Flow rate limitation and UARS - seeking clarification of criteria

[Coffee Man]

I was reading a thread about UARS and wanted to know if there is a definition for flow rate limitation that explains what would be significant? Sleepyhead allows for specifying flow rate limitation and duration as a way to setup user flags. But, I'm trying to find out if there is a definition of what percent flow rate limitation is significant i(in terms of UARS), and also what length of time (in terms of seconds) is clinically significant...excluding apneas and hypopneas which are detected by the APAP.  I would prefer this definition to be referenced to some published medical journal article, rather than someone's best guess. Thanks in advance. 

If this information is clearly presented in another thread, a link to that thread would be appreciated.

[sheepless]

I am interested in the answer to this question as well.

in addition, while I've asked before I've not received an answer to the question of whether resmed airsense and aircurve machines and sleepyhead accurately calculate and report flow limitations. eyeballing the chart, I am suspicious of the usual reports of 0.0 median and 0.0 95% flow limitations, but no way to check without seeing the underlying tabular data.

[Sleeprider]

Flow limitation in CPAP machines, at least the Resmed, are relative to the baseline inspiratory flow rate. I don't know the period of time or rolling time interval for the baseline flow, but it is reported to be determined on a breath by breath basis (as in ASV) and used in flow limitation and hypopnea calculations. Let's start with Resmed's published information on Flow limitations and how an Autoset machine responds to them:

1. Flow Limitation

What is flow limitation?

A physiological change in the status of the upper airway
Causes a spectrum of closure, from subtle narrowing to partial collapse

2. What is the significance of flow limitation?

It usually precedes snoring and apnea. It is therefore, usually, the earliest sign of impending airway collapse3
Airway changes limit flow despite respiratory effort and in fact, cause increased respiratory effort. This may lead to arousal

2. How do I observe flow limitation?

The inspiratory flow time curve represents inspiratory flow limitation
The shape of the curve indicates the status of the upper airway. A normal upper airway is observed as a bell-shaped curve. The curve flattens with increasing flow limitation and airway narrowing/ closure

3. How does a ResMed AutoSet device measure flow limitation?
It calculates flow limitation on a breath-by-breath basis, detecting and responding to subtle changes

4. How does a ResMed AutoSet device respond to flow limitation?

If flow limitation is detected, AutoSet Spirit responds by gradually increasing pressure to bring the airway back to normal. Typically, this helps prevent snoring and apneas4
If no further events occur, AutoSet Spirit gently decreases the pressure, towards the minimum set pressure

I'll keep looking for more research on the topic, but it is important to understand that some people have flow limitation on every single breath, and it is visible in their flow rate graphs. The Resmed algorithm responds to changes in the inspiratory flow rate structure, and that is a proprietary algorithm. For people that begin sleep without flow limitation, it seems to be pretty accurate, and will raise pressure based on the detection of flow limitation; however for people with chronic flow limitation, like COPD or severe UARS, the "relative" change in flow may not be detected on the same scale. These people may experience a constant 50% flow limitation, and only trigger pressure increases when that increases beyond baseline.

I think there is an excellent article hosted in the Apnea Board wiki regarding UARS, flow limitation and the use of bilevel pressure devices. http://www.apneaboard.com/wiki/index.php..._and_BiPAP

I'd like to help, but flow limitation is not a heavily researched and published subject. The Wiki article Beginner's Guide to Sleepyhead has some of the best information on the internet...why not, I wrote a lot of it http://www.apneaboard.com/wiki/index.php...SleepyHead . I also wrote a Flow Limitation Wiki http://www.apneaboard.com/wiki/index.php...Limitation Let's start with this, and see if more information is needed.

[Coffee Man]

I read the information from Krakow, but nowhere does it define what flow rate limitation is significant in terms of percentage change from other flow and nowhere does it say how long a flow rate limitation needs to be. This is, again, in reference to UARS.

[Sleeprider]

Coffee Man, it is very individual how FL not only affects you, but the baseline against which it is measured. If you're seeing the inspiratory flow wave form flatten out or go to a downward slope, that is significant. The metrics used by the machines as percent are only relative you your own baseline, and that is not a parameter that can, or has been studied. If you have characteristics of inspiratory flow limitation then the solution is either increased pressure, pressure support or both depending on what you have available to work with. In your case, there is only 3 EPR that can be employed to combat the problem. You have not posted a graph, so hard to say. What I am saying is that the published literature may not answer your question because flow limitation metrics (percent) is based on the individual's baseline, and researchers have not picked up the ball and studied it other than to evaluate response to pressure and pressure support.

[Coffee Man]

SleepRider,

I appreciate your input. I was trying to see if there were set parameters for change in flow and duration to use as a filter for SleepyHead to look for and flag such episodes. If there are no specific parameters, then I won’t worry about trying to setup such a user flag in SleepyHead. There are 2 predetermined users flags that can be set in SleepyHead, but I don’t know how or why they have the settings listed.

[Sleeprider]

If you want to be arbitrary (and I am), then user flags at 33%, and 66% might make sense as levels of significance. I said this is an individual issue, so if you have FL levels that lead to other issues, then by all means use that. Personally, I'd pursue getting FL below 30% consistently and making it a non-issue in therapy, but I would start with a study of the flow chart and whether FL is chronic or occasional. It's significant when most breathing is flow-limited, and that is more apparent than the percent of inspiratory flow reduction from baseline. If you start with FL, and then see increases, that is clearly more significant than if you have a nice rounded inspiratory wave form and then develop some FL from time to time at low levels. The problem for the Resmed Autoset is that it only responds to changes in FL, not a chronic FL present all night long.