How does the cpap machine figure the apnea index?I can sleep the clinic with the 21 {or was it 300?} wires knowiing -- but how does the machine with no wires figure it out? How does it know?

Easy.  Every machine has a flow meter, therefor it knows how much air is going in & out of the mask.  If you stop breathing (apnea) or decrease your breathing (hypopnea), more air comes out of the machine.

The wires at the sleep clinic were mostly for brain activity, eye movement, snoring, leg movement, etc.  There was also one near your nose & mouth to measure flow, just like your CPAP's flow meter.

Flow sensors in the machine detect your breathing. (This is how EPR or C-Flex can synchronize to your breathing.) It can also detect lack of breathing (apnea). In the lab, the cannula in your nose detects breathing (or lack of) in a similar manner.

What the machine can't determine is the type of apnea (central, obstructive, or mixed). The straps around your chest and abdomen in the lab allow that determination by sensing breathing effort. It also can't determine sleep stages, heart rate, or limb movements.


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Respironics Auto M Series, Pressure: 12 cm
Fisher & Paykel Opus 360 Nasal Pillows
My Sleep Blog

So, I guess your actual AHI is a little bit higher than what the CPAP reports because it cannot tell when you are awake versus when you are asleep.  It must figure the AHI for total time that you are wearing the mask rather than just sleep time.

"It must figure the AHI for total time that you are wearing the mask rather than just sleep time."

That is true, and because of the way that autoPAPs detect apneas and hypopneas, it could measure that you have a higher AHI while you are awake than while you are asleep.  

I don't wear my mask for a significant length of time while not sleeping, and I don't use an autoPAP very much, so I have no personal experience with this.  I have read posts in various forums from people who were quite concerned or even alarmed because their graphs showed that their AHI was higher, sometimes much higher, while they were awake than while they were asleep.

I do not know if autoPAPs will read lower AHIs for some people while they are awake, I haven't read anything about this.

Bearded One wrote:

That is true, and because of the way that autoPAPs detect apneas and hypopneas, it could measure that you have a higher AHI while you are awake than while you are asleep.

Where can I find more info on this?


_________________
Respironics Auto M Series, Pressure: 12 cm
Fisher & Paykel Opus 360 Nasal Pillows
My Sleep Blog

All I have seen are posts from people who were concerned about AH events being recorded during times while they believed that they were awake.  At least one person believed that they were watching TV at the time that the graph indicated high AHI.  I wonder if talking or or possibly even snacking may be causing the machine to falsely detect an event, perhaps as snoring.

There are people who wear their CPAP while they watch TV or read -- and those activities are very likely to affect the average AHI that is recorded.