The Role of Alveolar Edema in COVID-19

The coronavirus disease 2019 (COVID-19) has spread over the world for more than one year. COVID-19 often develops life-threatening hypoxemia. Endothelial injury caused by the viral infection leads to intravascular coagulation and ventilation-perfusion mismatch. However, besides above pathogenic mechanisms, the role of alveolar edema in the disease progression has not been discussed comprehensively. Since the exudation of pulmonary edema fluid was extremely serious in COVID-19 patients, we bring out a hypothesis that severity of alveolar edema may determine the size of poorly-ventilated area and the blood oxygen content. Treatments to pulmonary edema (conservative fluid management, exogenous surfactant replacements and ethanol–oxygen vapor therapy hypothetically) may be greatly helpful for reducing the occurrences of severe cases. Given that late mechanical ventilation may cause mucus (edema fluid) to be blown deep into the small airways, oxygen therapy should be given at the early stages. The optimal time and blood oxygen saturation (SpO2) threshold for oxygen therapy are also discussed.


Introduction
Since the outbreak of novel SARS-like coronavirus (SARS-CoV-2), over 181,521,067 confirmed cases and 3,937,437 confirmed deaths have been reported globally by the World Health Organization (WHO) as of 1 July 2021. Acute respiratory distress syndrome (ARDS) and the serious complications (mainly multiple organ failure) are the most frequent causes of death [1][2][3][4]. However, the main cause of death was not direct ARDS, but the multiple organ failure, such as RNAemia, acute cardiac injury, acute renal injury and septic shock [1][2][3][4]. ARDS is a type of respiratory failure whose definition is based on a ratio of arterial oxygen tension to fractional inspired oxygen (P a O 2 :F i O 2 ) of less than 300 mm Hg despite a positive end-expiratory pressure of more than 5 cm H 2 O [5,6], which means that hypoxemia is a necessary condition for the patient to have an ARDS. A longer-time hypoxemia may result in cardiac ischemia, renal ischemia or RNAemia/septic shock, indicating a multiple organ ischemia [1][2][3][4]. On the other hand, some features of COVID-19 pneumonia distinguish it from typical ARDS. The patients at early stages often display little breathlessness, despite profound hypoxemia, a symptom referred to as "happy hypoxemia". A report showed dyspnea in only 18.7% of 1099 hospitalized COVID-19 patients, despite their low ratios of partial pressure of arterial oxygen to percentage of inspired oxygen; in contrast, as high as 86% of the patients showed abnormal computerized tomography scans [7]. Although the profound hypoxemia was associated with a large intrapulmonary shunt (also different from typical ARDS), alveolar cells are well preserved in COVID-19 relative to typical ARDS [8]. also proved to be coordinated with nuclear division via the cellular process named mitotic fission. Increased mitotic fission in HPV, driven by enhanced fission but repressed fusion, accelerates the cell cycle and increases apoptosis resistance [20][21][22][23][24]. Thus, Warburg metabolisms sustain energy homeostasis through inhibiting oxidative metabolism, which may reduce mitochondrial apoptosis, and lead to uncontrolled cell division. Mitochondrial dynamic and metabolic disorders combine to show the hyper-proliferative and apoptosis-resistant phenotypes in pulmonary artery smooth muscle cells [20][21][22][23][24].
The renin-angiotensin-aldosterone system (RAAS) also plays a key role in endothelial injuries. RAAS is a key regulatory system of electrolyte homeostasis and pulmonary artery status and functions through angiotensin-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II type 1 (AT1) receptor axis and angiotensin-converting enzyme 2 (ACE2)/ angiotensin-1-7 Ang-(1-7)/MAS receptor axis. RAAS dysfunction has been proved to be related to the occurrence and development of acute pulmonary injuries and ARDS and may cause a serious prognosis and even death [25][26][27][28][29]. A large number of studies suggest that ACE2 works as the cellular receptor for SARS-CoV-2 entry [30][31][32]. Similar to other families of Coronaviridae viruses, the protein interactions between the virus and the membrane-bound ACE2 requires the cleavage of the spike glycoprotein S into two subunits. After the binding of the S1 receptor-binding domain (RBD) to ACE2, the transmembrane protease/serine subfamily 2 (TMPRSS2) cleaves the S2 subunit and facilitates membrane fusion [30][31][32]. It has been confirmed that SARS-CoV-2 cell entry results in decline of the ACE2 protein, which converts Ang II into Ang-(1-7). Ang II induces proinflammatory cascades when binding to the AT1 receptor, while Ang-(1-7) generates antiinflammatory effects through the interaction with the MAS receptor. Thus, down-regulation of ACE2 would result in pulmonary injury and vasoconstriction [25][26][27][28][29]33]. Besides these mechanisms, relative ACE2 deficiency may also lead to enhanced and protracted tissues, and vessel exposure to Ang II, which then enhances thrombosis, cell proliferation and recruitment, increases tissue permeability, cytokine production and results in inflammation (Table 1) [33,34]. Table 1. Pulmonary pathological changes in COVID-19.

