[CPAP] Questions about CPAP pressure algorithms about apnea

[charlie4691]

Hello board, 

I have a problem with waking up in the middle of the night and not getting a good night's sleep after that.
I've been using a CPAP for about 10 days because a sleep study(PSG) found that I wake up after 30 seconds of apnea. 

However, I'm still waking up in the middle of the night, even with the CPAP, so I'm looking at my OSCAR data. 
Please note that some of the data is in Korean. I can't change the language. 

I had a 15-second apnea at 4 a.m. The pressure went up well, but I woke up (as evidenced by the sharp increase in respiratory rate). 

But when I zoom in, I see that the pressure is increasing after 15 seconds of apnea. I think the pressure should increase as soon as there is even 5 seconds of apnea. 

Do other CPAP devices also increase the pressure after a long period of apnea? 
Is there a CPAP device that increases pressure sooner? 

If other CPAPs are the same, is increasing the minimum pressure the answer?

Any tips or opinion would really help.

[EddyDee]

In the closeup of the OA event, you can see that there is a brief expiration after the OA event ends. The machine raises the pressure exactly when that expiration finishes.

I think your question is: why does the machine wait until the OA event is over before raising the pressure? Why does it not raise the pressure as soon as the OA event starts, so that the increased pressure will "blow open the airway" and make the OA event stop?

If I understood your question correctly, the answer is that this is not how CPAP machines work. A CPAP machine raises pressure so that the airway stays open, and so an OA event will not start. But once the OA event has started, this means that the airway has already closed. More pressure cannot force the airway open again.

So instead, the machine waits until the OA event, and any expiration afterwards, have finished. Then it immediately raises the pressure, so that on the next breath the airway will not collapse again.

So your machine is doing exactly the right thing.

Now in terms of your therapy: you say you are waking up in the middle of the night. Do you say this because this is what you remember in the morning? Because in your OSCAR chart there does not seem to be evidence of this. In fact the chart shows nearly 8 hours of continuous mask usage (so you did not seem to actually get out of bed during the night).

Before 4am everything actually looks almost perfect. After 4am the tidal volume becomes more variable, and there are 2 OA events during this period. Do you remember what times you woke up in the night? Were you awake or asleep after 4am?

[Gideon]

Welcome to the forum. On Algorithms.

Originally A CPAP delivers a single pressure, this is set at a pressure that will prevent most Apneas by providing an positive pressure in the airway to splint/hold it open. This is a fundamental principle of all pap machines.

Next came a BiLevel with two fixed pressures, one for inhale, one for exhale. This proved to be effective at treating a wider variety of situations. To implement this an algorithm was developed to detect when a patient spontaneously shifted from inhale to exhale. "S" or Spontaneous mode on a BiLevel. Initially this was an extremely sharp square wave, IMHO very uncomfortable. Algorithms were added to smooth this transition and developed into easy breathe.

Later a mechanism was added to allow it to be easier to exhale called Pressure relief, EPR on ResMed. This was copied from the BiLevel with a limited range.

CPAPs we're now much more capable and an Auto mode was added. This mode sensed if the airway is physically blocked (obstructive) and if so applies a temporary pressure increase. In other words it applies a correction AFTER an Obstructive event is detected. Once an hour has passed the effect of that pressure increase is removed. Successive events build on each other until the max pressure setting is reached.. these algorithms are programmed to not react to a central/ non-constructive event.

Are there devices that behave differently? Yes, these are based on your breathing volume such as ASV and IVAPS. ASV maintains your 90 second average volume, IVAPS is set with a target volume. These modes react on the breath that is causing an event preventing the dip in volume an event typically causes

Does that help?

[charlie4691]

To EddyDee
I misunderstood the behavior of the CPAP, thanks for the detailed explanation. 
And if I wake up and can't get back to sleep, I check the time on my smartwatch. I woke up at 4am and recognized it.

To Gideon
From EddyDee's comment, it seems that if an apnea occurs and the airway is blocked, increasing the pressure will do no good. 

Do ASV and IVAPS devices detect a slight decrease in breathing and raise the pressure before an apnea occurs? 
And do they succeed in preventing apnea from occurring?

It's hard to find ASV and IVAPS devices in my country. I'd like to borrow one and try it out. 

Thank you for your detailed explanation and comments.

[Gideon]

That depends on what has caused the event. Common apnea will increase pressure to where the restriction does not occur, though it may take a number of events, 1 or greater, for that to occur. Should the cause be from a kink in the hose we call an airway, also called positional apnea but different than the front,side, back doctors call positional, then there is a very good probability that pressure will not open the airway.

A screenshot from OSCAR of your daily charts will tell us a lot.

[Gideon]

ASV and IVAPS work on the principle of maintaining volume. They use a difference in pressure relief to cause this. If volume is constant events do not occur. Look at the tidal volume and minute vent shortly after an event, you will see a dip. ASV and IVAPS correct this on the same breath it occurs on.