key: cord-0688355-ukne2nh0
authors: Ali, Mohammad A.M.; Spinler, Sarah A.
title: COVID-19 and thrombosis: From bench to bedside
date: 2020-12-16
journal: Trends Cardiovasc Med
DOI: 10.1016/j.tcm.2020.12.004
sha: 9b8f097e7b8e1e5f85e3d96d92ec59238bffbdff
doc_id: 688355
cord_uid: ukne2nh0

Coronavirus disease of 2019 (COVID-19) is the respiratory viral infection caused by the coronavirus SARS-CoV2 (Severe Acute Respiratory Syndrome Coronavirus 2). Despite being a respiratory illness, COVID-19 is found to increase the risk of venous and arterial thromboembolic events. Indeed, the link between COVID-19 and thrombosis is attracting attention from the broad scientific community. In this review we will analyze the current available knowledge of the association between COVID-19 and thrombosis. We will highlight mechanisms at both molecular and cellular levels that may explain this association. In addition, the article will review the antithrombotic properties of agents currently utilized or being studied in COVID-19 management. Finally, we will discuss current professional association guidance on prevention and treatment of thromboembolism associated with COVID-19.

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV2) resulted in global pandemic in early 2020. The World Health Organization (WHO) designated the name COVID-19 (Coronavirus disease of 2019) as the official name of the diseases caused by SARS-CoV2. First reported in Wuhan China in late 2019, the total confirmed COVID-19 cases worldwide have surpassed 49 million with more than a million of global death in eleven months (coronavirus.jhu.edu/map.html). From the name of its causative virus, COVID-19 is a primarily respiratory disease that, in severe cases, can lead to pneumonia or acute respiratory distress syndrome (ARDS). However, several non-respiratory presentations are being manifested in COVID-19 patients among which are venous and arterial thromboembolic events.

Thromboembolic complications have been reported in COVID-19 patients by different groups (just to mention few, [1] [2] [3] [4] ). Importantly, various organs are affected by COVID-19 induced coagulopathy, including the vasculature of lungs [1] , legs [3] , spleen [2] , heart [5] and brain [6] . These complications are usually associated with multiorgan failure and high mortality in severe cases of the diseases [7] . The current clinical data indicate that both pulmonary embolism (PE) and deep vein thrombosis (DVT) are the most frequently noted thrombotic events in COVID-19 [4] . Intriguingly, the risk of venous thromboembolism (VTE) remains to be high in hospitalized patients despite anticoagulation prophylaxis [8] [9] [10] [11] [12] . Indeed, the data are rapidly emerging and at the time of writing this article, the rate of venous thromboembolic events was estimated as high as 25 to 30%, particularly in critically ill and mechanically ventilated patients [13] . Other thrombotic complications were also reported including stroke, acute limb ischemia and acute coronary syndromes [ 6 , 14 , 15 ] . Both acute ischemic stroke and myocardial injury are reported in up to 5% and 20% in hospitalized patients, respectively [ 16 , 17 ] . The multisystem inflammatory syndrome in children (MIS-C) as a result of SARS-CoV2 infection may also increase, alarmingly, the risk of coagulopathy in the pediatric population [18] . In this review, we will summarize a number of mechanisms at the molecular/cellular level by which SARS-CoV2 infection may cause thrombosis. Also, we will review the current pharmacotherapy and professional association guidance on prevention and treatment of thromboembolism associated with COVID-19.

