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The AHI has been calculated a bunch of different ways, as you've probably noticed. In SHHS, something like RDI4P includes obstructive apneas, central apneas, and hypopneas, yet all the events require a >=4% desaturation.
For OAHI and other, newer variables (like the ICSD-3 set of AHI variables), the trend seems to be toward using "all apneas regardless of desaturation" (obstructive or central or both depending on the index of interest), instead of using a consistent desaturation requirement across both apneas and hypopneas.
So If I want to use current definitions, the more appropriate variables would be OAHI and AHI (ahi_a0h4)?
I looked at RDI4P, it doesn't seem to include RERA.
Your comment about the more appropriate variables is correct. The OAHI and ahi_a0h4 variables would give3 all apneas regardless of desaturation and hypopnea events with at least a 4% desaturation. The rdi4p variable uses only events with a 4% desaturation. Regarding RERAs, those events were not scored when this study was performed. Hope this helps.
Also, how was the apnea defined? I think current guideline is at least 90% reduction of airflow for 10s...
But not sure about the shhs dataset. Is it "airflow drop to 25% of baseline, for at least 10s"?
The Ns are different on those variables because "oahi" had some sort of filtering applied to it based upon the quality of the sleep study signals (e.g. oximqual). "ahi_a0h4" did not have any filtering applied. I don't know the exact set of filters used on the older variables. "ahi_a0h4" was created more recently.
Thanks for clarifying. The only difference between oahi and ahi_a0h4 seems to be the filtering, and it makes sense to me to include the filtering.
Do you know if there's a similar RDI/AHI variable with the filtering applied?
Variables like "oai0p" and "cai0p" also have filtering. The primary AHI/RDI variables used in SHHS1, like "rdi3p", do not appear to always have filtering present.
The mop file is very useful, thank you. I think I now understand what the filtering is for. Some notes below for others to check!
**Arousals in REM unreliable: when EMG is artifactual or absent during all or REM portion of the study.
Respiratory events/RDI unreliable: when due to the technical quality of the respiratory signals,
distinctions between hypopneas and normal breaths are equivocal in over 20 % of scored events ; also when the quality of the oximetry signal raises doubts about actual magnitude of desaturation linked with over 20 % of respiratory events (unstable baseline).
Apnea/hypopnea unreliable – when airflow signal is artifactual or absent for over 20 % of scored respiratory events.**
I am not sure which variables were used for filtering; but it seems like this filter "airqual >2" was used for OAHI
Thanks again Mike! Now I can address the reviewer's comments!