flow limitations expressed as an index?

[sheepless]

thanks for the useful comments. keep 'em coming!

I'm aware that there are uncertainties, faults and variations between manufacturers with respect to reporting flow limitations. way too complicated and I don't want to get hung up on what could be better.

at this point all I'm suggesting is to take whatever a machine reports and display it as an index or percentage (of density or frequency, duration per hour) for purposes of evaluating degree of flow limitation (in addition to severity, in whatever way that's determined) and comparing nights and settings. it doesn't have to be more accurate for comparison purposes. ideally, it would be a jumping off point for more but I think useful by itself.

[TBMx]

Could you explain a little bit more what exactly you believe these flowlimitations do to you?
Might sound like a silly question to you - we (meaning multicast and me) have this as an ongoing 'digression'. ResMed is the only one manufacturer, which overrates their 'flowlimitations' and has constructed a whole APAP-Algorithm around it. To no surprise every bench-study out there comes to the same conclusion: ResMed overtreats (meaning: the AutoSet-Algorithm produces a higher than necessary pressure). (EPR can also be seen as a necessity to compensate for that.)

The Sao Paulo Cohort did show that a completely normal and healthy population sleeps up to 30% of their sleep time with flowlimitations - so that seems to be quite normal. (An AutoSet would go crazy on those)

As the AutoSet-Algorithm reacts only on obstructive Apneas and their flowlimitations you already have the density of your flowlimitations (in the absence of remaining OAs) presented by your machine: the pressure!

I haven't found any indication that flowlimitations play a key role (or any role at all) with OSAS. If you are suffering from non-hypoxic SRBD that would be a different case - but so far you can't have both.
Maybe just try a lower upper pressure or a more gentle pressure-reaction and see if not ResMed's FLs disturb your sleep but the AutoSet-Reation to those^^ (most people sleep better with lower average pressure - assuming that their minimum pressure needs are met ... in the end: that is all that any APAP does: provide a lower average pressure than CPAP would (as CPAP would need to treat all or the majority of the obstructions meaning higher pressure all the time))

[sheepless]

I'm not sure I understand your comments well enough to respond intelligently. first, it isn't necessary to get into the weeds for what I'm looking for at the moment: simply a means of comparison to judge whether flow limitations are better or worse. we can currently compare a measure of severity but not duration (as far as I know) which strikes me as just as important.

I have experience only with resmed machines and yes they react to flow limitations. I haven't mentioned it in this thread but I have periodic limb movement. my theory is that the jolting movements produce, among other things, relatively flow limited breaths. whatever the mechanism, resmed machines raise pressure in the midst of my plm breathing. again, my theory, plm induced flow limitations are not passive (think of what happens to the throat and by association the airway when we grunt), so like chin tucking, pap pressure will not overcome these. the consequence is runaway pressure with apap, dramatic cycling of pressure support with asv.

but that's just one example and one that doesn't apply that widely. many people are sensitive to flow limitations, fluctuating or high pressure and attendant leaks and aerophagia. many people seem to suffer uars.

I don't know - not sure anyone does - where the thresholds of significance lie with respect to flow limitations. at present it seems pretty subjective. hence, perhaps, the need for more ways to characterize them empirically.

but for now, all I'm suggesting is to report whatever the machine data tells us about density/frequency/duration. maybe that'll reflect pressure as you suggest but I'd like to be able to demonstrate the association.

[cathyf]

To no surprise every bench-study out there comes to the same conclusion: ResMed overtreats (meaning: the AutoSet-Algorithm produces a higher than necessary pressure). (EPR can also be seen as a necessity to compensate for that.)

I've definitely struggled with understanding this for 6.5 years of using my ResMed machine. Looking at my data, I seem to have 3 states when the machine is on:

1. Awake, with the machine not registering any FLs, and my breath waveforms look rounded.
   
2. Asleep, with the machine logging non-zero FLs, and the waveforms look flow-limited.
   
3. Asleep, having flow-limited breaths that the machine either isn't detecting, or is detecting but not logging, or OAs which the machine doesn't flag as flow limitations, or wildly-erratic recovery breathing which the machine doesn't flag as flow limitations.
   

#3 means that the machine massively undercounts my FLs, so I can have a 95% FL of around .5, but the median is still 0.00, 0.01, 0.02.

