imagine this experience falls under heading: accuracy of the sleep lab's AHI. The lab's titration study found my AHI was brought down to zero when they reached a pressure of 13 (all OSAs). Follow up at-home use of that pressure first with a straight CPAP and then with an S9 Autoset found that it was actually too low, and it took higher pressure (in the range of 5-7 higher) to bring the AHI under 5! The lab seems fully equipped with all the right stuff. It's run by the large, local hospital. The sleep doc checked with the lab tech who ran the study to ask questions including whether I was on my back during the night as I am at home. Yes, yes and yes. The doc, a diplomate of the American Board of Sleep Medicine, was, and remains, stumped by the lab's missing it.
11-04-2012 04:40 PM
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SuperSleeper Offline

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Post: #39
RE: Is AHI misleading?
The more I read from you lv3101, the more strange this gets.

From looking at my admin logs, it's looking like two different people are using your username or else something strange is going on here. Are you in California as your profile says or somewhere else?'

Your account registration is listed from a foreign country.. did you register from outside the U.S.?

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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.

11-05-2012 07:46 AM
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zimlich Offline

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Post: #40
RE: Is AHI misleading?
I don't think the AHI is necessarily misleading unless you consider it as the whole story and the only parameter you look at-eg if O2 sats are fine I would not be as concerned with a higher than desired AHI. Also consider how you feel. I think the trend is away from regarding the AHI as most important.
Good Luck,
Mary
11-05-2012 09:26 AM
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