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Pressure, Flow, and Leaks
#11
(05-25-2016, 12:08 AM)chill Wrote: I've seen a lot of posts that say these CPAP machines increase pressure in response to leaks. That does not make sense to the part of my brain that remembers a bit of my university physics. If there is a leak pressure should drop, the same as punctured tire. The machine would increase flow (volume of air) to compensate for the leak and maintain therapy pressure (up to the point where it can't provide more flow which I think is the "large leak" point).

Right, except I think the threshold for "Large Leak" is usually much lower than where the machine can't provide more flow.

(05-25-2016, 04:05 AM)FrankNichols Wrote: Flow rate is whatever the flow rate needs (up to the capacity of the turbine) to be to maintain the pressure.

That is my understanding.

The pressure which is being maintained is the target Mask Pressure. The blower pressure is maintained at a higher pressure than the target Mask Pressure, by the amount of pressure loss across the hose.

The pressure dropped crossed the hose is a function of the airflow rate (which is measured by the blower) and of the tube type (diameter and length). This is why the correct tube type should be entered into the machine's settings by the equipment provider or by the user.

If we use the fat tube (not slimline) and if our Mask Pressure is fairly low, and if unintended Leak is fairly low, I think not much error would be caused by the tube type being incorrectly set.

But if we use the narrow tube, and if unintended Leak is very high, or if our Pressure is very high (leading to high intended leak through the mask vent holes), then the Mask Pressure may be surprisingly much lower than what the machine thinks/reports it is, if the tube type is incorrectly set.

(05-25-2016, 04:05 AM)FrankNichols Wrote: I have read (meaning I don't know what they do) that machines increase pressure (flow) in response to leaks ...

Usually, people who think "machines increase pressure (flow) in response to leaks" don't understand the difference between pressure and flow.

Pressure is measured in units of force per unit area, for example pounds per square inch.

Flow is a rate and is measured in units of volume per unit time, for example Liters per minute).

(05-25-2016, 05:00 AM)justMongo Wrote: There are two definitions of flow that one must consider. There is the absolute flow from the generator. And, there is the flow in/out of the person's airway.

Right. The "Flow" waveform in ResScan and SleepyHead represents the estimated rate of flow in our airway (lungs).

(05-25-2016, 05:00 AM)justMongo Wrote: There is apparently a middle ground where the leak causes the machine to be unable to properly detect the flow to/from the person. Thinking the flow has diminished, the machine raises pressure.

I have noticed in my Flow waveforms that during periods of high Leak the Flow plotted by the machine is usually much lower than when there is not high leak. I think the Flow estimated during high leak is surely much lower than reality.

So if high leakage made it seem like the Flow has dropped more than 40% or 50%, perhaps the machine could misinterpret the simple high leak as being, instead, an hypopnea accompanied by high Leak.

And you are saying, in other words, that during times of high leakage perhaps the machine may incorrectly think an hypopnea is occurring and, in response, may eventually raise the Mask Pressure? Seems like this may be possible, but I think neither ResMed or Philips Respironics has confirmed their machines could respond that way, so I'm not sure.

And if an ASV or other ventilator class of machine is used, during times of high leakage perhaps Pressure Support may be raised immediately in an attempt to end the apparent hypopnea? Again, it seems like this may be possible, but I'm not sure.
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#12
chill - would you mind if I moved this thread from the Advisory Members forum into the Main Forum?
SuperSleeper
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#13
I have no objections, there is a lot of good information here -- more than I expected! I posted here originally as my focus was on "I want to tell people the correct thing".
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