Flow limitation questions (ResMed S9 Autoset)

Pugsy wrote:Flow limitation Index is only reported on Respironics machines. ResMed machines do not report Flow limitations like the Respironics machines do. You get the graph and that is it.

I had a couple of questions about flow limitation (as reported by SleepyHead) and did a search, found Pugsy's above post. In addition to the FL graph SleepyHead does report Flow Limit numbers from my S9 Autoset -- my question is, what do these numbers actually represent? I understand what flow limitation is, just not sure what the numbers are measuring.

The SH graph range is 0.0 to 1.0 and SH also reports Flow Limit Min, Med, 95%, and Max numbers within this range in the Statistics box, but I don't know the significance of these numbers (if there is significance to them). So my first question: Are these numbers representing an actual measure of something and is there a number we're trying to stay below (like with the red line for leaks)?

Second question: Is it possible that reducing EPR could somehow increase flow limitation? I had a curious result from last night when I changed my EPR from 2 to 1. I had an AHI of zero for the night, yet my FL numbers were the highest I've seen so far. I know one night does not make a trend and could just be a fluke. Conceptually it doesn't make sense to me that EPR could affect FL one way or the other, but I really don't know enough about it.

I'm more interested in the first question than the second since last night may just be an oddity, but thought I'd ask both.

Re: your second question, there is both inspiratory flow limitation and expiratory flow limitation. If you have the latter, reducing EPR might not keep the airway fully open, so the waveform would show increased flow limitation. You can tell which flow limitation you have by examining your flow chart. Inspiratory flow limitation usually shows up as flat-topped flow peaks. Expiratory flow limitation waves look like a chair, with the back to the left side. Air inspiration is normal, but as you exhale, at some point the airway closes, which produces the "seat" of the chair in the wave as displayed on the flow chart. It's very distinctive. I wish I had some peaks to post, but I can't find them at the moment.

Respironics auto adjusting machines report FLs in the events graph. Makes it easier to spot on the reports.
That's what I meant about the difference. With ResMed you get only the graph that you have to put under the microscope.

kaiasgram wrote:

Pugsy wrote:Flow limitation Index is only reported on Respironics machines. ResMed machines do not report Flow limitations like the Respironics machines do. You get the graph and that is it.

-- my question is, what do these numbers actually represent? I understand what flow limitation is, just not sure what the numbers are measuring.

The SH graph range is 0.0 to 1.0 and SH also reports Flow Limit Min, Med, 95%, and Max numbers within this range in the Statistics box, but I don't know the significance of these numbers (if there is significance to them). So my first question: Are these numbers representing an actual measure of something and is there a number..........

Maybe I just don't get what is in front of my face, but what do the numbers represent? kaiasgram good question.

lilly747 wrote: but what do the numbers represent?

SleepyHead has numbers on the left side of the graph. ResScan doesn't but if I compare the actual flow limitation graph from SleepyHead with ResScan graph...they are identical.
ResScan's clinical guide only says " In the inspiratory flow limitation graph, the taller the bar, the more severe the flow limitation.

I don't think the numbers mean much of anything other than a scale measurement. I rarely have any flow limitations to speak of and my FL graph goes up in small increments to 1.0. Someone else that has a large number of severe flow limitations may have a different numerical scale on the left.
It doesn't appear to be a numerical value as in number of FLs...only a measurement as to severity.

Slartybartfast wrote:Re: your second question, there is both inspiratory flow limitation and expiratory flow limitation. If you have the latter, reducing EPR might not keep the airway fully open, so the waveform would show increased flow limitation. You can tell which flow limitation you have by examining your flow chart. Inspiratory flow limitation usually shows up as flat-topped flow peaks. Expiratory flow limitation waves look like a chair, with the back to the left side. Air inspiration is normal, but as you exhale, at some point the airway closes, which produces the "seat" of the chair in the wave as displayed on the flow chart. It's very distinctive. I wish I had some peaks to post, but I can't find them at the moment.

Thanks, that clears up a question I didn't ask directly but was wondering about, i.e., whether the FL graph can show both inspiratory and expiratory limitations. If you do come across some exemplary peaks I'd love to see them and I imagine it would be of interest to others too. How would you interpret the waveform you see here with this flow limitation -- I didn't include the other graphs but both leak line and snore line were zero, so no other influences other than the flow limitation in this chunk of time.



