How does the cpap read AHI? Is it accurate?

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How does the cpap read AHI? Is it accurate?

Postby jtd00123 » Sat Jun 09, 2012 8:42 am

Hello, I'm curious to see how accurate is the built-in AHI indicator found on many CPAPs. I currently use a Resmed S9, and according to it, my AHI is 5.3 at a pressure setting of 12, despite the fact that this setting eliminated my AHI in the sleep study.

Is it common to get misreadings from the CPAP? I ask because I tend to breathe out with my mouth with the CPAP machine on. I guess my question is how does the cpap read AHI to begin with? I was under the impression that detecting apneas requires some fairly sophisticated equipment. Any answers would be appreciated, especially those with strong knowledge of the technicalities of current paps.
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Re: How does the cpap read AHI? Is it accurate?

Postby greatunclebill » Sat Jun 09, 2012 9:12 pm

if you're breathing out your mouth while using nasal mask or equipment, proper treatment is not taking place and your ahi is probably real. if you can't keep your mouth closed and stop breathing out your mouth you may need to look into getting a full face mask. simply put, air escaping out your mouth means the machine is not maintaining the proper pressure required to treat your apnea's.

ahi is the total of all obstructive apneas, hypopneas and clear or central apneas divided by hours on the mask. example: ahi of ten over a six hour sleep means 60 events in 6 hrs = ahi of 10.
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Re: How does the cpap read AHI? Is it accurate?

Postby Bons » Sun Jun 10, 2012 3:41 pm

Cpaps don't measure as accurately as a hospital titration because the cpap only measures what it senses. It doesn't know if you are awake or asleep, for example, or whether or not you are holding your breath because you are in the middle of turning over in bed. The sleep tech can see these things and so s/he can decide whether or not to consider that event as an event.

Cpaps note resistance to the airflow and register everyone that is senses. That can partly explain why you have a higher AHI at home than you did in the lab.

Are your events obstuctive or clear airway? The clear airway ones are the ones most often "picked up" inaccurately by the pap.

And yes, if you are finding yourself mouthbreathing, be sure to use a chin strap or a full face mask.
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Re: How does the cpap read AHI? Is it accurate?

Postby jtd00123 » Mon Jun 11, 2012 8:09 am

Thanks guys. The vast majority are clear airway, yet I had only one central in the lab. Sometimes, I wake up completely refreshed, yet am shocked when I find out how high my AHI is. (sometimes worse than my first sleep study, which can't be right) I also have restless legs syndrome which causes me to move around a lot in my sleep, so I think that is also giving me an inaccurate reading.

Does mouthbreathing also give an inaccurate reading as well?
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Re: How does the cpap read AHI? Is it accurate?

Postby zephyr » Mon Jun 11, 2012 2:45 pm

I think mouth breathing with nasal mask will show up as leaks.

You AIH may go up if your having too many mouth leaks because its not maintaining pressure. I use a chin strap to control mouth leaks at night.
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Re: How does the cpap read AHI? Is it accurate?

Postby robysue » Mon Jun 11, 2012 4:08 pm

To give you a more technical answer:

Your CPAP machine detects apneas and hypopneas by analyzing the airflow into/out of your lungs. It tracks each breath to establish a running "baseline" for your normal breathing pattern during the last five or ten minutes or so. Whenever the airflow drops by 50% from the running baseline for at least 10 seconds, the machine will flag a hypopnea. Whenever the airflow drops by 80% from the running baseline for at least 10 seconds the machine will score an apnea.

Since you are using a Resmed S9, if the machine detects very little or no airflow (ie airflow has dropped by 80% from baseline) for about 6 seconds or so, the machine will use a proprietary algorithm called FOT to determine the patency of your upper airway. In other words, the machine will start oscillating the pressure at a set frequency and analyze what happens to the air flow. The air flow signal from the FOT is different when your airway is clear (ie NOT obstructed) instead of when your airway is obstructed (ie your airway has collapsed). If the pause in airflow is sufficiently long enough to score an apnea and the FOT detects an obstructed airway, the machine will score an OA. If the pause in airflow is sufficiently long enough to score an apnea and the FOT detects that the airway is clear, the machine will score a CA. Because the machine has detected no evidence of an obstruction, CAs are often thought to be central apneas, but the definition of a central apnea on a PSG does not involve testing the patency of the airway---Central apneas on PSGs are scored when there's no air moving into/out of your lungs for 10 seconds or more and the belts around your chest and abdomen indicate you are making NO effort to breathe.

