Here is how to measure your exact intentional leak rate SLPM

[feeling_better]

The leak rate for masks are published by the manufacturers in a table or as a graph which gives the leak rate for a particular pressure. One usually subtracts that value from the leak rates reported by our machines (some machine can save this value internally and automatically subtract it) to get the unintentional or unwanted leak when one uses the xPAP.

However, the published leak value may not be what your particular machine may be coming up with for a number of reasons:

* The actual pressure at the mask may be a bit different than the published value the manufacturer used, because of drops in the tube

* There will be manufacturing variations from mask to mask of even a particular model

* This may be the largest --- there is significant measurement accuracy and variation from machine to machine.

Please note that the if your machine is in AUTO mode, the intended and unwanted leak varies from pressure to pressure, but to describe the procedure, the following assumes the leak determination at a particular pressure.

It is relatively easy for you to determine the exact leak for your particular mask, as it would be measured and reported by your particular machine. Again I am assuming your machine has someway of reporting the leak to you.

1. Select a particular fixed pressure for this measurement.

2. Remove the nasal or other removable irregular interface that contacts your nose or face (the nasal cushion, etc).

3. Seal the open end of the tube end that would mate with the above removed interface airtight. This can easily be done by covering the tube end hole with a thin latex or vynil glove, or balloon and using a rubber band slipped over it in a number of turns. [Select another material if you are allergic to any of these!]

4. Run the machine and check carefully there is no leak from around the tube end you just sealed. There would be a constant air flow (intended leak) from the exhaust out port.

5. Most machines can be run for sometime to get a reading as a session of this test time only. You would have to select the time of doing this separated from your normal sessions by 4 hours or more for some machines like the M series.

6. Turn the machine off normally, and read the leak from the above test session. The maximum, average and 90% all should be the same value, since the leak was constant. This number is the intended leak rate for the pressure you selelcted.

Of course you can repeat the test for other pressures, if needed.

Here is what I measured with my M series Auto machine, the results are a bit surprising, because the deviation for two masks are in opposite directions!

Pressure Setting 6cm.

For Optilife mask L size

measured: 23.25. Listed value from the spec sheet graph: 21 (approx)

For AEIOMed Headrest L size

measured: 18.00. Listed value from the spec sheet graph: 18.5 (approx)

Please note that the above measured value is 'accurate' only for our own purpose for subtracting from the total to find leaks. It may not be absolutely accurate in that our machine may be measuring it inaccurately.

I would be interested in finding what variations people here see with their masks.

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EDITED JULY 12, 2008

On further thought, the above is the accurate intentional leak for a fixed pressure when using straight CPAP without exhale pressure relief by methods such as CFLEX or AFLEX.

When running Cflex, for example, for a given pressure setting say 6, the effective average pressure would be a bit less because the machine reduces the pressure during the exhale period. I do not know the exact algorithms used by CFLEX. Even if it is known, the average pressure will depend on the ratio of the inspire and expire which varies with the individual user. [I have not verified this with modeling or reading technical papers on the subject.]

The net effect of this would be, if you are using cflex or aflex, the intended leak rate for a given mask will be less than for the fixed pressure published. So the observed or calculated unwanted (preventable) leaks will be under reported. My assumption here is that most all machines simply measure the average flow from the machine in reporting the leaks (or those machines which have that feature, after subtracting a fixed amount based on the type of mask or value entered into its settings.)

end of Edit
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CPAPopedia Keywords Contained In This Post (Click For Definition): AEIOmed, seal, auto

[dsm]

Excellent idea & a good way to get accurate data.

If I am given 3 such readings (say 8CMs + 12 CMs + 16CMs) I can calculate all the other values and am willing to put up a new leak rate chart for any mask machine combo.

DSM

[-SWS]

I think that's an excellent write up.

I tend to think of this method as a viable way to help us "normalize" our mask leak data with respect to our individual machines---which potentially contain significant measurement variations from machine to machine. And that is slightly different than the objective of attaining an accurate universal measurement of flow leak for any given mask or interface (while disregarding xPAP or other unique extraneous factors).

I personally suspect that individual mask models will entail less leak variation from production run to production run, compared to xPAP machines that are more likely to entail leak-measurement variations from model to model---or even production run to production run of identical models. I say that simply because the density or complexity of opportunity for error injection should be significantly greater for complex machines (which are component dense or subsystem dense systems) than highly basic mask orifice and mating surface parameters. Regardless, there is room for leak variation here.

Regarding production variations in those mask orifices: identically milled orifices will always leak at the same fixed rate or function thanks to non-variant orifice-based flow physics. When there is significant manufacturing orifice variation here, the mask is significantly out of spec. Interface joints and gaskets can be significantly out of leak spec as well, warranting replacement or repair.

Regarding production variations in xPAP machines: from end-to-end there are significantly more opportunities for both internal leak occurrences and measurement variation than leak parameters associated with designed orifices and mating-surface interfaces at the mask IMHO. Additional non-production leak variations are also potentially injected in that system during end-to-end assembly and maintenance by us good old hoseheads. Again, we're considering only fixed leaks and resident leak measurement errors as opposed to spontaneous leaks that occur as we sleep.

So I suspect that we would see even more machine-to-machine leak measurement variation than we would see while comparing (machine factored or disregarded) leak variation among identical masks.

My other comment is that if any algorithm happens to rely on comparing inspired volumes against expired volumes to ascertain spontaneous leaks (different than baselining fixed leak rates), then this method may not work for lack of being able to source equivalent volumes back into the machine for volume comparison during baselining. We pretty much expire the same volume of gas that we inspire. However, I doubt any of the current algorithms happen to rely on dual-directional volume comparisons to indirectly facilitate the baselining or calculation of fixed leak rates---but haven't really checked.