Injuries
Pathogenic Mechanisms Refs.

Alveolar endothelial injury
Endothelial barrier disruption induces intrapulmonary shunt, hypoxia, intravascular coagulation and the release of pro-inflammatory factors. [1,5,35] ACE2-decline-induced pulmonary injury ACE2 deficiency leads to enhanced and protracted tissues, and vessel exposure to Ang II, which then enhances thrombosis and cell proliferation, increases tissue permeability, cytokine production and inflammation.

Ventilation-Perfusion Mismatch and Intravascular Coagulation in COVID-19
HPV is thought to optimize gas exchange through shunting blood from poorly ventilated areas to those rich in oxygen. However, a general pulmonary vasoconstriction happens in both normoxic lung competencies and hypoxic lung competencies, which may lead to the pulmonary hypertension (PH) and a risk of right-heart failure subsequently, just as observed in patients with acute altitude sickness [19,35,36]. HPV has been observed in COVID-19 patients [37,38]. Moreover, the pulmonary hypertension and the subsequent right ventricular dysfunction also have been confirmed in severe COVID-19 patients (Table 1) [39,40]. Significant alternations in monocyte size, lymphocyte stiffness, neutrophil size and deformability, and heterogeneity of erythrocyte size and deformation in COVID-19 patients also indicated great changes in pulmonary blood stream dynamics [41].
Nevertheless, drugs to inhibit HPV, such as acetazolamide, calcium channel blockers and phosphodiesterase-5 inhibitors, should be avoided for COVID-19 patients, given that the consequences of COVID-19 may be exacerbated by loss of HPV, whether because of the destructive effects of the virus on mitochondria or the ability of endotoxin and inflammatory stimuli to eliminate HPV [38]. HPV attenuating drugs may exacerbate hypoxemia in COVID-19 pneumonia [38].
In some cases, dysregulated HPV may also cause mismatched blood flow and alveolar ventilation, which may restore the oxygen supply to the poorly-ventilated alveolar cells and result in life-threatening hypoxemia [35]. Recently, Herrmann et al. [42] modeled lung perfusion abnormalities and suggested that early COVID-19 hypoxemia may be mainly attributed to severe ventilation-perfusion mismatch. Their model predicted that calculated shunt fractions in excess of three times the injured fractions, which could be explained by (a) extensive perfusion defect, (b) perfusion defect combined with ventilationperfusion mismatching in the non-injured fraction, or (c) hyper-perfusion of the small injured regions, with up to 3-fold increases in regional perfusion to the afflicted fraction [42]. Clinical features of COVID-19 pneumonia also confirmed the malfunction of oxygensensing responses including the ventilation-perfusion mismatch (Table 1) [43].
Pulmonary intravascular coagulation also plays an important role in the disease progression of COVID-19. After alveolar injuries, resident alveolar macrophages are activated, causing the release of potent proinflammatory mediators and chemokines that promote the accumulation of neutrophils and monocytes, such as vascular endothelial growth factor (VEGF), angiotensin II (Ang II), glycosaminoglycans (GAGs), von Willebrand factor (vWF) and soluble intercellular adhesion molecule (sICAM-1) [5]. Activated neutrophils further contribute to injuries through releasing toxic mediators. On the other hand, intravascular coagulation leads to platelet aggregation and micro-thrombi formation, which may aggravate the pulmonary injuries [5]. Intravascular coagulation primarily leads to increased dead space (increased wasted ventilation and less efficient carbon dioxide removal). Whereas inflammatory mediators from endothelial injury may worsen hypoxemia through exacerbate ventilation-perfusion mismatching (Table 1) [35,44]. Coagulation and thrombosis have been identified clinically as prominent symptoms of COVID-19 [45,46].
Nevertheless, neither intravascular coagulation nor ventilation-perfusion mismatch could be easily corrected. Oxygen supply is a common treatment to COVID-19 patients. Increasing inspired oxygen results in enhanced oxygenation but however does not improve the ratio of arterial oxygen tension to fractional inspired oxygen (P a O 2 :F i O 2 ) [42]. Inhaled nitric oxide decreases total pulmonary vascular resistance [47,48] but however may increase blood flow to these low ventilation-perfusion regions, which causes further arterial desaturation (Table 2) [35]. Table 2. Putative drugs and treatments to SARS-CoV-2 pneumonia.