How accurate is a CPAP's AHI? Well it's accurate enough for trending purposes. But it's not 100% on the spot accurate for any given night. To understand this, you need to know the following two definitions:

  • In-lab PSG AHI = (number of events scored while you are asleep)/(total time you are asleep) where "you are asleep" is detected by the EEG readings.
  • CPAP machine AHI = (number of events recorded during the night)/(total time the machine was running during the night)

Where do the errors in the machine AHI occur?

As others have noted, a CPAP machine cannot tell when you are asleep and when you are awake. Since the CPAP computes the AHI by simply dividing the number of detected events by the total time the machine was running for the night, that introduces two kinds of obvious errors as compared to an in lab PSG where the EEG is used to determine when you are asleep.

  • False events are "apneas" and "hypopneas" that the machine scores when you are actually awake. On a PSG, any "event" that occurs during a time period when the EEG indicates that you are actually awake is NOT counted towards the AHI. The reason is simple: Sleep apnea is a breathing problem that occurs when you are asleep. The control of our sleep breathing is different than the control of our wake breathing. And our wakeful breathing is much less regular than our sleep breathing. We sigh. We hold our breath momentarily when concentrating on something as simple (or complex) as turning over in bed while managing a six foot hose attached to our nose. We consciously take several deep breaths in a row to relax ourselves and then breath much more shallowly for a few more breaths. And some of us have a bit of a tendency to not breathe for a bit when we're transitioning from Wake to Sleep as our body resets the control of breathing. On an in-lab PSG, none of these kinds of "events" count because we're awake when they start. But a CPAP doesn't know that and scores them anyway. The net result of these "false" events is to make the AHI increase a bit because there can be a few more events scored than should/would have been scored on a PSG. And so the numerator of the AHI fraction is higher than it should be, which makes the computed AHI go up.

  • Inaccurate estimate of sleep time affects the denominator of the AHI fraction. The presumption by the CPAP makers is that your sleep time and the machine run time will be close enough to each other that the error introduced by using machine run time will be insignificant. And if you are one of those people who falls asleep within 5-15 minutes of going to be and doesn't spend much time awake during the night, that's actually a pretty valid assumption. But if you've got major insomnia and you're lying in bed with the mask on and the machine running for several hours each night, using machine run time instead of total sleep time to compute the AHI can increase the size of the denominator enough to reduce the AHI, perhaps significantly. (If two fractions have the same numerator, the fraction with the larger denominator will be the smaller of the two fractions---i.e. 29/8 is smaller than 29/6.)

For most people on most nights, the minor errors caused by the machine recording a few false events and using the run time instead of the sleep time more or less cancel each other out. And so the nightly machine AHI is, for most people a decent approximation of the true AHI on most nights, and so the machine numbers are useful for looking at trends.

In addition to the two obvious kinds of errors discussed above, there are also algorithm errors. Sometimes the machine will miss what looks like an obvious event---perhaps because the machine thinks it's not quite 10 seconds long. Or perhaps the drop from the current baseline is not quite enough. Sometimes the machine will score an event and you'll look at the data and not see anything that even vaguely resembles an apnea/hypopnea. These errors pop up often enough that if you zoom in on your wave flow data every single night, you're sure to see a few of them each week, but you probably wont see them every night. These random missed events and random extra events tend to cancel each other out in the long run. And so they don't tend to affect the overall trend of the AHI data over the course of several days or weeks.

And so when we consider the machine AHI data as trending data, it is considered accurate enough to be significant, even if any individual night might be off.

For example, if you have one night with an AHI that is up around 9, but your typical AHI is usually between 1.0 and 3.0, that one night is likely just an outlier: It may have been simply been a bad night for your OSA. Or it could be that the data for that night is particularly inaccurate---perhaps because you were very, very restless all night long. But that one night with an AHI of around 9 doesn't really indicate that there's something wrong with your overall therapy because the trend of data is that your machine AHI is consistently below 3.0, which clearly indicates that your real AHI is consistently well below the desired 5.0.

But on the other hand, if you are seeing machine AHI's that are consistently above a 5.0 night after night for several weeks, then there's a good chance that something is not yet "optimized" about your therapy---particularly if you continue to experience daytime sleep apnea symptoms. It could be that your pressure is set too low and too many events are still occurring. It could be that you are in the unlucky 10% of new CPAPers who wind up developing problems with pressure induced central apneas It could mean that your leaks are not under control and you are losing too much pressure for your therapy to be effective. In any case, an AHI that is consistently above 5.0 for a period of several weeks should be brought to your sleep doc's attention.