For 2 years I ran my pressures at 7/14. I had high FLs and low AHI. The machine wandered up and down between about 11 & 14 all night. I tried an experiment of setting the machine to 14/20. The pressure wandered around all night between about 15 & 20 -- but I had the same high number of FLs and low AHI. In other words, if I set the max at 14 the machine was happy to keep my pressures below 14 for much of the night, but if I set the min pressure at 14, then the machine would decide that I needed pressures higher than 14 for most of the night. But the whatever the pressures, I still had lots of flow limitations and only rarely had events. (Except for every once in a blue moon when I'd have a big cluster of events which I now know was chin tucking.)

But TBMx, what I see is that the machine will chase the flow limitations that it detects with higher pressure, and lower the pressure when it misses the flow limitations that are going on, but it doesn't matter and I won't have events as long as I keep my chin up.

[2SleepBetta]

For starters, would an index number derived as follows be of any value? Write code for OSCAR to sum the areas under the indicated rectangular FL peaks and divide the total by total sleep session time. Intuitively we react to the overall density and heights we see portrayed in the session view (as well as to expanded views). This naive method, if feasible, would assign a number to what we see in total, leaving us to probe deeper. (Please pardon my ignorance if this has already been suggested and rejected, though I'd llike to know reasons why.)

Having an overall metric might help us assess in shorter time the efficacy of, say, our one-at-a-time (grudging or welcome) changes in any all matters pertinent to health, sleep and sleep hygiene.

The ResMed patent applicaton of 8/25/2011 I scanned last night has a wealth of background detail on how Resmed and others have dealt with FL. The pdf application can be downloaded by clicking a download button here https://patents.google.com/patent/US20110203588A1/en . There is a rendering of the application at the linked site, but no illustrations.  I copied some of that text rendering below in support of matter in the next paragraph. 



A BIT OFF TOPIC:
Near the end of the application there is a tangent that adds a bit to our understanding of how WAKE can be distinguished from SLEEP according to some research. Conceptually, divide the total exhalation time for a number  breaths into their times from start-to-peak of exhalations. For sleep the ratio is 0.0 to 0.3, for awake it is >0.5, as shown below.

• 0325]
  A normalized expiratory peak location (NEPL) is a good indicator of a transition from sleep or obstructed sleep flow waveforms to waveforms indicative of wake, arousal or other unnatural, out-of-the-ordinary or unrecognizable events such as severe mouth leak. Thus, an apparatus of the present technology may implement a measure of arousal based on a peak expiratory flow. For example, depending on the location of a peak expiratory flow or normalized peak expiratory flow within an expiratory portion of a respiratory cycle, an arousal may be assessed. In such an embodiment, the time of the expiratory portion may be defined by ranges and the occurrence of the peak within the defined ranges defines an index that is indicative of arousal. The index may be indicative of arousal if the expiratory peak occurs in a latter time portion or range of the expiratory portion of the respiratory cycle.
  
  
• [0326]
  Such an index may be calculated as follows:
  
  
• [0327]
  1. Frame up breaths.
  
  
• [0328]
  2. Isolate the expiratory portion of each breath.
  
  
• [0329]
  3. Locate the time at which peak expiratory flow occurred.
  
  
• [0330]
  4. Divide the time from the beginning of the expiration to the peak by the total expiratory time, e.g., the index is in the range [0.0:1.0].
  
  
• [0331]
  Ordinarily when a patient is asleep the NEPL lies in the range zero to 0.3. For example, consider the histograms of FIG. 38 which show a comparison of three datasets corresponding to NEPL:
  
  
• [0332]
  (1) Essen—OSA patients on treatment,
  
  
• [0333]
  (2) Concord—patients being titrated, and
  
  
• [0334]
  (3) Awake—people breathing on an AutoSet Spirit airway pressure device available from ResMed.
  
  
• [0335]
  The graph in the right-hand side of FIG. 38 shows that awake breathers have far more breaths with values greater than (>) 0.5 than the asleep patients who have few. Thus, a system can be implemented with a de-weighting function for a flow limitation measure based out-of-the-ordinary, unnatural or unexpected events with respect to sleep. A suitable function based on a calculated or determined normalized inspiratory peak location value is illustrated in the graph of FIG. 39. The function is used to increase the strength of the flow-limitation measure required to cause a rise in treatment pressure as the breath becomes more unnatural, out-of-the-ordinary or unrecognizable (e.g., weird).
  

[TBMx]

@sheepless thanks a lot for your post - very interesting  
You have to auto-fill-in the language gap.

If I understand you correctly, you want to use the 'new FL index / score' as a surrogate for PLM-Detection?

