Explain upper solid horiz line in Fitbit visualization of "estimated oxygen variability" graph

Question

I am healthy and I wear a smartwatch while sleeping. Fitbit generated the following visualization of changes in estimated blood oxygen saturation variability (EOV). I think it is only the first derivative of some relative value that the device measured.

But what does the visualization even mean? YOu can see this curve shows a small interval where the line is yellow.

• Maybe the device was poorly aligned to my wrist during this interval.
• Maybe I was snoring or breathing less frequently
• perhaps it is normal for healthy persons, and somewhat expected, that the green line randomly crosses the dashed line once per night (the placement is somewhat arbitrary)

I have read the Fitbit EOV documentation (followed the "More info" link on phone app).
But my question is: There is a horizontal solid line at the top.
What is the significance of that solid horizontal line?
Is this a line where it is definitely unhealthy if the green line reaches that level? Does it turn orange into red at that level? Is this a purely cosmetic line to improve the graphic design?

Has anyone here ever seen a EOV graph going above that upper solid line?

Answer 1

The original graph didn't have that line at all.

It appears they added top and bottom lines to indicate the maximum and minimum values to the graph and you can see how close you are to them.

As far as what the line means, many high variations could indicate sleep apnea. Few variations like yours are normal and may even be recording errors.