Drugs or Treatments
Therapeutic Mechanisms Therapeutic Effects Refs.
Oxygen inhalation Oxygenation enhancement Alleviate hypoxia; however, high-flow oxygen lead to pulmonary toxic effects. [35] Mechanical ventilation Oxygenation enhancement Alleviate hypoxia; however, mechanical ventilation may cause mucus to be blown deep into the small airways, which then aggravates alveolar hypoxia. [9] ECMO Oxygenation enhancement Alleviate hypoxia; however, patients receiving ECMO still showed a high mortality rate. [9] Inhaled nitric oxide Decrease pulmonary vascular resistance Improve ventilation-perfusion ratio; however, may cause further arterial desaturation (hypoxia  [76,77] Aspirin, the most commonly used anti-platelet agent, is a cyclooxygenase-1 inhibitor and considered as a mild to moderate inhibitor of platelet function [49,50]. Hereby, aspirin has been suggested for COVID-19 patients [49,50]. Aspirin administration had an association with less mechanical ventilation and reduced ICU cases, but showed no apparent association with the fatality rate (Table 2) [51]. The therapeutic effects of low molecular weight heparin have also been investigated comprehensively [52][53][54][55][56][57][58]. Heparin may reduce the risk of in-hospital mortality and decrease the occurrence of severe cases. However, critically-ill COVID-19 patients still had high incidences of venous thromboembolism and worse outcomes, despite the heparin administration at the prophylactic dosage [52][53][54][55][56][57][58]. Moreover, besides its anticoagulation effect, heparin relieves the symptoms through complex mechanisms. SARS-CoV rolls onto the cell membrane by binding to cell-surface heparan sulfate proteoglycans (HSPGs) and scans for the specific entry receptor ACE2 [59,60]. In addition, heparin may enhance the open conformation of the subsequent ACE2 binding. Thus, heparin potently blocks both viral adhesion and spike protein binding with the host cell plasma membrane [59,60]. On the other hand, an heparin-binding sequence immediately upstream of the S1/S2 cleavage site has been found on SARS-CoV-2 S protein, indicating that heparin may promote the S1/S2 cleavage, induce exposure of the optimal epitope, and therefore accelerate the virus clearance [61]. These assumptions have been proved by a serological study that adding 10 µM heparins into the sera from COVID-19 patients led to a four-fold increase in antibody titers [62].