May I ask what kind of PLM you have? Meaning: did you had PLM BEFORE CPAP? Did the PLM-Index went down or up with CPAP (or in the titration night)? and finally: do you take a medication against PLMS (L-dopa and so on)? (and if so: is that working for you? - at least to a certain degree)

I'm not so sure if ResMed's flowlimitations are a really good tool for what you might be interested in. ... I try to explain where this is coming from:
a) ResMed does not really score flowlimitations in the clinical sense - It really is more of a flattening index. Automatically scoring flowlimitations is obviously quite complex - there have been approaches up to machine learning and neural networks with varying results.
Flattenings they can detect quite good - Flowlimitations with the spike at the beginning followed by a clear flatting as well.
If one takes the respiratory rate detected by the machine into account it is possible to see the double (or more) spikes as well. The respiratory rate doubles on those - resulting in 2 breaths seen by the machine which than of course look without any flowlimitation to the machine. (everything above 25 or double of your normal breathing rate is a very good cut off point for those)
b) Scoring flowlimitations gets even more complex with varying pressure (as in EPR / BiPAP) - the machines don't see your respiratory flow - that one is calculated (or more estimated) from the total flow which includes the ventflow from your mask and each and every leak - the pressure is measured in the machine itself - the actual pressure in the mask (and therefore the actual ventflow) is only estimated - and this changes a lot during the different phases of the inhalation / exhalation. Flowlimitations can obviously be detected in the actual flow but those are very subtle changes which may or may not get lost if you just miss the actual flow by a few ml. (this can be observed in patients with mostly hypopneas. ResMed only scores hypopneas if there is at least 1 obstructed breath in them - meaning their FLs ... introducing even little EPR on those always 'rounds' the flow (while lowering the EPAP) and the hypopneas are no more scored. Of course Pressure Support is a way to treat hypopneas - but as little as 1 or 2 cmH2O is in most cases simply not enough for that as can often be verified if you look closely enough at the resulting flow where 'out of seemingly nowhere' breathing patterns which look awfully a lot like 'recovery breaths' can be observed)
(... well and than there are all the artefacts - heart rate beeing one. ... could always have been the heart or a very subtle flowlimitation or both^^)

Basically we are toalking so far about artefacts in the flow - movement artefacts coming from the PLM.

As PLMs come in patterns I would assume that you are not solely interested in the amount or index of those but the index and length of recurring (isolated) flowlimitations. If you have 1 (or maybe 2) isolated flowlimited breath (maybe with a very low FL-score) that looks like nothing - if that repeats (usually around 30 seconds apart) over a longer period (minutes not hours) that might be a good indication of a PLM episode. Only the index of the FLs would be misleading for that - the index of those phases (let's call them epochs) would most likely yield more value.


There is another kind of PLMS: breathing induced Limb-Movement. Usually in a PSG you see that those are factored out - the timing of the breathing disturbance and the occurence of the movement are crucial for that. (Depending on what and by whom you read about that those are attributed to arousals and so on.)
Depending on the PSG and if you get the (detailled) titration report there is usually not much focus on PLMS during the CPAP titration or afterwards if that didn't show up in the baseline night.
But: there is such a thing as 'treatment emergent' PLMs. (maybe even additionally or they interchange each other)
I don't know what the underlying mechanism is ... the machine itself creates a resistance for your breathing - the most prominent case would be air starving - maybe its the very same mechanism. (before I said "assuming that their minimum pressure needs are met" - those things are included in that^^)
That additional resistance somehow deminishes with higher pressure.

I consider talking about UARS or "Flowlimitations" regarding these "breathing induced PLM" a little bit misleading - although in the end that is or might be just it.
The "restriction" or "limitation" in the breathing (aka flow) might be well under any medical treshold - persistent (or emerging) PLM even with L-Dopa seems to be a good indicator for that. "At home measurement" of PLM is not the easiest thing to accomplish - so called "sleep trackers" that only measure micromovement are good (or not that bad) as an indicator (most with PLM have more leg than arm movement - there are those "dumb" trackers out there which can be worn everywhere - even on the (foot) ankle).