SARS-CoV2 attaches to human cells through binding of its spike protein (S-protein) to the human angiotensin converting enzyme 2 (ACE2) receptor [19] . Interestingly, ACE2 expression is found to be higher in the ciliated cells of the nose compared to bronchi indicating that the nose is most likely the initial site of viral entry [20] . However, ACE2 is not only expressed in nasopharyngeal and lung cells but also in blood vessels, heart, kidney, testicle and brain [21] . In fact, this carboxypeptidase, ACE2, was first cloned from human heart and found to be highly expressed throughout the endothelia of coronary and renal blood vessels [22] . This may suggest important biological functions of ACE2 in cardiovascular system. Indeed, a major role of ACE2 among others is to inactivate angiotensin II by proteolytically converting it to angiotensin 1-7 (reviewed in [23] ). This thus places ACE2 in a critical position as a negative regulator of the renin-angiotensin-aldosterone system (RAAS). https://doi.org/10.1016/j.tcm.2020.12.004 1050-1738/Published by Elsevier Inc. 19 . SARS-CoV2 has higher affinity to ACE2 receptor compared to other coronaviruses. ACE2 is a carboxypeptidase that converts angiotensin II (Ang II) to angiotensin 1-7. The binding between the spike protein (S-protein) of the virus and ACE2 is associated with downregulation of ACE2 activity. In its turn, this will lead to augmentation of Ang II signaling and pro-thrombotic pathways. On the other hand, the angiotensin 1-7 signaling which mediates anti-thrombotic pathways is diminished.

Many reports indicate that SARS-CoV2 entry is associated with downregulating ACE2 activity (reviewed in [24] ), suggesting that RAAS may be augmented in COVID-19 patients ( Fig. 1 ) . Consistently, two clinical trials using angiotensin II receptor (AT 1 ) blocker (losartan) to ameliorate COVID-19 related complications are underway (clinicaltrials.gov NCT04312009 and NCT04311177). The higher affinity of SARS-CoV2 to human ACE2 compared to other coronaviruses [25] , and the downregulation of ACE2 in COVID-19 may explain the peculiar cardiovascular manifestations seen in vulnerable patients [26] . On the other hand, ACE2 expression may be upregulated in patients taking ACE inhibitors or AT 1 blockers [ 27 , 28 ] , thus raising the concern of facilitating SARS-Cov2 entry in these patients. However, the BRACE CORONA trial has provided clinical data showing that there is no clinical benefit in routinely withholding these agents in hospitalized patients with mild to moderate infection [ 29 , 30 ] .

Intriguingly, some COVID-19 patients presented with thrombotic events had no or mild respiratory symptoms [ 6 , 26 ] . These cardiovascular sequalae led some experts to consider COVID-19, even in the absence of respiratory symptoms, in the differential diagnosis of thromboembolism and acute coronary syndrome [31] .

The strong association between COVID-19 and vascular coagulopathy may suggest that there are multiple molecular pathways that are dysregulated during the clinical progression of the diseases and thus contributing to the associated thrombosis. Clearly RAAS augmentation, as a consequence of ACE2 downregulation, is an important factor [32] . In addition, the dysregulated immune/inflammatory response is a well-studied risk factor for blood coagulation [33] . One cannot rule out a "double whammy" effect in which both factors may additively or synergistically increase thrombosis risk in COVID-19 patients. Below we will summarize some potential mechanisms that may explain the association between COVID-19 and coagulopathy at molecular/cellular levels. However, more investigations are warranted in order to exploit these findings in therapeutics.

As described above, ACE2 converts angiotensin II to angiotensin 1-7. SARS-CoV2 uses ACE2 to internalize human cells and thus may lead to reduce ACE2 activity [24] . Consequently, this will result in increased angiotensin II and decreased angiotensin 1-7. It is worth noting that whereas angiotensin II has pro-inflammatory and prothrombotic effects, angiotensin 1-7 is now recognized as an important anti-inflammatory and anti-thrombotic peptide [ 34 , 35 ] . Angiotensin 1-7 binds Mas receptors on endothelium and increases nitric oxide and prostacyclin production, thus inhibits platelets activation [36] ( Fig. 1 ) . It has been shown that ACE2 is widely expressed on endothelial cells and direct SARS-CoV2 infection of endothelia is possible [37] . Therefore, dysregulated RAAS in vasculature of COVID-19 patients may initiate a cascade of events that lead to increased coagulopathy, as summarized below.