Alveolar Edema in COVID-19
Besides above pathogenic mechanisms, alveolar edema also plays a key role in the disease progression. Diffuse alveolar damage (DAD) is the histopathological pattern commonly described in COVID-19 (Table 1) [63][64][65]. Endothelial barrier disruption induces interstitial flooding via activation of the actin-myosin contractile apparatus [5]. Then alveolar edema leads to hypoxia at the injured alveolar units [5]. Hypoxia in turn inhibits edema fluid clearance, due in part to the disassembly of the keratin intermediate filament network, a fundamental element of the cellular cytoskeleton, therefore destructing the epithelial barrier [66]. Therefore, a long-term hypoxia aggravates the disease by inducing more alveolar edema, which forms a vicious circle (Table 1).
Impaired alveolar surfactant production may be another molecular mechanism of alveolar edema in COVID-19 in that the limitation of alveolus superficial active substance would increase alveolar surface tension and hamper alveolar fluid resorption (Table 1) [67][68][69][70][71]. Thus, treatments to alveolar edema may help to both reduce the size of poorly ventilated area and increase the blood oxygen content (Figure 1). Although exogenous surfactant replacements in animal models of ARDS and neonatal respiratory distress syndrome (RDS) showed consistent improvement in gas exchange and survivals, some adult studies have shown only improved oxygenation but no survival benefits [70]. Moreover, a few exogenous pulmonary surfactants have been currently authorized to treat pulmonary permeability edema in COVID-19 patients [70]. Virus infections cause alveolar edema and the hypoxic pulmonary vasoconstriction (HPV), which is an essential protection mechanism of the lung that directs blood perfusion from badly-ventilated to wellventilated alveoli to optimize gas exchange. However, SARS-CoV-2 infections may cause mismatched blood flow and alveolar ventilation, and may result in life-threatening hypoxemia. Treatments to pulmonary edema might promote lung edema resorption and show good therapeutic effects. When the disease develops into late stages, systemic alveolar edema and severe ventilation-perfusion mismatch occurs, and the blood oxygen will decline sharply. Then the high-flow oxygen therapy will be required. However, high-flow oxygen inhalation may not correct ventilation-perfusion mismatch and cause several adverse effects. The blue to red gradient bar shows blood oxygen saturation (SpO 2 ).
Besides exogenous surfactant replacements, other treatments that counter-balance the inhibition of edema clearance during hypoxia or improve the lung's ability to clear alveolar edema should also be adopted. For example, a relatively high partial pressure of O 2 in the alveolar gas facilitates alveolar fluid resorption by activating Na + transport across the alveolar epithelium, which makes an osmotic gradient responsible for the lung edema clearance [66]. Hereby, appropriate oxygen inhalation would accelerate the resorption of pulmonary edema fluid. However, simple oxygen therapy would not achieve adequate effects on alveolar edema clearance. Ethanol/butanol-oxygen vapor therapy (oxygen inhalation with 20% ethanol or butanol as humidifying agent) is a common treatment to pulmonary edema with a long history of safety [72][73][74][75]. Ethanol/butanol vapor decreases the surface tension of the foam inside the pulmonary alveoli and therefore alleviating alveolar edema (Table 2) [72][73][74][75]. Moreover, conservative fluid management, in which diuretics may be administered and intravenous fluid administration is minimized, would reduce hydrostatic pressure and enhance serum oncotic pressure, and therefore limit the development of pulmonary edema potentially ( Table 2) [76][77][78]. Unfortunately, these common therapies to pulmonary edema have not attracted enough attention in COVID-19 clinical practice.
When the disease develops into late stages, systemic alveolar edema and severe ventilation-perfusion mismatch occur, and the blood oxygen will decline sharply. Then the high flow oxygen therapy and mechanical ventilation will be required ( Figure 1). However, high-flow oxygen inhalation inhibits HPV because of the reversed diffusion of oxygen, that is, if enough oxygen could bind the receptor in the small alveolar-capillary-arteriole space, the vessels will not vasoconstrict [83]. Upon a high-flow and high-concentration oxygen inhalation, the reflex stimulation to respiration by hypoxia will disappear, resulting in a more serious retention of CO 2 , which may lead to the hypercapnic encephalopathy (hypercapnic coma) or even a respiratory arrest [35]. Furthermore, high inspired oxygen concentrations could lead to pulmonary-specific toxic effects, such as denitrogenation phenomena, inhibition of surfactant production and severe acidosis, and therefore may worsen ventilation-perfusion mismatch or induce a degree of hypoventilation (Table 2) [84,85]. On the other hand, mechanical ventilation may cause mucus to be blown into the deep of the small airways, which then aggravates intrapulmonary shunt and alveolar hypoxia ( Table 2) [86]. These deleterious effects may be an important reason for the high mortality after high-flow oxygen inhalation and mechanical ventilation (The ICU mortality rate among those who required non-invasive ventilation was 79% and among those who required invasive mechanical ventilation was 86%.) [9]. Therefore, nebulized heparin treatments, ethanol-oxygen vapor therapy should be given at the early stages of COVID-19.