Now in your case, you say the machine is recording a fair number of CAs, while your in-lab titration study only record one CA. If those CAs are clustered around times that you vaguely remember being awake/restless or occur just as you are falling asleep or occur just as you are waking up, then as Bons says, there's a pretty good chance they are false events---i.e. events that would not have been scored in the lab.

And yes, the restless legs might trigger inaccurate readings. And finally, if you are doing a lot of mouth breathing, that too can affect the accuracy of the data. You need to look at the leak data as well as the AHI data if you're doing a lot of mouth breathing.

I'd suggest that you have a printout of some typical night's detailed data with you when you have your follow up visit with your sleep doc. And get his/her take on what might be going on.
current settings Min EPAP = 4, Max IPAP = 8 and Rise time = 3

8/1/2010 sleep study results:
AHI = 3.9 [AHI = (#OA +#CA + #H w/desat) per hour]
RDI = 23.4 [RDI = (#OA +CA + #H w/desat + #H w/arousal) per hour]
Dx: Moderate OSA
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Re: How does the cpap read AHI? Is it accurate?

Postby CraigB » Mon Jun 11, 2012 4:23 pm

Wow! An amazing post, robysue. Thanks for that!
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Re: How does the cpap read AHI? Is it accurate?

Postby momma6c » Sat Jun 16, 2012 1:26 pm

Thanks for the useful info. I'm an old RN but very new to OSA. I'm 2 weeks into therapy with bipap settings of 13/8. Initial AHI was 52 but is now still showing a 7 night average of 19. I'm thinking this is too high and looking for possible causes. Pretty sure I'm not mouth breathing with the nasal pillows. Currently using a respironics auto bipap while I wait for my S9 vpap machine to arrive.
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Re: How does the cpap read AHI? Is it accurate?

Postby robysue » Sat Jun 16, 2012 6:30 pm

momma6c wrote:Thanks for the useful info. I'm an old RN but very new to OSA. I'm 2 weeks into therapy with bipap settings of 13/8. Initial AHI was 52 but is now still showing a 7 night average of 19. I'm thinking this is too high and looking for possible causes. Pretty sure I'm not mouth breathing with the nasal pillows. Currently using a respironics auto bipap while I wait for my S9 vpap machine to arrive.

Yes, a 7-day average AHI = 19 is still way too high. (On a diagnostic PSG, an untreated AHI = 19 lands you well into the range of moderate apnea and an invitation to Hoseheadland.)

Have you downloaded SleepyHead and looked at your detailed daily data? SleepyHead will work with both the PR System One Auto BiPAP and the Resmed S9 VPAP. Moreover, SleepyHead gracefully deals with a user who uses more than one machine.

The only really good way I know for eliminating mouth breathing as a possible cause for leaks leading to less than optimal therapy is to look at the leak data in either Encore or SleepyHead. The "Large Leak %" number on the System One's LCD is largely meaningless: Not only is "Large Leak" an undefined term, but also 1% of 7 nights data is a pretty substantial amount of time. But looking at the leak line in SleepyHead or Encore and comparing it to the expected leak rate for your mask at your pressure settings gives a pretty good indication of whether or not mouth breathing might be occurring when you're deep in sleep.
current settings Min EPAP = 4, Max IPAP = 8 and Rise time = 3

8/1/2010 sleep study results:
AHI = 3.9 [AHI = (#OA +#CA + #H w/desat) per hour]
RDI = 23.4 [RDI = (#OA +CA + #H w/desat + #H w/arousal) per hour]
Dx: Moderate OSA
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Re: How does the cpap read AHI? Is it accurate?

Postby Sleepster » Sat Jun 16, 2012 10:14 pm

jtd00123 wrote:Hello, I'm curious to see how accurate is the built-in AHI indicator found on many CPAPs. I currently use a Resmed S9, and according to it, my AHI is 5.3 at a pressure setting of 12, despite the fact that this setting eliminated my AHI in the sleep study.

Is it common to get misreadings from the CPAP? I ask because I tend to breathe out with my mouth with the CPAP machine on. I guess my question is how does the cpap read AHI to begin with? I was under the impression that detecting apneas requires some fairly sophisticated equipment. Any answers would be appreciated, especially those with strong knowledge of the technicalities of current paps.


The machine has a low-inertia flow meter, and also a pressure sensor. As others have described in detail, when these are combined with logic and control circuitry, you can get some pretty amazing data.

