COVID-19 May Cause Dangerously Low Oxygen Levels With Few Other Symptoms

Woman receiving inhalation treatment in quarantine.

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Key Takeaways

  • Happy hypoxia describes a situation in which a person’s blood oxygen levels are low but they feel fine. 
  • Researchers have discovered that happy hypoxia in COVID-19 patients has several causes.
  • These causes include impaired blood flow and blood oxygenation in the lungs.

Contrary to what its name might suggest, happy hypoxia is no laughing matter. The term refers to hypoxemia without dyspnea, or the ability to respirate normally without sufficient blood oxygen. Happy hypoxia has been observed since “time immemorial,” according to the EMCrit Project, but it only became a hot topic recently when reports of the condition in COVID-19 patients began flooding in from all over the world.

In a recent study on this bizarre medical phenomenon, researchers at Boston University in Boston, Massachusetts concluded that there are three contributing factors. The September study was published in Nature Communications. 

Happy hypoxia, also known as silent hypoxia, represents an early stage in COVID-19 infection. 

About one in five people who has to be hospitalized for COVID-19 will experience happy hypoxia. Manny Gonzalez-Brito, DO, a pediatrician who has seen many kids with happy hypoxia in his time, tells Verywell, this may occur for reasons both typical of respiratory illnesses and unique to the virus itself. 

“The way that the coronavirus and every respiratory virus predominantly changes your blood oxygen levels is by causing inflammation in the lungs," he says. "You can’t readily get air into your air sacs and the blood being perfused through your lungs is not getting the air that you're breathing. Now we've also discovered, with COVID-19, that there are other mechanisms [by which] this can happen."

It was these mechanisms that the Boston University researchers aimed to learn more about. By using biomedical computer modeling to evaluate the plausibility of several different physiological scenarios, they determined that the likely causes of happy hypoxia are a mix of:

  • Pulmonary embolism
  • Ventilation-perfusion mismatching in the non-injured lung
  • Normal perfusion of the relatively small fraction of injured lung

Dense as these terms are, they refer to relatively simple concepts. Perfusion typically refers to the supplying of blood to an organ or tissue and ventilation is the exchange of oxygen between the lungs and the atmosphere. Therefore, ventilation-perfusion mismatching occurs when the lungs receive blood but not oxygen or vice versa. Pulmonary embolism refers to blood clotting in the lungs and is a common cause of ventilation-perfusion mismatching. 

What This Means For You

If you notice or someone close to you notices that you are breathing abnormally, it could be a sign of hypoxia—and you should call your primary care doctor or get emergency care as soon as possible. 

A Mix of 3 Factors

The researchers began their study by closely examining blood flow direction in the lungs. They found that in many people with COVID-19, blood was flowing into regions so damaged they were unable to retain oxygen—normal perfusion of the small fraction, or part, of the compromised lung. 

Next, they turned their attention to the impact of blood clotting on blood flow. They discovered that clots too small for most medical equipment to pick up—pulmonary embolisms—were forming in the lungs as a result of blood vessel lining inflammation. 

Finally, they studied the ratio of air-to-blood flow in the lungs. Their model indicated that a ratio change—ventilation-perfusion mismatching—might be occurring in parts of the lungs that appeared healthy in scans. 

By themselves, none of these factors were enough to cause blood oxygen to drop to the levels recorded in COVID-19 patients and induce happy hypoxia. Taken together, however, they were. 

These findings shed light on the biological mechanisms underpinning the condition, Gonzalez-Brito says, potentially resolving—or at least taking a step toward resolving—one of COVID-19’s many mysteries.  

The fact that patients “don’t feel the shortness of breath makes sense...because it's not a lung issue, it's a plumbing issue with not getting the blood through there, so your lungs are not going to feel any sensory feedback, if you will, there,” Gonzalez-Brito says. 

How Is Happy Hypoxia Biologically Possible? 

As you might expect, blood oxygen levels measure the amount of oxygen present in your blood.

Blood oxygen levels between 95% and 100% are normal and blood oxygen levels below 92% are cause for medical concern. Incredibly, people with happy hypoxia sometimes even have blood oxygen levels so low as to be “incompatible with life,” according to Boston University’s The Brink, but feel no ill effects. How is this possible? 

“That's a fantastic question, and it really doesn't have a 100% ironclad answer,” Gonzalez-Brito says. However, he adds, “the shortness or the feeling of shortness of breath doesn't correlate well, in general, with someone's level of oxygenation, believe it or not.” For all intents and purposes, needing more air than you’re currently taking in is a subjective sensation rather than an objective one. Hyperventilating from anxiety is an example. 

How Can You Tell if You Have Happy Hypoxia? 

People with happy hypoxia can have some symptoms. Whether they realize it or not, Gonzalez-Brito says, they will likely be breathing more deeply and quickly. 

“The respiratory rate—how many times you breathe in a minute—would be probably the most objective thing that you can keep track of," Gonzalez-Brito says. "And if you're wondering if you're breathing too fast, this is something that you can easily ask your own physician for guidance [on because] I know there's people out there that might have different medical conditions."

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Farkas, J. PulmCrit – Understanding happy hypoxemia physiology: how COVID taught me to treat pneumococcus. EmCrit. April 15, 2020.

  2. Herrmann J, Mori V, Bates JHT, et al. Modeling lung perfusion abnormalities to explain early COVID-19 hypoxemia. Nat Commun. 2020;11:4883. doi:10.1038/s41467-020-18672-6

  3. Shiel, W.C. Medical definition of perfusion. MedicineNet. Last reviewed on December 31, 2018.

  4. Shiel WC. Medical definition of ventilation. MedicineNet. Last reviewed on December 27, 2018.

  5. Cleveland Clinic. Pulmonary embolism. Last reviewed on February 26, 2019.

  6. Colarossi, J. Three reasons why COVID-19 can cause silent hypoxia. The Brink. October 8, 2020.

By Caroline Tien
Caroline Tien is a journalist with degrees in English and biology. She has previously written for publications including Insider and Cancer Health.