According to a study by Brazilians, the main reason for the worsening of the disease is damage to small pulmonary vessels
Thromboses in small vessels (capillaries) in the lungs are one of the first consequences of a severe Covid-19 illness, even before breathing difficulties caused by so-called diffuse alveolar damage. This was confirmed by a Brazilian study published in October 2023 Journal of Applied Physiology. During the autopsy of 9 patients who died after the development of the severe form of the disease, a very characteristic picture with changes in pulmonary vascularization and thrombosis could be observed.
In the work, the researchers describe for the first time aspects related to endothelial damage and the formation of thrombi caused by the infection. The findings – such as the observation that in Covid-19 the focus is on the formation of thrombi in the pulmonary microcirculation – contribute to understanding the pathophysiology of the disease and to the development of new therapeutic strategies.
“This study was definitive proof of what we had been warning about since the start of the pandemic: severe Covid-19 is a thrombotic disease. The SARS-CoV-2 virus has a tropism for the endothelium [é atraído para esse tecido], the layer of cells that lines blood vessels. Therefore, when it penetrates endothelial cells, it first affects the microcirculation. The problem starts in the capillaries of the lungs and then clumps up the larger vessels and can potentially reach every other organ.” says pulmonologist Elnara Negri, professor at the Faculty of Medicine of the University of São Paulo (FM-USP), co-author of the article and one of the first researchers in the world to work with this hypothesis.
In the study supported by FAPESP, the authors used electron microscopy to observe the effect of the virus on the lung endothelial cells of patients who died of severe Covid-19 at FM-USP’s Hospital das Clínicas in São Paulo.
Through a minimally invasive autopsy, the high prevalence of thrombotic microangiopathy – a generalized occlusion of the microcirculation due to thrombosis – was observed in all samples. The samples analyzed come from patients who were hospitalized from March to May 2020.
All patients were intubated, required intensive care, and died of refractory hypoxia (respiratory failure). No patient included in the study was treated with anticoagulants as this was not the guideline for the treatment of Covid-19 at the time. Vaccines were also not yet available at that time.
Negri states that the endothelium is lined by a glycoprotein layer called the glycocalyx, which allows blood to flow naturally through arteries, veins and capillaries without clotting.
“Some previous studies by Helena Nader at Unifesp showed that the virus mainly binds to the ACE-2 receptor to enter the cell [proteína encontrada na superfície de diversas células do corpo, inclusive nas do epitélio e endotélio do sistema respiratório]. However, it first binds to heparan sulfate [um polissacarídeo] associated with the membrane of endothelial cells, forming the glycocalyx. Therefore, when SARS-CoV-2 penetrates the endothelium, it detaches it and destroys the glycocalyx. This results in tissue exposure and intravascular coagulation starting in the microcirculation.” explained Negri.
Because the virus initially affects the microcirculation of the lungs, the problem was never detected early in contrast tests to examine the presence of thrombi in larger vessels, which were then carried out in patients with severe Covid-19.
However, Negri states that endothelial dysfunction is a key phenomenon in Covid-19 because it is directly linked to the activation of the inflammatory response characteristic of the disease: “The massive invasion of the virus and the destruction of the endothelium favor the rupture of the endothelial barrier and the recruitment of circulating immune cells, thereby activating pathways associated with thrombogenesis and inflammation.”.
In their work, the researchers found that endothelium damage tends to precede two typical features of respiratory distress:
significant leakage of the alveolar-capillary membrane of the lungs;
Accumulation of fibrin polymers (protein associated with clotting and the healing process) in the pulmonary alveoli.
Work from the same FM-USP group led by Thais Mauad, including analysis of the transcriptome (set of RNA molecules expressed in a tissue), showed that several pathways associated with coagulation, platelet activation and thrombus formation were active early in the Lungs were activated from patients with alveolar damage, previous inflammation.
The analysis also confirmed that this is not an ordinary clotting event triggered by the activation of clotting factors. “In Covid-19, clotting occurs due to endothelial damage and is exacerbated by grandchildren’s disease [mecanismo imune que consiste na saída do material genético contido no núcleo dos neutrófilos em forma de redes –as NETs– na tentativa de prender e matar o patógeno] and by damage to red blood cells with dimorphism [uma alteração morfológica das células vermelhas do sangue] and platelet activation. Therefore, there is a whole structure that causes the blood to become denser and causes so many complications.” He specifies.
