(06-15-2014, 01:21 PM)johnalus Wrote: My primary CPAP is a Resmed Elite S9 and my AHI reading is usually a low 0.3 plus-or-minus 0.2. I have a travel CPAP, a Trascend EZEX, and with the same mask it shows my overnight AHI in the whole number range of 3.0, plus or minus, with most of the events on the Transcend being hypopnea interrupts.
I have OSA. Both machines are set for 12.0ml and I use the Resmed Mirage FX standard mask for both as well.
Should I be concerned about the diffence between the two different AHI readings? Is the difference significant?
Each manufacturer has their own algorithm(s) for scoring events. And, yes, those algorithms can make a (big) difference in the reported AHI. This is part of why the "official" medical care people tend to say that full data machines are good for trending
data, but should not be thought of as being 100% accurate every single night.
My guess is that the definition for scoring a hypopnea on the Transcend is different than that on the S9.
I'm working from memory here, so I may get this WRONG, but let me climb out on the limb anyway:
All full data machines use some kind of a "threshold" for how low the air flow into the lungs must drop before a hypopnea can be scored. Many machines, including Resmed if I recall correctly, require the airflow to drop by 50% for at least 10 seconds for a hypopnea to be scored. But that threshold is more than just a fixed number: The drop has to be 50% of something
, and the something
is usually referred to as the baseline
The thing is, different machines use different ways of computing the baseline
airflow into and out of the lungs. Most use some version of a "moving average" where the machine is focusing on just the most recent data. Some machines use 5-minute windows, replacing the data one minute at a time. Some machines use 5-minute windows with continuous updating of the window. Some machines use a certain number of breaths, regardless of how long or short those breaths were.
So whether or not a particular stretch of breathing will be flagged as a hypopnea by a particular brand of CPAP depends strongly on how the machine determines the baseline breathing pattern AND the threshold amount the airflow must drop before a hypopnea can be scored.
My guess is that the Transcend's algorithm for scoring hypopneas is more "sensitive" than the S9's. And my guess is that some of the Transcend hypopneas show up as peaks in the Flow Limitation graph on the S9 because the S9 considers them to be "not quite bad enough" to be scored as a hypopnea.
And how much you should worry about the fact that the Transcend's AHI is roughly 3.0 and the S9's is 0.3? I'd say that as long as you are feeling decently enough regardless of which machine you are using, you shouldn't lose any sleep worrying about which machine is more accurate in recording the hypopneas.