Flow Limitation

[super7pilot]

So on the two apnea forums I belong to. I see much talk of the FL's.
And everytime I start reading about it. It is long winded, overly tech based. And I want to go drown myself after having read the thread and walked away with my eyes glazed over and a headache.
Seems it's some proprietary secret with Resmed as to what the reading on the oscar scale even means. Does a .25 FL mean that there is a 25% restriction. Does .50 mean a 50% restriction? So if no one knows what the scale means, why does anyone even pay attention to it or make pressure adjustments on an unknown?

In a recent phone convo with my sleep Dr. I asked about FL's, she said to ignore it and it means nothing unless I was on some sort of Bipap or equivalent. And that APAP isn't one of those.

It would be nice to have a CPAP for dummies simple explanation.

Here is my chart from last night. I had my P30i on and switched to my F40i at 1:39 because my nose was going stuffy.

[Miss Emerita]

Flow limitations are what they sound like: restrictions of the flow of air somewhere in your airway. The scale from 0 to 10 is really just a way of saying "no restriction" to "a lot of restriction." And the way the machine figures out whether there's a restriction is by analyzing the shape of the flow-rate curve when you're inhaling. If there's a dent in the curve, or a flat-top, it'll flag it as an FL.

For some people, heavy FLs make no difference to their sleep. For other people, FLs have a negative effect on their sleep.

The best tool we have for reducing FLs is EPR. It lowers your pressure when you exhale, and by the same token it increases your pressure when you inhale, giving you a little boost. If you want to experiment, you could try raising your EPR to 3.

[dataq1]

I'd add a bit of nuance/correction to Miss Emerita post.

The scale is 0 to 1.0 wherein 0 is supposedly great and 1.0 is horribly bad. It is an arbitrary scale, and the your instantanous values determined by Resmed and others are indeed proprietary (and perhaps defy any coherent explanation).

As I understand it, the principle factor (just as Miss Emerita said) is the shape of your inhalation curve. In a perfect world, your inhalation curve would be perfectly rounded curve, that reflects the movement (expansion) of your chest wall (assuming that your chest expands in a smooth regular fashion.
If however, your chest does not rise and fall smoothly, for whatever reason, the air flow will not be smooth and regular even with a completely open airway.

What the proprietary Flow index does is assumes that any slight interruption in the smooth intake of air is the result of some force that partially blocks the air flow. So let's call the blockage a limitation to flow hence "flow limitation"

BTW, my pulmonologist also says to ignore the flow limitation ranking (unless consistently above 0.4), as it is his opinion that this arbitrary determination is principally used to validate a reason to increase pressure when using an titrating (auto) cpap.

[super7pilot]

And I doubt there are many folks (even those without Apnea) that have a pure sinewave breathing pattern.

Yes! The car analogy was an odd one for sure. So I'll restate it.

We measure our body temps in F or C. But imagine having a thermometer that went from 0-1. So what's a body temp of .5 mean? Should I take two aspirin or fill the tub with ice?

[dataq1]

And I doubt there are many folks (even those without Apnea) that have a pure sinewave breathing pattern.

Yes, absolutely.
Now to complicate the explanation (and perhaps give some credit to the software engineers), the evaluation for air flow pattern not only looks at the most immediate shape, but also at how that current shape may have changed over the past few minutes. If the current shape is different from the prior shape, the algorithm may flag this as a flow limitation, and indeed it may be.
Consider the shape may change if suddenly one nostril closed off (definitely a obstruction to flow), but also consider how your chest wall is impacted if you roll over in bed, so that now the chest muscles have to overcome the weight of your thoracic region. It's easy to see how the air flow curve will change, but adding a bit of pressure boost (because the FL algorithm calls for it) wont overcome the gravity of your spine and back muscles.
Bottom line (for me) is don't attach extraordinary importance to FL, unless you know exactly what is causing it.

