In December 2019, a new coronavirus, known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged in Wuhan, China and spread throughout the world.

In COVID-19, pulmonary edema has been described; however, the dominant paradigm is focused on cytokine storm as responsible for lung injury and subsequent acute respiratory distress syndrome (ARDS). Not everyone agreed with this paradigm. Sinha et al. challenged the role of this cytokine storm given that median interleukin-6 (IL-6) levels in non-COVID patients ARDS were up to 200 times higher than in patients with severe COVID-19. Gattinoni et al. maintained that COVID-19 presented as an "atypical form" of ARDS. On the other hand, Kuba et al. and Imai et al. reported that angiotensin-converting enzyme 2 (ACE2) levels during a SARS-CoV infection are decreased. Furthermore, in patients with COVID-19, plasma levels of Angiotensin II are higher than in healthy population and stimulate an upregulation of aldosterone level, triggering sodium and water retention. SARS-CoV-2 enters through ACE2 and downregulates ACE2 expression so that this enzyme is unable to exert its protective effects. The dysregulated activity of the renin angiotensin aldosterone system (RAAS) is partly responsible for pulmonary edema in COVID-19. ACE2 is known for its effect as the main counter-regulatory mechanism for the renin-angiotensin aldosterone system (RAAS), which is an essential player in blood pressure control by retaining sodium and water and increasing the intravascular fluid volume. SARS-CoV-2 binds ACE2 and accelerates the degradation of ACE2, and thus decreases the counteraction of ACE2 on RAAS. The final effect is increasing reabsorption of sodium and water and subsequent volume overload. The RAAS can be envisioned as a dual function system in which the vasoconstrictor/proliferative or vasodilator/antiproliferative actions are primarily driven by the ACE-ACE2 balance. According to that, an increased ACE/ACE2 activity ratio generated by the downregulation action of SARS-CoV-2 on ACE2 will lead to increased Angiotensin II and increased catabolism of Angiotensin 1-7, towards vasoconstriction, endothelial dysfunction, prothrombosis, proinflammatory, and antinatriuretic effect.

Acute pulmonary edema is caused mostly by one of the following mechanisms: pulmonary venous pressure elevation-volume overload-or augmentation of the alveolar capillary membrane permeability-inflammation. In fact, both mechanisms sometimes coexist and the distinction is irrelevant. There are bibliographic references of pulmonary edema in COVID-19, as well as evidence of volume overload in COVID-19: Lang et al. describes frequent and pronounced vasculature in affected lung areas that may be suggestive of disordered vasoregulation. Eslami et al. observed increased cardiothoracic ratio and it is also described as right ventricular dilatation. In this setting, a different approach emerged: moderate or severe COVID-19 could experience a severe acute pulmonary edema with a "dual hit". A "first hit" of pneumonitis-augmentation of the alveolar capillary membrane permeability-can lead to low hydrostatic pressure pulmonary edema. The "second hit" is high pressure pulmonary edema, caused by increase of hydrostatic pressure secondary to volume overload, a result of dysregulation of the RAAS. This results in a "dual hit" that triggers severe acute pulmonary edema. If this edema is not solved, then a "third hit" appears with secondary inflammation, superinfection, fibrosis and finally the typical ARDS. Consequently, the study group looked for patients with pulmonary edema before ARDS was triggered. Cases of moderate and severe COVID-19 were searched, with tomographic evidence of pulmonary edema: dilated superior vena cava, large pulmonary arteries, diffuse interstitial infiltrates and dilated right ventricle. At the detection of pulmonary edema in patients with COVID-19, a standard treatment consisting of oral hydric restriction and diuretics (NEGBAL approach) was initiated. The effects of furosemide on pulmonary edema were well established decades ago. To date there is no evidence that had suggested the model of pulmonary edema and volume overload secondary to the dysregulation of the renin angiotensin aldosterone system in COVID-19. The objective of this study is to compare patients with COVID-19 undergoing NEGBAL approach with a control group of patients with COVID-19 of similar demographic, clinical, gasometric and tomographic characteristics. Medical records of 120 adult patients were reviewed: demographic; clinical, laboratory; Pro b-type natriuretic peptide (pro-BNP): (negative: below 125 pg/mL); high-sensitivity cardiac troponin (hs-cTnT): (negative: \<14 ng/L); blood gas; chest tomography (CT); oxygen therapy support and mechanical ventilation (MV) requirements, all of which were reviewed and recorded by investigators. With the purpose of knowing the patient's basal hematocrit before COVID-19, prior hematocrit, if any, defined as hematocrit previous to COVID-19 infection (hematocrit prior to admission to NEGBAL) was also reviewed.

Once the NEGBAL strategy began, the clinical, tomographic, gasometric, biochemical evolution was evaluated until the 8th day (until the 12th day for tomography), ICU stay, hospital stay and morbidity and mortality until the 30th day.