The Optimal Time and SpO 2 Threshold for Oxygen Therapy
In a retrospective case report that included 69 adults in Wuhan, China, 29% of patients showed dyspnea and 20% of cases (14 patients) showed oxygen saturation SpO 2 <90% [oxygen index (OI) < 110 mmHg] during admission [87]. In their report, as of 4 February 2020, 18 (26.9%) of 67 patients had been discharged, and five patients had died, with a mortality rate of 7.5%. Noticeably, all five deaths occurred in the SpO 2 <90% group [87]. The median time from onset of symptoms to admission was six days (inter quartile range 4-9 days). However in the SpO 2 <90% group, the median occurrence time of lowest SpO 2 was only one day (inter quartile range 0-2 days) after admission [87]. In other words, SpO 2 of some patients at admission were already very low, which may develop severe ARDS subsequently (Table 3). Therefore, it may be too late for the patients to take oxygen therapy after admission. The best window period of oxygen therapy may be the six days from onset of symptoms to admission.
Dai et al. [88] classified COVID-19 patients into four stages according to the CT (Computed tomography, which uses X-rays to produce cross-sectional images) performances. Stage I: one or more lesions, in irregularly patchy or round shapes, generally showing ground-glass opacity with vascular enlargements. Stage II: more area lesions, found in bilateral lobes mainly at the sub-pleural areas, in irregularly patchy, round or reverse-butterfly shapes, diffused or scattered patches occasionally fusing into a large patch with a high density, vascular enlargements, reticular signs and bronchial wall thickening, sometimes with little fibrosis and atelectasis in sub-segments. Stage III: some lesions diminished or absorbed, the focus could be entirely absorbed, showing residual fibers. Stage IV: bilateral diffuse inhibitions, over half of the lung areas involved, occasionally extended to the entire lung and defined as the white lung, implying the systemic alveolar edema. However, the patients in the stage IV group showed only slightly-declined SpO 2 (94.70 ± 0.20%). While the stage II patients with only a small proportion of lung injury and HPV showed relatively high SpO 2 (97.2 ± 0.91%) [88]. SpO 2 <95% may indicate late infection stages (Table 3) [88].
Moreover, a recent study demonstrated a high correlation between decreased SpO 2 and severe cases that 78.0% (32/41) of the patients with SpO 2 ≤95% would develop into severe diseases (Table 3) [89]. The risk threshold of SpO 2 was 95%.
WHO and BMJ Best Practice suggested SpO 2 ≤90% or signs of severe respiratory distress, central cyanosis, shock, coma and/or convulsions as diagnostic standards for the severe pulmonary infections [90,91]. In addition, according to the novel coronavirus pneumonia diagnosis and treatment plan (trial version 7) published by the National Health Committee of China, either respiratory rate (RR) ≥30 times per minute, or resting state SpO 2 ≤93%, or OI ≤300 mmHg was defined as the severe condition, and then the oxygen therapy was given (Table 3) [92]. However, the median SpO 2 /respiratory rate value was significantly higher in COVID-19 patients than in non-COVID-19 patients, which implies that a normal breathing rate could mask profound hypoxia and make severity assessment in COVID-19 patients more difficult in out-of-hospital settings [93]. Besides, based on the above analysis, the diagnostic standard of SpO 2 either ≤90% or ≤93% may be too low to take the oxygen therapy in time. Table 3. Blood oxygen saturation (SpO 2 ) lower limit and upper limit for oxygen therapy.

SpO 2 Limits
Corresponding Disease Stage Refs.

