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Is it a CA or an OA regardless of how the machine classifies it?
#1
Question 
I have seen the statement that what is scored by the CPAP as an OA is really a CA and vice versa. How do we make these determinations? How does the machine make these determinations? Yes, I know that the Resmed machines use the FOT and the PR machines use a pulse but what is the response that the machine looks for to determine the type. Can we come up with some unifying set of observations that we can all use to make this a little more straightforward and maybe a little less confusing. Any takers? Can we have a conversation and possibly come to some conclusions?

Please respond with your thoughts.

Best Regards,

PaytonA

Admin Note:
PaytonA passed away in September 2017
Click HERE to read his Memorial Thread

~ Rest in Peace ~
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#2
Irrespective of the machine or the manufacturer they all use the same method to distinguish between "central" and obstructive apneas.
All machines have only 2 sensors (well, actually just kind of 2): they measure the pressure and the flow. (although some machines seem to determine the flow based on the turbine speed needed to keep the pressure up / steady and seem to not have any kind of real flow-sensor) - Everything else is derived from these two values.

Central apneas are defined by the missing respiratory effort - this can not be measured by the xPAP devices - so they use a missing obstruction to determine that - Clear Airway. If no obstruction is present it cannot be an OA - but on the other hand the airways may collaps during a real central apnea. (If this happens the machine raises the pressure further and in result it is likely that another or more central apneas might occur)

All machines use an oscillating pressure curve to determine these 2 - so they raise the pressure a bit for a very short time. (depending on the trademarks they are labeled differently)
The physics behind that is the "compliance" of the lung. If the pressure is raised and the airways are open a little bit more air is pumped into the lungs - if the pressure is lowered that little air is flowing out of the lungs. (basically that's the lung volume which is not available for "breathing" or oxigenation with xPAP)
The devices look for a corresponding oscillation in the flowcurve during these pressure pulses or oscillating pressure. If the flow stays "flat lined" the airways are obstructed - if there is movement in the flow the airways are open.

So that is what you have to look for if you want that distinction "by hand" - depending on the resolution of the curves this can be pretty hard - Resmed has very high resolution. It migh be a good idea to change the scaling of the pressure and flow graph in sleepyhead so that the oscillation / pulsation of the flow curve are easier to see.


Works usually fine .... but there are some circumstances where this doesn't work "as expected":
If the machine estimates the deadspace wrong (mostly underestimates) and the pressure is too low, OAs can falsely be determined as CAs.
- this is more common on devices which use a very small FOT / OPP / PP (like under 0.4 cmH20) and FFM. If the "deadspace" is "big enough" the compression of that air may be enough to show a pulsation in the flow. Raising the pressure generally resolves that. (Weinmann's Somno-Series is one example where this really happens and can be observed - the higher the pressure the more accurate the distinction^^)

another problem are leaks:
Leaks are - naturally - pressure dependent. If you already have a leak and raise the pressure more air leaks. (aka corresponding higher flow^^)
I personally do not trust any distinction / events made by the devices around leaks - not even small leaks!

I know there is a lot of talk about "acceptable" leaks for the devices. The CO2-Ventilation flow is no problem and can be pretty accurate determined by the devices - but they nearly use the same technique to determine leaks - if you have a long enough leak it is regarded as normal ventilation flow. I'm talking about leaks - in other words "small" leaks - not mouthbreathing with a nasal mask^^ (yeah - really! - some devices are better than others - but in the end the physics behind that are for all devices the same!)
These values of ~24 L/min (or 35 for FFM) are the corresponding values for the maximum turbine speed! - these values tell you at what point the machine can no longer keep the pressure up! Any - and I really mean ANY - event scored under leaks is a wild guess by the machine. (Mostly I consider them 'BS'! - if one eliminates the leaks completely most of these events vanish!)
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#3
TBMx,

I have several questions for you if you would be so kind.
  • First, how and why does the machine estimate "dead space"?
  • How and why would the airway collapse during a central apnea when using a CPAP? There is still pressure in the airway attempting to hold it open.
  • What part of the apnea and oscillating pressure curve does the machine use to determine whether the airway is clear or obstructed?
  • Why do you not consider mouth breathing for nasal masks a leak and why does the machine not just consider that normal vent flow if it lasts long enough?
  • Why do you say that the turbine speed can not keep up with leaks over 24 l/min? I have seen them keep up well over that figure.
  • Why would there be a higher max turbine speed for an FFM than a nasal mask ( 35 l/min vs 24 l/min)?

