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Flow limitation in Sleepyhead
I am now downloading my chip into sleepyhead. I have been reading RobySue's information on how to read the results, and I'm doing fairly well, just takes some practice. I do not however, understand how to read the "flow limitation" graph. I don't know what the numbers mean on the vertical axis of the graph, ie. .53, etc. Doesn't make any sense to me. As I think that flow limitation is really key, I would love to know how to interpret these numbers in the graph.
Any insight would be appreciated!
Pam (getting braver)
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The ResMed machine examines the high rate flow waveform data during inhalation. The inhalation waveform should rise and fall in a nice smooth curve.
When it flattens on top, then the machine scores a flow limitation. It reports the degree of flattening (flow limitation) on a scale of 0 to 1.0
That's why you see decimal fraction numbers that indicate the degree of flow limitation. 0 is no restriction, 1.0 is the highest degree of flow limitation it can score. (i.e. -- 0 is best, the higher the number, the worse the flow limitation.)

The ResMed Auto machine raise pressure in response to Flow limitation, snoring, and scored obstructive apnea.
It does this for snore and flow limitation as they are predictors of impending obstructive apnea.
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OK, I think I might be finally starting to understand this. Respiration can be represented by a sinusoidal waveform, where inspiration levels off gradually as your lungs get full, and expiration levels off gradually as your lungs become empty. The flow is greatest in the middle of inspiration or expiration, which can be represented by the rising and falling signature of the sine wave. The top peak represents when your lungs are essentially full, and the bottom peak or nadir represents when your lungs are essentially empty.

So a single breath can be represented as a single cycle of a sine wave (maybe not exactly a sine wave, but something close to that, maybe fatter at the top and skinnier at the bottom). My best guess is that the xPAP does this by constantly measuring the back pressure from the patient's breathing system against what it knows the therapeutic pressure to be at any particular moment (even with a steady or fixed therapeutic pressure, as you breath in the system pressure naturally lowers a bit, and as you breath out it increases a bit), and that can be graphed as a respiration waveform.

If during inspiration the top of the sine wave clips, or the waveform appears comparatively shallower, that represents something limiting the inspirational flow. If the xPAP sees that sine wave response to be clipped or flat-topped, it knows to then flag that as something limiting the flow, which is something that should not happen during a perfectly healthy breath. But rather than simply flagging that as an event, it plots it along a sliding scale, so that the intensity of the limitation is represented, and that data can then be indexed between 0 and 1 as a flow limitation graph.

An APAP and other machines can then respond to that limitation by changing pressure, forcing a breath, etc., depending on how severe the limitation is, or what sort of limitation is represented by characteristics of the change to the sine wave.

So that is how I am interpreting this. Do I have any of this right?

I am an engineer so I have to look at everything as an engineer, and this is the only way I can understand this. My apologies for that; your explanation is much more elegant and simple. I'm just not sure if I understand this properly. My hope is that we both said the same thing, but from slightly different perspectives (except that I am only guessing here).
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Ok justMongo and Tyrone,
I am not an engineer nor am I as brilliant as justMongo seems to be, but I am also trying to understand Flow Limitation graphs. I understand that our CPAP machines react to Flow Limitations and snoring by raising pressure... but what causes the Flow Limitations? Are they the start of an apnea? Does snoring cause Flow Limitations or does Flow Limitations cause snoring?
Also, does setting EPR or AFLEX, which gives relief on exhaling, affect flow limitations or make them worse?
Sorry if I am rambling, just trying to figure it out. Coffee
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One thing for sure, flow limitations (considered as the event reported by the xPAP) do not cause snoring. Flow limitations are flags of detected instantaneous conditions, conditions which cause the flow to be limited. Snoring is a condition when soft tissue vibrates acoustically, and that only happens when flow is limited. So the thing that happens (limitation of flow) causes the response (a snore) which is detected and interpreted as and logged as an event, in the "flow limitation" category. It is only detected because it has the ability to sense vibration, but that turns out to be a valuable clue in reporting and auto-responding to changes in respiration. But respiration "flow being limited" does cause snoring, and "flow being limited" can actually be an indicator of why a snore occurs.

So it is kind of how you define it. "Flow limitation" can be considered an event flagged by xPAP, which does not cause a snore, and is based on flow being limited (hence the name). It is only a data point. But an actual "limitation of flow", an actual physiological limitation of the flow of air during respiration, can often be associated with a snore. Often the limitation is what causes the snore, and the actual act of snoring can even further aggravate the limitation. So they are pretty well entwined, enough to make explaining it a little difficult. A data point and an actual physiological event are two different things, even if the name might be similar.