Again, what a great write up, feeling_better! Very useful, indeed. Thanks!

[feeling_better]

While I was doing leak measurement experiments, as specified here
I came across some unexpected results.

For a given mask, as I gradually increased the pressure, say in steps of 1cm, the measured intended leak did not change at all, for many steps. But it was definitely different for two different masks I had tried, and for both masks at some pressure the leak values were in the right ballpark as in their specification.

For a detailed explanation of this please see:
viewtopic.php?p=287621#287621

[crossfit]

can this be added to the yellow lightbulb? I was just looking for something like this today and it wasn't there.

I also have a question. How do you account for apap? Do you simply go with the intentional leak rate for the >90 pressure number? Or for the max pressure?

[feeling_better]

can this be added to the yellow lightbulb? I was just looking for something like this today and it wasn't there.

I also have a question. How do you account for apap? Do you simply go with the intentional leak rate for the >90 pressure number? Or for the max pressure?

crossfit, I have just started thinking about how to determine the cflex and aflex correction to standard intended leak. By reading the patents you just get only a what, not the exact numbers. I think it can be determined emperically by a number of experiments.

I just started on this for cflex this afternoon. I had to first have an accurate manometer that can measure while you are using the machine. I made one. With that I tried my M-series machine under cflex. This is what I found for just one pressure setting:

At my 5.5 pressure, I could not see practically any change at all between cflex settings 1,2 or 3. This was my feeling prior to the measurement and confirmation.

When I exhale it drops from 5.5 to 4 cm almost all thru my exhale. In the last 10 or so of the exhale the pressure starts to rise back to 5.5. I have done very very slow breathing in/out and fast one, the results are the same. If I do a very quick forced exhale, things are all over the place as expected. The pressure really goes up by 2 cm, and then it really comes down, but that is not really my normal breathing; probably similar for most people.

Now you need to know one more factor, which your way of breathing to determine the correction, again for one pressure setting. For me my exhale period is approximately 1/2 that of the inhale, while I am awake. You need some recording to determine this while sleeping.

So as an example, for me, I can find out my cflex intended leak at 5.5 by

(Leak @ 5.5 + 1/2 * leak @ 4.0*0.9 + 1/2 * leak @ 5.5 * .1) / 1.5.

I think this will give a fairly accurate value for me for cflex at 5.5.

Similar experiments have to be done with aflex, which I plan to do.

[feeling_better]

A few additional notes and tips on measuring the actual leak yourself. Please read the post at the beginning of the thread first.

There are some minor difficulties when one tries to follow my procedure above.

1. The lcd on M-series machines will not show any leak number at all if you run the machine for a short time. It may also need to detect somebody is breathing before it starts collecting any leak data that is displayed on the lcd. So you need the respironics software to look at the leak numbers during the measurements.

2. Some masks are not easy to cover up as described in the procedure above, to block the airflow while you are not wearing it, yet allow the air to leak out through the out ports. If there is a place where the nasal pillow can be removed, such as in the Headrest, then that tube end can be blocked with some membrane as I described above. However it may be easier to block almost all types of nasal pillow interfaces simply by inserting two noise reducing rubber ear plugs into them. If the those expandable rubber ear plugs are slightly larger than the pillow opening, you would get an amazingly air tight plug, at most pressures!

For full nasal, and full face masks you have to device your own method of closing up the long opening, if there is no tube to disconnect. Those who have successful methods, such as perhaps covering up with a balloon and then closing it with rubber bands, etc, can share their ideas here

3. It takes a bit of concentration to see the leak measurement data you did during the daytime. It is easier if you have a spare data card, or you can delete the data from the card when you download the data into Epro in the morning. So starting with a 'data deleted' card, you make one leak measurement as mentioned at athe beginning of this thread. Let it run for at least half an hour. Turn the machine off, wait about a minute, take the datacard to your computer.

It is better to create a new patient for downloading into say Leak Experiment. That way you would have very little data for that patient.

Download the data and view the daily detail. Respironics will not plot the leak graph for the half hour, because no breathing was detected! But you will have a little blip of blue at the left most point where the graph would start. At right side the Average Leak will show some value, which the leak value you are interested in as the result of your last measurement.

Note that since your leak should be really constant, --- assuming there was no other variable leaks from leaking hose joints, nasal openings, etc -- it should be reported as a whole number, no fraction! One of those 4, 11, 18, 25, 32, ... in steps of 7. If there is a fractional part, then you had a varying leak during the half hour test!!

4. Now you delete the data from the card (you can do that as you were downloading, or doing a second download just to delete it.) and do another measurement at another pressure. Well, this process is time consuming

5. Multiple measurements can be done before downloading, but then you need some other software to look inside the database, the distinct value for each pressure is recorded there, but the Epro software will only show one number for the average of all the measurements.

Has anybody got bold enough to try this experiment yet ?

[major_works]

Outstanding post. I've been wondering just what the *real* intentional leakage is from my OptiLife mask. I know what the graph in the user's manual says it *should* be for my prescribed pressure but I hadn't yet figured out how to realistically compare that number to what Encore Pro is reporting.

I will try this experiment as soon as I can; perhaps over the weekend. Thanks!

[jdm2857]

Does anyone know which machines automatically subtract intentional mask leak from the reported leak number and which do not?

I'm guessing my machine adjusts the leak because it has settings for various masks. But I am not sure. The wording in the Clinician's Manual is a bit vague, of course.

[Slinky]

The Resmeds.

If using a non-Resmed mask, you are to use the Standard mask selection and the Resmeds will deduct 0.4 L/s (24 L/M) regardless of the set pressure.

If using a Resmed mask, the mask selection should be set for that particular mask and then the device will subtract the allowed vent rate for that mask at your set pressure.