Reduced Cerebrovascular Oxygenation in Individuals with Post-Acute COVID-19 Syndrome PACS “long COVID”, 2023, Adingupu et al.

26% in the symptomatic group isn't exactly overwhelming, although I'd be quite happy believing that the hypoxia is something that comes and goes, depending on the demands placed on the body or specifically on the brain. I'd also be willing to believe that there are groups under the broad category of Long Covid for whom hypoxia is a significant symptom.

But having 12% of the asymptomatic group being similarly hypoxic seems like quite a problem to the hypothesis that brain hypoxia is a feature of (a type of) Long covid. I wonder how the authors explain this phenomenon of hypoxia but no symptoms.

Does the mean time after acute infection of only 2.5 months for the asymptomatic people mean that maybe some people were measured quite soon after the acute infection? So, maybe they still had the effects of the acute disease, but were feeling relatively good compared to having the infection? That doesn't sound like a very good explanation to me. But why else would apparently healthy people appear to have hypoxia? Are there problems in the reliability of the measurement approach?

This article gives a bit of background on the technique:
Frequency-domain vs continuous-wave near-infrared spectroscopy devices: a comparison of clinically viable monitors in controlled hypoxia 2017

 

If I'm understanding correctly the 2017 paper linked above, the technique is ok for identifying changes in brain oxygenation in a person. But, it's not good for saying 'this person clearly has a problem with brain oxygenation and this person doesn't'; it's not good for comparing between people. Perhaps things have improved since 2017. But it is sounding as though the technology may not be good enough for this purpose it has been put to.

I wonder if a measurement of the change in oxygenation when oxygen intake is restricted might be more useful? As in, the healthy oxygenated brain will show a big decrease, but a brain that is already rather de-oxygenated might not show much of a change.

I really want to know if there are differences in brain oxygenation. It feels as though there should be and it would help us know what is going on. I think ME/CFS is a lot like operating at high altitude.

I assume the measurements were done with the participants supine. I wonder what would happen with the participant upright.

 

That sounds like quite a lot. In that 2017 paper, oxygen saturations dropped 15% from baseline when the participants had oxygen restriction similar to being 5000 m above sea level. That's just a bit less than the altitude where supplementary oxygen is typically used (and that's after a careful acclimatisation process to the lower oxygen levels, conducted over days).

For sure there is something weird going on with blood supply. Yesterday my son was doing a trick where he held his arm up and it would immediately blanch white, and then turn grey, as if it was dead. It was as if there was a tap and the blood was gushing out of the arm into the body. Then he would lower the arm and it would rapidly turn bright red. If that is happening in an arm, it would not be at all surprising that oxygen supply is not right in the brain either.