Thread by @Laconic_doc

@MAM12CP @timstorrie 1. It's a well-recognised (almost defining) phenomenon of Covid-19

2. This severe oxygen deficit seen is worse than with 'normal' pneumonia, both wrt the lack of symptoms, & wrt the cause

3. There's a mis-match betw blood flow & air flow in the lungs (as yet unexplained)

@MAM12CP @timstorrie a) ref for picture: bestpractice.bmj.com/topics/en-gb/3…

(an excellent, if a little technical, review of how doctors should assess patients with suspected Covid-19) 2/n

@MAM12CP @timstorrie b) re 3. above - initially the problem was thought to be the many micro clots found clogging up the tiny blood vessels adjacent to the alveoli (air sacs) in the lungs. This would stop blood getting to the air sacs & absorbing oxygen.
However, blood-thinning meds didn't help!

3/n

@MAM12CP @timstorrie More recently, it has been found there seems to be a failure of 'auto-regulation' (i.e. self adjustment) of the tiny airways leading to the alveoli.
Normally, in alveoli without air flowing to them, the relevant tiny blood vessels shut down. 4/n

@MAM12CP @timstorrie This prevents non-oxygenated blood getting through to the heart - all the blood is diverted to alveoli that *are* getting air in them.

The more blood that gets through the lungs without picking up oxygen [O2], the lower the overall blood oxygen - even if breathing 100% O2!
5/n

@MAM12CP @timstorrie This auto-regulatory mechanism seems to have failed in Covid-19 pneumonia (as seen in many other types of so called 'acute respiratory distress syndrome' - ARDS)

It if not yet understood what makes this happen in Covid-19 (or indeed, other ARDSs) 6/n

@MAM12CP @timstorrie It remains totally unexplained how people with v low blood O2 levels remain without symptoms, though - a fascinating conundrum that may help us to understand lung & breathing physiology, once it's eventually unravelled! /END