First: Don't panic.
If your AHI is very low, your blower therapy is probably doing its job.
Please see my replies interspersed below.
(07-22-2013, 09:50 AM)Gatnus Wrote: ... During both, I had 0 central apneas. According to the S9 however, I can't go a night without having 2-4 centrals per hour. ...
I have had nothing but centrals since I started the S9. You are not alone. Actually, I might have one or two obstructives a year.
(07-22-2013, 09:50 AM)Gatnus Wrote: ...Any idea if the S9 can be reporting false positives?...
It is unlikely they are reporting false positives for the very reason that you can see the FOT curve in the flow graph. Please view the following image:
This is a portion of your original image. Notice the very tiny waves in the section highlighted in red. These are Forced Oscillation Technique (FOT) pulses that kick in when the blower recognizes that you haven't been breathing for a few seconds. Since your airway is unobstructed, the pulses are considerably dampened in the mask pressure graph (below your flow graph in your original image). It's not exactly clear without comparison to an obstructive apnea. If you were having an obstructive apnea, the airway would be blocked, and the FOT in the Mask Pressure would be more eratic because the volume of the lungs isn't open to dampen the feedback. Check out this image comparing an obstrucive to a central apnea from a recent night for me:
Notice that with an obstructive, you can see no heartbeat secondary-order waves inside the area of FOT (highlighted in red in the flow rate). However, in the open-airway (central) apnea flow rate, you see the heartbeat (as indicated by the blue arrows) because the pressure of the heart's pulse feeds back into the graph. The machine is that sensitive. Notice that the FOT in the mask pressure is more regular because it is dampened by the volume of the lungs, so there a smaller feedback to the mask sensor. These two things together fairly assure airway patency (openness): heartbeat in Flow Rate and dampened FOT in Mask Pressure. There may be more things ResMed's algorithm looks for, but for the most part, these are the things that show very clearly when the airway is unobstructed.
(07-22-2013, 09:50 AM)Gatnus Wrote: ...Is this something I should be concerned about?
It all depends. If your AHI is low (below 5, but preferrably below 2), then medicare would say your therapy is going well. If you still feel like you're not getting enough sleep or you still have massive headaches when you wake in the morning, there may be other factors. For instance, you can have an AHI of .5, but if your centrals are often 36 seconds long, that's something to worry about. You should see your sleep doctor. If you don't see many central apneas above 10 to 15 seconds in length with an AHI of .5, well heck, if you're reasonably fit enough to swim, you probably hold your breath longer than that underwater, so it's likely less problematic.
(07-22-2013, 09:50 AM)Gatnus Wrote: ...Why would a hospital and a free standing clinic would both put me at 0 CAs per night but the S9 has me having 15-30 episodes per night? Are they setting different standards for what a CA is?
Sleep studies use thoracic belts that measure effort during apnea made by your chest/belly. (You may remember having to strap a belt or two around your midsection for your sleep study.) This is the standard. FOT is considered a valid measure of airway patency, but for some reason (not well understood by me) it is not really used in validating obstructive apnea. Yet.
Believe it or not, sleep medicine is still young. Less than ten years ago, some people were still using machines the size of a small canister vacuum. And they were loud. A lot has changed since then. It takes a while for new technology to filter into the sleep lab.
If your AHI is low, your centrals are not very long, your leak line is relatively flat, and you have some zest in your day, you're probably doing okay. If you have any doubts it's always a good idea to talk with your doctor to see if you should be checked out for complex sleep apnea or periodic breathing. In either case, you'll be prescribed a more expensive machine, the ResMed VPAP ASV or the PRS1 BiPAP AutoSV Advanced.