It is a mystery that has left doctors questioning the basic tenets of biology: Covid-19 patients who are talking and apparently not in distress, but who have oxygen levels low enough to typically cause unconsciousness or even death. The phenomenon, known by some as “happy hypoxia” (some prefer the term “silent”) is raising questions about exactly how the virus attacks the lungs and whether there could be more effective ways of treating such patients. A healthy person would be expected to have an oxygen saturation of at least 95%. But doctors are reporting patients attending A&E with oxygen percentage levels in the 80s or 70s, with some drastic cases below 50%. “It’s intriguing to see so many people coming in, quite how hypoxic they are,” said Dr Jonathan Bannard-Smith, a consultant in critical care and anaesthesia at Manchester Royal Infirmary. “We’re seeing oxygen saturations that are very low and they’re unaware of that. We wouldn’t usually see this phenomenon in influenza or community-acquired pneumonia. It’s very much more profound and an example of very abnormal physiology going on before our eyes.” Dr Mike Charlesworth, an anaesthetist at Wythenshawe hospital in Manchester, said that while other lung conditions could cause severe hypoxia, these patients would normally appear extremely ill. “With pneumonia or a pulmonary embolism they wouldn’t be sat up in bed talking to you,” he said. “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.” Charlesworth had a personal experience of the issue while suffering from Covid-19 in March. After becoming unwell with a cough and fever, he spent 48 hours in bed, during which there were signs he was hypoxic, he said. “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. My wife commented that my lips were very dusky. But I was probably hypoxic and my brain probably wasn’t working very well.” He recovered after a few days in bed, but he and others are conscious that not all cases have positive outcomes. An anaesthetist at a London hospital, who spoke anonymously, recalled one patient who attended A&E saying she felt cold. “When we put the stats probe on her, her saturation was 30% on air,” he said. “We obviously thought that was wrong, as usually patients are likely to have hypoxic cardiac arrests.” But when a blood sample was taken, her blood was very dark and had oxygen levels equivalent to those seen in people acclimatised to high altitudes. The patient was placed on a ventilator and survived for about a week before dying. “I have had a few patients like this,” the doctor said. “Sadly, their outcomes tend to be bad in my experience.” Conventional medical wisdom is that as oxygen supplies fall, the heart, brain and other vital organs are placed at risk – and the effect is thought to be cumulative. Typically patients would lose consciousness below an oxygen saturation of 75%. However, it is not the fall in oxygen levels itself that leaves people feeling breathless. Instead, the body senses the rising levels of carbon dioxide that typically occur simultaneously as the lungs are unable to clear gas as efficiently. But in some Covid-19 patients, this response does not appear to be kicking in. “I don’t think any of us expect that what we’re seeing can be explained by one process,” said Bannard-Smith. Swelling and inflammation in the lungs is likely to make it difficult for oxygen to enter the bloodstream. There is also emerging evidence that Covid-19 can cause blood clotting. The vessels in the lungs that collect oxygen and transfer it into the wider bloodstream are so tiny they can become blocked with the smallest of clots. Several clinical trials are looking at whether blood thinners could prevent or treat complications of Covid-19, including respiratory problems and low blood oxygen. Some have suggested that, since people are often oblivious to falling oxygen levels, those with Covid-19 symptoms or a positive test result should be given pulse oximeters, a simple device that clips on to the finger and can be used to detect oxygen levels at home. However, as yet there is no evidence that early detection of hypoxia would help avoid severe outcomes and Charlesworth said the practicalities would be difficult. “Transportation of the devices would put more people on the road,” he said. “Then there are issues around people buying them on the internet and whether they [have proper safety certificates] … If you’re at the point of needing your oxygen levels monitored that’s the time to go to hospital.”