What makes a machine say CA (central apnea / clear airway)?

[dsws]

When I was getting acquainted with my machine (it's an ibreeze, in case the auto text isn't working), trying it out while awake, I wanted to understand what it was responding to.

I could generate as many "obstructive apnea" flags as I wanted, just by pausing for fourteen seconds between breaths. Usually, ten seconds is enough, but when I think I stop and start a breath isn't always the same as when the machine does. Fourteen seconds is a pause that's short enough to be easy to repeat with every breath, and long enough to guarantee that every single breath will be flagged as an obstructive apnea event. Hypopnea flags and flow-limitation flags aren't as easy to generate consistently, but they show up.

No matter what I did, I could not get the machine to flag anything as a "central apnea". I pause between breaths with my airway open, breathing through my nose as usual, and it's marked as obstructive. I pause between breaths with my mouth open, and it's still marked as obstructive. Nor have I ever seen it flag an event as "central" that happens while I'm asleep.

I thought I found a description some time, but now I can't find it. I think it was something to do with percentage of baseline amplitude over the previous however-many seconds, same general idea as for flagging events as obstructive, just with some difference in details. I've also seen stuff that seems to say it's about the flow in response to the pressure fluctuations that the machine makes after I haven't been breathing for a few seconds.

Anyone know?

[ChicagoGranny]

The iBreeze creates small pressure oscillations (pulses) that flow through the tubing and into the mask. The algorithm then compares this pressure with the corresponding flow response.

[dsws]

Thank you. That's what I thought -- as far as it goes. But what kind of flow response is it looking for, and what can cause that? As I said, i tried holding my breath while keeping my airway open, with all the variations I could think of, and none of them got flagged as a "central apnea" by the machine.

[ChicagoGranny]

i tried holding my breath while keeping my airway open

Bet you can't do it.

The vocal folds, along with the surrounding tissues, form a structure known as the glottis. When holding your breath, the vocal folds adduct (move closer together), effectively closing the glottis and preventing air from entering or leaving the lungs.

The algorithm is on to your attempted trickery.

[dsws]

When I say "hold my breath with my airway open", think of it as breathing in or out very, very slowly until I can't tell which I'm doing from one fraction of a second to the next. The air flow absolutely is stopped only by trying to hold my diaphragm as still as possible. It's been a while (I didn't see the reply when it was posted, mea culpa), so I don't remember whether I was breathing in very slowly, breathing out very slowly, or aiming for completely still. But I sort of think I could do either, at will (i.e. generate apnea flags entirely while breathing in slowly or entirely while breathing out slowly).

[ChicagoGranny]

Don't try to fool Mother FOT.

[dsws]

FOT?

[ozij]

Frequency Oscillation Technology - that's what the machine uses to recognize open vs. closed airway.

[dsws]

Thank you.

I'm not trying to fool it. I'm trying to get it to accurately detect that my airway is open, when my airway is open. And of course, now I was trying to make sure I haven't been fooling myself into thinking my airway is open when it's closed. And no, it was open. I was inhaling, which I couldn't have done through a closed airway.

I just did a few breaths where I inhaled as slowly as I could while definitely inhaling, and as steadily as I could while keeping it that slow. Then I looked at the graph. (I have an iBreeze, so I can only look at the graph on the manufacturer's proprietary software. Its data format isn't compatible with OSCAR or SleepHQ.)

When the machine makes the graph, it uses the steady flow as its baseline, i.e. what it thinks is zero flow. In other words, it assumes that the little bit of air I'm inhaling is just leakage, and it puts the average of the low-flow part right on zero liters per minute. But on one of the breaths it turned expiratory pressure relief on and off during the part that it centered on zero. So apparently it's using a different algorithm to detect breaths in real time than it is when it makes the graph and flags apneas and hypopneas.

Btw, it flagged the same breath as both an apnea and a hypopnea, on a couple of them. Should it really be doing that?

[Miss Emerita]

I'm really not sure why you're worried about this or trying the things you're trying. But to emulate a CA, I think you want to exhale and then pause for at least 10 seconds before you inhale.

[Dog Slobber]

Trying to gage CPAP accuracy by creating what you believe to be Centrals, is silly.

[ChicagoGranny]

Should it really be doing that? s

What is behind this? What are you really concerned about? Do you feel energetic during the day? Do you have excess sleepiness during the day? Just before bedtime, do you feel tired but not stressed?

Reading this forum, one might think the objective is to achieve fine statistics and pretty line graphs with nice marks. That's not the objective. Breathing well and being healthy is the objective.

[Nocibur]

If you're looking for something to do come help me rake leaves.

[dsws]

What is behind this? What are you really concerned about? Do you feel energetic during the day? Do you have excess sleepiness during the day? Just before bedtime, do you feel tired but not stressed?

Reading this forum, one might think the objective is to achieve fine statistics and pretty line graphs with nice marks. That's not the objective. Breathing well and being healthy is the objective.

I have obesity (extreme, but down substantially from my all-time high), diabetes (well controlled with medication, diet, and exercise), fatty liver (moderate), kidney stones (only twice), gallstones (I don't know how severe, because apparently there's no such thing as an elective ultrasound in the US), and high blood pressure (according to the numbers, anyway: the doctors don't seem to care, given the range of other issues competing for their attention, and given that the numbers aren't that far out of line).

However, I feel fine, aside from the few times I've had pain from the kidney stones and gallstones, or the many times my feet hurt because I decided to stand for more than about an hour per day (or walk for more than two, or use the arc trainer for more than three). (Yes, for a while I was literally doing three hours a day of calorie-burning activity at the gym, some days, to lose weight. Compared to that, CPAP is a breeze, at least for me.)

It feels as though I sleep just fine, although it can take me a long time to get to sleep. I wake without needing an alarm, and I don't fall asleep during the day unless I decide to intentionally take a nap. At the primary-care medical appointment, he just asked the questions about daytime sleepiness and whether I wake up gasping, and we left it at that. At the appointment with the obesity doctor, she asked the same questions and got the same answers. But when I commented that I thought sleep apnea wasn't always that obvious, she agreed and recommended a home sleep study, which said moderate obstructive apnea. I also bought a logging pulse oximeter, which shows desaturations even when the CPAP machine thinks I'm doing ok.

From what I've read, and from what I've heard from medical people, my understanding is that low blood oxygen can contribute to a bunch of the medical issues I have -- even if it resolves fairly quickly each time, without waking the person up enough to cause daytime sleepiness. So yes, being healthy is the objective. And because all my health issues are chronic rather than acute, the only way to try to deal with them is to try to understand what's going on, and make the appropriate adjustments to my lifestyle, medication, and CPAP use.

[ozij]

So yes, being healthy is the objective. And because all my health issues are chronic rather than acute, the only way to try to deal with them is to try to understand what's going on, and make the appropriate adjustments to my lifestyle, medication, and CPAP use.

That sounds like what you should do. If you want to learn about how your CPAP works you have to go to the company's site and look for relevant info. And there are many academic papers about how Auto CPAP's function and fail. Different machines respond according to different algorithms.

I have obesity (extreme, but down substantially from my all-time high)...
[...]
I also bought a logging pulse oximeter, which shows desaturations even when the CPAP machine thinks I'm doing ok.

Ever heard of OHS = Obesity Hypoventilation Syndrome? Could be the machine you're trying to get acquainted with isn't good for the job of making sure your oxygen is high enough and stable all nigtht through.

Definitely discuss your low oxygen with the doctor who prescribed the machine, and if he/she doesn't trust your oximeter, ask for a nightly oximetry recording with a professional oximeter.