[super7pilot]

Consider the shape may change if suddenly one nostril closed off (definitely a obstruction to flow), but also consider how your chest wall is impacted if you roll over in bed, so that now the chest muscles have to overcome the weight of your thoracic region. It's easy to see how the air flow curve will change, but adding a bit of pressure boost (because the FL algorithm calls for it) wont overcome the gravity of your spine and back muscles.
Bottom line (for me) is don't attach extraordinary importance to FL, unless you know exactly what is causing it.

And yet, The Flow Limitation oscar chart is talked about by a lot of folks in general. So, we (the forums) are def assigning great importance to the FL readouts that are actually secretive in nature and can't be quantified. Color me confused.

But I for one am going to just ignore or not even track the FL's anymore.

[Miss Emerita]

It's fine to ignore FLs if you want to. On the other hand, if you are not yet sleeping well, and you want to experiment, you could try using more EPR to see whether that made you sleep better. -- Notice I said nothing about the numbers there!

[super7pilot]

Flow limitations are what they sound like: restrictions of the flow of air somewhere in your airway. The scale from 0 to 10 is really just a way of saying "no restriction" to "a lot of restriction." And the way the machine figures out whether there's a restriction is by analyzing the shape of the flow-rate curve when you're inhaling. If there's a dent in the curve, or a flat-top, it'll flag it as an FL.

For some people, heavy FLs make no difference to their sleep. For other people, FLs have a negative effect on their sleep.

The best tool we have for reducing FLs is EPR. It lowers your pressure when you exhale, and by the same token it increases your pressure when you inhale, giving you a little boost. If you want to experiment, you could try raising your EPR to 3.

Not much change to the FL's (from 99.5% .13 @ EPR-2 to 99.5% .09 @ EPR-3) Centrals picked up in number though

screenshot-20241125-111652.png (519.61 KiB) Viewed 1719 times

[Miss Emerita]

Your 95% number went from .03 to .00, which is a significant change. I wouldn't worry about the CAs. The key question is whether you have more restful sleep with EPR of 3. Give it at least 3 or 4 nights before you try to answer that question.

[super7pilot]

I have been overall more rested as I get used to the therapy.
When I started to back off on the EPR setting and also raise the min pressure to the 8-10 range. My CA's (mostly) went away.

From day one with a setting of 4-20 to my current setting of 11-16. My OA's have been pretty much nonexistent at about 10-11.5cm. It has been the CA's that have been the most active apnea class (didn't have CA's in my lab) then Hypopnea's, then a few Rera's. I think last night was the first UA.

Now that I'm improving I seem to be chasing after that last very elusive mosquito that I can hear but can't see.

Like my post in the other forum eluded to. After working in electronics for nuke weapons systems. I am used to dealing with absolutes. This equals that, full stop. None of this mysterious "it depends' 'might be'

[robysue1]

Seems it's some proprietary secret with Resmed as to what the reading on the oscar scale even means. Does a .25 FL mean that there is a 25% restriction. Does .50 mean a 50% restriction?

Nobody knows what the numbers on the flow limitation graph in Oscar mean because Resmed hasn't published any units for the vertical scale of the flow limitation graph. Indeed, Resmed has made very little technical information about that graph public. And it's important to remember that Oscar is the result of sleep apnea patients programming their own program to read CPAP data. (The initial work was done by a (former) CPAPtalk member by the name of JediMark when he was frustrated with a whole bunch of things.) Resmed, PR, and other CPAP manufactures have never sanctioned any of the work done on the Oscar project, and none of the manufacturers have shared proprietary information with any of the Oscar programmers.

In ResScan, Resmed's own software for reading the data off a Resmed CPAP's SD card, there is a flow limitation graph, but instead of a numerical "scale" for the vertical axis, there are three icons: A very round inhalation icon that corresponds to 0.0 on the Oscar graph, a somewhat distorted inhalation icon that corresponds to 0.5 on the Oscar graph, and a very distorted inhalation icon with a flat top that corresponds to 1.0 on the Oscar graph. The Resmed documentation for interpreting the ResScan flow limitation graph essentially says that the higher the graph is, the more distorted the shape of the inhalations are. In other words, the vertical scale of the graph is about how distorted the shape of the inhalations are rather than whether there is a measurable drop in air flow into the lungs during inhalation.

