It is common to hear from health professionals that a patient arrived talking on a cell phone when attending Covid-19 even though low oxygen saturation was soon afterwards. This symptom, which has become classic and representative of the disease, has been called hypoxemia or silent hypoxia, which is not a medical technical term, but which appeared on a daily basis.
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In these cases, the patient does not complain about anything, does not have an increased respiratory rate, on physical examination he does not show any great signs that there is something wrong. “However, when assessing oxygen saturation, when measuring the amount of oxygen in the peripheral blood, in the capillaries, the saturation is very low”, says Maria Vera Castellano, coordinator of the Scientific Committee for Advanced Pulmonary Disease of the Brazilian Society of Pulmonology and Phthisiology.
Although there is no consensual definition for this condition by the scientific community, it is known, says CHC-UFPR pulmonologist Josiane Marchioro, that this presentation, which is unusual and probably happens with other respiratory viruses, has become more evident due to the magnitude of the pandemic we are experiencing.
And cases like this are worrying, especially if the saturation is very low. “Then you have to act quickly, you have to supplement oxygen, enter with non-invasive or invasive ventilation to correct it, because the low amount of oxygen will impair the functioning of the heart, kidneys and the body in general,” says Maria Vera.
There are some possible explanations for silent hypoxemia that are still being studied. “One of them is that the virus desensitizes receptors – which are in the vessels (as in the carotid arteries) – responsible for detecting the low level of oxygen in the blood and for communicating this to the brain. Without this communication, there is no command for us to breathe faster, that is, to increase the respiratory rate, making the central nervous system not recognize hypoxemia and not generate the perception of dyspnea ”, says Josiane. “The virus would interfere with these alert mechanisms responsible for increasing the respiratory rate and the recruitment of respiratory muscles for greater efforts”, adds Maria Vera.
Another hypothesis focuses on a mechanism common to other causes of respiratory failure, which does not occur frequently in Covid-19: the accumulation of CO2 in the blood, which makes it more acidic, and which is the greatest mechanism to stimulate the increase in the speed of breathing.
The SBPT coordinator cites yet another hypothesis, that, as Covid-19 courses with thrombus formation in the circulation in general – and even in the pulmonary circulation – that this can cause a ventilation / perfusion disturbance, causing there to be areas poorly perfused ventilators, this being another possible cause of oxygen drop.
In addition to these probable causes, there is the perception of shortness of breath, which is quite subjective and varies greatly from patient to patient. “This perception is reduced especially in the elderly and diabetics, who are at risk for developing severe Covid-19,” says Josiane. “This group is hypoperceptible to dyspnea, usually it takes time to show signs and to mention dyspnea, so the association of the probable actions of the viruses with this characteristic of this risk group makes them end up suffering the effects of Covid-19 in a worse way, explaining how this silent hypoxemia would occur, ”says Maria Vera.
The observation of repeated cases of severe hypoxemia not accompanied by shortness of breath (dyspnea) at the clinic was important, according to Josiane, as it led doctors to pay more attention to patients initially classified as mild cases of the disease.
How ‘normal’ hypoxia occurs
When oxygen in the body decreases, among the signs that doctors perceive is the patient’s great discomfort. The control of this supply is made by the ventilatory drive, related to the functioning of the central nervous system.
“There are, in an area in the brain, the bulb and the bridge, where this control is located, in addition to peripheral and central chemoreceptors that respond to variations in the concentration of oxygen carbon dioxide and that emit signals to the organism, causing to increase the respiratory rate and the intensity of the effort of the respiratory muscles ”, says Maria Vera Castellano, coordinator of the Scientific Committee on Advanced Pulmonary Disease of the Brazilian Society of Pulmonology and Tisiology (SBPT).
“This is how the doctor perceives hypoxemia normally, due to the high respiratory rate and the different use of respiratory muscles, with the need for an extra respiratory effort, due to thoracoabdominal dyssynchrony, the breathing that normally happens in a calm way it will happen in an accelerated way and with a lot of use of accessory muscles, which we call circulation. So these are signs that the doctor perceives and that in these cases of silent hypoxia do not appear, ”she says.
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