How can you have an Obstructive Apnea without a Flow Limit?

Hi everyone. I thought an obstructive apnea would automatically cause a Flow Limit because that's what an obstruction would do. So I don't understand why I often see events labeled "Obstructive Apneas" and yet they're not happening at the same time as Flow Limits. Am I misunderstanding something about Flow Limits? Thanks for any insights. Here's an example.

Chris33022 wrote: ↑

Mon May 16, 2022 11:04 am

I thought an obstructive apnea would automatically cause a Flow Limit because that's what an obstruction would do.

In layman's terms, an apnea is a cessation of flow, and a flow limitation is a reduction of flow below the normal level of flow. The following discussion is quite comprehensive - http://www.apneaboard.com/wiki/index.ph ... limitation

Hi Chris,
I would agree that one would reasonably expect that a Obstructive Apnea should be preceded by some indication of a flow limitation. After all, we don't really expect that the airway would collapse completely between breaths.
One might speculate that the speed of a complete collapse (OA) occurs so rapidly that the flow limitation algorithm is unable to detect a partial obstruction before it become complete.

Here is a nice example of a flagged flow limitation (of course on your machine they don't flag the limits, but grade then 0-1. Now closely examine your flow pattern just prior to your obstructive apnea and see if the machine software just missed detecting the FL.

Useful question, thanks

dataq1 wrote: ↑

Mon May 16, 2022 12:23 pm

I would agree that one would reasonably expect that a Obstructive Apnea should be preceded by some indication of a flow limitation.

That's not what he asked. You have a habit of misquoting members.

Re: OA and Flow Limit Question

Palerider gave me this info when I first started this journey. When I read this his post and went to the link he gave me, it was like a light came on for me. Hard to explain, but everything just fell into place.

He is one of the best sources of information on here.

Re: OA and Flow Limit Question

jimbud wrote: ↑
Tue Feb 26, 2019 12:33 pm
I am smart enough to know I know a whole lot less about this Obstructive Breathing business than I do know. So here is a question : How can I have a 81 second OA event and show no Flow Limit. (as per Sleepyhead). JPB

by palerider » Tue Feb 26, 2019 5:29 pm

Irregardless of what bonjour says (which may (though I'm far from sure) be 'technically right, is actually wrong in practice as far as the CPAP is concerned) , flow limitations are a restriction of the *rate* of airflow, while hypopneas are a restriction on the *volume*.

ChicagoGranny has a nice picture, hopefully she'll post it. With that you can see that the rate of flow of (the height of the flow curve on the trace) is flattened, and lengthened , which can result in the same volume of air inhaled, but you have to work harder to get it, and this disturbs sleep.

https://www.youtube.com/watch?v=-gie2dhqP2c

Thanks again to Palerider.

JPB

jimbud wrote: ↑

Mon May 16, 2022 2:08 pm

Re: OA and Flow Limit Question

Palerider gave me this info ......

Yes, that made sense to me (that there was a restriction PR, uses a straw example) until I was shot in the chest (while wearing Kevlar) and it hurt to take a nice continuous breath. I had heaps of flow limitations, none of which could be associated with restrictions in my airway, but were associated with muscle wall tenderness.

Brother observed the same thing when he fractured a rib, breathing is just not as nice and smooth causing slight hesitation in inhalation and reflected in flat-topped or mountainous inhalation curves that get reported as flow limitations.

FWIW I have always had OAs flagged with no FL flagging going along with it. In fact I rarely get any FL flagging and it has always been that way with both Respironics machines and ResMed machines.
Never really thought much about it one way or the other. It falls into the category of mine "can't do anything about it so won't worry about it". Which means I never bothered to investigate it. I always just assumed I didn't meet whatever criteria is needed for a flagging.

wrote: flow limitations are a restriction of the *rate* of airflow

It's more complicated than that. From the link above,

From the S9 onward, flow limitation is calculated using a combination of flatness index, breath shape index, ventilation change, and breath duty cycle. Ventilation change is the ratio of the current breath ventilation to recent 3-minute ventilation. Breath duty cycle is the ratio of current breath time of inspiration to total breath time of recent 5 minutes. If a breath is severely flow limited, the flow limitation index will be closer to one and when the breath is normal or round, the flow limitation index will be zero.

In other words, there is a restriction in the airway.

Chris33022 wrote: ↑

Mon May 16, 2022 11:04 am

. . . I thought an obstructive apnea would automatically cause a Flow Limit because that's what an obstruction would do. . . .

Not to be persnickity, but it is really the opposite.

I know what you mean: "why am I getting this flag without getting this flag'? But still, the reality is more that a flow limitation can cause an apnea.

A researcher or or manufacturer may create an arbitrary definition for what to flag based on proprietary considerations and on how something is being measured and evaluated, depending on context.

