Why 'Silent Hypoxia' Can Make Some Patients with COVID-19 Critically Ill

This lung condition can sneak up on patients with COVID-19 without obvious symptoms.

One of the most serious symptoms of COVID-19 infections is a form of oxygen deprivation called "silent hypoxia."

In this dangerous condition, a patient's blood oxygen saturation levels drop extremely low, indicating they aren't getting enough oxygen to their lungs. At the same time, the patient does not experience breathing difficulties or breathlessness, hence the term silent hypoxia.

By contrast, when a patient experiences typical pneumonia, hypoxemia leads to shortness of breath and chest pain.

Unfortunately, hypoxia, silent hypoxia, and the need for supplementary oxygen are all predictors of worse outcomes in COVID-19 patients.

This baffling condition was first highlighted during the COVID-19 pandemic by emergency physician Richard Levitan of New York City's Bellevue Hospital. Levitan described that he was seeing a significant number of COVID-19 patients admitted to the emergency department with profound hypoxemia but no breathing difficulties. Some of these patients with pneumonia had blood oxygen saturation levels as low as 50%, proving how "silent" silent hypoxia really is.

For comparison, a near-normal blood oxygen saturation level is more than 90%, with 94-100% considered normal, explained critical care pulmonology expert Vandana A. Patel, MD, FCCP, a clinical advisor for the online pharmacy Cabinet. 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.

We may have come to know about silent hypoxia from the COVID-19 pandemic, but it isn't a new phenomenon. "It's been seen in high altitude sickness," said 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."

Many patients hospitalized with acute COVID-19 have severe hypoxia. 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," said 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 develop breathlessness, significant pneumonia may have already set in—and severe 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," said 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, told Health. If and at what point this happens depends heavily on how healthy the patient is, explained Dr. Kaufman. Healthy young people can tolerate low oxygen saturation for a long time without difficulty, said 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," explained 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."

As the scientific community's understanding of COVID-19 continues to evolve, the question of why some patients infected with COVID-19 are asymptomatic, and others develop severe diseases such as hypoxia is an active area of investigation. While 41% of patients with COVID-19 are completely asymptomatic, severe disease will develop in approximately 13% to 20% of infected individuals. Experts believe the severity of the disease depends on risk factors such as age, race, ethnic background, or sex; underlying health conditions and other factors such as viral dose, viral strain, the intensity of exposure, immunization status, and socioeconomic factors.

Because hypoxemia in COVID-19 can be debilitating and even fatal to a patient, early detection and prompt treatment are essential to prevent potential complications.

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 said positioning patients in the upright or semirecumbent position (where the head and torso are 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, added Dr. Patel.

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, said Dr. Patel. Ultimately, this could help them avoid becoming extremely sick and needing the most invasive treatments.

Health experts believe that monitoring oxygen saturation levels in primary care and home settings is a good way to detect hypoxemia in patients with chronic lung disease, asthma, and pneumonia. Home monitoring of oxygen saturation levels using portable pulse oximetry (Spo2) has been used as part of a package of care for the early detection of hypoxemia in individuals who have a higher risk of mortality and morbidity with COVID-19.

Guidance published in November 2021 by the World Health Organization (WHO) recommends the use of pulse oximetry monitoring at home as part of a package of care, including patient and provider education and appropriate follow-up for symptomatic patients with COVID-19 and patients with risk factors for severe disease.

The pandemic has caused an increase in the use of pulse oximeters. The US Food and Drug Administration (FDA) warns patients and health care providers that, although pulse oximetry is useful for estimating blood oxygen levels, the devices have limitations. According to the FDA, pulse oximeters may be less accurate in people with dark skin pigmentation. The agency recommends that patients with conditions such as COVID-19 who monitor their condition at home should pay attention to all signs and symptoms and communicate any concerns to their health care provider.

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 CDC, WHO, and their local public health department as resources.

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