All xPAPs work the same way in how they calculate the AHI:
Each manufacturer has an algorithm that uses the flow rate to determine when an apnea or hypopnea
may have occurred. The apnea algorithm is usually pretty simple: Little or no air flow into or out of the lungs has been detected for at least 10 seconds. Machines like the Resmed then use proprietary technique called FOT to determine whether the airway appears to be open/clear or obstructed and then classifies the apnea as a CA (clear airway) or OA (obstructed airway) depending on the results of the FOT algorithm.
To detect hypopneas, the machine has to calculate a running "baseline" of what the airflow look likes, and if the airflow drops by something like 50% from the running baseline for more than 10 seconds, a hypopnea is scored.
To calculate the AHI, the machine simply divides the total number of events scored by the run time (in hours) in which the events were scored.
Now all of this works reasonably accurately
if the person using the machine is
asleep for the vast majority the machine is on.
Problems happen, however, when a person uses the machine for an extended period of time
while awake or
while drifting in and out of real sleep. Wake breathing is simply not as regular as sleep breathing, mainly because different parts of the nervous system control breathing when we're awake versus when we're asleep. And when we're awake, there are all kinds of natural,
normal breathing patterns (that we may not even be fully aware of) that can look like apneas or hypopneas in sleep breathing. That's why during a sleep test there should be some kind of mechanism (such as EEG leads) for determining whether the person is actually
asleep or is
awake: On a sleep test, an apnea or a hypopnea cannot be scored during any epoch where the person is
awake.
Forgot one important detail. I wore the mask for another 20 minutes or so, but didn't fall asleep. AHI was then 19.
Because you were awake, that AHI of 19 is genuinely
meaningless.
I took a 45 minute nap today and AHI read as 57.5. Is this accurate? I kind of freaked out since I never had a number that high. My AHI without treatment is normally between 19-22.
I suspect (but obviously can't prove) that you were most likely bouncing back and forth between
wake and
sleep during the nap---i.e. there may be a lot of what rubicon refers to as "sleep-wake-junk" (SWJ) during that 45-minute nap.
Background: Been using the Air Touch F20 (Foam) full face mask since being diagnosed with sleep apnea a few years ago. The Air Touch has always worked well and AHI was 0.3 - 1.0. Recently (last couple of months) the cushions seem to be leaking a lot (even new ones) and I've noticed AHI increasing,
If you've been noticing a lot of leaking with the Air Touch cushions, that might be causing you to arouse all the way to wake to fiddle with the cushion in an attempt to fix the leak. And while you're fiddling with the cushion, you may be unconsciously holding your breath (while awake) just long enough for the machine to score apneas. And it's worth noting, whether a machine scores an CA or an OA when you're holding your breath while awake seems to depend on exactly how you physically hold your breath. In my case, the machine will almost always score an OA, and I suspect that I use my epiglottis when I hold my breath in the same way that I do when I swallow food.
Joined as robysue on 9/18/10. Forgot my password & the email I used was on a machine that has long since died & gone to computer heaven.
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