This lung condition is sneaking up on COVID-19 patients without obvious symptoms.

By Claire Gillespie
April 23, 2020
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Doctors are noticing a worrying trend in COVID-19 patients: Their blood oxygen saturation levels are extremely low, indicating they aren't getting enough oxygen to their lungs. Yet they’re showing no signs of breathlessness. 

“This is a phenomenon known as silent hypoxia,” critical care pulmonology expert Vandana A. Patel, MD, FCCP, a clinical advisor for the online pharmacy Cabinet, tells Health. “Despite having low blood oxygen saturation levels in their body due to COVID-19, some people do not feel any sensation of breathlessness.” 

A near-normal blood oxygen saturation level is more than 90%, with 94-100% considered normal, explains Dr. Patel. If a patient registers a number lower than this, the brain might not get the oxygen it needs, leading to confusion and lethargy. If the level drops as far as the low 80s, there’s a real danger of damage to vital organs and even death. 

Richard Levitan, MD, an emergency doctor at Bellevue Hospital in New York City, published an op-ed in The New York Times on April 20 about his experience with COVID-19 patients suffering from silent hypoxia. He said he had been seeing patients whose lungs were filled with fluid or pus, but they weren't experiencing breathing difficulties expected with these symptoms until the day they arrived at the hospital. 

Some of the coronavirus patients Dr. Levitan saw with pneumonia had blood oxygen saturation levels as low as 50%, proving how “silent” silent hypoxia really is. 

It may be a talking point right now, but silent hypoxia isn’t a new phenomenon. “It’s been seen in high altitude sickness,” says Dr. Patel. “Any condition that causes damage to the lungs can cause it, although it’s more common in chronic conditions like COPD (chronic obstructive pulmonary disease) and pulmonary fibrosis, where the lungs are chronically damaged, than in pneumonia.” 

In many COVID-19 cases, the virus silently causes injury to air sacs in the lungs. “The coronavirus affects the air sacs and causes pneumonia, which leads to impairment in the diffusion of oxygen through its membrane,” says Dr. Patel. “Initially the lungs remain compliant and can expel carbon dioxide, so people cannot feel any sensation of being breathless.”

By the time patients do develop breathlessness, significant pneumonia may have already set in—and serious damage may have been done. “The air sac injury caused by the virus can rapidly progress to cause dangerously low levels of oxygen, which can result in further tissue damage in various organs, including the lungs, heart, liver, kidney, and brain,” says Dr. Patel. The worst case scenario is multiple organ failure, which can be fatal. 

"Silent hypoxia could be serious if the body’s organs (for example, the heart, liver, kidneys, brain) do not receive enough oxygen for them to function normally," David Kaufman, MD, pulmonologist and director of the medical ICU at Tisch Hospital/NYU Langone, tells Health. If and at what point this happens depends heavily on how healthy the patient is. Healthy young people can tolerate low oxygen saturation for a long time without difficulty, says Dr. Kaufman.

"People with underlying medical problems like heart disease, lung disease, or kidney disease might begin to experience difficulty due to low oxygen saturation sooner," explains Dr. Kaufman, "but the oxygen level when this occurs differs from one person to the next and depends on many factors, like whether the person has anemia or impaired blood flow."

If doctors recognize silent hypoxia early enough, it can be treated with oxygen therapy (through nasal tubes, a face mask, or a tube placed in the windpipe). Dr. Patel says positioning patients in the upright or semirecumbent position (where the head and torso is at an angle of 45 degrees), or in the prone position (lying on their belly) might help. Close monitoring via blood tests is also important, she adds. 

To prevent silent hypoxia, the lung damage that causes it has to be prevented first. However, an oxygen-monitoring device called a pulse oximeter can help to detect low oxygen levels and alert people to seek early medical care, says Dr. Patel. Ultimately, this could help them avoid becoming extremely sick and in need of the most invasive treatments. 

Dr. Levitan believes pulse oximeters are the way forward when it comes to detecting and treating the coronavirus. “Widespread pulse oximetry screening for COVID pneumonia—whether people check themselves on home devices or go to clinics or doctors’ offices—could provide an early warning system for the kinds of breathing problems associated with COVID pneumonia,” he wrote. 

He added that all patients who test positive for the coronavirus should have pulse oximetry monitoring for two weeks, as this is the period during which COVID pneumonia typically develops.

“All persons with cough, fatigue, and fevers should also have pulse oximeter monitoring even if they have not had virus testing, or even if their swab test was negative, because those tests are only about 70% accurate,” wrote Dr. Levitan. “A vast majority of Americans who have been exposed to the virus don’t know it.”  

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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