In addition to its immediate physiological effect in vasoconstriction, angiotensin II has been shown to be a potent mediator of oxidative stress damage through rapid generation of reactive oxygen species mediated by NADPH oxidases (reviewed in [35] ). On the other hand, angiotensin 1-7 may play a pivotal antioxidant role by induction of nitric oxide synthesis/release from endothelial cells [36] . Both accumulation of reactive oxygen species and deficiency of nitric oxide are expected to have various detrimental effects on endothelium. Despite limited clinical data in COVID-19, oxidative stress damage to the infected endothelium may play a key role in severe SARS-CoV2 infection. Vitamin C, a potent antioxidant, has emerged as a potential therapy due to its potential benefits in COVID-19 [38] . However, the evidence whether these benefits are directly related to vitamin C antioxidant properties or other undefined mechanisms is still lacking.

Dysregulated RAAS can not only cause endothelial damage via oxidative stress as described above, but also through various pathways including overexpression of LOX-1, COX-2 and VEGF in the endothelium (just to name a few) [39] . There is a strong relationship between endothelial dysfunction and thrombotic events in various pathologies (reviewed in [40] . The glycocalyx in healthy endothelium plays an important role in preventing the clotting cascades and glycocalyx degradation, seen in endothelial dysfunction, may trigger various clotting cascades [41] . Endothelial dysfunction is also associated with endothelial expression of many prothrombotic molecules and receptors including P-selectins [42] , angiopoietin-2 [43] and endothelin-1 [44] , which are active players in thrombosis. In addition, several studies suggest that endothelial damage/dysfunction is a critical component of thrombin generation and activation via the release of the procoagulant factor, fVIII [45] [46] [47] .

Ackerman et al., examined lungs from COVID-19 and influenza A patients who died from associated ARDS [48] . This comparison revealed very interesting findings and showed distinctive vascular features between SARS-CoV2 and influenza H1N1 infections. COVID-19 lungs displayed more severe endothelial damage and disrupted endothelial membranes in comparison to those from influenza. Histological analyses of blood vessels showed widespread thrombosis and microangiopathy. Despite ARDS was the common cause of death in both groups, angiogenesis and alveolar capillary microthrombi in COVID-19 were 3 to 9 times as prevalent as in influenza, respectively [48] . Moreover, epidemiological data show that COVID-19 patients with conditions associated with preexisting endothelial dysfunction such as aging, hypertension, obesity and diabetes are more frequently admitted to intensive care units and have poor prognosis [49] . These observations strongly suggest that endothelial dysfunction and subsequent activation of the clotting cascade are more common to the pathogenesis of COVID-19.

Endothelial damage, in general, seems to contribute to the pathophysiology of COVID-19. This may also suggest a potential role of von Willebrand Factor (vWF) in COVID-19 associated coagulopathy. In addition to being in plasma, vWF is deposited on subendothelial spaces where it is associated with type VI collagen [50] . Upon endothelial damage, subendothelial vWF is released, further multimerized by disulfide bonds and activated by exposing both platelet-binding and collagen-binding domains [43] . Therefore, active vWF multimers act as molecular glues that stick platelets and subendothelial collagens together, activating platelets aggregation and thrombosis.

In a single-center cross-sectional study, vWF antigen and activity were found to be three times higher in non-intensive care unit (ICU) COVID-19 patients compared to control group [51] . In ICU COVID-19 patients, vWF concentration/activity were further elevated in comparison to non-ICU cohort. Soluble thrombomodulin concentrations, another marker of endothelial damage, were also found to be associated with poor clinical outcomes and survival [51] . Despite some limitations, this study reports that vWF is also elevated in non-critically ill patients with COVID-19. Consequently, both critically and non-critically ill patients with COVID-19 may have higher thromboembolic risks [ 52 , 53 ] . The roles of dysregulated RAAS and vWF in COVID-19 associated thrombosis open potential therapeutic avenues for agents that interfere with this pathway. For example, N-acetylcysteine is reported to reduce intrachain disulfide bonds in large vWF multimers inside thrombi, thereby leading to their dissolution [54] . In fact, searching clinicaltrials.gov revealed three recruiting (NCT04374461, NCT04370288, NCT04279197) and two not-yet recruiting (NCT04455243, NCT04419025) clinical trials using Nacetylcysteine as a potential therapy to improve COVID-19 outcomes. Future will tell whether N-acetylcysteine is effective in improving COVID-19 outcomes by reducing the risk of thrombosis.

