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Difference between central and clear airway apnea
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Sleepster Offline
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Post: #31
RE: Difference between central and clear airway apnea
The CPAP machine has a pressure sensor and a flow rate meter. When the pressure changes there's a response: the flow rate changes. Pressure pulses are just rapid changes in pressure, for example 4 Hz would represent 4 cycles per second, each cycle lasting for 0.25 seconds and consisting of, for example, high pressure followed by low pressure. The response (reading on flow meter) is used to make a determination as to whether the airway is open or closed.

I don't know of any independent studies that have been done to investigate the reliability of the determination.

One thing's for sure, manufacturers are getting pretty darn good at using the determination so that their automatic pressure adjustment algorithms can do a very good job of keeping us comfortable with lower pressure when they can, and treating obstructions with higher pressure when necessary.

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05-17-2016 06:35 PM
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archangle Offline
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Post: #32
RE: Difference between central and clear airway apnea
(05-17-2016 01:51 PM)PaytonA Wrote:  FOT is a little more than an attempt. There are over 20 years of data backing it up in other ventilation scenarios. Chest belts are a secondary measurement also and are not 100% accurate.

Chest effort belts are the gold standard, no if's, and's or but's. They measure the "real" symptom, which is lack of respiratory.

FOT is remarkable, but a distant second compared to a chest belt.

First, it detects clear airway, not central apnea. Central apnea is the thing of concern. You can have a central apnea with an obstructed airway. The difference matters because central apnea has different causes and treatment than obstructive apnea.

FOT is also problematic because it's a very indirect measurement. The machine is trying to "see" the state of the airway by "looking" through the humidifier, hose, mask, and nasal passages. The mask matters. Airflow restrictions in the airway matter. The compressibility and stretchiness of the tissue around the lungs matters. Leaks also complicate things.

FOT is useful information, especially since you get it "for free" with out extra sensors, just the CPAP interface to the patient. In particular, I think when the machine says "CA," it probably is a central, unless you're awake. However, I think it will sometimes misclassify a CA as obstructive or UA.

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05-18-2016 04:05 AM
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FrankNichols Offline

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Post: #33
RE: Difference between central and clear airway apnea
(05-18-2016 04:05 AM)archangle Wrote:  In particular, I think when the machine says "CA," it probably is a central, unless you're awake.

Based on the definition I have seen presented in this thread, I would think if I am laying in bed awake and I intentionally stop breathing for 10 seconds or longer, it should record a CA - since the airway is not obstructed, my brain is simply not sending the signal to breathe. On the other hand maybe if the brain decides not to breathe it also instructs other parts of the body to ensure that I do not breathe by closing off the airway. My point being, it is not JUST the brain doesn't say to breathe. There is more physiology going on during an actual CA.

I just tried it (twice) and both breathing interruptions were recorded as OAs.

[Image: TwoOAs.png]

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(This post was last modified: 05-18-2016 05:08 AM by FrankNichols.)
05-18-2016 05:06 AM
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Post: #34
RE: Difference between central and clear airway apnea
(05-18-2016 05:06 AM)FrankNichols Wrote:  Based on the definition I have seen presented in this thread, I would think if I am laying in bed awake and I intentionally stop breathing for 10 seconds or longer, it should record a CA - since the airway is not obstructed, my brain is simply not sending the signal to breathe.
....
I just tried it (twice) and both breathing interruptions were recorded as OAs.
This happens to me as well. I think when I hold my breath intentionally my epiglottis covers the top of the windpipe the same way it does when I'm swallowing. (That is the purpose of the epiglottis: It seals off the windpipe when you are swallowing so that food or liquid goes down the esophagus instead of the windpipe.)
If the epiglottis is in position to block the windpipe, then the airway is technically obstructed and the CA test can easily detect that blockage, but it can't detect that its cause is the epiglottis in the normal position for swallowing.

Quote:On the other hand maybe if the brain decides not to breathe it also instructs other parts of the body to ensure that I do not breathe by closing off the airway. My point being, it is not JUST the brain doesn't say to breathe. There is more physiology going on during an actual CA.

I think you may be onto something here. This is why a CA in a lab is scored based on the the belts. In a lab, they do NOT make any effort to determine the patencey of the airway during an apnea. The belts are what is used to determine if the apnea is an obstructive apnea, a central apnea, or a mixed apnea:
  • If the belts show evidence of effort to breathe, the apnea is scored as an obstructive apnea because the assumption is that since an effort to breathe is present, the problem must be an obstructed airway.
  • If the belts show that NO effort to breathe is being made, the apnea is scored as a central apnea because the problem with caused by the brain failing to send a "Inhale Now" signal. The lab does not determine nor care about the patencey of the airway because the "real" problem is that no effort to breathe is being made.
  • If the belts show NO effort to breathe is being made at the start of the apnea, but the flow rate shows that no air is getting into the lungs after the effort to breathe resumes, the apnea is scored as a mixed apnea: The apnea starts off looking like a central apnea, but instead of the airflow resuming as soon as the brain starts sending "Inhale Now" signals, the airway is blocked and so the apnea ends looking like an obstructive apnea.

