How AHI is determined

[Jeremy H]

If you turn over in your sleep several times a night, adjust pillow, mask, etc., does that translate into an AHI reading? I rarely sleep through out the night. I wake up from time to time. How does the machine read when you stop breathing? What about when I take the mask off? I'm trying to figure out how and why.

[mesenteria]

Any time the machine's programming directs it to ramp up pressure due to what it thinks is an obstructive event, or if it feels a flow limitation, or if it sounds and gets what it thinks is an 'open airway', it should record the event.  It if is a matter of record, then it ought to be calculated in the night's AHI.  

So, it would be somewhat, not completely, unusual for the machine to record such short-duration events, unless they are followed by an obstruction, or by a CA, that ends up longer than about 10 seconds, at which it will record it and duly include it.

[Dormeo]

As Mesenteria says, pauses in breathing of less than 10 seconds will not be flagged as OAs or CAs. The machine can't tell whether you're awake or asleep, so if you pause between breaths for 10 + seconds, it will drop a flag. We breathe with less regularity when we're awake, so it's not uncommon to see some events flagged during wakeful periods. They will usually be CAs, though it's not impossible for them to be read as OAs.

[SarcasticDave94]

Agreed with the others. Unless I'm getting it backwards, I'm thinking it's ResMed that uses FOT-forced oscillation technique, to determine clear airway/Central Apnea. Respironics uses something else, but basically pressure pulses and sensing resistance to flow to flag events.

[Sleepster]

If you turn over in your sleep several times a night, adjust pillow, mask, etc., does that translate into an AHI reading?

No.

I rarely sleep through out the night. I wake up from time to time. How does the machine read when you stop breathing? What about when I take the mask off? I'm trying to figure out how and why.

No one sleeps through the night. We wake up, but often have no memory of it, so we think we slept through the night.

If you stop, or nearly stop, breathing for at least 10 seconds, that is an apnea. If your breathing is restricted below a certain level for at least 10 seconds, that is a hypopnea. Add up the number of apneas and hypopneas, divide by the total number of hours during which the apneas and hypopneas were counted, and that gives you the apnea-hypopnea index (AHI).

Example: Let's say that between midnight and 5:00 am you had 7 apneas and 3 hypopneas. 7 + 3 = 10. So you had 10 events during a 5-hour period. 10 divided by 5 gives you a AHI of 2. In other words, you had, on average, 2 events per hour.

Taking the mask off has no effect on this. The machine has a flow rate meter in it, and that's how it "knows" if you're breathing or not, or how much you're breathing or how much you're leaking.

[Jeremy H]

I can't believe that I stop breathing for 10 seconds. I don't use an auto to ramp up pressure. Thanks for the info.

[Jeremy H]

Last night with cpap, I help my breath for 10 seconds. No way that could happen while sleeping. It's easy to hold your breath while sitting up in a chair. Laying down it's more pronounced. Can't see that happening. I did try an auto machine once, a resmed. By the time I woke up, the pressure was at 9 and my lungs were filled up like balloons. Needless to say it was not a good nights sleep. I can't get my AHI below 10. If I did, it would be way to much pressure.

[Sleeprider]

Jeremy, time for a daily details chart. 10 seconds is not that long. The average respiratory cycle is only 5-seconds, so skipping a breath is as easy as turning over. It matters the kind, duration and frequency pattern that your events present. The chart can tell a story. "No way" is simply denial. Look at the flow rate.

[OpalRose]

I can't get my AHI below 10. If I did, it would be way to much pressure.

If your AHI is really at 10, you need to post a chart or two. That is just too high!

[Gideon]

Just to give you an idea of how much pressure an CPAP machine produces try this.
First a CPAP can fill a balloon with air, it cannot inflate one.
Also I am not saying you are not having issues.

Get a tall glass of water and a straw. With the straw just off the bottom blow bubbles. I'll assume you had no trouble. You just exhaled against the maximum pressure a CPAP/APAP can produce, 8 inches of water, in metric 20 cm of water. The 10 you mentioned above is only 4 inches of water.

I'm always amazed at what a low pressure machine our CPAPs are.

Please try the above.

A CPAP can move quite a volume of air, take the mask off your hose and see how much. This is flow, not pressure. We can get a lot of Flow from leaks.

Now please post your nightly chart so we can identify the issue.