Best Regards,

PaytonA

Admin Note:
PaytonA passed away in September 2017
Click HERE to read his Memorial Thread

~ Rest in Peace ~
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#4
(04-16-2017, 11:40 AM)PaytonA Wrote: First, how and why does the machine estimate "dead space"?

from what I believe; through the settings. In most modern devices you can select what type of tube you are using (normal one or small one) and in some what kind of mask you are using.
(but I guess you are more interested in the 3rd answer - as it is more accurate in combination with this ... I used "estimation of dead space" more as a rhetorical phrase - in the end what counts really is the dead space - but they do calculate conductance which in turn is influenced by the dead space and the dimensions of the tube and the mask)


(04-16-2017, 11:40 AM)PaytonA Wrote: How and why would the airway collapse during a central apnea when using a CPAP? There is still pressure in the airway attempting to hold it open.

we are talking here basically about APAP - not CPAP! A CPAP device does score events but to no end. CPAP does nothing than keeping the pressure steady - event scoring is of no importance.
With that in mind one possibility is: pressure sinks to the lower threshold ... central apnea occurs ... and of course than the airway can collaps. (the higher the pressure the more unlikely this becomes)

(04-16-2017, 11:40 AM)PaytonA Wrote: Why do you not consider mouth breathing for nasal masks a leak and why does the machine not just consider that normal vent flow if it lasts long enough?

The devices use something like "resistance" or more accurate the conductance to "measure" the leaks. Mouthbreathing is a very sudden change in that because it is a sudden large leak - and it goes as sudden as it comes. This results in very high canges in the cunductances and therefore it is highly unlikely that this gets interpreted as "CO2 Vent Flow". "Normal" leaks are not that big and are not that constant high - so the conductance over time changes and therefore can be misinterpreted as "CO2 Vent Flow".

Resmed usually estimates that pretty accurate - but: take a look at this picture
[Image: jBmU08K.png]

look around 23:03 or 23:13:30 or 23:20 ... you will see the "baseline" of the flow jumping down under the 0-line. That only happens if the machine miscalculated the leak rate.
That's why I also say: everything - and I really mean EVERYTHING - around or under leaks is a very rough estimate - or basically BS!


(04-16-2017, 11:40 AM)PaytonA Wrote: Why do you say that the turbine speed can not keep up with leaks over 24 l/min? I have seen them keep up well over that figure.

The numbers didn't come from me - It's what I read multiple times over here, in some manuals and pretty much everywhere aorund.

But If you do the math ... let's take the upper end of around 20 cmH2O (If these numbers are out they have to hold for everyone!)
the masks (FFM) have a CO2 Vent Flow there about 50l/min ... than we have to consider the "normal" breathing - 20 per minute / 500ml per breath --> 20 l/min this makes something around 70ish l/min.
The devices have something around 120 liters / minute in this pressure-range as maximum flow. so this leaves something in the 50ies.
If you factor in the measurement error you get something around 35 l/min as "safe" leak.


(04-16-2017, 11:40 AM)PaytonA Wrote: Why would there be a higher max turbine speed for an FFM than a nasal mask ( 35 l/min vs 24 l/min)?

That's actually a very good question - I'm interested in a solid, profound answer as well.

From what I would make out of it: (wild guess)
If a nasal mask leaks there are not that many options. The postion of the mask can change a bit or the mouth can open or the mask gets loose entirely. In the last two cases there is no way for the machine to keep up the pressure in the airways.
There might be some limitation in the flow due to "narrow" (or whatever) airways of the patient so that the flow is not that much higher than 24 l/min - but if the mouth is open the air esacapes there and the lower airways are no longer stabilized. So the information of 35 liters per minute (as an example) might be misleading to the patient if he or she has very narrow nasal airways and the "maxiumum" nasal flow lies under that value - the pressure can in no way be compensated!

