Case of mine: Young patient, O2sat <50% room air; 80% on 15 L NRM. Completely normal mental status, no shortness of breath, no increased WOB, no tachypnea. Pt feeling ok and looked happy. (1/4) 
I was perplexed by the lack of symptoms. Very low PaO2 confirmed by ABG. I even ruled out R-L shunt by #POCUS (agitated saline). Diffuse B-Lines seen. CT Scan showed diffuse ground glass opacities. Hypoxemia persisted, patient treated with NIV on CPAP and improved! (2/4)
Final diagnosis was influenza. This was 2 yeas ago. This is when I learned about happy hypoxemia. Remembering this now makes me wonder why are we so surprised this is happening now with COVID? I certainly was perplexed then (enough to look for intracardiac shunt 🤣). (3/4)
Before that case, I’ve never seen anything like it (mind I’m young and unexperienced). Now seeing it everyday! Certainly everyone is talking about it (even experienced clinicians), suggesting this presentation is way more prevalent in COVID19. But surely not something new.. (4/4)
For many of you asking, this was not methemoglobinemia: 1) Methemoglobin levels were normal, 2) also in methemoglobinemia (as far as I understand) O2Sat% gets "clamped" at 85% no matter the methemoglobin concentration. As for the mechanism of happy hypoxemia, ill share 2 threads:
Brilliant explanation of physiology:
Excellent threat with such insight:
Since many people keep asking: I wrote I was “perplexed by lack of symptoms”. I should have wrote “severe symptoms”, pt did complain of fever, malaise, headache. It is not as if pt was totally unaware, This was influenza! The point being surprisingly no respiratory distress.