* Point of confusion: If I reduce EPR, wouldn't I expect to see decreased FL rather than increased FL -- I'm not letting the pressure drop as much when I reduce EPR so I would have thought that keeping the exhalation pressure up more would help keep the airway more open.

Pugsy wrote:

lilly747 wrote: but what do the numbers represent?

SleepyHead has numbers on the left side of the graph. ResScan doesn't but if I compare the actual flow limitation graph from SleepyHead with ResScan graph...they are identical.
ResScan's clinical guide only says " In the inspiratory flow limitation graph, the taller the bar, the more severe the flow limitation.
I don't think the numbers mean much of anything other than a scale measurement. I rarely have any flow limitations to speak of and my FL graph goes up in small increments to 1.0. Someone else that has a large number of severe flow limitations may have a different numerical scale on the left. It doesn't appear to be a numerical value as in number of FLs...only a measurement as to severity.

Thanks Pugsy. I hadn't thought about the possibility that the parameters could be different for someone who has more severe flow limitations, I assumed that the range went from 0.0 to 1.0 for everybody. Can you help me make sense of this definition of Flow Limitation Index from the SleepyHead glossary:

Flow Limitation Index
Changes in flow limitation are recorded as events. The Flow Limitation Index is calculated by the total number of flow limitation events per night divided by the hours of use. Note: The average is calculated by taking the total number of events divided by the number of therapy days. This can be used to indicate if there has been a significant degradation in the flow signal, resulting in a pressure increase. This value is only reported on auto pressure machines.

Does this definition pertain to both Respironics and ResMed machines or just Respironics?

BTW, interesting to read that you usually have no FLs. I have very low AHI all the time, but I also have FLs all the time -- my questions in this thread are from trying to understand if my FLs are significant enough to be affecting my quality of sleep and if so, how to fine tune my pressures and EPR. Plus it's just interesting to study!

kaiasgram wrote:

Slartybartfast wrote:Re: your second question, there is both inspiratory flow limitation and expiratory flow limitation. If you have the latter, reducing EPR might not keep the airway fully open, so the waveform would show increased flow limitation. You can tell which flow limitation you have by examining your flow chart. Inspiratory flow limitation usually shows up as flat-topped flow peaks. Expiratory flow limitation waves look like a chair, with the back to the left side. Air inspiration is normal, but as you exhale, at some point the airway closes, which produces the "seat" of the chair in the wave as displayed on the flow chart. It's very distinctive. I wish I had some peaks to post, but I can't find them at the moment.

Thanks, that clears up a question I didn't ask directly but was wondering about, i.e., whether the FL graph can show both inspiratory and expiratory limitations. If you do come across some exemplary peaks I'd love to see them and I imagine it would be of interest to others too. How would you interpret the waveform you see here with this flow limitation -- I didn't include the other graphs but both leak line and snore line were zero, so no other influences other than the flow limitation in this chunk of time.



* Point of confusion: If I reduce EPR, wouldn't I expect to see decreased FL rather than increased FL -- I'm not letting the pressure drop as much when I reduce EPR so I would have thought that keeping the exhalation pressure up more would help keep the airway more open.

Yes, that's what'cha call a brain-phart. That's a highly technical term for when the fingers type what the interface between the chair and the keyboard MEANT them to type, but it didn't make it to the keyboard. I meant to say that if you reduce pressure (by EPR) during expiration, your airway might not stay fully open and in that way (an increase in) EPR (which is a reduction in pressure) can contribute to expiratory flow limitation. Reduce or turn off EPR and see whether that results in a reduction or elimination of the events. Some doctors will insist you not use it because your sleep study indicated that was a possibility.

On your chart, if you grab the left axis of the chart and drag it up, you should be able to decrease the vertical scale and see the chair-like profile of those peaks better.

kaiasgram wrote:Flow Limitation Index
Changes in flow limitation are recorded as events. The Flow Limitation Index is calculated by the total number of flow limitation events per night divided by the hours of use. Note: The average is calculated by taking the total number of events divided by the number of therapy days. This can be used to indicate if there has been a significant degradation in the flow signal, resulting in a pressure increase. This value is only reported on auto pressure machines.

