Hypopnea - Resmed scoring

[Old Mike]

Based on personal experience with more than one machine brand, I reckon that Resmed under-reports hypopneas. When I switched from a Devilbiss Blue to a Resmed 10 Autoset a couple of years ago, my AHI dropped immediately from an average of about 3 to 1.5 (with the same mask and pressure settings). Most of the drop was in hypopneas. While the Devilbiss had a tendency to slightly over-report hypopneas, visual inspection of Oscar data suggested that the Resmed 10 definitely missed quite a few of them.

[Gideon]

That hypopnea is preceded by an arrousal which is usually more important. It correctly noted the reduction in volume.
I am missing context of the full night

[BoxcarPete]

The flow rate chart represents what should be a closed system, if volumes are different where and why did it go?

It's not a closed system, not even close. Airflow in an Airsense 10 like mine comes in the intake, past a single flow sensor, through the blower, humidifier, and tube. This is where things split off. Most of the air goes straight out the mask vent unless you have a mask leak greater than the vent flow. A normal mask vent should blow about 20L/min IIRC. Leaks can easily exceed this, so now we're looking at something like 50 L/min passing through the system, and only 6-7L/min going into and out of your lungs before mixing with the flow stream and going out the vent like everything else.

So as you can see, it's very far from a closed system, with a steady intended flow that's over 3x higher than the signal, and a variable source of noise that can be twice as high as that sometimes, too. You can try fooling it yourself by taking as deep a breath as you can, as slowly as possible. Then exhale as slowly as possible as well. If you do this properly over a period longer than 10 seconds, you may see a mask leak appear and disappear, with the event flagged as a CA in your chart. The machine has a hard job to estimate this simply because it is not at all a closed system.

[Gideon]

Do you not breathe out everything you breath in. The system I was referring to is your respiratory system. Your respiratory system is a closed system. And I did say you need to examine any time they are not matching.

[BoxcarPete]

I actually don't, and neither do you 

The reason we do this whole breathing thing is so that we can make use of atmospheric oxygen to metabolize hydrocarbon fuel sources. The H2O is mostly retained by the body, but we have no use for the CO2, so we breathe that back out, but it's not all the O2 we take in. Now I'm wondering about what level of external humidity deficit is required to make up the difference.

But either way, it's quite a small fraction of the amount we breathe, so it's of course useful to be skeptical of any section of our charts that appear imbalanced in this way.

Also, there's no good emoji to express insufferable pedantry, which is a bummer.

[Gideon]

Chopped liver here, signing off this thread

[Crimson Nape]

BoxcarPete - I'm having a problem understanding your post about the relationship between water versus carbon dioxide.

- Red

[BoxcarPete]

We use the oxygen we breathe to metabolize sugars and fatty acid for energy, therefore a small amount of the air we breathe is converted from O2 to CO2 and H2O. Water is of course used by the body for a variety of things, but CO2 needs to get purged, and in fact this is what drives our desire to breathe.

So the mass and/or volume of what we breathe in is different from what we exhale because the gases are exchanged and used in the body. It's also interesting to note that if you lose a lot of weight by "burning" fat, where did it all go? You breathed it out!

The other variable is water. There's a humidity level of the air we breathe in, and of course when it gets into our lungs it's humidified by either our nose on the way in or the lungs themselves. Then it's exhaled, so if it's still wet, we can lose mass this way also, and make our exhaled breath greater than what we inhaled in the first place.

I'm genuinely curious as to how these effects can influence our machines' attempts to balance the airflow readings without any awareness of the situation. The resmed machine has its sensor prior to the machine humidifier as well, so it has absolutely no clue about the mass flow  or composition of gas at the mask, let alone the lungs.

[Opus1]

@boxcar
Don't you think that your discussion is WAY off topic from my original question/issue? 

At the outset of this thread, I wanted to know if any forum member knows what period of time (or breaths) Resmed uses to establish the baseline for a hypopnea determination. Gideon suggested it was a moving average, and that makes sense, but it does not answer the question "moving average" over what period. 

Secondarily, given the snippet of flow rate chart, should/would a competent polysomnographic technologist edit out  that specific hypopnea determination, either because it appears that a "sigh" preceded it or that the reduction was likely post arousal.




 

[BoxcarPete]

Might be a bit off in the weeds, but definitely on the topic because it's directly related to the estimates your machine is making to flow rate, which is the way a hypopnoea is determined, and can be influenced by things like big, deep breaths.

If the average reading is falsely suppressed by something like a mask leak, or you fill your lungs deeper and don't expel it all and the machine wrongly accounts for the missing air somehow, it can result in false flags. The body's CO2 ventilation vs air intake is admittedly probably not a huge factor in this.