The same is not true for a FFM. There is more "room" for leaks before the mask completely goes sideways.


(04-16-2017, 11:40 AM)PaytonA Wrote: What part of the apnea and oscillating pressure curve does the machine use to determine whether the airway is clear or obstructed?

I'm not sure if I understand you here. I'm from germany - so please forgive me.
The oscillating pressure is a change in the pressure. ... if you zoom in on a CA - you will notice a curve in the pressure around some seconds in the apnea. (look on the mask-pressure - not the IPAP / EPAP curve)
than look on the flow-curve - you will see the same pattern in the flow-curve.

... do the same with an OA - the same oscialting curve in the pressure curve - but the flow-curve will stay "flatlined" around the 0-line. (you might see some kind of oscilation - but it should be clearly not as high as during the CA! - that's most likely than the dead-space-compression)

... if you could post some zoomed in pictures of CAs and OAs - I can edit the pics to make it more clear. (I can't really use the pics from my machine because the resolution is not that grate for showing this - it would be more of an educated guess^^ - the S9 should have a resolution around 10 or 20Hz in these curves - so you should see that very good)
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#5
(04-16-2017, 11:40 AM)PaytonA Wrote: First, how and why does the machine estimate "dead space"?

(04-16-2017, 03:24 PM)TBMx Wrote: from what I believe; through the settings. In most modern devices you can select what type of tube you are using (normal one or small one) and in some what kind of mask you are using.
(but I guess you are more interested in the 3rd answer - as it is more accurate in combination with this ... I used "estimation of dead space" more as a rhetorical phrase - in the end what counts really is the dead space - but they do calculate conductance which in turn is influenced by the dead space and the dimensions of the tube and the mask)

The reason for the question is that I see little value in the knowledge of the amount of "dead space" in the system. The effect it has is on the timing and strength of the pressure signal. It has been my understanding and it makes sense to me that the tube and mask settings are to give the machine a good estimate of the amount of pressure drop before the air reaches the patient so that that pressure drop can be compensated for in supplying the prescribed pressure. Important in CPAP but not so much in APAP.


(04-16-2017, 11:40 AM)PaytonA Wrote: How and why would the airway collapse during a central apnea when using a CPAP? There is still pressure in the airway attempting to hold it open.

(04-16-2017, 03:24 PM)TBMx Wrote: we are talking here basically about APAP - not CPAP! A CPAP device does score events but to no end. CPAP does nothing than keeping the pressure steady - event scoring is of no importance.
With that in mind one possibility is: pressure sinks to the lower threshold ... central apnea occurs ... and of course than the airway can collaps. (the higher the pressure the more unlikely this becomes)

I used the term CPAP in a general way. I agree that a straight CPAP machine does not need to distinguish between types of apnea except when trying to tweak the pressure setting to improve results. So with an APAP we have set the min. EPAP pressure to prevent obstructive apneas and the max.IPAP pressure to minimize hypopneas and flow limitations. Now along comes a clear airway apnea. With the min. EPAP set correctly to prevent OAs, how does the airway collapse during a clear airway apnea?

(04-16-2017, 11:40 AM)PaytonA Wrote: Why do you not consider mouth breathing for nasal masks a leak and why does the machine not just consider that normal vent flow if it lasts long enough?

(04-16-2017, 03:24 PM)TBMx Wrote: The devices use something like "resistance" or more accurate the conductance to "measure" the leaks. Mouthbreathing is a very sudden change in that because it is a sudden large leak - and it goes as sudden as it comes. This results in very high canges in the cunductances and therefore it is highly unlikely that this gets interpreted as "CO2 Vent Flow". "Normal" leaks are not that big and are not that constant high - so the conductance over time changes and therefore can be misinterpreted as "CO2 Vent Flow".

Resmed usually estimates that pretty accurate - but: take a look at this picture
[Image: jBmU08K.png]

look around 23:03 or 23:13:30 or 23:20 ... you will see the "baseline" of the flow jumping down under the 0-line. That only happens if the machine miscalculated the leak rate.
That's why I also say: everything - and I really mean EVERYTHING - around or under leaks is a very rough estimate - or basically BS!

