EPAPmin and minimum EPAP

[tmoody]

I thought I saw a thread on this very question, but now I can't find it, so...

If my EPAPmin is set to 8.5 (it's never been lower than 8), how is it that SH often reports nightly EPAP minimums lower than 8, sometimes lower than 7?

My flex setting is 1, if that's relevant.

[FrankNichols]

I don't use your machine and I don't have a clue what I am talking about, but... that never stopped me before.

Could it be that that machine sets it at 8.5, but you then do a big exhale (sigh) and that drives the pressure up briefly and then you stop suddenly leaving the machine with a lower than it should be EPaP pressure, and there is a lag while it ramps back up. Ie. does the lower point appear to be a spike/transient or a long duration below the set pressure.

[justMongo]

...but you then do a big exhale (sigh) and that drives the pressure up briefly...

Sorry Frank: Just doesn't work that way. The flow generator is always pushing air down the tube. On exhale, the flow generator reduces it's flow to maintain pressure.

I don't have an answer to the OP question.

[FrankNichols]

...but you then do a big exhale (sigh) and that drives the pressure up briefly...

Sorry Frank: Just doesn't work that way. The flow generator is always pushing air down the tube. On exhale, the flow generator reduces it's flow to maintain pressure.

I don't have an answer to the OP question.

justMongo, you just said the same thing I said leaving out the inertia/lag built into the system. The amount of air it is pushing down the tube (work) is a function of how much air is leaking out the intentional leaks (vents) and the unintentional leaks (leaks). That total of those (and your inhalation) must equal (leaks and pump flow) or something really really bad will happen

When the system senses the pressure going up above EPAP as a result of your exhaling and adding a negative flow, it will reduce it's pressure/flow to try to maintain it at the EPAP setting (there may be builtin delays to prevent the system from correcting for things it shouldn't - this results in a positive swing in the pressure above the EPAP pressure) - this happens every time you exhale.

This then results in the amount of "flow" (work) the machine has to do being reduced slightly and the waveform will drop back down to the EPAP setting. If it did not reduce the pressure would stay above the EPAP while you exhale. (the reverse happens every time you inhale and you see a positive spike on the pressure)

Then when you stop exhaling there is a brief instantaneous moment while the machine senses the pressure is too low (below EPAP) because you are no longer contributing to the pressure (negative/outbound flow) and begins ramping up it's flow/pressure to compensate. At that point in time the pressure is below EPAP. What I don't know is how long it takes the system to begin compensating - it could be so brief that you can't see it on the charts, or it could have a built is delay to prevent oscillations - a damping factor in which case you will see a negative spike in pressure on each exhale. I don't know.

Note this will show up on the mask pressure and not the flow rate, since flow rate is a rate and not an actual instantaneous flow volume. Also, I am not looking at a BiPAP waveform right now, since mine is a CPAP, and so I can't see just how fast the BiPAP will react - the spikes may be very small if the machine is very fast and is able to ramp pressure rapidly up and down. I see examples of fast changes in the waveform when it is testing for Centrals - I believe it is able to change the pressure at rates above 4 cycles per second. But, even that fast there should be some small amount of time for the spikes i am referring to, to be visible.

I designed process control systems for a living for a while that did something very similar, just not for CPAP machines, so, while the CPAP may work completely differently I expect it is at least similar to what I said.

[PaytonA]

Maybe we are overthinking this. It may be as simple as the flex setting or ramp allowing the pressure to drop below the set minimum. I am not sure for the PR machines. Sleepyhead picks things like that up.

Best Regards,

PaytonA

[FrankNichols]

Maybe we are overthinking this. It may be as simple as the flex setting or ramp allowing the pressure to drop below the set minimum. I am not sure for the PR machines. Sleepyhead picks things like that up.

Best Regards,

PaytonA

[justMongo]

Frank: If I understand your point, then we could look at the system like a linear, series Voltage regulator. As we draw more current, there is a slight change in the voltage across Vsense. That change is a function of the resistor dividers and the gain of the closed loop system.

Or, am I overthinking your answer?

I'm think that the observed effect in the OP is in line with Payton's thought. A natural response to the way a PR machine operates with given settings.

The OP is, so far, best answered by Payton. We need a P-R person who knows more about the flex settings to really put a pin in it.
Hey Sleeprider, where are you?

[FrankNichols]

Frank: If I understand your point, then we could look at the system like a linear, series Voltage regulator. As we draw more current, there is a slight change in the voltage across Vsense. That change is a function of the resistor dividers and the gain of the closed loop system.

Or, am I overthinking your answer?

I'm think that the observed effect in the OP is in line with Payton's thought. A natural response to the way a PR machine operates with given settings.

The OP is, so far, best answered by Payton. We need a P-R person who knows more about the flex settings to really put a pin in it.
Hey Sleeprider, where are you?

Yup, agree.

[palerider]

I thought I saw a thread on this very question, but now I can't find it, so...

If my EPAPmin is set to 8.5 (it's never been lower than 8), how is it that SH often reports nightly EPAP minimums lower than 8, sometimes lower than 7?

My flex setting is 1, if that's relevant.

flex lowers your epap.

[palerider]

I designed process control systems for a living for a while that did something very similar, just not for CPAP machines, so, while the CPAP may work completely differently I expect it is at least similar to what I said.

what I think you're missing here is that the machines are very sophisticated.

they measure the pressure and flow many times per second, 25 per in the case of resmed, respironics is probably similar.

breathing is very very slow, by comparison.

also, the machines are looking for specific things, changes in flow that signify what is happening, you don't instantly change from a big inhale to a sudden exhale in your sleep, you inhale, normally in a nice bellshape curve of flow volume, which drops off to zero, the machine is adjusting pressure all during the inhale, and depending on its settings, starts to ramp pressure down as you finish the inhale, sometimes before. (this was a minor problem for me until i tuned my machine). as you start exhaling, the blower slows, keeping pressure to what it's set to be.

having measured the pressure at the back of my machine, and at the mask with a differential manometer, I can tell you that my machine is quite accurate at keeping the mask pressure at what it should be, even though the pressure at the machine itself varies quite a lot, as it does it's little dance.

besides, your idea of the pressure going up too high on exhale and resulting in a higher epap wouldn't coincide with the original question.... which was why is my epap being reported lower than the minimum set.