Confusion over PS equation

[David Clark]

Everywhere you look the formula for determining the Pressure Support setting is  (PS) ≈ IPAP – EPAP, so if IPAP is 15, EPAP is 9 then PS 'should' be 6 according to that. Or IPAP 20, EPAP 10 then PS 10. But you don't really see anyone setting the PS higher than 5...most of the time 3 or 4 regardless of what the IPAP/EPAP settings are. I just don't get why that formula is what it is if people don't actually set the machine that way. Even when I first got my machine is was set IPAP 25 EPAP 5 PS 5...I don't actually know how high the PS can be set, I've never tried turning it up past 6.

[Crimson Nape]

Resmed VAuto's Pressure Support can go up to 10 cm. Recently, we had a new member that had a PS of 8 cm. I believe he had a medical condition that supported this pressure.

The PS is added to the EPAP value and is limited by the IPAP maximum setting. This can get confusing because the AirSense CPAP just lists a pressure range or setting and an EPR value. Since the AS10's pressure setting is the IPAP pressure, the EPR is subtracted from this pressure. In short, PS is added to EPAP on a Bi-level and EPR is subtracted from the IPAP on a CPAP.

An example of making a VAuto and an AS10 pressures match, both having a range of EPAP 8 to 12 and an IPAP of 11 to 15.
AS10's setting would be a pressure setting of Min=11 and Max=15 with an EPR of 3.
VAuto's matching setting would be, EPAP=8 IPAP max=15 and a PS=3.

Have I been confusing enough?

[David Clark]

Have I been confusing enough?

Yep...perfectly enough. 

[David Clark]

I'm not really too confused on how PS functions, just on why the 'recommended' setting is ipap-epap when most of the time it is never set that way, or even can be set that high, in most cases. I just don't know how they came up with ipap-epap is what it should be.

[Gideon]

How you set PS actually depends on the machine
This can be
Set IPAP, EPAP the difference is PS
Set Min EPAP, Max IPAP, PS = x

And then it gets crazy in a CPAP with EPR

[Crimson Nape]

I can only provide a basic answer. The EPAP pressure controls hypopnea and OAs. You would raise the EPAP until these cease. The PS can correct inhalation flow limitations. The limitations will show up on the Flow Rate graph on the top of the inhalation arc. For practical purposes, a normal arc will mimic a sine wave (on the top). Flow limitations will display as a plateau or (to me) like a cursive "r". To overcome the flow limitations, you keep increasing the PS until the waveform looks close to normal. It sounds easy, but the PS is a double-edged sword. Increasing PS will cause CO2 washout, thus causing CAs. So the bottom-line is to use as much pressure that is needed that is below the CA threshold.

Did I answer your question?

[Geer1]

You have to think of it from basics point of view. EPAP is exhalation pressure. IPAP is inhalation pressure. PS is the difference between them.

EPAP and PS are the important variables that control treatment. IPAP doesn't have a therapeutic effect (in other words you would never target a certain IPAP other than to target a specific EPAP/PS). Different machines are programmed differently but the basics are always the same.

[David Clark]

Thank you Crimson Nape, Gideon and Geer1 for you explanations of the correlation between EPAP, IPAP and PS. I understand in a rudimentary way how they work, but I really don't have a grasp on how I should set my machine to get what most of us want...lower numbers. I get...high OA, turn up EPAP, high flow limits turn up IPAP/PS unless you get CA's then turn it down some. At least I guess that's the general idea.
What I'm really asking about though is the formula itself... PS=IPAP-EPAP so if IPAP is 20 and EPAP is 10 then PS should be set to 10 according to that formula. But if you set it at 10 that would make me think you're going to start having CA's. And if the PS can only be set to 10 anyway then in some cases you can't even follow the formula if you wanted to. I just don't understand how whoever came up with that formula came up with it.
I'm not getting horrible numbers right now. I still get clusters of CA's right after I go to bed, maybe one or two throughout the night and clusters if/when I wake up to go to the bathroom and again when I lay back down. Since those clusters I feel are either awake or kind of awake readings I'm not overly concerned with those and since I seem to only get maybe a few during that night, that may or may not even been actual CA's, I'm not too concerned with them either.
I get a few OA's in there and really I'm not even sure those are all actual OA's because of the way the breathing looks before and during they look more like arousal breathing, maybe a position change, so I'm not even sure how many of those count.
I haven't been posting anything in my therapy thread because, honestly, I feel like you all may be getting a little tired of me trying to chase better numbers by tweaking a setting here and there.
I feel like I sleep decent. I don't have just energy to burn but I think I might have a little more than I did before I started therapy.

[Gideon]

There is a big difference between min EPAP and EPAP.
EPAP typically refers to actual pressure.

Also there is a big difference between max IPAP and IPAP.

A typical PS is something around 4. A PS of 10 is extremely unusual.

We try to lead you toward good settings. To do this we need to see how your body reacts to the settings we suggest. If you choose to use different settings we have to continually adjust to the changes you choose to make which often obscures the results we are looking for. If you don't understand why/what we are doing ask.

[Geer1]

You seem to be confused by something but I am having trouble understanding what it is. You are correct that if IPAP is 20 and EPAP is 10 then PS would be 10 and unless you have extremely severe lung/breathing issues yes you would almost certainly induce central apnea.

Your comment that if PS can only be set to 10 then in some cases you can't follow the formula makes zero sense. The formula is a definition and it always holds. PS is always equal to IPAP - EPAP and similarly IPAP always equals EPAP plus PS. Some machines are set up by setting EPAP and PS and some are set by setting IPAP and PS (EPR) but formula never changes and you cannot break it because it is literally the definition of PS.

If I had to guess you are confused by auto adjusting pressures and min/max ranges. For example perhaps you are confused about seeing ranges like the one in your CPAP pressure (profile) that states EPAP 9 - IPAP 15 PS 3. That is actually min EPAP 9 and max IPAP 15 with a PS of 3. The machine starts at EPAP 9 and PS 3 which gives IPAP of 12 at the beginning of the night. If there are breathing issues the machine increases both EPAP and IPAP at the same rate but it will not increase over EPAP 12 and IPAP 15. PS of 3 is always maintained because this is a fixed PS machine. 

The reason we have not been responding much to you isn't because we are tired of your chasing better numbers it is because there is nothing to recommend to improve your numbers. You are hoping for more but the reality is that your results look pretty much as good as I believe they are going to get. Your AHI is mostly driven by those odd flurries of sleep transition central apnea and there is nothing you can or should do to try and treat them unless they get significantly worse, those centrals can be ignored and removed from your AHI calculation. The majority of your other CAs and OAs like you say are questionable if they are real apnea or not, any that occur after an obvious arousal again should get ignored and removed from AHI calculation. If you went through and removed all these apnea your AHI is probably under 1 most nights indicating the CPAP machine is already doing what it can/is supposed to do. Some level of apnea is normal, no one breaths perfectly all night and chasing 0 is unrealistic and usually harmful to treatment. Imo the only remaining breathing issue you may have are RERAs but we can't diagnose those without EEG data and even if they are occurring all you can do is try increasing EPAP or PS by minor amounts to see if that helps. This is why we say you have to focus on how you feel and not on the data because the data in your case is pretty much useless because it no longer indicates obvious issues. When you get to this point and you feel you sleep decent but still lack energy it is probably because either you need to give your body more time to adapt to CPAP or because breathing issues aren't causing your lack of energy (which can be caused by numerous things).

I believe I had already mentioned most of this in your therapy thread but literally the only thing left you can to do is try minor tweaks to pressure, PS or sensitivity settings to see if they make you feel better.