I can not make out much detail in those time periods but it looks to me like some ragged breathing. The flow jumps down below the 0 line at the end of every inhalation and with rapid, ragged breaths, it can look like it is diving right from the maximum inhalation flow to the maximum exhalation flow.

I am quite sure that the Resmed machines do not use resistance or conductance to "measure" leaks (estimate leaks). It is a fairly complex calculation but it does compare current results with 5 previous results. The tubing type and mask type settings are for the purpose of providing and estimate of the pressure drop between the machine and the patient. This is so that the machine can provide the correct pressure at the patient. This is not so important with an APAP since it will adjust to your needs regardless of the pressure reading except in the case of limits.

(04-16-2017, 11:40 AM)PaytonA Wrote: Why do you say that the turbine speed can not keep up with leaks over 24 l/min? I have seen them keep up well over that figure.

(04-16-2017, 03:24 PM)TBMx Wrote: The numbers didn't come from me - It's what I read multiple times over here, in some manuals and pretty much everywhere aorund.

But If you do the math ... let's take the upper end of around 20 cmH2O (If these numbers are out they have to hold for everyone!)
the masks (FFM) have a CO2 Vent Flow there about 50l/min ... than we have to consider the "normal" breathing - 20 per minute / 500ml per breath --> 20 l/min this makes something around 70ish l/min.
The devices have something around 120 liters / minute in this pressure-range as maximum flow. so this leaves something in the 50ies.
If you factor in the measurement error you get something around 35 l/min as "safe" leak.

I have not looked at your numbers closely but what I am telling you is that with my Resmed machine at 20 cm/H2O and a FFM, I have had leaks in the 60-80 l/min range for a period of time and the pressure never decreased


(04-16-2017, 11:40 AM)PaytonA Wrote: Why would there be a higher max turbine speed for an FFM than a nasal mask ( 35 l/min vs 24 l/min)?

(04-16-2017, 03:24 PM)TBMx Wrote: That's actually a very good question - I'm interested in a solid, profound answer as well.

From what I would make out of it: (wild guess)
If a nasal mask leaks there are not that many options. The postion of the mask can change a bit or the mouth can open or the mask gets loose entirely. In the last two cases there is no way for the machine to keep up the pressure in the airways.
There might be some limitation in the flow due to "narrow" (or whatever) airways of the patient so that the flow is not that much higher than 24 l/min - but if the mouth is open the air esacapes there and the lower airways are no longer stabilized. So the information of 35 liters per minute (as an example) might be misleading to the patient if he or she has very narrow nasal airways and the "maxiumum" nasal flow lies under that value - the pressure can in no way be compensated!

The same is not true for a FFM. There is more "room" for leaks before the mask completely goes sideways.

Resmed claims that up to 24 l/min their data is good and there is no pressure problem and that is for all masks. There is a fairly knowledgeable person on one of the other forums that claims that35 works fine for her. I tend to stick with the 24 l/min.

(04-16-2017, 11:40 AM)PaytonA Wrote: What part of the apnea and oscillating pressure curve does the machine use to determine whether the airway is clear or obstructed?

(04-16-2017, 03:24 PM)TBMx Wrote: I'm not sure if I understand you here. I'm from germany - so please forgive me.
The oscillating pressure is a change in the pressure. ... if you zoom in on a CA - you will notice a curve in the pressure around some seconds in the apnea. (look on the mask-pressure - not the IPAP / EPAP curve)
than look on the flow-curve - you will see the same pattern in the flow-curve.

... do the same with an OA - the same oscialting curve in the pressure curve - but the flow-curve will stay "flatlined" around the 0-line. (you might see some kind of oscilation - but it should be clearly not as high as during the CA! - that's most likely than the dead-space-compression)

... if you could post some zoomed in pictures of CAs and OAs - I can edit the pics to make it more clear. (I can't really use the pics from my machine because the resolution is not that grate for showing this - it would be more of an educated guess^^ - the S9 should have a resolution around 10 or 20Hz in these curves - so you should see that very good)

What I was asking here is as follows. If you have an apnea that starts as a clear airway apnea and the FOT or whatever starts and identifies it as a clear airway and then before the apnea ends the airway collapses and it becomes abstructive,does the machine classify it as an obstructive apnea. So does the machine make the determination just prior to the end of the apnea or does it determine which type of apnea lasted longer and score it for that or is it something else?