The immune response to SARS-CoV2 infection has not been fully understood. Several studies highlight changes in both innate and adaptive immunity in COVID-19 patients (reviewed in [55] ). In severe cases, dysregulated innate immune response and the subsequent massive release of pro-inflammatory cytokines "cytokine storm" clearly participate in the pathogenesis of the disease. In its turn, dysregulated innate immune response results in subsequent activation of various pathways "immunothrombosis" that may lead to blood coagulation ( Fig. 2 ) . Below we will summarize some of these pathways that have been described in COVID-19.

The complement system is an integral part of the innate immune response. In SARS-CoV1 murine model, a study reported increased complement activation and found that mice lacking the complement factor C3 exhibited less respiratory inflammation and better function [56] . Similar findings were also reported in mice infected with the Middle East Respiratory Syndrome Coronavirus (MERS-CoV) [57] . Despite lacking experimental data in SARS-CoV2, the previous studies suggest that excessive complement activation contributes to the dysregulated immune response seen in coronaviruses infection.

There is mounting evidence of a crosstalk between the complement system and coagulation pathways. Atypical hemolytic uremic syndrome, a rare genetic disorder of uncontrolled complement activation, is also characterized by thrombotic microangiopathy [58] . In fact, the complement system is capable of activating coagulation cascades through multiple mechanisms. The complement factors C3 and MAC directly activate platelets and induce platelet aggregation [59] . Similarly, complement factor C5a has been shown to stimulate the expression of plasminogen activator inhibitor-1, thereby promoting thrombosis [60] . Interestingly, plasminogen activator inhibitor-1 was found to be elevated in both non-critically and critically ill patients with COVID-19 [51] . Last but not least, C5a increases tissue factor activity, which initiates the intrinsic pathway of coagulation [61] . In a preprint report of an ongoing study, COVID-19 patients with ARDS began to recover after treatment with recombinant anti-C5a antibody [62] . These observations suggest that further studies are warranted to investigate whether complement activation is involved in COVID-19 associated thrombosis. Accordingly, complement inhibition could be a potential target in COVID-19 treatment.

Neutrophils are crucial players in innate immune response. One of their recently discovered and yet not fully understood function is releasing neutrophil extracellular traps (NETs) in response to infection [63] . These structures comprise of extracellular scaffold of chromatin containing microcidal proteins designed to impede the dissemination of microbes in the blood [63] . Despite being efficient strategy to trap and kill microorganisms, excessive NETs (NETosis) formation can be harmful to the host. Several studies have implicated NETosis in various human pathologies including sepsis, vasculitis and thrombosis (reviewed in [64] ).

The role of NETosis in inducing thrombosis is well established. For example, the NETs themselves contain various prothrombotic molecules, such as tissue factor, protein disulphide isomerase, factor XII, vWF and fibrinogen [65] . In addition, the extracellular DNAs released from NETs can directly activate platelets and lead to thrombus formation [65] . Circulating histones (major components of NETs) were also found to activate Toll-like receptors on platelets and promote thrombin generation [66] . In general, NETs tend to form larger aggregates that drive thrombosis by providing a scaf- Innate immunity plays critical role as an early defense mechanism against microbial infection including SARS-CoV2. However, uncontrolled innate immune response elicited by overactivated neutrophils will initiate coagulopathic pathways. These pathways may include excessive complement activation, cytokine storm and NETosis, each of which may cause thrombosis by various mechanisms.

fold for binding of activated platelets and erythrocytes and set up a vicious cycle propagating thrombus formation [67] .

