Pillow vs FFM, pressure conversion formula?

[kiwii]

The perceived pressure when using a FFM is much different than the pressure when using pillows.

My pressures are perfectly comfortable when using the SimPlus FFM. The same pressure delivered directly into my nasal passages by the Swift FX is significantly stronger (as in, unusable for me).

Before anyone asks, yes, I did change the mask setting.

Is there some formula to convert a FFM pressure (14/8) into an equivalent nasal pressure?

I've ordered the AirFit P10 and also the Brevida and it seems like I ought to be able to use them at a lower pressure, if necessary, and still receive effective therapy.

[DeepBreathing]

I don't believe there is any formula as such, but there is a lot of anecdotal evidence that you can run pillows at a lower pressure than a full face mask to achieve the same efficacy. I think it's a case of trial and error.

[kiwii]

I would be interested to read some of that anecdotal evidence but my searching is proving ineffective.

[kiwii]

Does anyone know how to find the velocity of the air through the hose?

With that plus the change in volume, I've got the equation. Well, my engineer husband has the equation.

[DeepBreathing]

The velocity is constantly changing as you inhale, then drops to zero as you exhale and pause between breaths. During the exhale, the expired air forces the flap valve to close and is expelled through the mask vent, so the air in the tube won't be moving. You could calculate the velocity at various stages by looking at the flow rate curve in SleepyHead. I'm not sure that this exercise would produce anything approaching an exact pressure number - why not just drop your pressure a couple of cm H2O and go from there?

[kiwii]

...why not just drop your pressure a couple of cm H2O and go from there?

Because I promised the 2nd sleep doctor that I wouldn't change the pressure - that I would allow him to be in charge. Okay, the next part isn't rational: because of the promise, I feel perfectly okay with making a calculated adjustment to allow for the difference between the FFM and the pillows. I do not feel okay with experimenting; it feels like I am defying his authority, which is a huge problem for me. Gads. It is very frustrating to be so constrained by emotional issues.

I am not keeping this doctor. So that could be a work-around for me. However the appointment for the next potential doctor isn't until Jan 24th. I haven't cancelled the next appointment with Doctor #2, which is also in Jan. For that matter, I haven't cancelled the next appointment with Doctor #1, which is also in Jan.

Geez, come to think of it, I haven't even had a second appointment with any of them (doctor #1 was supposed to have seen me about 6 weeks after I started, in October, but he went out of town and rescheduled my appointment until January). My freaking settings haven't even been adjusted by the freaking sleep doctor yet.

[Sleep2Snore]

It is an interesting question, when I started using CPAp a lot longer than I care to admit, we were just given a mask, then when we changed we were just handed Nasal Pillows.  It is only now that the machines come out with a setting have they stopped to think about it.  The old machines didn't have any setting for this and the Sleep Clinics didn't think about changing pressures!  So if you went the other way round, the FFM would not have enough pressure?

I have not seen any formula for this.

ResMed do publish charts for pressure vs flow rates for their masks.

[kiwii]

The velocity is constantly changing as you inhale, then drops to zero as you exhale and pause between breaths. During the exhale, the expired air forces the flap valve to close and is expelled through the mask vent, so the air in the tube won't be moving. You could calculate the velocity at various stages by looking at the flow rate curve in SleepyHead. I'm not sure that this exercise would produce anything approaching an exact pressure number - why not just drop your pressure a couple of cm H2O and go from there?

After thinking about it some more, what I am really looking for is the velocity at the highest pressure setting. This would ensure that I could tolerate any of the changes. It should (I think) be a ratio that I could figure out based on a bogus velocity, if I really had to come up with a number. If only my brain would work.

There is also the complication of any additional hosing between the regular hose and mask. I know that the Swift FX has a short, lightweight hose of its own. I don't know about the P10 and Brevida yet.

Even an approximation would help though.

For the emotional issues - it is from ptsd, which was triggered early on with xPAP treatment. It doesn't affect my ability to use the machine, thank goodness, but it does affect my ability to trust authority figures and ask for help. Basically, anyone a 7-year old would view as an authority figure is a problem for me and it can be overwhelmingly fearful. It isn't rational, and I cannot control it.

[DeepBreathing]

After thinking about it some more, what I am really looking for is the velocity at the highest pressure setting.

I'm a retired engineer but I specialised in transport systems and it's a long (too long!) time since I did any fluid mechanics. But I think in this instance the pressure isn't a determinant of the velocity. It's going to be a function of how fast you inhale, which is shown by the flow chart in #SleepyHead. The chart is calibrated in L/min, so if you zoom in on some high flows and place your cursor at the peak of an inhale waveform you'll get the instantaneous maximum flow rate. Knowing this and the diameter of the hose (15 or 22mm are the standards) it will be easy enough to calculate the instantaneous velocity. However it will be complicated by the short section of small diameter hose at the mask end (which the P10 does have). However you can use the same process to calculate the velocity through the smaller hose and even that coming through the pillows themselves. However I have no idea how to convert that to a pressure difference for different mask types.

Here is the link to the pressure v flow rate charts that Sleep2Snore mentioned: https://www.resmed.com/au/dam/documents/...nz_eng.pdf

I mentioned anecdotal evidence earlier - it's scattered through various threads on this forum over the years where people have mentioned that changing between pillows and FFM (or vice versa) required a small change in pressure to achieve the same efficacy. I myself know that when I changed from one FFM (Quattro Air) to another (Simplus) I had to raise my min EPAP by 1.0 to get the same efficacy.

I really think it's going to be a matter of try it and see, as I suggested earlier. (Happy to be proved wrong). I understand the complications with your doctors but I'm not really sure what to suggest in that regard. Are you in a position to ask one or the other (or their staff) if they have a standard recommendation for pressure adjustment for a mask type change?

[Sleep2Snore]

It would come down to how much flow rate comes out of the vent in the mask, Nasal Pillows tend to have less of a vent than FFMs and Nasal Masks.  So with less flow getting out of the vent the pressure will be greater to try to breath against.
I've seen two mask vent rate profile charts, from ResMed, that were published at different times, and they show *different* leak amounts... implying that either one or both of the charts are wrong, or ResMed has changed the mask vent rates...or the masks vents!
Don't know if this will help you or not, I would tend to just change to the other mask and use Sleepyhead to see how it goes.  If the pressure appears to high, drop it a bit and look at the data next day.


https://www.resmed.com/in/dam/documents/...ow_eng.pdf