Best Regards,

PaytonA

Admin Note:
PaytonA passed away in September 2017
Click HERE to read his Memorial Thread

~ Rest in Peace ~
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#6
this is (to my knowledge) the most current patent from ResMed for leak calculation (hasn't changed that much except from phrasing from the first one back at the end of the 90ies or '00ies): https://www.google.ch/patents/US7661428
(The "ResMed-magic" lies in the LP-function and the choosing of the (changing) time-constant RC which seems to be not that fixed as they describe it in the patent^^)

back to the "acceptable" leaks. We are not talking about the same.
There are 2 pressures. 1 at the mask - or the one applied to the patient (that is the one that really matters to us!)
The other one is the pressure measured at the end of the device (that is the only pressure the devices know of).

These 2 pressures are only at one given point in time equal: in the tiny timeframe between (the end of) exhale and the (beginning of the) next inhale. (where the respiratory flow is 0)
If you start the inhalation you take air out of the "system" (system meaning device + tube + mask) this results in a pressure drop (we are talking about mmH2O .. otherwise the AAV would kick in) - the device compensates. If you exhale you put air into the system --> pressure rises ... machine compensates by reducing turbine speed. (this is actually the most important point where quality of the turbine matters ... if you have a poor turbine it is not able to reduce the speed fast enough, which results in slower pressure drop and therefore you might need the exhalation relief (EPR on ResMed-Devices) ... all the exhalation relief systems are build (historically speaking) just for this very reason^^)

I don't see how dead space is factored in in this situations ... all that matters here is the pressure.

the picture from the other thread which I showed: If you open it up in a new tab and zoom in you can there clearly see how the machine got the leakrate wrong. As I already linked the patent: look at the 3rd or the 8th picture - in figure 1f or 4h you can see very clear what I was talking about. (It really is not that important for an APAP-Device to get the baseline right - they (mostly) use the derivations from the flowcurve for there interpretations and calculation magic)

As mentioned earlier the machine measures the pressure at the end of (or in) the device. There is something called dynamic pressure. That is pressure that builds up simply from flowspeed. It's not something tiny - it is the very thing that keeps airplanes up in the sky! (yeah, really!)
If you have leaks you reach the upper end of the turbine speed. There we actually reach pretty amazing flow-speed and dynamic pressure really becomes a factor^^
(to my knowledge the flow dynamics inside the tubes are pretty much unknown (way toooo complex) and all devices calculate with a very rough estimation of the total resistance (or cunductance for that matter) - if the flowspeed and therefore the turbulences insinde the tube increase the resistance does as well - resulting in a drop of conductance)

I took the upper end of APAP devices (once again: I'm talking APAP - not bilevel, not ASV, not whatsoever - plain APAP! - no IPAP no EPAP - just (variable) PAP!) because there the "useable" flow is the lowest. A big part of the turbine speed in that area goes into keeping up and/or building the pressure.
If you personally do not reach the end of the pressure capabilities of your device, the leak rate, which the machine should be able to compensate, is accordingly higher.
Once again: If you talk about leaks around 70ish liter per minute + the vent-flow from your mask + your breahting - we end up somewhere around 120-150 liters per minute - take a look at the diameter of your tube - do the math and calculate the flowspeed of the air! (awesome isn't it? - now imagine that as water (yeah I know - liquids have a total different (flow-)dynamic) .. time to clean the terrace?)

I would not trust the value of the pressure sensor there!


... but I believe all my techno-blabla went way beyond your original question - I only mentioned that because I always find it important to understand what the devices do and to what end before putting too much faith in what the devices show us.

as far as it goes for the "obstruction during a central apnea": it simply is a medical fact - it can happen - if your therapy is set right it should not happen.
Real central Apneas can only be determined by missing breathing effort - you really need the belts from the PG or PSG (in the sleep lab) to get that right.
Depending on the algorithm used and the manufacturer and the model they all operate as follow:
breathing (seems to be) stopped ... start the FOT (for ResMed I believe this starts after 4 seconds in the apnea) ... if the device sees a corresponding oscillation in the flow-curve it is a central one in any other case it is an obstruction.

so to answer your last question: I can only say that definetely for my machine - as I tested that (as far as I can or could - but I tried some crazy stuff to understand my machine^^)
My machine does score it as central - if at some point (seems to at least or longer than 2 seconds (not 2s in total; this has to be continuous) there is an oscillation in the flow-curve it is central - in any other case it is an obstruction.

