A dangerous symptom of the coronavirus that can cause a patient to fall unconscious or even die is known as hypoxia — when the body’s tissues do not receive enough oxygen.
Shortness of breath often accompanies hypoxia as an indicator that the body’s oxygen levels have dropped, said Dr. Ron Damant, a professor at the University of Alberta who specializes in pulmonary medicine.
This can lead to death if it’s severe enough, he said.
But a phenomenon called “silent hypoxia”, when oxygen levels fall but an individual feels no symptoms or signs that something is wrong, is an effect of the coronavirus that some doctors are seeing in patients.
What is silent hypoxia?
Dr. Richard Levitan, an emergency doctor working in New York City, wrote for the New York Times at the end of April that he has seen COVID-19 patients with “alarmingly low” oxygen levels, but no shortness of breath. He describes this as “silent hypoxia”.
These patients had oxygen saturation levels as low as 50 per cent when normal levels are usually at 94 to 100 per cent at sea level, Levitan explained.
What concerns him is patients were only arriving at hospital when they started to experience symptoms, and at that point they were already in critical condition.
The purpose of the lungs is to bring the air that we breathe into contact with our blood. When the respiratory system is functioning properly, that process goes smoothly, said Damant.
The lungs become inflamed and stiff, leading to breathlessness, he said.
But for some patients with the novel coronavirus, they go through a phase where the lungs are inflamed, but the receptors in the lungs that signal to the brain that something is off aren’t working properly, he explained. This is why they might not feel the shortness of breath that comes with hypoxia.
“They’re not really feeling that until later on, maybe, when the lungs start to become stiffer and harder to move, and then the person perceives breathlessness,” he said.
Another reason for silent hypoxia in COVID-19 patients could be a slow and steady decline of oxygen, where the patient hasn’t noticed decreased lung function as it might be subtle, said Damant.
It’s similar to how mountaineers slowly acclimatize to higher altitudes by getting used to reduced oxygen so they don’t black out, he said.
Silent hypoxia is not a new phenomenon and those with COVID-19 who are experiencing this will eventually feel the shortness of breath associated with the disease, as the lung damage becomes worse, he said. If the lung damage is getting worse, they are likely to feel symptoms at that point, he explained.
In order to determine the severity of oxygen loss, doctors use monitors called an oximeter that clip onto the fingers or ear lobes to assess the saturation of oxygen in the blood, Damant explained. Sometimes they may have to poke the arteries to engage in a test called an arterial blood gas to check an individual’s lung function.
Are there long-term consequences of silent hypoxia?
Dr. Rupert Kaul, director of the division of infectious diseases at the University of Toronto, says silent hypoxia has been observed with COVID-19 patients at outpatient clinics in Toronto.
But it’s not something that’s unique to COVID-19, he said. Walking pneumonia, for instance, is one illness where people feel completely fine, but a chest x-ray shows they have severe pneumonia, he said.
For people who do well against a coronavirus infection — younger, healthier people — it wouldn’t be a surprise that some might have low oxygen levels but did not develop symptoms at all, because their body was able to fight off the illness before it got worse, he said.
Those who had oxygen levels dip with no symptoms wouldn’t need to get to the hospital, he added, as if the disease is worsening and requires attention, they will experience symptoms.
Doctors are concerned about individuals who have COVID-19 but present no symptoms overall, as they could infect others without knowing, said Kaul. However, low oxygen levels that never develop into worsening symptoms likely won’t have any serious, long-term consequences, he explained.
If a physician uses an oximeter and notices a patient has low oxygen levels, they can tell them to monitor for symptoms, he said. Kaul says if an infection is worsening to the point where a patient’s condition is serious, symptoms will present and they will be in hospital at that point.
Many with possible silent hypoxia could clear the infection on their own, he said.
“I still think that we probably would see many people have those transiently low oxygen levels that are able to clear the infection, without actually getting clinically ill,” said Kaul.
Questions about COVID-19? Here are some things you need to know:
Health officials caution against all international travel. Returning travellers are legally obligated to self-isolate for 14 days, beginning March 26, in case they develop symptoms and to prevent spreading the virus to others. Some provinces and territories have also implemented additional recommendations or enforcement measures to ensure those returning to the area self-isolate.
Symptoms can include fever, cough and difficulty breathing — very similar to a cold or flu. Some people can develop a more severe illness. People most at risk of this include older adults and people with severe chronic medical conditions like heart, lung or kidney disease. If you develop symptoms, contact public health authorities.
To prevent the virus from spreading, experts recommend frequent handwashing and coughing into your sleeve. They also recommend minimizing contact with others, staying home as much as possible and maintaining a distance of two metres from other people if you go out.
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