According to Resmed and various journal articles, the more distorted the shape of the inhalation part of the flow rate graph, the more likely it is that the upper airway is becoming unstable and hence the more at risk the upper airway is to collapsing enough to restrict the air flow enough to warrant flagging a hypopnea or an obstructive apnea. The only thing is, we have no idea what factors Resmed's algorithm is using to judge the degree of distortion in the shape of the inhalation part of the the flow rate graph, nor do we know what their weighting of those factors is. It's also apparent from what little documentation is out there, Resmed uses some kind of running "average" over the last several minutes of breathing when as part of the computation in how "distorted" the inhalation part of the flow rate graph appears to be.

Intuitively, based on the icons the ResScan software uses, we would expect a significant degree of flow limitation to be scored when the tops of the inhalations are flat and square shaped. In practice, however, if you scroll through a lot of flow rate graphs while simultaneously looking at the flow limitation graph, you see a lot of places where you're left scratching your head: Why was a flow limitation scored HERE, but not THERE? Why are these inhalations considered more "flow limited" than those inhalations, which look pretty much the same?

Then there's this fact: It's quite clear that in some people's data, flow limitations get scored when they're congested and the problem is not the upper airway, but the nose. And that kind of "flow limitation" isn't necessarily something that indicates the upper airway is at risk of collapse.

So if no one knows what the scale means, why does anyone even pay attention to it or make pressure adjustments on an unknown?

It is known that Resmed's Auto algorithm responds (and responds aggressively at times) to flow limitations. Indeed, activity in the flow limitation graph is responsible for the vast majority of pressure increases in many people's data. Once the machine is happy enough with the shape of the inhalations, it starts to decrease the pressure.

Since Resmed's AutoSet and VAuto increase the pressure when flow limitations are being scored, most people around here jump to the idea that flow limitations must be bad. And since some flow limitations get better with the increase in pressure, there are folks around here who kind of jump on the idea that eliminating as much activity from the flow limitation curve as possible will mean they are sleeping "better". And the way to attempt to eliminate or minimize flow limitations is to increase the minimum pressure on an APAP or the minimum EPAP on a VAuto (Resmed's bipap machine).

In addition, there are folks on this forum who either have or think they have UARS rather than OSA. The difference between UARS and OSA is a matter of where the sleep disordered breathing fits on a continuum. In UARS, people typically arouse before the airway has collapsed enough to score a hyponea, but there is evidence of increased respiratory effort---i.e. the person is working harder to get air into their lungs in the breathing right before the arousal. (This kind of arousal is called a RERA.) And on an in-lab sleep tests designed to measure UARS, its been noticed that there is a strong correlation between badly distorted inhalations and RERAs. So it's hypothesized that flow limitations (in the form of badly distorted inhalations) are what wakes people with UARS up. So people with UARS often try to minimize activity in the flow limitation graph, often because they are worried that the algorithm for scoring RERAs may not be picking up all of the RERAs that are still occurring while using CPAP.

But it's not always that simple: Sometimes flow limitations do not get better when the pressure is increased. Why? Nobody on this forum really knows, but a reasonable hypothesis is that if the flow limitations do not get better when the pressure is increased, then they're probably not caused by an airway that is becoming unstable and threatening to collapse due to OSA. Rather, maybe they're caused by nasal congestion or (particularly if they're persistent) a deviated septum. And then there's this lovely piece of the puzzle as well: Sometimes too much pressure makes the breathing unstable and those unstable breaths can look like and be scored as flow limitations by the machine.

In a recent phone convo with my sleep Dr. I asked about FL's, she said to ignore it and it means nothing unless I was on some sort of Bipap or equivalent. And that APAP isn't one of those.