I include a screenshot of a central apnea from my Weinmann SomnoBalance (please keep in mind - flowresolution is 5Hz - pressure 2Hz - so it's more a "educated guess from knowing the device" than real "seeing"!)

   

this is a "real" central apnea from STS ... had to search really hard and couldn't find a better one as I do not have CAs (other than from STS) ... my machine scored a lot CAs after my initial pressure settings from the sleep lab of 4-14cmH20 - but only in the pressure range from 4-5 cmH20 which where clearly no CAs but misinterpreted OAs .. that went to the point of beginning periodic breahting. (my sleep study also didn't show any indication of centrals)

The latter - of course - can happen if you have an obstruction due to too low pressure and the obstruction vanishes during the pressure rise from the FOT.


(but weinmann and resmed do things completely different ... WeinMann also 'has' central hypopneas - at least with the european firmware the resmed 10 series doesn't even score a length for hypopneas (or RERAs - but in europe only the "for her" does score RERAs at all) .. how funny is that?)

How do I determine whether a CA is 'real' or not?:
I take a look around ... first I look for the obvious: CA from STS (Sleep transitioning state - where a CA is allowed to happen) 3-4 CAs (in total for the night) - 1 every 1.5 to 2 hours ... I count them of as STS.
than I look around for the breathing pattern - might be REM-sleep, where (a few) CAs are allowed to happen as well.
than I look at the current pressure and zoom in on that (and by zooming I also mean changing the y-axis scaling to actually "see" the FOT) and take a look at the flowlimitation (or obstruction or whatever the device calls it) - if the pressure is too low it might be a misinterpreted OA - if there was no flowlimitation / obstruction before and after that - it might be a real CA.

look what happens after the machine correpsonded to that interpretation - if it drops the pressure and you get another OA after that it clearly was an OA. - if there is no raise in the flowlimits / obstruction it might have been a CA.

same goes for OAs: if the pressure rises and the next "OAs" happen everytime the pressure builds up it might have been a central one. (this is not so much of interest for Resmed as they basically only react to flowlimitations - but is important for Weinmann (at least the somno-Series ... haven't had the time so far to really look into the prisma-series) as they meanly react to obstructions and not so much to flowlimitations. (they have this handy (as in easy to interprete) "epoch-thingy"))

notice the "might" as - as I already pointed out - it's only a guess because the machines simply cannot measure the central part!
(I'm talking APAP - not ASV, not Bi-Level, not whatsoever ... there they simply start doing their job - the result is seen nealy instantly^^)
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#7
(04-16-2017, 10:05 PM)PaytonA Wrote: What I was asking here is as follows. If you have an apnea that starts as a clear airway apnea and the FOT or whatever starts and identifies it as a clear airway and then before the apnea ends the airway collapses and it becomes abstructive,does the machine classify it as an obstructive apnea. So does the machine make the determination just prior to the end of the apnea or does it determine which type of apnea lasted longer and score it for that or is it something else?

actually I have to correct my last statement regarding that. ResMed does have the unclassified apnea - if it is a mixed apnea it is scored as an "UA".

I found a better screenshot of the FOT at work (taken from a ResMed Airsense 10 For Her Autoset)

The blue curve is the mask-pressure (in high res) and the black one is the flowrate.

[Image: resmedfotafstv.jpg]

the 1st red box shows it clearer. The pressure starts "pulsing" and you can see the corresponding pulsation in the flowcure - airways must be clear - if they would have been obstructed the flow would not pulse (that much).

(the flow from the 2nd apnea gets a bit distorted during the end - clearly the user was awake and so it is likely that the distortion comes from leak-flow due to movement. (the leak scored by the machine was zero during the whole time - but the devices can't measure the leakrate that fast))

you would see that much clearer if you would rescale the y-axis.
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#8
Thank you, good information.
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