A mysterious, potentially deadly side effect of COVID-19 is baffling doctors.
The condition has been dubbed "happy hypoxia" and describes a number of patients who are experiencing oxygen saturation levels well below what's normal - but say they are still relatively comfortable.
A normal blood-oxygen saturation level is around 95 percent and if these levels drop then patients will be breathless and uncomfortable. People with respiratory infections are more susceptible to a drop in blood-oxygen saturation.
Once levels drop to around 75 percent, patients usually become unconscious.
However, in the early stages of COVID-19 doctors are saying some patients have levels as low as 50 but are still able to comfortably chat.
"We just don't understand it," Dr Mike Charlesworth, an anaesthetist at Wythenshawe hospital in Manchester, told The Guardian.
"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."
Patients are presenting with the strange side effect across the world, with an emergency doctor from New York saying it's completely mystifying.
"There is a mismatch [between] what we see on the monitor and what the patient looks like in front of us," said Reuben Strayer, an emergency physician at Maimonides Medical Center in New York City.
One theory proposed to Science Magazine by San Paulo pulmonologist Elnara Marcia Negri is that subtle blood clotting may begin early in the lungs which in turn prevents the blood from becoming properly oxygenated.
Strayer says it is reasonable to imagine that hypoxia emerges because "small blood vessels of the lung are being showered with clots."
His hospital and others are beginning to test COVID-19 patients for excess clotting and treating those who show it with blood thinners.
However despite testing and treating it's still not known whether the clotting results in happy hypoxia or not.
