Just trying to understand this stuff

[Domino96]

My polysomnography report shows that during the night, I had a total of 6 central apneas, 0 mixed and 13 obstructive. I had 228 hypopneas and 12 RERAs. My AHI was reported as 46.2 and RDI as 48.4. OSCAR reports clear airway apneas rather than central. My doctor said that central apneas are the result of the brain not sending the signal to breath, but that the cause is not fully understood.


Clear and central apneas sound similar, but are they in fact the same? If not, what are the differences?

[Domino96]

I have noticed that the clear airway apneas almost never line up with flow limitations on the OSCAR charts. Some obstructive apneas do overlap with flow limitation on the charts. However, most obstructive apneas seem to occur between flow limitations (see screenshot example below). Hypopneas and RERAs always, at least partly, overlap a flow limitation, though frequently the hypopneas or RERAs appear to begin before the nearest flow limitation begins (see screenshot example below).


(a) Am I correct in assuming, that obstructive apneas, hypopneas and RERAs are caused by, or at least associated in some way with, flow limitations?
(b) If so, how is it that obstructive apneas appear between, but do not overlap, the nearest flow limitations? 
© And how is it that obstructive hypopneas and RERAs can begin before the nearest flow limitation?
(d) Or can obstructive apneas, hypopneas and RERAs occur when there is no flow limitation?


I'm just trying to understand this stuff. Any explanations of what's going on here will be appreciated.

[Sleeprider]

Your doctor gave a far over-simplified explanation of the mechanisms of central apnea focused only on neurological origin. There are a lot of possibilities, including the most common, which is simply labeled idiopathic central apnea (without known cause). A good start at understanding the complexity of the issue would be to read our wiki articles on:
Respiratory Drive wiki https://www.apneaboard.com/wiki/index.ph...tory_Drive
Central Sleep Apnea https://www.apneaboard.com/wiki/index.ph...pnea_(CSA)

We have many members of this forum that use the Adaptive Servo Ventilator (ASV) which is a form of bilevel positive air pressure device that resolves both obstructive and central apnea, as well as all forms of hypopnea, periodic breathing (a.k.a. Cheyne Stokes Respiration), and other problems. This is the go-to device for complex mixed or central apnea. Based on your doctor's explanation, he will not be of any help, and you should identify a practitioner that has knowledge of central and complex apnea and experience in diagnosing and treating it. If your doctor is actually qualified, he is condescending and insulting with his explanation and lack of any intention to address it. Anyway, back to ASV. You can read about it here starting on page 28, and note that central events are not uncommon, and there is an effective and commonly used treatment. https://document.resmed.com/en-us/docume...er_eng.pdf Also read our wiki on ASV https://www.apneaboard.com/wiki/index.ph...tion_(ASV)

Once your read these materials you may have more questions, or at least more that are your concerns. Your diagnostic test had more obstructive apnea, than central, so it's not a surprise to see some CA events in your results. Central hypopnea is central when the respiratory drive is low resulting in lower respiratory volume in the absence of obstruction. Similarly central apnea is simply a pause in breathing. The most common cause of this is a feedback loop of hyperventilation and hypoventilation in response to blood levels of carbon dioxide, and the neurological feedback mechanism that monitors them, do not react quickly enough to maintain an even respiratory rate.

Based on your charts, your CA component is not very severe with CPAP therapy. Your charts show some periodic breathing and minor clusters of CA as discussed here in our Optimizing Therapy wiki: https://www.apneaboard.com/wiki/index.ph...y_Clusters The majority of the events in your posted charts are obstructive, and you have some fairly high flow limitation. Just my opinion, but I don't think you will end up needed ASV, and if we can resolve the positional source of your obstructive apnea and flow limitation, then lower pressure may and lack of destabilizing flow limits may resolve the issue. We could have done this in your therapy thread.

[Domino96]

Thanks for your detailed reply and references, Sleeprider.

I'm working through the references and will likely have more questions on the topic.  I only discovered OSCAR this past December and I'm trying to absorb as much as I can, after blindly bobbling along during the first 10 months of cpap use with frustratingly inconsistent results. Right now, it's the best it has been. I'm just trying to get as much out of therapy as I can.

[Domino96]

Mask pressure during apneas and hypopneas:

I understand that mask pressure is sampled 25 times per second while pressure is sampled one time per second with the result that the mask pressure shows breath by breath.  In the absence of an event, the mask pressure generally reflects the ipap and epap pressures.  I'm using a variable pressure of 11 - 16 with the EPR set to 3.  During Hypopneas and RERAs, there is only slight, if any, variation in the pressure graph and the mask pressure graph (see example below).  However, during apneas (whether obstructive or clear airway) the mask pressure graph flattens out and is close to the epap pressure (see example below).  Why is this and what does it mean? If mask pressure is affected by respiratory effort, why isn't this, though to a smaller degree, also reflected during hypopneas?

[Sleeprider]

The pressure always falls back to EPAP and only goes to IPAP for a limited time when inspiratory pressure is triggered. This is normal function for the Resmed. Some machines are designed to return to IPAP or CPAP set pressure before inspiration begins, but the Resmed Autoset algorithm for EPR works identically to the Aircurve algorithm for PS. This is why you will often see us refer to the Autoset as a bilevel machine, with a strong consideration of using EPAP to control obstructive events, and treating IPAP the same we we do with a bilevel to support inspiration and control flow limits. This is both a fault and a feature. It is a fault because people expect CPAP to provide a set pressure that is titrated to prevent OA. With the Resmed, we have to account for the loss of pressure with EPR, which is why, we usually recommend the minimum pressure be set to at least 4-cm plus the EPR setting. Similarly, if a higher pressure is needed to prevent OA, the minimum pressure needs to be the titrated pressure plus EPR.

In the images you posted, the hypopnea shows enough inspiratory effort to trigger IPAP. this is often sufficient to stimulate a normal spontaneous breath, but in this case it is low volume breathing with greater than 50% flow reduction resulting in the hypopnea flag. The OA event appears to show some pressure fluctuations that look like breathing attempts, but they don't trigger IPAP. With the Aircurve bilevel, we can increase the trigger sensitivity, and often stimulate normal breathing where light inspiratory effort is sufficient to trigger IPAP, and of course we have more than 3 cm of pressure support to work with. That is why, we often suggest the Vauto to individuals with central apnea event rates that are well below the need for ASV. We can actually cancel those apnea in many cases.