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

[HalfAsleep]

I notice that my APAP pressure stays pretty steady all night long. 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? So you breathe.... 

Example below. Pressure never budges. That would be an unidentified apnea all right; more like the APAP is sleeping on the job, doing diddly. My settings here are 5-15, so the pressure was free to move around.





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

[Walla Walla]

It didn't move probably because you just had a CA event and this event was a UA. Not knowing what it was the machine's pressure didn't react.

[OpalRose]

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.

[TASmart]

Your machine does not use the leak status to change pressures. Its programming is to increase pressure based on the sleep obstruction status, not the pressure or leak status. Further, if you are having a leak, does no good to raise the pressure, it would just leak more ( not for an absolute fact, some leaks may be corrected by increasing pressure but not all, depends on the cause of the leak).

[Sleeprider]

One last comment related to the olympic breath-holding in your chart above. Not only does your machine not raise pressure for an event like this, it actually stays in EPAP. Take a look at the mask pressure. This is one of the drawbacks of EPR and bilevel without backup. The machine waits for you to spontaneously initiate a breath and follows that with IPAP pressure. Until you breath, you were stuck at whatever your current pressure minus EPR happened to be.

[HalfAsleep]

Long UA's seem to be my thing.....



The longest one is the 77 seconder in the top post. I had a 55 seconder in my diagnostic sleep study and no one said anything....
I could be an opera star if I could do this in real life! An Ironman? an Everest climber? An underwater explosives expert? An octopus?

[Reznik]

I think that you're misunderstanding what you're reading, and you're also asking the wrong question.

The reason that your CPAP doesn't increase the pressure when you have a leak is that its not supposed to increase the pressure.  It's supposed to keep the pressure constant at whatever setting you have it at.  And it does that, even if you have a leak.  If you have a leak, the machine does work harder to accommodate the leak and to keep the pressure where it is supposed to be.  Obviously, at a certain point, that becomes impossible.

So, the question you should be asking is:  When I have a mask leak, does the machine work harder to keep the pressure where it is supposed to be?  And the answer is:  Yes, it does.  The fan speeds up and pushes more air through the system, so as to maintain the pressure where it is supposed to be.

You can verify that by listening to the sound the fan makes when you block the hose opening with your fingers and when you unblock it partially, or completely.  As you unblock the opening, the fan ramps up to replace the air that it spilling out, in an effort to maintain the fixed pressure inside the system.  When the hose is wide open, the fan makes a lot of noise, but it cannot keep up.  On the ResMed series, the machine will detect that it cannot keep up, give you an error message, and shut down.

[HalfAsleep]

One last comment related to the olympic breath-holding in your chart above. Not only does your machine not raise pressure for an event like this, it actually stays in EPAP.  Take a look at the mask pressure.   This is one of the drawbacks of EPR and bilevel without backup.  The machine waits for you to spontaneously initiate a breath and follows that with IPAP pressure.  Until you breath, you were stuck at whatever your current pressure minus EPR happened to be.

I'm not advanced enough yet to understand EPR and EPAP. Give me a few more days....

Also, the micro jaggies in my submarine breathing stunts..... are those puffs of air? It's not exactly flatlined, so something is going on...

-----------

I'm a bit more understanding the leak issue. Pressure is about OA's. Duh...good reminder.

[Reznik]

The micro jaggies are deliberately sent out by the ResMed machines as part of their effort to detect whether you are having a central or obstructive apnea.  See Page 8 of the Clinical Manual:

"The device detects both obstructive and central sleep apneas (CSA). CSA detection uses the Forced
Oscillation Technique (FOT) to determine the state of the patient’s airway during an apnea. When an
apnea has been detected, small oscillations in pressure (1 cm H2O peak-to-peak at 4 Hz) are added
to the current device pressure. The CSA detection algorithm uses the resulting flow and pressure
(determined at the mask) to measure the airway patency."

The Clinical Manual also has a a nice picture below the text that I quoted showing what it looks like on the flow graph, which shows that the pulses start after 4 seconds of no breathing and continue until the apnea is over.  If you hold your breath for four seconds while wearing a nasal mask, you can feel the pulsing in your nasal passages.

Honestly, I don't think its very accurate.  I've deliberately held my breath while using the machine and had it report both central and obstructive apneas as various times. It is fairer to say that if the machine says you were having a central apnea, you might have been having a central apnea. And if it says you were having an obstructive apnea, then you might have been having an obstructive apnea.

[HalfAsleep]

I think that you're misunderstanding what you're reading, and you're also asking the wrong question.

The reason that your CPAP doesn't increase the pressure when you have a leak is that its not supposed to increase the pressure.  It's supposed to keep the pressure constant at whatever setting you have it at.  And it does that, even if you have a leak.  If you have a leak, the machine does work harder to accommodate the leak and to keep the pressure where it is supposed to be.  Obviously, at a certain point, that becomes impossible.

So, the question you should be asking is:  When I have a mask leak, does the machine work harder to keep the pressure where it is supposed to be?  And the answer is:  Yes, it does.  The fan speeds up and pushes more air through the system, so as to maintain the pressure where it is supposed to be.

You can verify that by listening to the sound the fan makes when you block the hose opening with your fingers and when you unblock it partially, or completely.  As you unblock the opening, the fan ramps up to replace the air that it spilling out, in an effort to maintain the fixed pressure inside the system.  When the hose is wide open, the fan makes a lot of noise, but it cannot keep up.  On the ResMed series, the machine will detect that it cannot keep up, give you an error message, and shut down.

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)?

But it does change pressure to offset obstruction? That's the whole point. But it doesn't lower pressure to accommodate more air flowing through the system? In that case, why would the pressure ever go down when the obstruction is eliminated?




My APAP pressure is set from 5-15. It doesn't use the upper range of that. Unless, of course, someone steps in with a correction on that.