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Accuracy of Resmed AS10 vs Sleep Study Type
Hey guys,

I'm wondering - how accurate is the AHI measurement of the Resmed AS10? Is it more comparable to a home sleep study number or an in-lab number?

I ask because my doctor explained that home studies are notoriously inaccurate and only generally indicative, and this was proven out in my own case. While the home sleep study showed an AHI of ~9, the in-lab study showed ~38.

So, I'm wondering whether, when my CPAP machine reports an AHI of 3, it's actually 3 or could be significantly more due to the measuring method.

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These machines are not nearly as accurate as an in-lab test. The number they report might be higher or lower. I think the AHI is usually higher than the lab test would be as it will include events while you are going to sleep or waking up, the lab test would exclude these.

The important thing is not the number, but how you are feeling the next day.
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How is the machine measuring differently than a home sleep study (which drastically under-reports)?
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(10-28-2016, 05:25 PM)IMSleeplessInSeattle Wrote: How is the machine measuring differently than a home sleep study (which drastically under-reports)?
It really depends on how the home sleep study was set up for detecting the air flow into and out of your lungs. It also depends on how the home sleep study was set up to determine when you were asleep and when you were awake. If your untreated OSA results in a lot of mini-arousals/wakes that are not caught as "mini-arousals/wakes" by the home sleep test, it is possible that a home sleep study could over estimate the amount of SLEEP time, which would could then under estimate the true AHI.

The way your CPAP does that is through the back pressure at the machine's end of things. The microchip on the machine knows how much air it is pumping into the system and it can measure the changes in the airflow that are caused by your breathing by measuring the differences in the back pressure. When you inhale, the machine has to blow more air into the system to keep the pressure constant, and it knows this because at the start of the inhalation, the back pressure will drop. It can tell when you start exhaling because the back pressure will increase. Because the machine knows how much air it must blow into your upper airway at all times to keep the pressure at the desired amount, it can track your breathing pretty precisely. If you happen to have a Resmed AirSense 10 Elite or AutoSet, the machine actually records every breath you take all night long.

Your machine will be able to determine with a pretty good accuracy every time you stop breathing for more than 10 seconds and it will score an apnea. The criteria for scoring hypopneas is less accurate, but still pretty decent: Hypopneas are scored when the airflow drops by 50% from the running baseline established over the last couple of minutes of normal breathing as I recall. But establishing that baseline can be a bit tricky if you are spending a lot of time in drifting in and out of sleep. And because WAKE breathing is often much more ragged than high quality SLEEP breathing, the machine may have a tendency to score "events" during your WAKE breathing that would not be scored on an in-lab sleep test. Those "events" scored in WAKE are not real events and so the more time you spend lying in bed awake with the machine on, the more likely those events may artificially inflate the machine scored AHI.

In general CPAP manufacturers say the data is best looked at over longer periods of time than a single individual night. The data is extremely good for trending: If your AHI starts to increase and stays elevated over several weeks or a few months, then it is very, very probable that your treated apnea has gotten worse and you need a pressure adjustment of some sort. On the other hand, if your AHI takes a few weeks to drop down and settle at a level that is below 5, that usually means it's just taken a while for your body to adjust to CPAP. If your AHI consistently runs below 5.0 night after night, that is a good indication that your CPAP is doing its job. Some people do find that they're more sensitive to slightly higher machine scored AHI's than others. In other words, some people find that they feel best if they have their machine set so that the machine scored AHI is consistently staying below 2.5 or 2.0.

A bad night here and there in terms of the machine scored AHI may mean nothing more than you had a bad night's sleep over all. Or that you're coming down with a head cold. You only need to worry about machine scored AHIs that are above 5 if you start to get enough of them where you would say that your AHI is no longer below 5 almost all the time.
Questions about SleepyHead?  
See my Guide to SleepyHead
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You might find these referenced articles useful from an old thread and post. See post #28


Happy Pappin'
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Robysue's Beginner's Guide to Sleepyhead

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Robeysue, I found your explanation about how the machine records AHIs so interesting and informative. So lucky to have you on the board. Thanks, it was news I can use.
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