MJS_ wrote: ↑Sun Apr 09, 2023 3:59 pm
Thanks again! I suppose it makes sense that I'm having "flow limitations" since my (perhaps ill-advised) goal has been to approach the minimum air pressure needed to prevent OAs. If Hs and OAs are not occurring, then is it common or widely accepted knowledge (among apnea experts) that the air pressure should be increased to eliminate flow limitations? How harmful are air flow limitations in the absence of Hs and OAs?
As palerider has pointed out, flow limitations can be thought of as "hypopnea wannabes" in the sense that it is difficult to get enough air through a partially compromised airway. What I mean by this is that flow limitations can indicate the airway is as a real risk of collapsing. And in that case, more pressure helps smooth out the flow limitation by doing a better job of preemptively preventing the airway from collapsing enough for an H or an OA to be scored.
Having said that, I also want to add that for
some people flow limitations get scored because of other reasons. Some people find that severe congestion (which is NOT part of sleep disordered breathing) can cause the machine to score a lot of flow limitations. Some people with deviated septums find that their machines seem to score lots of flow limitations regardless of how high they set the pressure; so the assumption is that the deviated septum may be distorting the shapes of the inhalations in the flow rate curve, and all the pressure in the world isn't going to fix a deviated septum. For a
small number of PAPers, restless SWJ breathing can get unstable enough to cause some flow limitations to be scored; for some of these folks, more pressure can lead to more flow limitations, even though that's counterintuitive.
So around here the general consensus seems to be: If you have a lot of flow limitations and you want to experiment, try more pressure to see if the flow limitations start to smooth out. If they get better with more pressure, then that's a good bet that the flow limitations scored by the machine are, in fact, examples where the breathing is becoming unstable specifically because the airway is starting to collapse. If there's no improvement in the flow limitations AND if you're feeling good when you wake up AND your AHI is nice and low, then that can be evidence that your flow limitations might not be tied to an airway that is threatening to collapse, and if you want to cap the max pressure to limit the machine's ability to increase pressure up and up and up in response to the flow limitations, that probably won't compromise the efficacy of your therapy.
Also around here, the general consensus seems to be: If (you are NOT yet feeling good when you wake up OR if you are still dealing with excess daytime sleepiness) AND your AHI is well under 5 AND there's a lot of activity in the Flow Limitation graph, then there's a pretty good chance that your flow limitations are real---i.e. they probably are being scored when your airway is still threatening to collapse and that compromised airway may be disrupting your sleep even though the disruptions are not rising to full-fledged RERAs, Hs, and OAs. And since you're not feeling good on waking or you're still dealing with a lot of daytime sleepiness, it's well worth experimenting with allowing the machine to go where it wants to go in terms of increasing the pressure---as long as you're not dealing with aerophagia.
In my case, I don't typically have a whole lot of flow limitations and I do have a high risk of aerophagia. So I do cap my max IPAP at 9cm to prevent the aerophagia. In the past I have experimented (on a PR machine, not a Resmed) with increasing the max IPAP to as high as 12 or 13cm. And every time, the machine would find at least one point during the night where there were just enough flow limitations being scored to make the pressure go up to the max. But the overall number of flow limitations never went down, the aerophagia raised its ugly head, and I felt worse in the morning. So in my case, I've concluded that one of two things is going on: Either the flow limitations are not caused by an unstable airway OR the flow limitations are real, but I have to make a working compromise between being able to get a good night's sleep (with no aerophagia) and the pressure needed to "fix" the last remaining flow limitations.
What is the normal range for the Flow Limitation summary statistics (median, 95%, 99.5%)?
Nobody knows. I'm serious about that: Nobody knows whether the summary statistics have any real meaning in terms of clinical significance. And nobody knows what a "normal" range for those summary statistics would be.
While Oscar reports the Flow Limitation data numerically, the versions of ResScan that I'm familiar with (from many years ago) had a vertical axis with no numbers on it. Instead, the vertical scale on the Flow Limitation graph consisted of three images: One was a stylized shape of an inhalation with a fully open airway---a nice round hump. At the other end was a stylized shape of a very badly flow limited breath---it looked like a table-top with legs. In the middle was a stylized shape of a possibly flow limited breath that was more angular than the "fully open airway" icon and less like a table-top than the one used to indicate a severely flow limited breath. We do know enough about the flow limitation graphs to know that when the Flow Limitation graph is at 0.0 in Oscar, the Flow Limitation graph in ResScan was at the icon for the stylized shape of an inhalation with a fully open airway. Likewise, when the Flow Limitation graph is near 1.0 in Oscar, it was near the icon for the stylized shape of a very badly flow limited breath.
Since ResScan didn't report
numbers on those old Flow Limitation graphs, ResScan never reported summary statistics on for the flow limitation graph. And hence I doubt that even Resmed's engineers have any ideas of what a typical PAPer's median, 95%, 99.5% flow limitation statistics would look like.
Wild ass-guessing would say that if the 95% flow limitation number is 0, you had very few or no breaths with any flow limitation that could be detected by the Resmed software. Wild-ass guessing would also say that if your median flow limitation is above 0.25 (first WAG) or 0.5 (second WAG), then you've probably got some problems with residual flow limitations. And you might just feel better if you increased the pressure enough to smooth those flow limitations out---particularly if you are NOT feeling well when you get up in the morning OR if you are still dealing with an unacceptable level of sleepiness in the daytime or early evening.
Complicating the interpretation of Resmed's flow limitation data even further is this: PR machines score flow limitations in an entirely different fashion: They are scored more like OAs and Hs---i.e. there are "tick marks" with a time duration; so they are scored for specific short stretches of breathing, but PR does not attempt to quantify the "severity" of the flow limitation by assigning it a number between 0 and 1. On a PR machine, OSCAR can display a Flow Limitation Index = (# of FL)/(Run Time), which is gives the average number of flow limitation ticks for each hour the machine is run. How high is an acceptable FLI on a PR machine? Nobody knows. Should the FLI be added to the RDI? Nobody knows.
) my Flow Limitation statistics are from the average,