Help me understand individual breaths

avi123 · Post by **avi123** » Thu Feb 13, 2014 10:16 am

Setj wrote:
Sludge wrote: That's "FOT".
That stumped me until I googled this explanation on this forum.
Forced Oscillation Technique. It's how the data capable S9s attempt to detect central apnea. It's a rapid series of small puffs of air. The machine measures the airflow changes to see if the air is reaching your lungs or not. (Simplified version)
So is it safe to assume the sawtooth in my usual waveform is FOT?

I am beginning to think this therapy is about optimized for me and it is time to move on to other things. I would continue to check the display every morning. I would only do a data download if the numbers on the display looked bad or if I felt bad.

Comment,

I have similar flow graph shapes as that of Setj:

But I can see the FOT, more clearly, in the graph during an event:

Same thing with Setj's graph. Those are all FOT oscillations. In Setj's flow- graph the vertical axis is scaled more precisely and because of it you can see the FOT's waves of 1 cm H2O, in amplitude, also enveloping the respirations waves. :

archangle · Post by **archangle** » Thu Feb 20, 2014 1:52 am

The average airflow displayed should always be 0. When SleepyHead (or other program) shows something different, it's a bug somewhere.

The machine (and software) can't measure actual airflow into your lungs, it can only see how much air is flowing down the hose, which will include the intentional and unintentional leaks. i.e. if it showed the actual airflow, the average airflow would be above 0.

I believe all CPAP machines and software handle this problem by subtracting out a "moving average" of the airflow for the past x seconds and this will keep the airflow reading centered around zero.

Sir NoddinOff · Post by **Sir NoddinOff** » Thu Feb 20, 2014 5:42 pm

archangle wrote:The average airflow displayed should always be 0. When SleepyHead (or other program) shows something different, it's a bug somewhere.

The machine (and software) can't measure actual airflow into your lungs, it can only see how much air is flowing down the hose, which will include the intentional and unintentional leaks. i.e. if it showed the actual airflow, the average airflow would be above 0.

I believe all CPAP machines and software handle this problem by subtracting out a "moving average" of the airflow for the past x seconds and this will keep the airflow reading centered around zero.

Archangle. That's the best explanation of FOT and the 'zero' line issue that I've read yet. It would be nice if a ResMed or PR engineer (or tech) could weigh in on how exactly their machine data is adjusted for intentional leaks and unintentional leaks. They are probably sworn to secrecy, therefore It's not gonna happen Given all that, I'm still allowed to wish.

Sludge · Post by **Sludge** » Fri Feb 21, 2014 5:24 am

Sir NoddinOff wrote:It would be nice if a ResMed or PR engineer (or tech) could weigh in on how exactly their machine data is adjusted for intentional leaks and unintentional leaks. They are probably sworn to secrecy, therefore It's not gonna happen Given all that, I'm still allowed to wish.

Wish granted:

https://www.google.com/patents/US6945248

http://www.google.it/patents/US5313937

If the estimated leak flow rate signal from low pass filter 38' is equal to the true system leak flow rate, and if the patient's inhaled and exhaled volumes are identical for a given breath (i.e. total positive patient flow equals total negative patient flow for a given breath), then the integral calculated by integrator 60 will be zero and no adjustment of estimated leak flow rate will result. When the integral calculated by integrator 60 is non-zero, the integral value in the form of an output signal from integrator 60 is provided as indicated at 64 to a sample and hold module 66. Of course, even with a zero value integral, an output signal may be provided to module 66, but the ultimate result will be no adjustment of the estimated leak flow rate signal.

A non-zero integral value provided to module 66 is further provided to module 38' as indicated at 68 with each patient breath by operative action of the IPAP/EPAP state signal upon module 66 as indicated at 70. The effect of a non-zero integral value provided to module 38' is an adjustment of the estimated leak flow rate signal proportional to the integral value and in the direction which would reduce the integral value towards zero on the next breath if all other conditions remain the same.