90%
In the SpO 2 <90% group, the median occurrence time of lowest SpO 2 was only one day after admission, indicating a very late stage. [87] 94.7% Stage IV: bilateral diffuse inhibitions, over half of the lung areas involved, occasionally extended to the entire lung and defined as the white lung. [

Four Clinical Comments
Based on the above analysis, we propose four comments to prevent SARS-CoV-2 patients from developing into severe hypoxemia: (a) For the suspected cases with symptoms, finger SpO 2 (with finger oximeter ideally) should be measured at each time of nucleic acid test sampling and daily after symptom onset. However, finger SpO 2 varies greatly with the altitude and the age [35], and the finger oximeter itself may have a large deviation, so it is recommended that each oximeter should be calibrated with several healthy people of different ages to get the reference value. If the patient's SpO 2 was lower than the reference value by 3% or more (e.g., if the reference value was 98%, then ≤95% is the threshold for oxygen therapy), it is suggested that the patients were hospitalized immediately for standard low-flow oxygen inhalation possibly combined with 20% ethanol as humidifier. If immediate hospitalization was not possible, the patient was recommended to take oxygen in the home, such as with a portable oxygen respirator. During the in-home oxygen therapy, finger SpO 2 should be monitored continually to assure that SpO 2 has been restored to 96%, but not higher than that (Table 3). This is because saturation above this level likely causes an increased risk of death without plausible benefits [94,95]. This upper limit may be lower for the patients with chronic respiratory diseases. For instance, the oxygen treatment goal should be 88-92% for patients with chronic type II respiratory failure (Table 3) [92]. Nevertheless, if SpO 2 cannot be enhanced afterwards, the patient should seek medical advice or go to the hospital in time. The in-home oxygen therapy may be of great significance for countries with a shortage of medical resources. The COVID-19 patients usually require oxygen long-term oxygen supplies. However, if humidification with 20% ethanol was adopted, long-time ethanol vapor inhalation may generate adverse effects to the respiratory system and the nervous system [96,97]. The optimal length of ethanol-oxygen vapor therapy needs to be investigated in clinical trials. (b) For the patients with very low SpO 2 , high-flow oxygen inhalation should be applied.
Nevertheless, humidification with 20% ethanol might be also recommended on this occasion. (c) Fluid management might be considered for all COVID-19 patients and conservative fluid management might be applied to severe cases. Some patients may be dehydrated with evolving acute kidney injury at hospital presentation for COVID-19 pneumonia. Therefore, conservative fluid management to these patients should be applied with caution. Detailed guidance of fluid administration in patients with COVID-19 has been discussed elsewhere [98]. For the in-home patients, appropriate reduction in water intake might be an expedient measure. (d) The prone position could reduce the risk of ventilation-associated lung injuries by the combined effects of more uniform distribution of breathing and less compression of the left lower pulmonary lobe by the heart [5,[99][100][101]. Therefore, patients with low SpO 2 are advised to use prone position as much as possible. In addition, the patients should avoid any vigorous activity that may increase respiratory rate and tidal volume because pulmonary injury will be worsened by the mechanical stretch during the strained breathing [5,[99][100][101]. The benefit of prone ventilation is larger than that for typical ARDS. HPV is regionally variable, resulting in heterogenous ventilationperfusion matching. Prone ventilation may minimize the heterogeneity and allow HPV to divert blood flow to the caudal/dorsal regions of the lung. Although HPV is considered to be weak in COVID-19, residual HPV might be optimized when prone [38].

Clinical Outlook
In summary, we suggest that in the beginning of the COVID-19 pulmonary involvement (decrease of SpO 2 of 3%) the patients should receive immediate oxygen therapy and pulmonary edema treatments. However, this is merely a concept paper that needs to be tested in controlled randomized trials. It should be noted that not only SpO 2 is important but also the patient's associated tachypnea or hyperpnea, and SpO 2 should be interpreted with caution as there is a left-sided shifting of the oxyhemoglobin dissociation curve due to tachypnea/hyperpnea induced by hypoxemia [102]. Thus, SpO 2 monitoring with a finger oximeter is just a stop-gap measure, and the CT performance is still a "golden rule".

Conflicts of Interest:
Si-Cong Jiang was employed by the Chengdu Kang Hong Pharmaceutical Group Comp. Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.