Does this definition pertain to both Respironics and ResMed machines or just Respironics?

There is no Flow limitation index for ResMed machines because they don't report a numerical value that can be divided by number of hours used to get an hourly index. That definition is only for Respironics machines.

ResMed machines just show the "severity" and not how many of them you had. If you are really good at looking at the flow graph you maybe can spot them and manually figure up an hourly average but I rarely can tell the difference in flow even with Respironics big FL flag stuck in there.
I don't have many flow limitations get flagged with my PR S1 machine either.
Let me see if I can find some examples for you.
The reason I like Respironics for FLs is they stick a big flag with what they are calling a flow limitation...easy to count.
So a numerical value only...severity we have to eyeball how long it lasted.
So Respironics gives us a number of FL occurrences with no level of severity. Only in Auto pressure mode.
ResMed gives you a level of severity but it is hard to figure out how many of them you had. Doesn't matter what mode.
So advantages and disadvantages to both. Nothing is ever perfect.

Have you seen the ResScan interpretation guide that explains what you see in the reports?
It still applies to SleepyHead even if things are visually shown a little different.
You have a Mac don't you?
Let me see if I can get you a copy of it. Give me a sec.

I think Resmed's algorithm sees FL more as a trend rather than a discrete event, so they chart it in ResScan as a continuous line rather than flagging it and assigning a value to it. Whenever FL is detected it immediately results in a pressure increase, so when you're reviewing your data and you see the pressure spike up, you can look at the FL chart and see whether it was caused by flow limitation, or a hypopnea or obstructive apnea. And if you want to obsess about it, you can zoom in on the flow peaks to about the 1 minute scale and look at their profile. It takes a little practice to identify the profile, but once you've seen it you'll recognize it in the future.

Slarty thanks for clarifying about the EPR. A brain phart sounds much more sophisticated than a plain old brain fart.

I tried stretching the flow rate graph up to exagerrate the peaks. For the most part, the back of my "chair" seems to be on the right side of the wave with the flattening coming before it on the left. Sometimes the back of the chair is more in the center of the wave, but rarely do I see it on the left side with flattening on the right side of it (I looked at quite a few nights of graphs). Now I'm brain pharting (bad night last night) -- so what should I conclude is my predominant flow limitation type, inspiratory or expiratory?

This isn't really good, but it's a chart I snatched from an academic paper that shows flow limitation. Second from the bottom where it says FLOW. You can see the peaks are truncated, not smooth peaks like the abdominal and rib belts directly above it show. What you're seeing with those three traces is that the abdomen and rib belts are showing smooth movement. Were there no obstruction, the flow peaks would appear similar to those. But something is stopping air flow hear the top of the peak. And that is inspiratory flow limitation. A couple of the flow peaks toward the right side of the trace show the general profile of expiratory flow limitation, but the chair back isn't really high enough and the seat isn't well enough established to say that's what it is. Still thinking where I saw a better example . . .

Geez, replying to my own post. That's not a good sign . . .

Here it is. Expiratory flow limitation from another paper. See the FLexp and the arrows? They're pointing to the chair profile in the flow chart (above) and the pressure/time chart (below). It sort of looks like a chair, doesn't it?

Note the heavy black marks on the trailing side of the FLexp peaks. That is snoring. Expiratory snoring. You'd expect to see that with expiratory flow limitation. Since there is no real good example of inspiratory flow limitation in this figure, there's also no inspiratory snoring. Usually you would expect to see snoring and flow limitation together.

So you can have inspiratory flow limitation and inspiratory snoring, and expiratory flow limitation and expiratory snoring.

Thanks so much for these examples. Thanks to both you and Pugsy. I generally have a pretty ugly-looking waveform even though I have very few OSAs or Hypops -- very few. But I might be skating the edge of events and/or having RERAs that continue to affect my sleep. Trying not to be obsessive but since our S9's don't flag RERAs or FL 'events' for us, I'm wanting to look a little more closely at whatever clues those graphs might hold to see if I could tweak my pressures or EPR a bit more and improve my therapy. Thanks again guys.