EPR is a comfort tweak to xPAP. You need the pressure when you are inhaling, because your throat may collapse or be obstructed on inhalation, because inhalation lowers the pressure differential between inside your throat and outside your throat (which is how your diaphragm fills your lungs). When you exhale, the pressure inside your throat is comparatively higher than outside your throat, meaning that your throat acts as if inflated like a balloon, keeping it open during exhalation. Which also explains why snores happen mostly (if not always) during inhalation.

So although the therapy comes historically from Continuous Positive Airway Pressure (which is where the acronym CPAP comes from), technically you really only need it during inhalation, and don't need it during exhalation. But early CPAP lacked the ability to know when you were inhaling, and constantly turning it on and off is impractical as who could ever sleep with pressure going on and off all the time?

But pressure going up and down a bit in concert with exhalation is a happy medium that does work for modern xPAP technology, so EPR is there to give a little pressure relief during exhalation. But it does not work for everyone, and it can add to the likelihood of events for some.

But basically, a flow limitation occurs during inhalation much more significantly than it might during exhalation, so EPR, which is not invoked during inhalation, will usually not contribute to it. Every patient is different, tho, so while it might be great for some, not so great for everyone.
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Thanks Tyrone,
Great explanation!
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The way the different machines treat flow limitations is quite different. Respironics auto adjusting machines report FLs in the events graph. Makes it easier to spot on the reports, and there is an immediate pressure response. With ResMed you get only the graph that you have to put under the microscope.

Respironics claims, While other devices typically respond only to flatness and shape, our auto algorithm analyzes changes in flatness, roundness, peak and shape. This precise recognition of unique patient flow patterns is the reason why the REMstar Auto reacts better than any other device on the market.

[Image: flow_limitation.jpg]

Recording flow limitations with a machine is not nearly as accurate as polysomnograpy. The presence of an abnormal contour on the inspiratory airflow waveform is indicative of flow limitation and noninvasively identifies increased upper airway resistance. This is interpreted as a precursor to Hypopnea or OA and pressure is increased.

Snoring can co-exist with flow limitation http://journal.publications.chestnet.org...28/685.pdf. Snores are another important trigger for pressure increases.

Something else, Respironics does randomly increase pressure 1.5 to 2 cm to evaluate whether a beneficial effect is achieved. I see these pressure increases on my graphs throughout the night. Normally they drop back to minimum pressure within 1-1/2 minutes.
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Hmm. "Claims", indeed. It seems to me that responding to "changes in flatness, roundness, peak and shape" isn't materially different than responding to "flatness and shape", since detecting a change in "shape" already implies detecting "roundness", and a change in peak is basic to any APAP algorithm. It seems like simply a more flowery way of saying exactly the same thing while passive-aggressively taking a shot at the competitor. The guy who wrote that should go to work for Sharper Image. Having "factory-air-conditioned air from our air-conditioned factory" in your car is not the same as having factory air conditioning in your car.

If it were that much better, wouldn't all the competitors be either very quickly doing the exact same thing or be out of business? Wouldn't the sleep docs all recommend their machine only? BTW, they don't.

With ResMed, you may only get "the graph you have to put under a microscope", but the machine doesn't need to do that. It still responds, still knows how and when to respond, and in much the same manner as any other APAP. Besides, I don't own nor need a microscope, and neither does an APAP.

And I know this because I also "see these pressure increases on my graphs throughout the night."
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Flow limitations and pressure algorithms are all tied up in patent claims and proprietary matters. They are quantifiable under sleep study conditions using multiple sensors. I think it's amazing the machines incorporate flow and pressure sensors that can remotely sense flow limitation at all, let alone its shape.

I like that respironics flags FL as an event. A lot of people come on the forum to ask questions and are not yet tuned into the Resmed FL graph. I have noticed that there regardless of manufacturer, the amount of FL has a very real impact on the sense "I slept well or not". It's also a very good queue that pressure is not optimal when a lot of flow limitation is appearing. Even in complex apnea, solving FL, H and OA while not aggravating CA is worth looking at and usually the way to get at it is the minimum EPAP pressure, which is somewhat analogous to the APAP minimum pressure.
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(02-22-2015, 01:20 PM)ppowers Wrote: I don't know what the numbers mean on the vertical axis of the graph, ie. .53, etc. Doesn't make any sense to me. As I think that flow limitation is really key, I would love to know how to interpret these numbers in the graph.

We don't know the units they're using in the measurement of flow limitation, and therefore have no idea what they are actually measuring.

We glean from their literature that it's an attempt at measuring factors that are likely to precede apneas and hypopneas. In other words, it's an attempt to predict that events are likely to occur, and therefore it's necessary to raise the pressure to prevent them.

What I want to learn when looking at my data is whether or not I've got my maximum pressure high enough. I feel I've got it right at 13.6 because rarely does the machine ever find it necessary to go that high. I was titrated at 13 and spend most of the time below 12, but on rare occasions I do go above 13.
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