“Silent hypoxia” could kill some COVID-19 patients. But there is hope.


As doctors see more and more COVID-19 patients, they notice a strange trend: patients whose blood oxygen saturation levels are extremely low but who are barely out of breath.

These patients are very sick, but their disease does not present itself as the typical acute respiratory distress syndrome (ARDS), a type of lung failure known since the 2003 epidemic of the SARS coronavirus and others. respiratory diseases. Their lungs are clearly not oxygenating the blood effectively, but these patients are alert and relatively well, although doctors may wonder if they should be intubated by placing a breathing tube in the throat.

The concern with this presentation, known as “silent hypoxia”, is that patients show up in hospital in poorer health than they think. But there could be a way to prevent that, according to one New York Times Op-Ed by emergency room physician Richard Levitan. If sick patients were given oxygen monitors called pulse oximeters to monitor their symptoms at home, perhaps they could seek medical treatment sooner, and ultimately avoid the more invasive treatments.

Related: Are ventilators overused in COVID-19 patients?

“This is not a new phenomenon,” said Dr. Marc Moss, head of the division of lung science and intensive care medicine at the University of Colorado Anschutz Medical Campus. There are other conditions in which patients are extremely low in oxygen but do not feel any sense of suffocation or lack of air, Moss told Live Science. For example, certain congenital heart defects cause circulation to bypass the lungs, which means that the blood is poorly oxygenated.

However, the increased understanding that people with COVID-19 may present with these atypical coronavirus symptoms changes the way doctors treat them.


Normal blood oxygen levels are around 97%, Moss said, and this becomes worrisome when the numbers drop below 90%. At levels below 90%, the brain may not have enough oxygen and patients may start to experience confusion, lethargy, or other delusions. As levels drop in the 1980s or below, the danger of damage to vital organs increases.

However, patients may not feel as dire as they do. Many coronavirus patients presented to the hospital with oxygen saturation in the 1980s, but appeared to be fairly comfortable and alert, said Dr. Astha Chichra, an intensive care physician at the Yale School of Medicine. They may be slightly breathless, but not in proportion to the lack of oxygen they receive.

There are three main reasons why people experience dyspnea or difficult breathing, Moss said. One is something that clogs the airways, which is not a problem in COVID-19. Another is when carbon dioxide builds up in the blood. A good example of this is during exercise: an increase in metabolism means greater production of carbon dioxide, leading to heavy breathing to exhale all of this CO2.

Related: Could genetics explain why some COVID-19 patients do worse than others?

A third phenomenon, particularly important in respiratory diseases, is the decrease in pulmonary compliance. Pulmonary compliance refers to the ease with which the lungs enter and exit with each breath. In pneumonia and in ARDS, the fluids in the lungs fill microscopic air sacs called air sacs, where oxygen from the air diffuses into the blood. As the lungs fill with fluid, they become tighter and stiffer, and the person’s chest and abdominal muscles must work harder to expand and contract the lungs in order to breathe.

This also occurs in the case of severe COVID-19. But in some patients, the buildup of fluid is not enough to make the lungs particularly rigid. Their oxygen levels may be low for an unknown reason that does not involve fluid build-up – and that does not trigger the body’s need to breathe.

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Work to breathe

What exactly is going on is still unknown.

Chichra said that some of these patients might just have fairly healthy lungs, and therefore have lung compliance (or elasticity) – so not much resistance in the lungs when a person breathes in and out – to feel like they ‘They were not short of air, even though their lungs became less efficient at diffusing oxygen into the blood. Others, especially geriatric patients, may have co-morbidities which means that they live regularly with low oxygen levels, so they are used to feeling somewhat lethargic or easily short of breath, she said.

Related: 11 surprising facts about the respiratory system

in the New York Times Op-Ed on the phenomenon, Levitan wrote that the lack of gasping could be due to a particular phase of the pulmonary insufficiency caused by COVID-19. When lung failure begins, he writes, the virus can attack lung cells that make surfactant, a fatty substance in the alveoli, which reduces surface tension in the lungs, thereby increasing their compliance. Without a surfactant, increased surface tension will cause the air cells to deflate, but if they’re not filled with fluid, they won’t feel stiff, writes Levitan. This could explain how the alveoli fail to oxygenate the blood without the patient noticing the need to gasp for more air.

The virus could also create hypoxia by damaging blood vessels that lead to the lungs, said Moss. Normally, when a patient has pneumonia, the tiny blood vessels around the fluid-filled areas of the lungs constrict (called hypoxic vasoconstriction): detecting a lack of oxygen in the damaged areas, the body transfers blood to other healthier parts of the lungs. Because pneumonia fills the lungs with fluid, the person will feel hungry for air and short of breath to breathe. But their vessels send blood to the least damaged parts of the lung, so their blood oxygenation remains relatively high, given the damage.

In COVID-19, this balance can be turned off. The lungs are not very full of fluid and rigid, but the blood vessels do not constrict and do not redirect the blood to the least damaged areas. People feel free to breathe in and out without resistance, but the blood is still trying to capture oxygen from damaged and ineffective alveoli.

“What most likely happens here is that the hypoxic vasoconstriction is lost for some reason, so the blood flows to places where there is damage to the lungs,” said Moss. It could also be a combination of factors, he added.

“I’m not going to say that the alveoli are normal and that the surfactant is normal, but when someone has disproportionate hypoxia compared to what you would see in the lung, it makes pulmonologists think that there is a problem on the side of the blood vessels, “he said.

In the New York Times, Levitan suggests that patients who are not sick enough to be admitted to the hospital should receive pulse oximeters, devices that attach to the finger to measure blood oxygenation. If their oxygen count begins to drop, it could be a harbinger of seeking medical treatment.

“It’s an intriguing possibility,” said Moss.

Even without widespread oxygen monitoring at home, doctors are now starting to differentiate between patients who have low oxygen levels and who work hard to breathe, and those who have low oxygen levels but who breathe. without distress, said Chichra. At the beginning of pandemic, knowing that COVID-19 patients can start to fail quickly, doctors have tended to quickly put people with hypoxia on ventilation. Now, said Chichra, it becomes clear that patients who don’t struggle to breathe often recover without being intubated. They can do well with oxygen delivered through the nasal tube or a mask without a recycler, which fits over the face to provide high concentrations of oxygen.

Hypoxic patients who breathe quickly and laboriously, with a high heart rate, tend to be those who require mechanical ventilation or non-invasive positive pressure ventilation, said Chichra. The latter is a method that uses a face mask instead of a tube in the throat, but also uses pressure to push air into the lungs.

“The main difference we have seen between these people is that the people who work hard to breathe are the ones who usually need to be intubated,” said Chichra.

Originally posted on Live Science.


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