A well-known symptom of the coronavirus is low oxygen levels in the body, aka hypoxia, which normally lead to shortness of breath. But some coronavirus patients with hypoxia don't experience this hallmark sign that their oxygen levels have dropped into dangerous territory.
The phenomenon is referred to as "silent" or "happy hypoxia," where the body has oxygen saturation below 90%, but the person can still breathe normally. Some doctors take issue with the term, though, because coronavirus patients with silent hypoxia often have other symptoms, such as fever or cough.
Silent hypoxia isn't new or specific to the coronavirus, interventional pulmonologist Dr. Udit Chaddha, assistant professor of pulmonary, critical care and sleep medicine at the Icahn School of Medicine at Mount Sinai in New York, told TODAY. It can happen with lung infections, viral or not.
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That said, some experts believe it's more common and looks different in COVID-19 versus other respiratory illnesses.
"If you'd told me two months ago that someone would be sitting in an ER comfortable and able to have a conversation, but have an oxygen concentration around 80%, I'd tell you that you were crazy," Dr. Eric Cioe-Pena, an emergency medicine physician at Northwell Health in New York City, told TODAY.
What is "silent hypoxia"?
Silent hypoxia occurs in coronavirus patients when "there's areas of the lung ... (where) ventilation is pretty normal, but there's enough disease that they're having lower oxygen levels," Dr. David Hill, a pulmonary specialist in Waterbury, Connecticut and spokesperson for the American Lung Association, told TODAY.
"(These patients) will still have good enough lung function in terms of how the lungs move that they're able to blow off their carbon dioxide well so they don't develop the shortness of breath," he added.
Chaddha said he's seen in it in "a handful" of his coronavirus patients. Of every 10, usually between two and four experience silent hypoxia to some extent, he estimated.
When patients with silent hypoxia seek medical care, they've usually had coronavirus symptoms for five to seven days, Hill said. Cioe-Pena added that the "worst" time for hypoxia, silent or not, in coronavirus patients is around day 10 of the infection.
All three experts agreed that there are no characteristics or criteria that determine whether a person is at higher risk of silent hypoxia. It can be treated with a ventilator, but usually other strategies come first, such as a CPAP machine (continuous positive airway pressure machine, which uses a hose and mask to deliver constant air pressure) or adjusting the position of the body.
What's the impact of silent hypoxia on the body?
Chaddha stressed that the presence of silent hypoxia doesn't determine a coronavirus patient's prognosis.
"Let's say patient A had silent (hypoxia), and patient B did not," he explained. "It doesn't necessarily mean that patient A will do bad or good, or patient b will do bad or good. If patient B is more short of breath, it probably means there are other things going on."
Silent hypoxia doesn't usually go untreated for extended periods in coronavirus patients because they usually develop shortness of breath eventually, which should prompt them to seek medical care, Chaddha said. Also, many of these patients experience other signs of the coronavirus before shortness of breath, which land them in the emergency room or a doctor's office.
Since most patients may have untreated hypoxia for just a few days, experts stress that "low oxygen levels by themselves don't cause a lot of damage," Hill said. "Long-term, if we're talking months to years, that's not something we expect to see with COVID ... I really don't believe there's a lot of people hanging out at home feeling well and suddenly showing up sick with COVID-19."
Cioe-Pena added that low oxygen levels are "cause for investigation (by a medical professional) but not necessarily ... a life-threatening condition." In fact, he discharged a patient with the condition because she was otherwise fine.
Young, healthy patients can tolerate low oxygen levels for longer, Hill added.
How can you protect yourself from silent hypoxia?
The only way to know if you have silent hypoxia without seeking medical care is through at-home monitoring, Chaddha said. The best device for this is a pulse oximeter, which measures the saturation of oxygen carried in a person's red blood cells. You attach it to your finger tip to get a reading.
They're available for purchase in pharmacies and online for between $20-$60, but many experts say this tool isn't appropriate for everyone.
Chaddha only recommends monitoring your oxygen levels at home if you have the coronavirus and previous low oxygen levels, and your condition is not yet improving.
Hill, meanwhile, called the idea that every COVID-19 patient get a pulse oximeter "premature." He suggested that if you're in a high-risk group, talk to your doctor about pulse oximetry. You should also ask for advice on which device to purchase and training on how to use it. Inaccurate readings only further overwhelm the health care system.
Ultimately, the existence of silent hypoxia isn't a reason to believe your health could be failing with little to no signs. As Chaddha explained, "Don't get worried about it, but be aware of it."