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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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.