The researcher also says that in this scenario, where the blood becomes very viscous and highly thrombogenic, the patient must remain hydrated, unlike what is recommended in the treatment of diffuse alveolar damage: “In addition, timing and strict control of anticoagulation are essential”.
It is no coincidence that another study by the same group, involving researchers Marisa Dolhnikoff and Elia Caldini, showed that lung damage in severe cases of Covid-19 is linked to the degree of illness. By analyzing lung autopsy tissue samples, scientists found that the higher the NET levels, the greater the patients’ lung damage.
Negri says he began to suspect the connection between Covid-19 and thrombosis at the beginning of the pandemic, when he noticed a condition very similar to that of heart patients he had treated more than 30 years ago. After cardiac surgery, these patients also developed vascular microcoagulation. But this happened because they were undergoing a treatment called extracorporeal circulation with bladder oxygenators – a device that is no longer used in medicine precisely because it causes endothelial damage.
“It was a technique that was widely used 30 years ago, but it causes lung damage very similar to that of Covid-19.” So I had already seen that. In addition to the lung damage, another similarity between the two cases is the occurrence of peripheral thrombotic phenomena, such as purple toe.” He specifies.
“Because in severe Covid-19 disease the initial drop in blood oxygenation is secondary to pulmonary capillary thrombosis and there is no initial accumulation of fluid in the lungs, the organ does not become “soaked” or lose its complacency. This means that the lungs of a patient with early severe Covid-19 do not look like a sponge full of fluid, as is the case in patients with respiratory distress syndrome. On the contrary: when respiratory insufficiency begins in connection with severe Covid-19 disease, the lungs are dehydrated and cannot enter the circulation due to the formation of blood clots, even though the air reaches the alveoli.” Explain.
Negri says that in these cases, the patient can easily fill the lungs with air, but oxygen cannot get into the blood because the capillaries are blocked. “That explains the call happy hipoxia [hipóxia feliz] This means that the patient does not feel that his oxygen supply is low and he does not suffer from shortness of breath.” it says.
When the researcher witnessed the intubation of a patient with severe Covid-19 disease, he realized that a treatment proposal was necessary that was completely different from what was made at the beginning of the pandemic.
“The secret to treating patients with severe Covid-19 disease is to keep them hydrated and use anticoagulants at the right dose. In a hospital environment, the appropriate anticoagulant level must be checked as soon as desaturation begins, that is, the oxygen level in the blood is low.” After that, it is necessary to check the therapeutic anticoagulation levels through blood tests daily, always in a hospital environment, to reduce the risk of bleeding, and to carry out prophylaxis for an average of four to six weeks after discharge, which corresponds to the period of reconstruction of the endothelium itself.”
The researcher explains that this protocol occurs with hydration and the use of anticoagulants because, unlike other acute respiratory syndromes (SARS), in which the problem of lack of oxygen passing from the lungs into the blood is mainly related to inflammation of the alveoli , In early severe Covid-19 disease, endothelial damage to the pulmonary capillaries predominates.
“It is precisely this difference between Covid-19 and other severe acute respiratory syndromes that was not known at the beginning of the pandemic. This is why so many patients died in intensive care units. [unidades de terapia intensiva] from Italy for example. The treatment protocol used back then was different.” He specifies.
Before publishing the work in Journal of Applied Physiology Already in 2020, Negri’s group had observed that the use of heparin (an anticoagulant) improved the oxygen supply of critically ill patients. The following year, in collaboration with colleagues from several countries, they conducted a randomized clinical trial in which they demonstrated that treatment with heparin reduced mortality in severe cases of Covid-19. The results were published in British Medical Journal.
“The study helped change treatment guidelines for Covid-19 around the world as we demonstrated a 78% reduction in the risk of mortality when anticoagulation was started in patients who required oxygen support but were not yet on it were in the intensive care unit.” says the researcher.
Negri states that in severe Covid-19 disease there is a rush to reverse endothelial dysfunction through the use of anticoagulants: “It is necessary to treat coagulation as quickly as possible, as this is crucial to prevent the development of acute respiratory syndrome and other sequelae of the disease, as is the case with the so-called long Covid.”.
Another study was recently published in Natural medicine by British scientists confirms the thrombus-forming nature of SARS-CoV-2. The work identified the only prognostic markers for Long Covid to be fibrinogen and D-dimer – two proteins associated with coagulation.
“The study shows that long COVID results from thrombosis that was not adequately treated. The microcirculation problem can persist in multiple organs, including the brain, heart and muscles, as if the patient had suffered a small heart attack.” explained Negri.
With information from Agência Fapesp.