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How does a cpap machine measure central apneas?
#1
How does a cpap machine measure central apneas?
The title of this thread already show's what i'm trying to ask.

How does a cpap machine 'know' the difference between obstructive apneas and central apneas?

I have both so i was wondering how a machine differentiates the two?

Obviously a machine doesn't know or feel any obstruction/brain not giving a signal to breathe but it still somehow knows when it's an obstructive apnea or a central one.

(my neurologist is trying to see if my central apneas are pressure related and hopefully i'll know more next time i see her on november 5th)

Maybe it's a silly question but it keeps me a little busy the last few days  Grin
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#2
RE: How does a cpap machine measure central apneas?
I'm thinking in part at least it's based on flow and restriction. No flow with restriction would equate an Obstructive event, while no flow with no restriction is a Central. Also, there's forced oscillation technique (FOT) to test for the clear airway state.
Dave

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#3
RE: How does a cpap machine measure central apneas?
Basically (very), the forced oscillation technique send a low frequency pulse down the tube and into the nasal passages. I the throat is obstructed, the pulse gets reflected back; if the throat is open it goes into the lungs and is absorbed.

That's my understanding, anyway.
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#4
RE: How does a cpap machine measure central apneas?
It doesn't because it can't.  What can be detected is an apnea without obstruction, that is a "Clear Airway", abbr. CA.  A CA may have different causes.
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#5
RE: How does a cpap machine measure central apneas?
A clear airway had been clinically shown to be the equivalent of a central apnea. The one thing no CPAP can do, and doesn't do is determine if you are sleeping. And by definition you cannot have 'sleep' apnea when you are awake.
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#6
RE: How does a cpap machine measure central apneas?
Well, actually the autoset algorithm can tell whether I'm asleep or awake -- if it's measuring flow limits or apneas, I'm asleep. No flow limitation, I'm awake.

Now of course I like my vauto which allows me to sleep without flow limitations, but it does make it more complicated not having sure signs in the data stream of whether I'm awake or asleep! Laugh-a-lot
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#7
RE: How does a cpap machine measure central apneas?
Resmed uses forced oscillation technique. When the start of an apnea is detected the machine supplies a small amplitude oscillating pressure wave. If your airway is obstructed then the pressure wave will only affect the small volume of air in the hose and upper airway and the machine will see high pressure fluctuations and will record little to no associated flow as there is little movement of the air. If your airway is open then your lungs etc increase the volume significantly and since air is compressible the fluctuations in pressure readings will be smaller and you will be able to see the oscillation transposed more clearly into flow rate chart because the air is moving in and out of your lungs in time with the oscillation.

If you look at mask pressure and flow rate charts beside each other for both obstructive and clear airway events you should be able to see what I am saying.

Philips Respironics machines do this differently. Can't remember exactly how they work but they send a pulse rather than an oscillating wave.
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