With this system, if the patient's net breathing cycle volume is zero, and if the system leak flow rate changes, the integrator circuit will compensate for the change in leak flow rate by incremental adjustments to the estimated leak flow rate within about ten patient breaths.

The integrator circuit 60 also will adjust the estimated leak flow rate signal in response to non-zero net volume in a patient breathing cycle. It is not unusual for a patient's breathing volume to be non-zero. For example, a patient may inhale slightly more on each breath than he exhales over several breathing cycles, and then follow with a deeper or fuller exhalation. In this case, the integrator circuit would adjust the estimated leak flow rate signal as if the actual system leak rate had changed; however, since the reference signal correction is only about one tenth as large as would be required to make the total correction in one breath, the reference signal will not change appreciably over just one or two breaths. Thus, the integrator circuit accommodates both changes in system leakage and normal variations in patient breathing patterns. The integrator circuit normally would be active, for example, during rapid patient breathing.

An end exhalation module 74 is operative to calculate another data component for use in estimating the system leak flow rate as follows. The module 74 monitors the slope of the instantaneous flow rate wave form. When the slope value is near zero during exhalation (as indicated by the state signal input 76) the indication is that the flow rate is not changing. If the slope of the instantaneous flow rate signal wave form remains small after more than one second into the respiratory phase, the indication is that exhalation has ended and that the net flow rate at this point thus is the leak flow rate. However, if estimated patient flow rate is non-zero at the same time, one component of the instantaneous flow rate signal must be patient flow.

When these conditions are met, the circuit adjusts the estimated leak flow rate slowly in a direction to move estimated patient flow rate toward zero to conform to instantaneous patient flow conditions expected at the end of exhalation. The adjustment to estimated leak flow rate is provided as an output from module 74 to low pass filter 38' as indicated at 80. When this control mechanism takes effect, it disables the breath by breath volume correction capability of integrator circuit 60 for that breath only.

Sir NoddinOff · Post by **Sir NoddinOff** » Fri Feb 21, 2014 10:46 am

Thanks Sludge... that's some good research. Wish fulfilled. Is there a translator in the house???

Ha, ha just kidding - I've got a degree in electronics (tho I haven't put it to use for many years). Hmmm... I'm sensing some integral calculus going on here from a quick glance at the images. I'll take some time in the next few days to read thru these patent documents and see what I can make out. I do find this stuff fascinating, plus I'd simply like to know what's strapped to my face all night long

Sludge · Post by **Sludge** » Sat Feb 22, 2014 4:23 am

I think the important part of this concept is in these graphs:

A leak is introduced into the system. The breathing waveform is filtered (smoothed) to reveal the leak beyond the baseline, and this value is subtracted from the waveform to center it.

robysue · Post by **robysue** » Sat Feb 22, 2014 11:34 am

Sir NoddinOff wrote:Thanks Sludge... that's some good research. Wish fulfilled. Is there a translator in the house???

Ha, ha just kidding - I've got a degree in electronics (tho I haven't put it to use for many years). Hmmm... I'm sensing some integral calculus going on here from a quick glance at the images. I'll take some time in the next few days to read thru these patent documents and see what I can make out. I do find this stuff fascinating, plus I'd simply like to know what's strapped to my face all night long

Yep, there's integral calculus going on. The main thing to remember from your freshman calculus course is this: The definite integral from t=a to t=b of f(t) dt is equal to the area trapped ABOVE the t-axis and below the graph MINUS the area trapped BELOW the t-axis and above the graph. In other words, the integral computes signed area.

The idea is that the signed area trapped by the wave flow data for one full breath cycle (one inhalation followed by one exhalation) should be 0 (or pretty close to 0) under most circumstances: In other words, the "area" for the exhalation should cancel out the "area" for the inhalation when a patient fully exhales. If the integral of the wave flow over one breath is NOT equal to 0, then the machine's algorithm is supposed to use the integral's numerical value to figure out where the corrected baseline ought to be drawn for the wave flow curve.