Our machines can only measure airflow and estimate the patency of our airways. If a central apnea occurs while the airway is open, it will be correctly scored as a CA. If a central apnea occurs while the airway is obstructed (for whatever reason), the apnea will be misscored as an OA. If a mixed apnea occurs, it will be misscored as an OA.

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05-18-2016 06:13 AM
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Post: #35
RE: Difference between central and clear airway apnea
Interesting subject...

Yesterday, when my alarm went off, I deceided to lay there still attached to machine and listed to the news. I know I was fully awake. Later, when looking at Sleepyhead, it had recorded two CA's during that 5 minutes that I was just laying there awake. Both were short...about 12 seconds, and I don't remember purposely holding my breath. Dont-know

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(This post was last modified: 05-18-2016 06:30 AM by OpalRose.)
05-18-2016 06:29 AM
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FrankNichols Offline

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Post: #36
RE: Difference between central and clear airway apnea
(05-18-2016 06:29 AM)OpalRose Wrote:  Interesting subject...

Yesterday, when my alarm went off, I deceided to lay there still attached to machine and listed to the news. I know I was fully awake. Later, when looking at Sleepyhead, it had recorded two CA's during that 5 minutes that I was just laying there awake. Both were short...about 12 seconds, and I don't remember purposely holding my breath. Dont-know

It must have been some special news to take your breath away Smile

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05-18-2016 06:52 AM
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FrankNichols Offline

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Post: #37
RE: Difference between central and clear airway apnea
(05-18-2016 06:13 AM)robysue Wrote:  This happens to me as well. I think when I hold my breath intentionally my epiglottis covers the top of the windpipe the same way it does when I'm swallowing. (That is the purpose of the epiglottis: It seals off the windpipe when you are swallowing so that food or liquid goes down the esophagus instead of the windpipe.)
If the epiglottis is in position to block the windpipe, then the airway is technically obstructed and the CA test can easily detect that blockage, but it can't detect that its cause is the epiglottis in the normal position for swallowing.

Interesting, I just played around stopping breathing a few times and I think (am probably wrong) that I was able to tell when I closed my epiglottis. If I maintain slowly breathing and just "pause" without trying to "hold" my breath I feel nothing in my throat, if I take a breath and "hold it" - say like I was going to dive into the pool - I feel my throat close.

Probably meaningless, but interesting. Your comment sounds right on!

I think the point is the FOT is a technique that gives some information for the algorithm to base adjustments on without requiring us to wear a belt around our abdomens. It is not a perfect technique - but it does work often enough to help.

My feeling is we should not get all worked up over absolute numbers reported, but should be more concerned with trends and feelings. How we feel is a good indicator of quality of sleep - not perfect, just good. And numbers that are trending down are better than numbers that are trending up...

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05-18-2016 06:59 AM
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Post: #38
RE: Difference between central and clear airway apnea
(05-18-2016 06:59 AM)FrankNichols Wrote:  I think the point is the FOT is a technique that gives some information for the algorithm to base adjustments on without requiring us to wear a belt around our abdomens. It is not a perfect technique - but it does work often enough to help.
Exactly! The FOT on the Resmed machines and the PP on the PR machines do a good job in determining which apneas are most likely to be CAs and which ones are least likely to be CAs. And that allows the machines to appropriately NOT adjust the pressure when the apnea(s) being scored have a high probability of being central apneas.

Quote:My feeling is we should not get all worked up over absolute numbers reported, but should be more concerned with trends and feelings. How we feel is a good indicator of quality of sleep - not perfect, just good. And numbers that are trending down are better than numbers that are trending up...
Exactly! The manufacturers have never claimed that their machines perfectly score every event all night long for every CPAP user.

But they do score enough things correctly most of the time for most users to provide high quality trending data. And the combination of the trending data and the subjective data on "how you feel" gives a very complete picture of the efficacy of therapy over the long run.

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05-18-2016 07:44 AM
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Post: #39
RE: Difference between central and clear airway apnea
(05-18-2016 06:29 AM)OpalRose Wrote:  I don't remember purposely holding my breath.

awake breathing is very irregular. if you wore a breathing monitor all day long, and tried to score it by sleep breathing standards, it'd look like absolute crap... however it's perfectly normal.
05-18-2016 01:54 PM
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Post: #40
RE: Difference between central and clear airway apnea
(05-18-2016 04:05 AM)archangle Wrote:  Chest effort belts are the gold standard, no if's, and's or but's. They measure the "real" symptom, which is lack of respiratory.

Chest effort Belts may be the gold standard but they do not measure the "real" symptom which is the brain not sending the signal to breathe per the definition of central apnea.
05-18-2016 02:00 PM
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