"The airway" isn't really just one location, despite the fact that the way the breathing is monitored and reported nudges us (tricks us) into thinking of it that way.

You have a stuffy nose one night, so your breathing effort increases. That added work and increase in negative pressure throughout the airway from that limitation at one end of the airway can cause a hypopnea or apnea at a completely different area of the airway. Or a RERA in general.

https://jcsm.aasm.org/doi/10.5664/jcsm.8270

Other apneas and hypopneas are not directly related to what is generally called, or thought of as, inspiratory flow limitation.

lazarus wrote: ↑

Mon May 16, 2022 5:56 pm

I agree with your persnickity, I took it for granted that Chris meant the other way around.

So, let me be a bit picky. Did your really mean to say that "a flow limitation can cause an apnea" ?

I generally agree with Pugsy's statement back in April that "flow limitations are early warning signs that the airway is trying to collapse." (that's what made Chris' question so interesting.)

However I reserve full endorsement based on my family's trauma experience (mentioned above). The preferred sinusoidal breathing curve (particularily on inspiration) seems to rely on the smooth expansion of the chest wall (to create the negative pressure in the lungs). If that smooth expansion is hampered by trauma or disease mechanism, the negative pressure will not develop smoothly, giving rise to a deformed inhalation curve. It is that deformed inhalation curve that is the principle driver for the flow limitation detection.

That is what was explained to me by my pulmonologist after I was shot, why I was having so many FLs but no obstructions. It it made sense to me.

OTOH, (isn't there always another hand), perhaps that OA (the only one that night) was misclassified and should have really been a CA (of which he had about 22 during the session)

Tec5 wrote: ↑

Mon May 16, 2022 10:00 pm

Did your really mean to say that "a flow limitation can cause an apnea" ?

Yes. As the link in my post explains.

When the extra effort to breathe against the limitation of flow occurs, sometimes (but not always) this decrease in pressure throughout the airway can cause narrowing and closing of the airway in locations away from where the flow limitation happens.

You can take a straw with a weakened spot in the middle and pinch it almost closed on the other end. If you increase suction, the straw can be made to collapse where it is weak. It is similar with the airway at times. The tongue and soft palate can be sucked down into the airway from the effects of the negative pressure from increased effort and narrowing sinuses, for example. That is how flow limitation caused in one spot can lead to closure in another. Thus, flow limitation "causing" apnea.

All that may be a distinction without a difference with PAP treatment, since PAP acts as a stent for the entire airway. But with other forms of treatment, it explains why "fixing" one part of the airway surgically doesn't fix the overall problem. A different part of the airway then becomes the weakest link and apneas just start happening there during negative pressure, "moving" the site of apnea away from where the procedure was done.

When a ResMed APAP increases the pressure because of detecting the start of flow limitation, it doesn't just prevent apnea at the location that the flow limitation is occurring but prevents it throughout the airway.

This is so whether other software flags flow limitation in a report or not based on other proprietary arbitrary definitions for reporting.

Of course, an apnea can happen without warning too. And sometimes people use the term "a flow limitation" as simply a way of describing what seems to be an event but that is less than a hypopnea. But (1) what happens, (2) what name is used to describe or label it, and (3) how (or even whether) it gets reported at a lab or by home-machine software are all often three very different things among clinicians, research docs, technicians, manufacturers, software developers, and patients.

lazarus wrote: ↑

Tue May 17, 2022 12:07 am

Very good discussion, thank you.
I was specialy impressed with your visualization of a straw collapsing (somewhere along it's length) when high suction (negative pressure) is applied and somewhere else in the lumen a restriction occurs. Perhaps an analogous (and common) example is really sucking hard on a straw in a milkshake. Maybe most can relate to that experience.

I'd be very interested in your observation relative to an inability to inhale smoothly due to chest musculature disability or trauma (as was in my case) that manifests itself distorted flow rate morphology.

Lastly, any thought on the possibility the the single OA the OP experienced was misidentified? (may actually be a CA?)

I do appreciate having this professional-like discussion, thanks

Thank you so much, everyone, for all these explanations. I didn't want to wait too much longer to thank you all, even though I haven't yet had time to really study every single link you sent me (including ChicagoGranny's, Lazarus's, and the old thread called "OA and Flow Limit Question" that jimbud sent). I look forward to reading those things, as soon as I get a chance.

Meanwhile, I found these two comments you made particularly helpful:

Tec5 wrote: ↑

Mon May 16, 2022 3:05 pm

I was shot in the chest (while wearing Kevlar) and it hurt to take a nice continuous breath. I had heaps of flow limitations, none of which could be associated with restrictions in my airway, but were associated with muscle wall tenderness.

and

lazarus wrote: ↑

Tue May 17, 2022 12:07 am

When the extra effort to breathe against the limitation of flow occurs, sometimes (but not always) this decrease in pressure throughout the airway can cause narrowing and closing of the airway in locations away from where the flow limitation happens.