My guess is that your sleep doc believes that you have a plain vanilla version of OSA and that as long as your AHI is nice and low and you are not reporting that you are waking up numerous times every single night and having trouble getting back to sleep, then the machine's Auto algorithm can deal with whatever flow limitations there are without needing to focus on them as a specific issue.

It would be nice to have a CPAP for dummies simple explanation.

15-20 years ago most patients were given straight CPAP machines that recorded nothing but how long you used the machine. You had to fight (and fight hard) to get a machine that recorded the efficacy data---i.e. the stuff you see in Oscar. Back then, most sleep docs and DMEs thought it was fine if a sleep apnea patient had no information at all about how well their machine was working in terms of treating their OSA. In other words, the sleep docs and DMEs wanted to treat us patients like mushrooms---something to be kept in the dark.

So there's been a change in how much information the average patient is now given access to (through things like MyAir and the fact that many more patients are now set up with AutoPAPs instead of dumb data-free CPAPs. But there's still a lot of folks out there in sleep medicine who seem to believe the average patient is just too dumb to understand any of what's going on. And that's why there's no CPAP for dummies simple explanations for a lot of things.

Here is my chart from last night. I had my P30i on and switched to my F40i at 1:39 because my nose was going stuffy.

For what it's worth: The flow limitation graphs on the two charts you've posted don't actually look that bad. If these were my charts, I would not be worried as much about the flow limitations as the raggedness in the flow rate graph itself: It is possible that you are having more spontaneous arousals than you realize. It is also possible that since some CAs seem to be mixed in with periods where flow limitations are being scored, that your flow limitations may be unstable breathing that is not being caused by an airway that is threatening to collapse.

I have been overall more rested as I get used to the therapy.
...
Now that I'm improving I seem to be chasing after that last very elusive mosquito that I can hear but can't see.

If I were you, I'd focus less on trying to eliminate the last of the flow limitations (i.e. the elusive mosquito) and more on how you are feeling. Sometimes when we try to chase the "perfect" flow rate curve with a 0.0 AHI and no flow limitations all night long, all we do is make it harder to just get some really nice, decent enough sleep that allows us to be at our best in the daytime.

In other words, don't let the perfect become the enemy of the good.

Like my post in the other forum eluded to. After working in electronics for nuke weapons systems. I am used to dealing with absolutes. This equals that, full stop. None of this mysterious "it depends' 'might be'

As a mathematician, I can understand a bit about what you mean by "dealing with absolutes"---you've either got a theorem proved or you don't.

But in my own long and very difficult journey to becoming a successful happy PAPer, I learned to think of dealing with CPAP/APAP/BiPAP/AutoBiPAP as a process. As in it's more like working with mathematical conjectures rather than the end result (a proved theorem). It takes a whole lot of mucking around playing with ideas that don't work out to turn a conjecture into a theorem, and part of that process is tweaking the hypotheses---i.e. the mysterious "it depends" and "might be's" of the proof-finding process.

[super7pilot]

Thanks for that info Robysue1, I have felt all along that a lot of my issues are with nasal congestion and a deviated septum on the right nostril. The septum was a recent revelation by an ENT. However, When I was on CPAP many years ago. I had some Radio freq reduction done on some turbinates to help with the sinus. But ultimately as I was young and not over weight. The Dr's said I was a good candidate for UPPP surgery. Which I did have done. And boom. Apnea no more. Well for 23 years at least.

And of course I also wonder if the ragged breathing (at least partially) is from just not being completely used to my therapy yet. My P30i mask seals great and is comfy. But it is noisy to the point of being bothersome.

My F40i is very quiet and comfy. But has proven to have an unreliable all night seal because that stupid (but stylish) curvy upper strap pulls the mask up so much that even with the nose cradle. It ends up waking me from nose pain. Any slack on the upper straps elicits leak city.

I have the airfit F30i on it's way courtesy of the VA. It's strap system looks like it will pull the mask into my face without the upwards pull. But noise from the hollow frame could still be an issue.

I freaking hate masks.

