Comparing AHI to O2 desaturation numbers

jaybee72 · Post by **jaybee72** » Mon Sep 10, 2007 8:31 pm

Ok, in the last 2 weeks I've read a TON of posts on here; I've been soaking this stuff up like a sponge. And yes, I think I've read about everything linked under the "collective wisdom." Now I may be missing a connection, or just read so much I'm forgetting some, but here is a question I don't think I've seen directly answered.

I'm wondering about how to compare desaturation numbers with AHI numbers. In other words, what does it mean if your AHI is not terribly high (16.2) yet the pulse oximeter readings were dropping into the 70's during the sleep study? According to the definitions that I believe snoredog generally posts that was a moderate OSA (bordering on mild) with a severe O2 desat... Would that mean the average length of the apneas and hypopneas are longer? Which tends to lead to more OSA symptoms like fatigue, etc - the AHI or desat?

As I read through posts, and I've been making sure to read all the newbie posts I see a wide range of disparity between the two measurements. For instance, one person will post an AHI of 90 and say they only dropped to 92% O2 saturation. I'm just trying to understand how they work together.

Maybe I'll have less questions if the sleep center decides to send the final results of the titration to my doc so I can finally meet with him and go over it all...

Slinky · Post by **Slinky** » Mon Sep 10, 2007 8:51 pm

Oh boy! I understand it ... but to EXPLAIN it ... besides I've forgotten the exact figures.

An apnea is defined as what? A 4% drop in 02 for 10 seconds or longer?

Okay, so if your baseline 02 is 95% and you experience an apnea that drops "just" 4%, your sats are still 91% and thus not below a normal 02 sleep level - BUT it does count as an apnea.

The AHI is the number of Apneas + Hypopneas per hour. You could have an apnea where your 02 sats dropped from 91% to 70% over a period of 30 seconds. That is just ONE apnea w/a drop to 70% 02 saturation.

So now we have 2 apneas for a total of 40 seconds of apnea in this hour. But we haven't counted how many hypopneas this hour. To make it easy on me lets say there were 14 hypopneas to arrive at that 14 + 2 = the 16 AHI for that hour. And say those 14 hypopneas never dropped your 02 below 90%, yes your sats dropped the required amount to qualify as a hypopnea but not below the accepted 02 saturation..

Is this clear as mud? Told you I can understand it but not explain it!!! I'm tired. I'm going to bed. This is too much brain work too late at night!!!

NightHawkeye · Post by **NightHawkeye** » Mon Sep 10, 2007 9:06 pm

Jaybee72, welcome to cpaptalk. Sounds like you're really doing your homework.

jaybee72 wrote:I'm wondering about how to compare desaturation numbers with AHI numbers.

My conclusion has been that they can't be directly compared. You've highlighted a significant deficiency in the way that apnea is scored. Which is more severe - a person with high AHI but mild desats, or a person with low AHI but deep desats? Both can be debilitating - the first person dies after falling asleep at the wheel, while the the other dies in bed from a heart attack.

As for the meaning of the depth of desats, you might be interested in an article I posted a while back dealing with PFO's (Patent Foramen Ovale, IIRC), a hole between the chambers in the heart which was found in at least one study to be associated with sleep apnea. The study associated individuals who had deep desats with significant PFO's. The PFO's allowed veinous blood to cross into the atrium and be sent back through the body without being oxygenated in the lungs first.

Hope this adds to your understanding. You should be able to do a Google search on the internet and find a few articles about it. The one I first read and posted about was in Science News, but there are others.

