Why doesn't APAP compensate with pressure when you have a leak?

[Reznik]

I think that what Sleeprider is trying to say is that you might want to consider turning off EPR because the impact of leaving it on is that you're getting a lower pressure during these events because of it.  But, I may be misinterpreting what he said.

[Reznik]

No, it doesn't change the pressure.

Yes, it does send more air into the system in an effort to maintain the pressure.

I think that you're confusing pressure with effort.  

It does work harder.  It does so because if it didn't work harder when there was a leak, the pressure would go down.

[Sleepster]

I notice that my APAP pressure stays pretty steady all night long.

Okay.

Sometimes, I have big leaks, sometimes small ones, or none at all....whatever. But why doesn't the APAP jump into the fray when there's a leak in order to compensate so the mask pressure stays even?

It does! You just said that it stays pretty steady. Isn't that the same thing as "stays even"?

Pressure never budges. That would be an unidentified apnea all right; more like the APAP is sleeping on the job, doing diddly.

Pressure rises in response to flow limitation. If you see signs of flow limitation but no corresponding rise in pressure then you might have an issue.

My mouth is wide open, and APAPette (A10A 4H) is unresponsive, same as me.

If you have a huge leak the CPAP machine will increase the flow rate in an attempt to keep the pressure constant. This is the engineering marvel of the CPAP machine, it keeps the pressure constant over a wide range of flow rates.

[jaswilliams]

You said ...

I sort of understand what you're saying (I have to grapple with it some more); it's about how hard the machine works to maintain a pressure and not how high the pressure goes to offset the leak (which doesn't happen)?

The machine does not change pressure during any event.


If you have an OA event or an increase in flow limitation The machine will increase pressure after it has classified the event. If you zoom in on an OA in your chart and look at the pressure graph you will see the pressure does not rise until after the OA event has been raised.



Jason 

[HalfAsleep]

No, it doesn't change the pressure.

Yes, it does send more air into the system in an effort to maintain the pressure.

I think that you're confusing pressure with effort.  

It does work harder.  It does so because if it didn't work harder when there was a leak, the pressure would go down.

I thought that's what you were saying, but I'm trying to wrap my head around it. I can only retain it for about 20 seconds.

[HalfAsleep]

I notice that my APAP pressure stays pretty steady all night long.

Okay.

Sometimes, I have big leaks, sometimes small ones, or none at all....whatever. But why doesn't the APAP jump into the fray when there's a leak in order to compensate so the mask pressure stays even?

It does! You just said that it stays pretty steady. Isn't that the same thing as "stays even"?

Pressure never budges. That would be an unidentified apnea all right; more like the APAP is sleeping on the job, doing diddly.

Pressure rises in response to flow limitation. If you see signs of flow limitation but no corresponding rise in pressure then you might have an issue.

My mouth is wide open, and APAPette (A10A 4H) is unresponsive, same as me.

If you have a huge leak the CPAP machine will increase the flow rate in an attempt to keep the pressure constant. This is the engineering marvel of the CPAP machine, it keeps the pressure constant over a wide range of flow rates.

With all these contributions from y'all, I'm starting to get a fix on this....

[HalfAsleep]

Walla Walla is correct in that your machine won’t raise pressure when you experience a CA.  

Your Cpap machine also is unable to compensate for large leaks, meaning it may be unable to keep up with the loss of air flow, and won’t raise pressure in response to large leaks, thus affecting your therapy.

Here is a small excerpt from The Beginners Guide to SleepyHead on Leaks:

Why are Large Leaks an Issue?

……why Large Leaks cause problems for our CPAP machines and how those problems may affect our therapy.
When we use a CPAP, our upper airway is part of a "semi-closed pressurized system" comprising the blower, the tube, the mask, and our upper airway. The system is "semi-closed" because there is an intentional leak: The system is designed to intentionally lose air through the exhaust vents. The desired pressure is maintained in the system by constantly supplying enough new air into the system to balance out the amount of air being lost through the leaks: Our CPAP must blow enough air into the "semi-closed pressurized system" to replace the air lost through the exhaust vents in the mask.

It may help to draw an analogy: We can think about the CPAP machine, hose, mask, and our upper airway as a leaky inner-tube with a tire pump attached to it. We can keep the inner-tube fully pressurized and inflated as long as we keep adding air to the tube by pumping as much air into the tube as is being lost through the leak. But if we start losing more air through the leaks than we can pump in with the tire pump, the inner-tube loses pressure and starts to collapse.

It's the same idea with our CPAP machines: As long as the excess leak rate is not too high, the CPAP machine has no real trouble blowing enough new air into the system to compensate for the total leak rate. In other words, the CPAP can compensate for additional, moderate unexpected excess leaks by simply blowing more air into the system in order to maintain the same amount of pressurization. But there are limits to how much air can be lost before the machine simply can't keep up with its job: Once the total leak rate crosses into official Large Leak territory, the CPAP will have trouble adding enough air to the system to balance out the air leaking out of the system. And hence the machine will have trouble maintaining the desired pressure level.

As you can see, if the machine cannot keep the desired pressure, then your therapy may be compromised.  This is why we “nag” so much about fixing leaks, whether it be from the mask or mouth.

Large leaks matter if lasting for a long period of time.  Small blips of leaks here and there won’t affect therapy.

For more information, read the Leak section in The Beginners Guide to SH.

[HalfAsleep]

I think that what Sleeprider is trying to say is that you might want to consider turning off EPR because the impact of leaving it on is that you're getting a lower pressure during these events because of it.  But, I may be misinterpreting what he said.

I thought that was the point, too. However, since my APAP is set with such a wide range (5-15), I wasn't sure whether or not I use EPR isn't maybe moot, since I don't ever seem to max out.

[Sleeprider]

EPR: exhale pressure relief. You appear to be using an EPR setting of 2. That results in the exhale pressure being 2-cm lower than the set pressure or the inhale pressure. If your inhale pressure (IPAP) is at 8 cm, then during exhale (EPAP) your pressure is only 6 cm (EPAP or exhale pressure). When you have an apnea, the pressure remains at the lower presssure.

You asked why the machine was not increasing pressure during an apnea. My point was that your pressure is lower than you think. If you look at the mask pressure chart during any apnea you can see this. For people at low pressures, EPR can really undermine the pressure needed to avoid obstructive apnea. EPR also tends to increase the event rate of people with central and complex apnea. We did do an experiment on your therapy, and found that EPR does not appear to make much difference.

[HalfAsleep]

EPR: exhale pressure relief.  You appear to be using an EPR setting of 2.  That results in the exhale pressure being 2-cm lower than the set pressure or the inhale pressure.   If your inhale pressure (IPAP) is at 8 cm, then during exhale (EPAP) your pressure is only 6 cm (EPAP or exhale pressure).  When you have an apnea, the pressure remains at the lower presssure.  

You asked why the machine was not increasing pressure during an apnea.  My point was that your pressure is lower than you think.  If you look at the mask pressure chart during any apnea you can see this.  For people at low pressures, EPR can really undermine the pressure needed to avoid obstructive apnea.  EPR also tends to increase the event rate of people with central and complex apnea.  We did do an experiment on your therapy, and found that EPR does not appear to make much difference.

Still learning..... I'll have to look at my charts to figure this out.


Yes, I remember we did try with the EPR off when I used 8-8. I think I've also tried EPR off at 5-15, but decided to revert as far as possible to prescribed defaults until I get through my first compliance tête à tête so I can minimize doctor fussing.