Sludge · Post by **Sludge** » Sat Feb 22, 2014 2:37 pm

robysue wrote:If the integral of the wave flow over one breath is NOT equal to 0, then the machine's algorithm is supposed to use the integral's numerical value to figure out where the corrected baseline ought to be drawn for the wave flow curve.

OK, but in practice, "IMHO" there may be very little reaction to single breath variability because it is so common (imagine the little blip leak at peak inspiration. E will be <I but no adjustment should be made).

Rather, leaks are corrected by the filtered waveform using a floating window (in this case 10 seconds). See explanation below that perhaps makes the Fig, 4 waveforms clearer:

robysue · Post by **robysue** » Sat Feb 22, 2014 6:46 pm

Sludge,

Of course you're right, the machine is not going to respond to each little discrepancy. Thing is: Mathematically speaking, if there's a problem with the 0 line that has to be corrected, it is going to affect all the breaths during the time frame roughly equally. Hence from a mathematical view it doesn't really matter all that much if you simplify (beyond the point of reasonableness perhaps) and talk about the area trapped by the trace for one breath or the trace for a series of breaths. If the 0 line is set correct, the integral for one breath and the integral for many breaths will both equal 0. If the zero line is not set correctly, the integral for one breath will be approximately equal to (1/n) times the integral for n breaths---assuming that all the breaths during the time frame are roughly equal like they should be in nice, regular sleep breathing.

Sludge · Post by **Sludge** » Sun Feb 23, 2014 5:01 am

robysue wrote:Mathematicallly...

If the 0 line is set correct, the integral for one breath and the integral for many breaths will both equal 0. If the zero line is not set correctly, the integral for one breath will be approximately equal to (1/n) times the integral for n breaths---assuming that all the breaths during the time frame are roughly equal like they should be in nice, regular sleep breathing.

Begging your mathematicalness, I believe there are two components that we are discussing, namely, are breaths intact (f = 0) and do we need to consider that, and is there inadvertent leak and need to adjust 0.

I believe your explanation is somewhat misleading in that it suggests a misplaced 0 line creates inaccurate integrals and is therefore the driving force to correction.

However, "IMHO" single-breath integrals are calculated based on their own individual quality:

to see if a correction needs to be made, then the algorithm goes on to calculate inadvertent leak through LP analysis.

Specifically, I would envision an f ≠ 0 leak as the aforementioned "blip" (although again, in practice, I just can't envision a lot of f ≠ 0 breaths, unless we're talking about BiPAP or some sort of expiratory relief such that Variable Leak Milieu is encouraged).

Sludge · Post by **Sludge** » Sun Feb 23, 2014 5:12 am

Sludge wrote:... unless we're talking about BiPAP or some sort of expiratory relief such that Variable Leak Milieu is encouraged...

Or like there's VLM that is restricted to end-inspiration/begin-expiration (evenly distributed) on traditional CPAP (an area that might create VLM because there's increased flow at that point), so the likelihood is that you have f = 0 again.

OhHelpMe · Post by **OhHelpMe** » Sun Feb 23, 2014 7:34 am

"See, the enemy is puffed up." - Habakkuk 2:4

Sludge · Post by **Sludge** » Sun Feb 23, 2014 9:22 am

OhHelpMe wrote:"See, the enemy is puffed up." - Habakkuk 2:4

Habakkukia ("air trapping")!

Of course!

Another potentially highly disruptive force to \int_a^b \! f(x)\,dx methodology!

Sleep2Die4 · Post by **Sleep2Die4** » Sun Feb 23, 2014 10:09 am

OhHelpMe wrote:"See, the enemy is puffed up." - Habakkuk 2:4

haha The area over the X axis was greater than the area under the axis.

CowFish · Post by **CowFish** » Sun Feb 23, 2014 10:11 am

Sleep2Die4 wrote:
OhHelpMe wrote:"See, the enemy is puffed up." - Habakkuk 2:4
haha The area over the X axis was greater than the area under the axis.

Funny, unless it is happening to me and sometimes it does.

Help me understand individual breaths

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