Regards,
Bill

Guest · Post by **Guest** » Mon Sep 10, 2007 9:47 pm

JB - Let me give it a try. There is no direct correlation between AHI and low o2 sats. AHI is simply the number of apneas (Apnea is when there is very little or no airflow through the body. There is no minimum desat number for an apnea) plus the number of hypopneas (Hypopnea is when your body is moving some air, but not enough to preserve sleep. The hypopnea is what slinky was talking about with the 4% desat rule) per hour that you are having. It's possible to have severe sleep apnea with minimum desats. Conversely, it's possible to have mild sleep apnea (or even upper airway resistance syndrome) with severe desats. I think the bottom line is: How healthy are your heart and lungs? Most people I put the CPAP on in the sleep lab do not require supplemental O2, even if their desats were horrible on the first test.

Ps- to add to this novel-long post , there are things called RERA's which are basically hypopneas that do not have the 4% sat drops. The RERA's are not figured in the AHI, but are typically included in the RDI. Does that help, or too much info in one post??

mckooi · Post by **mckooi** » Tue Sep 11, 2007 12:40 am

Hi,

This is like statistic 101, you need to know the exact breakdown of hours you sleep into each stages for each hoour inclduing the rem and non-rem stages. Then you will be able to nail down to the right point when you were having sleep deficency. Statistically speaking Mean, Mode, Median do not mean anything unless you fall into the bell shape curve. I be;lieve your sleep pattern is skewed. Overall, it might be that you have a Nrem stage at the end of your sleep pattern that could creat a apnea phenomeno that your oxygen level is very low. Check on your longest apnea and last 1 to 2 hours of your sleep data. It might tell more.....or how do you feel duirng the first few hours of the day and when you have lunch after it will tell more you sleeping overall efficiency. Hope that's help.

Mckooi

Guest · Post by **Guest** » Tue Sep 11, 2007 3:03 am

What?!? McKooi, I'm having trouble following your train of thought. The only way JB will see sleep staging is on the sleep report. Sleep staging means little to the layperson. Of course JB's sleep is skewed......He / she has sleep apnea.

Julie · Post by **Julie** » Tue Sep 11, 2007 3:10 am

Actually, to possibly the majority of OSA patients, sleep staging is quite a basic concept.

Goofproof · Post by **Goofproof** » Tue Sep 11, 2007 5:34 am

Apple - Oranges, you need XPAP treatment, period. You need to fill in your profile. You need your AHI under 5, be nice to be under 2. Time to put the slide rule in the drawer and get the treatment to work. Jim

jaybee72 · Post by **jaybee72** » Tue Sep 11, 2007 6:51 am

Thanks for all the responses! They were all helpful as I try to arm myself to be able to ask intelligent questions when I get to see the doctor to go over all the results. I must have lucked out on the first study since my doctor got the results the next day. This one is taking much longer in comparison; it's been a week now. But that's still much less than a lot of the posts I've read on here.

I guess I will just have to wait until I can see the actual study data before I'll be able to fully understand what is going on. Right now I have only been given a few key numbers by the nurse at my doctor's office who had no idea what she was looking at or giving me, and then a little better info from the tech just before starting the titration.

jaybee72 · Post by **jaybee72** » Tue Sep 11, 2007 6:56 am

Goofproof wrote:You need to fill in your profile.

I added my location; not sure what else you're looking for...

flylow · Post by **flylow** » Tue Sep 11, 2007 7:26 am

I had success getting the report by forcing the issue and making an appointment with the sleep doc. Even though the report was not finalized complete with interpretive doc statement at time of appointment, I was able to discuss the study and walk out with a prescription and move forward. Good luck.

Goofproof · Post by **Goofproof** » Tue Sep 11, 2007 8:56 am

jaybee72 wrote:
Goofproof wrote:You need to fill in your profile.
I added my location; not sure what else you're looking for...

The equiptment you use and pressure, I may have mistook things, you don't have the equiptment yet. welcome! Jim

Guest · Post by **Guest** » Tue Sep 11, 2007 8:17 pm

Hi Julie....I wasn't trying to snarky about the sleep staging. I just meant that it doesn't really matter whether a person has a apnea in stage2 or stage4. All that really matters is that the apnea is occuring, and needs to be treated. I guess Goofproof stated it a little more succinctly