CNN  — 

In hospitals around the world, doctors are shaking their heads in disbelief as they watch Covid-19 patients who should be comatose or “seizing” from hypoxia – a lack of oxygen in the body’s tissues – check social media, chat with nurses and barely complain of discomfort while breathing.

Some have dubbed them “happy hypoxics,” a terrible misnomer for what could be a long, slow recovery – or worse.

The proper medical term is “silent hypoxia.” It happens when people are unaware they are being deprived of oxygen and are therefore showing up to the hospital in much worse health than they realize.

Typically, these patients have experienced some Covid-19 symptoms for two to seven days before they show up at the hospital complaining of sudden chest tightness or an inability to breathe deeply, said Dr. Richard Levitan, who’s been an emergency room physician for some 30 years.

While he practices at Littleton Regional Healthcare in New Hampshire, Levitan recently spent almost two weeks volunteering in the emergency room of a New York City hospital near the epicenter of the city’s devastating outbreak.

There he watched patients come into the emergency room with blood oxygen levels as low as 50%, so low they should have been incoherent, even unconscious. Normal blood oxygen saturation is between 95% and 100%, and anything below 90% is considered abnormal.

In addition, Levitan said, scans of these patients’ lungs showed signs of pneumonia so severe they should be in terrible pain as they gasp for their next breath.

“Their X-ray’s looked awful, their oxygen was terrible, and yet they’re completely awake, alert on a cell phone, and they all said is they’ve been somewhat sick for days,” he said.

“And then only recently did they notice either shortness of breath or fatigue or something else,” Levitan added. “That’s what is so fascinating about this disease and also so terrible.”

It’s terrible because by the time a person realizes they are having trouble taking a deep breath and reaches out for help, they are already dangerously sick.

“Some may ultimately require a ventilator.” Levitan said, “As levels of carbon dioxide rise, fluid builds up in the air sacs and the lungs become stiff, leading to acute respiratory failure.”

How can this happen?

Doctors speculate that, for some people, Covid-19 lung problems progress in a way that isn’t immediately apparent. As patients focus on battling such symptoms as fever and diarrhea, the body begins fighting back against the lack of oxygen by speeding up breathing to compensate.

“Just imagine that you had a full glass of air, and now that cup becomes half full,” said critical care pulmonologist Dr. Cedric Rutland, a spokesperson for the American Lung Association.

“What are you naturally going to do? You’re going to try to fill it twice as fast because you lost half,” said Rutland, who is also a assistant clinical professor at the University of California, Riverside.

People may not be aware of their more rapid breathing rate and don’t seek help, yet blood oxygen levels continue to fall. In the meantime, the body slowly becomes somewhat adjusted to the lower levels of oxygen, much like what happens when a person travels to a higher altitude.

By the time these patients get to the hospital with crippled lungs and crashing oxygen levels, “this has been taking place for a bit of time.” Rutland said. “So your body is kind of used to it.”