As I said "misleading": normally BiPAP / EPR comes to mind ... but as this might include very subtle 'limitations' this is not always the route to go down. The devices need to identify the start of the inhalation - only when that is detected the pressure changes to the higher IPAP providing the needed pressure support. The problem can however be the first part - even if it are just some milliseconds - there you are on the lower EPAP which may be not enough to overcome the 'restriction' / 'obstruction' - others might be sensitive to intra-breath pressure changes (not the same but maybe comparable or in the meaning of the 'fighting' on BiPAP / PCV). Pressure-changes themself are or might be a problem on their own - more pressure means more breathing effort is needed - but that was the origin of the problem in the first place.
The resulting pressure - or to be precise lower pressure range / EPAP - might be higher - even much higher - than what would be needed to just overcome the cassical events of the breathing disturbances (including the non-hypoxic ones like labeled as UARS). (yes - there is no shame in CPAP)

Another completely different route is or would be: respiratory driving strength - That would mean EERS.
Usually PLMs are present in the lower sleep stages (N1/2) and not the deeper sleep stage or REM. I would expect a transitionphase and not a sudden impact.
What would the world be without pitfalls: Negative Effort Dependence would be such with the increased respiratory drive - in that case EERS should be counterproductive and result in more disturbed sleep / PLM.

Do you have by any chance screenshots somewhere of PLMs under CPAP / seen by the CPAP? I know what the breathing pattern looks like in a PSG or without CPAP - but I have never seen clear data of that from a PAP-device. (Flow and the high-res pressure chart are the ones I am most interested in ... ideal over a period of at least 2-3 minutes before and ofter the episode ... well that might be more than just 1 screenshot)

To sum it up: PLMS should be seen in the respiratory flow - but ResMed's "FL" do still not look like a promising route for that. (but I might be wrong)
In my eyes there is a big misconception about that floating around - their FLs are meant to be a tool for the decision if or if not to raise or lower the pressure. That is not a diagnostic tool - they don't have to get it 100% right - the overall result is the only important thing. (meaning: up the pressure preemptive to prevent possible obstructions and otherwise provide the lowest possible pressure ... and that evidently works if the lower and upper range have been titrated correctly)
... but in the end there is one thing with OSAS or SRBDs in general: those are all(!!) just surrogates. Yes! even the AHI is just a surrogate - meaningless in itself (well, not completely) but overall a good tool to diagnose OSAS and one key-parameter for treatment success - but not for all as it is just a surrogate (for the underlying real problem).
If you start and put too much meaning in those surrogate-values you end up barking up the wrong tree.  

[sheepless]

"If I understand you correctly, you want to use the 'new FL index / score' as a surrogate for PLM-Detection?"

before I wade into reading and responding to the last 3 posts, I want to be clear that this is NOT what I'm thinking. sorry if I gave that impression. I'll have to go back to figure out why I even mentioned plm.

you-all are understandably highlighting important issues like machines leaving obvious flow limitations undetected or unreported and the need for much greater depth of understanding them. these things concern me and I'd like to see progress in these regards, but they aren't particularly relevant to my current proposal (that's probably evolved over the course of this thread, so apologies if not clear).

all I'm suggesting at this point is for a statistic that reports density/frequency/duration to supplement the current report of severity.

how much of my night consists of flow limited breathing in whatever way it's reported by my machine? let's see duration as well as severity. simple as that. comprehensive or not, accurate or not, to the extent the machine methodology is consistently applied, and I think it's safe to say it is, it would still be useful for comparison purposes.

I'll come back to this after I've had time to digest the thoughtful comments above.

[sheepless]

a couple disclaimers:

I know next to nothing about flow limitations and related algorithms.

my 'knowledge', especially about periodic limb movement, but all things cpap/apnea related, is based primarily on my own experiences, my mostly subjective interpretations of them and a bunch of personal assumptions I can't prove. not a very sound basis but it's all I have.

okay,

cathyf, I think the things you describe are simply definitional. yes, a full on obstructive apnea is flow limitation but I think they're not called or counted as fl because they meet the definition of apnea. flow limitations are some definition of a lesser restriction; once the degree of restriction exceeds that, it's called hypopnea or apnea. a case could me made that adding h and oa to the cumulative value ascribed to flow limitations would confuse more that enlighten.

as for all the flattened and distorted waveforms that aren't flagged, as I understand it, the machines compare now to some previous period of time; if sufficiently distorted by some definition not rising to the level of hypopnea or apnea, it's reported as flow limitation. it appears, for example, that if one's breathing is flow limited 100% of the time, the machine would have nothing to compare and it would report zero flow limitations. clearly not ideal, but that's what we're dealing with for now.