[super7pilot]

Well, That was a regressive night. Went to bed with the nasal pillow P30i. Something I noticed that was strange. And this was the bouncing I could feel in the hollow mask frame. Could it be that with EPR-3 (at the start of the night) that as I exhale then right after that exhale starts the machine then drops the pressure. Thus creating a rebound/bouncing? It was very disconcerting to say the least

The first part of the night I was doing ok. Then the nose started in at just past 2am, went away and returned at 3:45 At about 4:05 I had a bathroom break. That lasted until 4:15 when I took off the P30i and put on the F40i and went to 100% mouth breathing and EPR-2. That went for about 20 minutes when I (purposely) for the first time in DECADES. Turned onto my back. And it all went off the rails. Although my numbers were still not nutty high at AHI 5.68 for the night. Had it not been for the craziness at 4:40 to 5:08. My numbers would have been just slightly over my recent average.

Very frustrating. But I'm quite sure the basis of my issues with not getting restful sleep is a combo of the nose congestion and perhaps an intolerance to the aggressive pressure changes being made. Esp on EPR. I'm not sure what to think now.

I really need to get into the ENT Dr. and have some sort of roto rooter on my nose to fix the deviation & congestion. Even though I don't like to, I may try a shot of afrin one night to see how that effects my restfulness.

I'm starting to fear that I will be one of those with a machine in the back of the closet if things don't improve. The improvements since starting therapy have been that I have returned "almost" to the same level as I was at before therapy. I'm just not so zombie like as when I first went on therapy.
And for extra insult. I do have a wicked case of dry eye in my left eye and it burns to no end. That certainly doesn't help with getting to and staying asleep.

[Miss Emerita]

Yes, you could do an Afrin experiment. Some people benefit from using Flonase, though it takes a week or two to kick in. And if you haven't already, you might get yourself tested for allergies.

[robysue1]

Thanks for that info Robysue1, I have felt all along that a lot of my issues are with nasal congestion and a deviated septum on the right nostril. The septum was a recent revelation by an ENT.

Given this info, I'd just ignore the small remaining amount of activity in your flow limitation graph.

My P30i mask seals great and is comfy. But it is noisy to the point of being bothersome.

My guess is that the noise issue is being caused by the fact that the P30i has the hose connection at the top and the air flows through hollow tubes that lie on the sides of your head. That means the sound of the air moving through those tubes can be conducted directly into your inner ear by your skull.

You might find a Resmed P10 nasal pillows mask to be just as comfy, but much quieter since the hose connection is at the front of the mask so the air doesn't flow down tubes that are lying on the sides of your head. The Resmed Swift FX also has very comfortable headgear that is more stable than the P10's, but the FX has a horrible exhaust vent design that blows a jet stream directly on the user's chest or arms. (I use a homemade, unauthorized "diffuser" when I use my FX because I just can't tolerate that jet stream exhaust flow.)

My F40i is very quiet and comfy. But has proven to have an unreliable all night seal because that stupid (but stylish) curvy upper strap pulls the mask up so much that even with the nose cradle. It ends up waking me from nose pain. Any slack on the upper straps elicits leak city.

Do you mean the F40? I can't find an F40i listed among cpap.com's masks. The F40 has the hose connection at the front, which would explain why it's quieter than the P30i. Given the design of the mask, I can see why you say it pushes the nasal cradle up into your nose and causes pain. Particularly since you can't loosen the straps without causing leaks. That is one of the downsides of full face masks with nasal cradles.

I have the airfit F30i on it's way courtesy of the VA. It's strap system looks like it will pull the mask into my face without the upwards pull. But noise from the hollow frame could still be an issue.

Yep. I bet this one will be just as noisy as the P30i.

But I have to ask: If the P30i works well (except for being noisy), why are you trying to find a full face mask? Why not try the P10 instead?

I freaking hate masks.

Truth be told, I think a lot of us hate masks, but we realize that putting up with the mask is a necessary evil if we want to get a really good night's sleep.