The mysterious condition that appears to defy basic biology is raising questions about how COVID-19 attacks the lungs, the Guardian reported.
While a healthy person's blood-oxygen saturation is at least 95 percent, doctors have reported some coronavirus-stricken patients with levels in the 80s or 70s - with some extreme cases below 50 percent, according to the outlet.
And yet these so-called "happy hypoxics" have been observed scrolling on their phones, chatting with their health care providers and describing themselves as generally comfortable, Science Magazine reported.
"There is a mismatch [between] what we see on the monitor and what the patient looks like in front of us," Dr. Reuben Strayer, an emergency physician at Maimonides Medical Center in Brooklyn, told the magazine from his home as he recovered from the illness himself.
Strayer said he and other doctors are seeking to understand the odd condition, which he first noticed in March as patients streamed into his ER, and how to treat it.
Dr. Jonathan Bannard-Smith, a critical care specialist at the Manchester Royal Infirmary in the UK, told the Guardian that some patients are unaware that their oxygen saturations are so low.
"We wouldn't usually see this phenomenon in influenza or community-acquired pneumonia," he said. "It's very much more profound and an example of very abnormal physiology going on before our eyes."
He added: "It's intriguing to see so many people coming in, quite how hypoxic they are."
Another British physician said people would ordinarily appear to be extremely ill with other lung conditions that could cause severe hypoxia.
"With pneumonia or a pulmonary embolism they wouldn't be sat up in bed talking to you," Dr. Mike Charlesworth of Wythenshawe hospital in Manchester told the Guardian.
"We just don't understand it. We don't know if it's causing organ damage that we're not able to detect. We don't understand if the body's compensating," added the doctor, who also was infected with COVID-19.
After coming down with a cough and fever, he spent 48 hours in bed, during which he said there were signs he was hypoxic.
"I was sending very strange messages on my phone. I was essentially delirious. Looking back, I probably should've come into hospital. I'm pretty sure my oxygen levels were low," he told the news outlet.
"My wife commented that my lips were very dusky. But I was probably hypoxic and my brain probably wasn't working very well," he added.
What causes people suffering from lung diseases to feel breathless is not the fall in oxygen levels but rather the body sensing the rising levels of carbon dioxide.
"The brain is tuned to monitoring the carbon dioxide with various sensors. We don't sense our oxygen levels," Paul Davenport, a respiratory physiologist at the University of Florida, told Science Magazine.
But among some coronavirus patients, this response does not appear to be kicking in.
During the early phase of COVID-19, low saturation levels aren't always accompanied with obvious respiratory difficulties.
Carbon dioxide levels can be normal, and deep breathing may be comfortable, Dr. Elnara Marcia Negri, a pulmonologist at Hospital Sirio-Libanes in Sao Paulo, Brazil, told the magazine.
"The lung is inflating so they feel OK," she said, though their oxygen saturation can be in the 70s, 60s, 50s or even lower.
Theories about what causes "happy hypoxia" are emerging, as many doctors recognize clotting as a major feature of severe COVID-19.
Negri believes slight clotting might start early in the lungs, perhaps a result of an inflammatory reaction in their fine blood vessels, which could trigger a cascade of proteins that prompts blood to clot.
Strayer also finds it reasonable to imagine that hypoxia may be caused when "small blood vessels of the lung are being showered with clots."
His hospital and others are beginning to test many coronavirus patients for markers of excess clotting and treat those who show it with blood thinners. Science Magazine reported.
But Strayer stressed that "it is simply not known" whether clotting causes "happy hypoxia."
Recent imaging of a hypoxic patient showed "almost waxy-looking film all around the lungs," Caputo said. "I don't know what is actually going on pathophysiologically down there."
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