Small AHI, Big Oxygen Drop!

[S. Manz]

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[attachment=36633]
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Please note:
I am not a medical pro and as far as I can tell by going back about 50 sessions, this graph does not represent a set pattern of any form, so please take my analysis as an "observation" only and for what its worth.
If you see similarities between my description of this graph and any of yours, please let us hear about it.

So I put on my oximeter at around midnight and get into bed for some reading.
1:11-I mask up and lay down.
1:21-Wake up for water 
1:23-Masked up and falling asleep fast
1:36-My SpO2 levels drop slightly but stabilize. This is normal as my breathing slows down (At 10 Breaths per minute I am a naturally slow breather anyway).
I also have low level COPD but not serious enough yet to be on medication for it, which explains the lower overall saturation levels.
2:59-Wake up for washroom and raiding the fridge for snacks...There has been no Hypopnea events so far.
3:18-Mask up and immediately fall back asleep.
3:23-Oxygen level starts to sink after being stabilized between 1:36 and 3:18
3:42-Hypopnea event #1 is followed by a sharp spike up in Oxygen and it immediately drops back down (sharp spike is due to deep breathing subsequent to Hypopnea #1)
4:41-Oxygen level has continued downwards and is now almost at it lowest level. NOTE the slight increase in heart rate. This I am told is normal as the heart tries to compensate for low oxygen levels.
4:42-Boom..Hypopnea event #2. NOte the sharp spike in heart rate immediately after, followed by a sharp spike in SpO2 levels, then dropping down to its lowest.
5:11-Oxygen saturation is stabilized at its lowest however this is when it gets a little weird because for not apparent reason the SpO2 starts elevating at this point. (DO I HAVE A BAD DEVICE AND BEEN WASTING MY TIME? I must check by using another device for comparison).
NOTE that the heart rate has been lowering at a shallow rate since 4:42 when the oxygen levels were at their lowest.
5:25-Hypopnea #3 followed by a sharp spike in both the heart rate and O2 levels which drop back but then continue to rise slowly until it reaches the 'median" and levels off.
6:00-I wake up and de-mask.

When I put this observation into context, it all makes some sense except for the fact that (as I mentioned at the beginning) I cannot see a definite pattern, going back quite a few sessions.
Conclusion:
I haven't got the foggiest idea what on god's green earth is happening here.
Any ideas?

[Sleeprider]

I'd like to see this chart with a customized Advanced View.  Just go to the Daily Details as usual, then select the menu View/Advanced. You may have to do some adjustments by dragging some charts. What I want to see are
Events,
Flow Rate,
Respiration Rate,
Minute Vent, and
Flow Limitation

This will show us exactly the mechanism of your hypopnea and why your SpO2 drops out.  It is a bit complicated to produce this graph, but it is really going to show us some important information to put this together.

[S. Manz]

There you go:

Full view:
[attachment=36634]
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Hyp #1
[attachment=36635]
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Hyp #2
[attachment=36636]
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[S. Manz]

Hyp 3
[attachment=36637]
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Hyp 1,2,3
[attachment=36638]

[Sleeprider]

Thanks for the graphs. The drop in SpO2 is not related to minute vent or its components, respiration rate and tidal volume. The nadir of SpO2 is at 4:45 and your respiratory volumes and rate do not suggest any change, and if anything, your respiratory volume is increased ahead of the desaturation. The event is immediately preceded by a large increase in minute vent caused by deeper breathing (tidal volume), followed by suppressed minute vent caused by shallower breathing, because respiration rate remains normal for you. The likely explanation is that the surge in minute vent purges CO2 and suppresses tidal volume for several breaths allowing SpO2 to drop. The hypopnea events themselves are not a reduction in respiratory volume from normal, but are artifacts from the significant increase in volume ahead of the event, and a return to normal.

The data does not explain why this happens, only that it does, and that hypopnea, and ensuing desaturation is unrelated to flow limitation. The solution to improve minimum oxygen levels is to increase EPAP which recruits lung volume and increases oxygen transfer at the alveolar level. There is nothing else I can suggest other than to increase EPAP min which will improve overall oxygen levels, but may or may not avoid the large increase in tidal volume ("sigh") that seems to be the trigger for hypopnea and desaturation.

[S. Manz]

That's the best explanation I could have hoped for and I agree with your suggestion to elevate EPAP.
I think I will try doing that by a small amount for a few nights and see if my oxygen levels will improve.
I can also try switching to ASV Auto which will then give me EPAP variability and see.
My only issue with elevated EPAP is that it used to keep me awake, which is why I went back to the minimum setting some time ago, however now that I am better  at it it, I should be able to tolerate elevated EPAP.
Thanks again.