Silent hypoxia and its role in COVID-19 detection
What is silent hypoxia and how can it occur?
Silent hypoxia is when a pulse oximetry check on a patient who does not appear to be short of breath, results in an oximetry finding lower than a physician would expect.
This occurs in a number of situations, but most recently it has made headlines because of the large number of incidences of this occurring in individuals diagnosed with COVID-19.
What is meant by the term ‘silent’ in silent hypoxia?
The term silent comes from the fact that the patient does not appear to be short of breath. They are not gasping, they do not have an increase in their respiratory rate, and they are not complaining of feeling air hunger or looking uncomfortable.
Most individuals, when their oxygen levels start to drop into the eighties or lower, will feel air hunger and start to breathe more rapidly and feel uncomfortable.
Breathing tends to be a very natural, easy thing for us to do. But when the oxygen level drops to that level, most individuals will have a sense of what is called dyspnea, or shortness of breath.
The reasons we become short of breath can include both the low oxygen level, as well as a high carbon dioxide level.
There are some physicians saying that the reason people are not appearing short of breath is that the carbon dioxide level may still be normal in these individuals. Therefore, it is not signaling any distress at that time.
Why is hypoxia thought to occur in some COVID-19 patients? Why is it occurring as silent hypoxia?
As with a lot of things regarding COVID-19, we are going to rely on more and more data and studies that show how many individuals develop silent hypoxia. There are probably many COVID-19 patients with hypoxemia who get missed because they are admitted and diagnosed for other reasons such as fever, cough, or other things of that nature.
I think we are going to need to look at a larger group of individuals to figure out why exactly the virus causes silent hypoxia, compared to other viruses like influenza, where it is not seen as often.
There is something different about this virus in the way that it affects the blood vessels and the airways. There are even some ideas that the virus is affecting the nervous system and affecting the actual mechanisms in our brain that help to regulate respiration.
The mechanism of why oxygen levels drop has to do with how well the blood flow through the lungs matches the airflow through the lungs. For some reason, which we cannot identify yet, in certain patients the virus affects both the blood vessels in the lungs, as well as air sacs in the lungs, mismatching the flow of blood and air, causing the oxygen levels to drop.
The reason that individuals are not feeling as short of breath or looking to be short of breath is still a bit of a mystery. As with a lot of things to do with COVID-19, this has not been determined yet.
Perhaps the virus is affecting the blood vessels lack of ability to contract and constrict the way they are supposed to, or perhaps there is actually some central effect on the brain where the respiratory effort is not picked up because the virus is affecting the brain's ability to recognize the hypoxia.
It may be that the oxygen level is dropping without changing much in the way of the lung's mechanics, meaning that the lung is not becoming stiffer and the patient is not feeling any distress when they are breathing in and out, because there is no increase in breathing work.
The lungs may not be sensing that the oxygen level is dropping, but the concern is that the other organs in the body are not getting as much oxygen as they need.
There are a lot of theories right now as to why this virus is acting differently, but answers are going to take time. We cannot identify how many people or which individuals go on to develop silent hypoxia at the moment.