There may already be some good responses relevant to this question posted at this site, but I'm having trouble finding them. If an experienced member can point me to posts that are especially relevant, I'd appreciate it. If good answers are not already available, then I'd like to solicit responses here. The question really is a general one but the particulars of the answer may be shaped by my personal background. For this reason, I'll start with some background about my own pattern of sleep disordered breathing and about my AHIs. Thanks very much.
My Personal Background: I have complex / mixed sleep apnea, with both obstructive events (especially during REM sleep) and central events (especially during Non-REM sleep). Many of these events seem to be hypopneas, but there also appear to be some frank apneas. I have some periodic breathing. I am currently using Auto CPAPs, set in the pressure range of 6-10 cm.
Machine and AHI Background: For the moment, I have both a Resmed AirSense 10 Auto and a Respironics DreamStation Auto. Though I so far have only a few days of relevant data to compare, it appears that the Resmed is giving me lower AHI values, in the range of 1.8 to 5.0, with an average of around 3.8. The Respironics under the same condition has been giving me numbers in the 8-9 range. I have heard and read that AHI data are not comparable between Resmed and Respironics, and that this data should be used only for trending purposes with a particular machine. I have also seen references at this site to the Respironics usually giving lower AHI values (I saw some mention of Resmeds double-counting hypopneas) but based on the limited data I have so far, my AHIs seem to be about twice as high on the Respironics.
In case it is relevant, I'll add that the AHI values I just cited all pertain to a short period during which I'm doing a particular therapeutic intervention. Specifically, I'm using so-called "enhanced expiratory rebreathing space [EERS]" (that is, a non-vented mask with the valve displaced a bit down the tube), which is an arrangement designed to increase the percentage of CO2 that I'm inspiring; this arrangement is part of an attempt to stimulate breathing and thus help overcome some of my central apneas and hypopneas. I'm also adding some supplemental oxygen to the mix, again with the objective of reducing the central events. (I'm doing all this under the direction of my sleep doctor.)
Question: I would like to better understand the issue of non-comparability between Resmed and Respironics, and to better understand why my AHIs might be higher on the Respironics (perhaps in general, perhaps with reference to my particular pattern of sleep disordered breathing and/or the intervention I'm doing at the moment). I welcome both reference to specific threads and new comments that would give me insight into this issue.
That's a pretty tough question in that the machines score events a bit differently. I think the PR machines generally score events more aggressively, particularly hypopnea events which seem to get scored for changes in flow volume over period of time that may not even be evident with examination of the waveform. An apnea is indicated if there is an 80%/75% (Respironics/ResMed) reduction in airflow for 10 seconds compared to the average airflow over an extended period of several minutes or if there is no airflow detected for 10 seconds. On the other hand for hypopnea, the reduction in air flow doesn't have to be as much to get a flag with the Respironics machine...only has to be 40% vs 50% for Resmed.
So it is likely that the PR S1 is flagging stuff that the S9 is ignoring.
The best way to see the difference for yourself, since you have both machines is to start looking very close-up at events in the flow waveform. See if you can pick out patterns that are flagged in the Respironics machine, that don't get flagged in Resmed. Also take a look at the duration of events that are occurring and do some comparing. Be careful about periods where you may be awake or aroused as these can often result in pauses in breathing that may or may not be "real" CA and H. The breakdown of events that cause the difference between the two machines is an important piece of information to consider.
Resmed has a different approach to exhale relief (EPR) than does Respironics (A-Flex/C-Flex), and this may make a difference for you. Patients with complex apnea are often treated on bilevel or ASV, and it's interesting that your doctors are taking the approach to change the mask venting to affect spontaneous respiratory triggers. In summary, the two machines score events a bit differently, but I would not have expected the magnitude of difference you have expressed, from the differences in the algorithm alone.
Thanks very much, Sleeprider.