You can take a straw with a weakened spot in the middle and pinch it almost closed on the other end. If you increase suction, the straw can be made to collapse where it is weak. It is similar with the airway at times. The tongue and soft palate can be sucked down into the airway from the effects of the negative pressure from increased effort and narrowing sinuses, for example. That is how flow limitation caused in one spot can lead to closure in another. Thus, flow limitation "causing" apnea.

All of this is so helpful, but I still don't think I understand it crystal-clearly yet. Thanks to Tec5's example, above, I now understand how a flow limit could be caused by lots of things other than Obstructive Apnea. But I still don’t understand the reverse—meaning, why Obstructive Apnea doesn’t get reported as the ultimate Flow Limit. After all, there is a total stop of “flow” when your airway closes and no air goes through (during OA). So why doesn’t that automatically register as a very strong Flow Limit? Same goes for Lazarus's great straw example. If you plug the end of the straw not partly, but completely, and you're still sucking on the straw, the weak middle part will remain collapsed during this time. So, isn't that a very very strong "flow limit"? Or does something only register as a flow limit if there is still some air passing through, but not a complete stoppage? If yes, then I would think that all hypopneas would be reported as flow limits. Are they? I haven't had time yet to go in my past Oscar graphs and check.

Tec5 wrote: ↑

Mon May 16, 2022 10:00 pm

OTOH, (isn't there always another hand), perhaps that OA (the only one that night) was misclassified and should have really been a CA

I don't know, based on how TheLankyLefty27 explains it in his YouTube videos, that one OA I had looks to me like a real OA because there is a strong increase in breathing right afterwards, as if it's maybe gasping. Jason says that one way to recognize a CA is that the breathing resumes exactly as it was beforehand, with no increase, no gasping.

Thanks again, everyone!

Chris33022 wrote: ↑

Tue May 17, 2022 12:30 pm

But I still don’t understand the reverse—meaning, why Obstructive Apnea doesn’t get reported as the ultimate Flow Limit.

Of course, an apnea is an ultimate flow limitation. However, there are naming conventions and definitions of each term in every field - science, business, religion, etc. In the science of sleep apnea, a flow limitation is not included in the definition of an apnea.

Anyway, what good would it do to report an apnea as also a flow limitation? It's already reported as an apnea. Reporting it as a flow limitation would only open the situation for confusion.

Tec5 wrote: ↑

Tue May 17, 2022 11:42 am

your visualization of a straw

Not originated by me. I first heard it from -SWS and then subsequently heard variations in a lecture at an A.W.A.K.E. meeeting and in various posts. But thanks.

Tec5 wrote: ↑

Tue May 17, 2022 11:42 am

inability to inhale smoothly due to chest musculature disability or trauma (as was in my case) that manifests itself distorted flow rate morphology.

The body and brain are amazing in what they can compensate for with other muscles/neurons! But an APAP may not know what to make of it all as far as translating that with an algorithm. Therefore, I might tend to opt for straight bilevel with a decently appropriate PS, not using any auto modes.

Tec5 wrote: ↑

Tue May 17, 2022 11:42 am

possibility the the single OA the OP experienced was misidentified? (may actually be a CA?)

Always a possibility. Home-machine-based flags and reports are guesstimates in all such matters. And one event can easily be a combination of central and obstructive despite our propensity to label it one or the other. For example, even with plain vanilla OSA, a shift in sleep stage may generate a harmless pause that precipitates a nearly instantaneous closure from the pause itself. A home machine will naturally report that as obstructive (the airway did not remain clear) despite the event not being obstructive at its root.

Tec5 wrote: ↑

Tue May 17, 2022 11:42 am

professional-like discussion,

I am NOT a pro.

Chris33022 wrote: ↑

Tue May 17, 2022 12:30 pm

why Obstructive Apnea doesn’t get reported as the ultimate Flow Limit.

Maybe because that wouldn't be helpful to double-label a single event. But the reporting mechanisms for FL in home machines and patient software are notoriously vague and proprietary and undocumented. I tend to ignore completely all home-machine reports of FL while taking lab reports of it very seriously. Hey, just me.

Chris33022 wrote: ↑

Tue May 17, 2022 12:30 pm

Jason says that one way to recognize a CA is that the breathing resumes exactly as it was beforehand, with no increase, no gasping.

I won't quibble with generalities since I'm as guilty as (possibility more guilty than) anyone. But such matters are clearer in a lab than at home, and a central CAN morph into an obstructive.

Sorry to muddy the waters and equivocate like that, but I'm just naturally a waters-muddier and equivocator. (Apologies to Granny; I know that drives her bananas.)