2SleepBetta, I'm always impressed by your posts. you've read about and given a lot more thought to these things than me. what you describe at the beginning of your post sounds much like what I have in mind. the latter part sounds promising but I haven't looked at your source. and for present purposes, it looks to go well beyond the simple metric I'm proposing for now.

TBMx, I'm interested from the standpoint of all flow limitations, not just related to plm. in fact, pressure rising during my plm episodes is completely ineffective and quite disturbing. the only thing I've found to do is to use vauto with what amounts to fixed epap and fixed ipap, otherwise, pressure would go as high as settings allow, right up to 25 cmw. rising pressure might tell me I have too many flow limitations, but recognizing that and capping max pressure removes it as an indicator. what do I look at then? right now, we look at the degree of severity currently reported. as I keep saying, duration strikes me as important as severity but as far as I know, that's not reported in oscar.

as for statistics and indices being surrogates, yes, of course they are. in every discipline such things are used to reduce a lot of stuff to something simpler for the sake of measurement and comparison. that's all I'm suggesting here: incorporate some measure of density to supplement the measure of severity. another descriptive tool to apply to the art of titration.

I'm intrigued by some of your comments on plm. the only means I have to document my plm is via the pap machine generated flow rate waveform. I can't prove it but all indications are that my plm is not pressure or treatment related. if you want to dig into this, let's do it in my plm thread here: http://www.apneaboard.com/forums/Thread-...g-movement

I've posted screenshots of my plm flow rate pattern all over the place on this site, including there.

[cathyf]

cathyf, I think the things you describe are simply definitional. yes, a full on obstructive apnea is flow limitation but I think they're not called or counted as fl because they meet the definition of apnea. flow limitations are some definition of a lesser restriction; once the degree of restriction exceeds that, it's called hypopnea or apnea. a case could me made that adding h and oa to the cumulative value ascribed to flow limitations would confuse more that enlighten.

as for all the flattened and distorted waveforms that aren't flagged, as I understand it, the machines compare now to some previous period of time; if sufficiently distorted by some definition not rising to the level of hypopnea or apnea, it's reported as flow limitation. it appears, for example, that if one's breathing is flow limited 100% of the time, the machine would have nothing to compare and it would report zero flow limitations. clearly not ideal, but that's what we're dealing with for now.

Ok, to back up here, a lot of what is disturbing me from a quantitative view is the inconsistency of characterizing & logging things -- in some cases bigger FL number means more flow-limited than smaller FL number, where in other cases a smaller FL number means more flow-limited than a bigger FL number.

We are all in a situation where we are most deeply familiar with our own data, and so I might actually have something interesting to add here because I've figured out how to radically change my data

Without a cervical collar, I have dramatically more flow limits than when I wear one, even on nights where I do not get into a nasty-ugly-AHI-75 chin-tuck scenario. I can see that without the collar I can get into severe episodes -- but even when NOT in severe episodes I have big ugly flow limitations. I'm thinking that is from dropping my chin some, but not too far.
So, anyway, I'm really familiar with three scenarios:

1. severe nasty flow limits interspersed in the in-betweens of nasty clusters.
   
2. ugly flow limits but no or almost no events or none near in time.
   
3. much more mild flow limits, and a nice low AHI.
   

What happens in #1 is that the FL curve is jaggy up and down, continually dropping to zero during apneas, recovery breathing, arousals. While #2 the FL curve comes up off the x-axis for sustained periods of no zeros. So #1 can actually have as many if not more periods of zero than #3 -- which tends to make the median zero in both cases.

(I don't have attachment space to add pictures. If anyone wants a picture, I'll try to clean out and get some space.)

[sheepless]

I'd like this deeper dive into flow limitations to continue but hope my much less complicated proposal doesn't get lost.

"in some cases bigger FL number means more flow-limited than smaller FL number, where in other cases a smaller FL number means more flow-limited than a bigger FL number."

probably just me being obtuse but I'm not sure I follow. are your references to the flow rate graph, the flow limitations graph under the Daily tab, the flow limitations graph under the Overview tab or the flow limitations stats under the Daily tab? like, when you refer to the FL curve, it sounds like you might mean the flow rate curve?

"Without a cervical collar, I have dramatically more flow limits than when I wear one"

IF I understand your post (emphasis on the IF), I still think it makes sense as a definitional issue. I'd say the collar is keeping the airway open enough to reduce the restrictions from 90 or 100% (whatever the definition of oa is) to 40 or 50% (whatever the definition of flow limitation is). partial opening of the airway producing fewer apnea, more flow limitations.