One important point that I didn't see others mention in their responses: The AHI recorded during your sleep study was just the AHI you had that one night. The AHI varies from night to night based on a lot of different factors, such as how much stress you're under, what medications you've taken, and how tired you are.

Also, that pressure setting of 12 was determined by a technician to minimize, not necessarily eliminate, all AHI events. For example, the typical OSA patient might suffer more OA's and HY's when the pressure is too low, and more CA's when the pressure is too high. Also, typically a larger pressure is needed when you're lying on your back. So, if you slept on your back during the sleep study, the pressure of 12 might have been necessary to eliminate most of your OA's during that period of time when you were sleeping on your back. But maybe you rarely sleep on your back when you're at home in your comfort zone. In this case the pressure of 12 is too high for you, and it induces enough CA's to raise your AHI above 5.

You can look at your Flow Rate graph and see how long these events last. If they last 10 seconds or so, they're not as damaging as if they last 90 seconds.

When I was switched from a CPAP at 13 to a BiPAP at 13/8, my OA index shot up to around 20. They fixed it by lowering the pressure to 11/8. I still don't understand why I was ok with a CPAP pressure of 13, but not with BiPAP pressures of 13 and 8. Apparently the lowering of the pressure during each exhalation tricked my brain into thinking it didn't need to signal my lungs to breathe!
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Re: How does the cpap read AHI? Is it accurate?

Postby DMDesiderata » Sat Aug 11, 2012 11:55 pm

I started using a CPAP in late March 2011 and all was going very well until I became ill with a very bad case of bronchitis in mid-April. I wasn't able to use my CPAP for at least 2 weeks due to having bronchitis. When I went back to using my CPAP, I never got back to the same feeling I had before getting bronchitis. So I started using a Nebulizer Machine to help clear out my lungs. So come mid-June the company that provided me with the CPAP picks up my SD card to download the data to send to my Doctor. A month later, my AHI numbers are rising above 4.0 and even reached 4.9. I let my Doctor know about this, so he now sends a script to the CPAP company to do another data download and they came to pick up the SD Card in mid-July. I just had my appointment with my Doctor today and he looked at the two reports and it shows that my AHI is around 2.5 (which is much better than above 4.0). I asked him what is the reason for such a huge difference and I don't think I really got a good answer to this question. I get home and look at the two reports and suddenly I know why there is a discrepancy. The first report showed data that went up to June 10th covering 28 days and the second report showed data that went up to June 12th covering 30 days. Since the two reports overlap with the exception of 2 days, they would obviously show very similar results. I needed the report that went from mid-June to mid-July!!!! So even though the second report was printed on July 20th, the data only went up to June 12th!!! Now I need to call the CPAP company and my Doctor to discuss this and hopefully the Doctor will agree to increase my pressure. I'm hoping that the CPAP company can still provide the data through mid-July so the Doctor can see this for himself. I'm seriously at the point where I just want the program they use and I can generate my own reports. It's really not a difficult thing to do. I have a very good Doctor whom I really like; this was an easy thing to miss.

Last night my AHI was 5.1 so there is definitely a trend going on here. I'm just not sure what to do about it. Mask Leak is zero so that is NOT the issue. I would like to get the software that gives a more detailed report than what I can just read off the machine. Which software is best and how do you get it?
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Re: How does the cpap read AHI? Is it accurate?

Postby Sleepster » Fri Aug 17, 2012 5:15 pm

Look at the on-screen display. It should show the averages for both the last 7 days and the last 30 days.
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Re: How does the cpap read AHI? Is it accurate?

Postby robysue » Fri Aug 17, 2012 9:21 pm

DMDesiderata wrote:Last night my AHI was 5.1 so there is definitely a trend going on here. I'm just not sure what to do about it. Mask Leak is zero so that is NOT the issue. I would like to get the software that gives a more detailed report than what I can just read off the machine. Which software is best and how do you get it?

I can help you with getting appropriate software so that you can look at the data yourself. But I need to know exactly which System One are you using.

The model number should be on a tag attached to the bottom of the blower unit. You can either post here or PM me and I'll help you find the software.
current settings Min EPAP = 4, Max IPAP = 8 and Rise time = 3

8/1/2010 sleep study results:
AHI = 3.9 [AHI = (#OA +#CA + #H w/desat) per hour]
RDI = 23.4 [RDI = (#OA +CA + #H w/desat + #H w/arousal) per hour]
Dx: Moderate OSA
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