How Resmed Determines Apnea Type

[Geer1]

I was looking the operator manual for humidity information and saw the section on how Resmed scores apneas as either central or obstructive. I found interesting and helpful and figured it was worth sharing although some of you probably already know this.

The device detects both obstructive and central sleep apneas (CSA). CSA detection uses the Forced Oscillation Technique (FOT) to determine the state of the patient’s airway during an apnea. When an apnea has been detected, small oscillations in pressure (1 cm H2O peak-to-peak at 4 Hz) are added to the current device pressure. The CSA detection algorithm uses the resulting flow and pressure (determined at the mask) to measure the airway patency.

In the first image you can see how during a central apnea the pressure fluctuations cause flow fluctuations.

screenshot-20191126-001312.png (108.65 KiB) Viewed 487 times

During an obstructive apnea the flow is unaffected by the fluctuating pressures.

screenshot-20191126-001326.png (108.52 KiB) Viewed 487 times

I found this interesting and helpful as it gives a person the ability to look at events that don't meet apnea criteria due to shorter length. For example the following obstruction that resulted in arousal after 7-8 seconds of obstruction.

screenshot-20191126-001243.png (110.57 KiB) Viewed 487 times

[Geer1]

Previously I have noticed I often have a shorter apnea like episode before a flagged central apnea, upon reviewing this it appears that these events are most likely sleep transition central apneas after an arousal due to obstruction. Here is an example.

screenshot-20191126-002554.png (110.14 KiB) Viewed 484 times

Overall this has helped me confirm that a number of what I believe are arousals are being caused by obstructions, as are some of my "central apneas". I may have AHI under 1 most nights but there may still be room for improvement.

[tyrinryan]

Would you not expect to see a flow limitation of some sort at the area where resmed charts an OA? In chart number 2?
What am I missing? I see the same things in my chart. If the FOC says there is an OA why doesn't it find a flow limitation too?

[Jas_williams]

Would you not expect to see a flow limitation of some sort at the area where resmed charts an OA? In chart number 2?
What am I missing? I see the same things in my chart. If the FOC says there is an OA why doesn't it find a flow limitation too?

A flow limitation can only occur where there is some flow to limit. There is no flow in these examples, they are 100% no flow apnoea of either obstructive or central as determined by the Resmed. Flow limitation only occurs in breaths that are reduced by pending obstruction based on the shape of the top of the waveform.

[Geer1]

I don’t know how ResMed calculates flow limitation but jas is right and it obviously requires a certain amount of flow. If you look at a high value flow limitation (near 1) you will note there is still some flow happening.

That is why I was interested to figure this out because it seems obvious that some of my shorter time events are obstruction related but due to current definitions they don’t get flagged as issues although a number of them appear to be related with arousals.

[palerider]

Or, as it is usually explained,

https://youtu.be/50NvHsUQr-M

[chunkyfrog]

Or, as it is usually explained,

https://youtu.be/50NvHsUQr-M

So simple even a frog can undersrand it.

[palerider]

Would you not expect to see a flow limitation of some sort at the area where resmed charts an OA? In chart number 2?
What am I missing? I see the same things in my chart. If the FOC says there is an OA why doesn't it find a flow limitation too?

"Flow limitations" are often misunderstood.

A FL is s restriction on airflow rate, whereas a hypopnea is a restriction on airflow volume.

You can have bad FLs, but still manage to take a full breath, it just takes more work and time to do it.... Try breathing through a straw, that's like a FL.

They're bad because they increase the work of breathing, and disturb rest, in and of themselves.