In the AHI discussion, we talked about that some Sleep Apnea Sufferers don’t have desaturations, or only a little desaturations.

I have been searching for an explanation for this for some time. I have found a lot of studies about this, but still mostly theories.

Probably there are a lot of reasons for this. Here is what I found who makes sense to me.

If there is a frequent event, not allowing for full resaturation, this is more likely to result in more severe desaturations.

A longer duration of events allows for longer time for desaturations.

But maybe the expiratory reserve volume (ERV) is the most important factor. (This is the lung volume of air/oxygen when you have exhaled).

A lot of studies shows, that SA sufferer have a less ERV than normal people. Also some studies shows there is a correlation between overweight and the ERV.

In my own case I have no desaturations, and if my first sleep study was done by a nocturnal oxygen measurement, then I would not have been diagnosed with SA.

I have also mentioned expiratory apneas, in an earlier discussion. This is very common, but a normal PSG study will not show this.

I found an article about sleep disorders in children. I know that I can’t compare SA in children and adults, but one thing does really make sense to me.

Here is a paragraph from the article:

“Obstructive expiratory apnea has received little attention. It is defined as absence of polysomnographically recorded nasal and oral airflow in the presence of continued expiratory effort against an occluded upper airway. Almost all expiratory apneas are preceded by an augmented breath or sigh. Upper airway occlusion may occur at any level, from the oropharynx to the larynx. Preliminary evidence suggests that in some children reflex glottic adduction occurs in the presence of continued diaphragmatic contraction. Increased parasympathetic tone may be present, affecting the diaphragm via the vagus nerve, as well as increasing vocal cord adduction via the recurrent laryngeal nerve. Significant heart rate deceleration occurs during the first third of the respiratory pause in a manner similar to that seen during the Valsalva maneuver. There is little change in the SaO2 in spite of the length of the apnea. This might be explained by a temporary increase in lung volume and positive expiratory pressure.”

Link to this article:
http://www.childsdoc.org/fall96/sheldon/sleepdis.asp

When you have a normal inhalation, then the lungs are filled with air. When exhalation and an obstruction occur you have a lot of air in the lungs. The positive pressure (Valsalva maneuver) will expand your lungs temporary (and you will have a large ERV), and you can have very long apnea before any desaturations.

I think this also explains why I always have had (and still have) a large lung volume, even I have been a heavy smoker for a lot of years.

Henning