One report revealed higher levels of cell-free DNA, myeloperoxidase-DNA complex and citrullinated histone 3 (indicative markers of NETs) in sera of patients with COVID-19 as compared with the control group [68] . Interestingly, cell-free DNA and to a lesser extent myeloperoxidase-DNA complex and citrullinated histone 3 demonstrated significant correlation with other predictive tests for the severity of COVID-19 (e.g. C-reactive protein, D-dimer). Patients requiring mechanical ventilation had significantly higher levels of cell-free DNA and myeloperoxidase-DNA complex [68] . Emerging evidence indicates that NETosis contribute to increased thrombotic risks in COVID-19 [69] . COVID-19 sera were found to have higher levels of NETs carrying active tissue factor and promote thrombosis. Both NETosis and complement inhibitions, in-vitro, attenuated thrombin activation by COVID-19 sera [69] . In addition, the same study showed that NETs release is positively correlated with in-vivo, thrombotic potency in COVID-19 [69] . Accordingly, one can speculate that NETosis is a major driver of COVID-19 thrombosis; however, future studies are needed to investigate the effects of NETs inhibition in improving COVID-19 outcomes.

The mitogen-activated protein kinases (MAPKs; ERK, p38 and JNK) are considered central kinases which are early activated in in-nate immune response. MAPKs activation, likely mediated by Tolllike receptors signaling, induces expression of multiple genes that altogether regulate the innate and inflammatory response [70] . Furthermore, the roles of MAPKs in coronaviruses replication/infection have been elucidated. In vitro, SARS-CoV1 infection was associated with activation of multiple MAPKs pathways [71] and inhibition of MAPKs has been shown to inhibit murine coronavirus replication [72] . Given the similarities between those coronaviruses and SARS-CoV2, it is possible that MAPKs are activated in COVID-19. It is worth noting that MAPKs can also be activated in COVID-19 downstream of dysregulated RAAS as a result of ACE2 inhibition [73] . On the other hand, MAPKs pathways are significantly upregulated in various cardiovascular pathologies including thrombosis [74] . For example, activation of p38 signaling induces expression of tissue factor, a major initiator of coagulation, and mediates thrombotic events in anti-phospholipid syndrome [75] . Therefore, chronic activation of MAPKs pathways may also contribute to elevating the risk of thrombosis, seen in COVID-19.

In summary, there seem to be numerous molecular/cellular pathways potentially explaining the high risk of thrombosis in COVID-19. In this review, we classified some of these pathways under two main categories; dysregulated RAAS and dysregulated immune response. However, it is important to emphasize that pathophysiologic mechanisms of COVID-19 induced thrombosis are often intermingled with multiple crosstalks between pathways during clinical progression of the disease. Future studies will re-veal whether one or more of these pathways can be exploited as therapeutic targets to improve thrombotic status in patients with COVID-19.

Clinically, evidence of inflammation and coagulopathy is commonly observed in patients infected with SARS-CoV2. Proinflammatory cytokines, including IL-6 and IL-8, are elevated in patients with COVID-19. Rapidly elevating D-dimer levels and elevated fibrinogen degradation products are observed, especially in non-survivors [8] . Also, patients with D-Dimer levels of more than 6 times the upper limit of normal experienced higher mortality in a recent study of almost 500 Chinese patients with COVID-19 [76] . These patients may also have evidence of antiphospholipid antibodies, prolonged activated partial thromboplastin time (aPTT), as well as mild prolongation of the prothrombin time (PT) [77] .

Up to 70% of the most severely ill patients with SARS-CoV2 have features of disseminated intravascular coagulation (DIC) [ 78 , 79 ] . Unlike sepsis-associated DIC, patients with COVID-19 associated DIC have relatively mild thrombocytopenia (100 × 10 9 /L to 150 × 10 9 /L) and would usually not meet classic ISTH criteria for DIC [79] . In addition, the levels of IL-6 seen in patients with severe COVID-19 are more than five times higher than those observed in bacterial sepsis [80] . Thrombotic microangiopathy, particularly in the lungs, has also been observed [79] . Renal and neurological dysfunction associated with SARS-CoV2 infection has also been postulated to be caused by microvascular thrombotic injury [79] . Finally, as discussed above, there is evidence for direct endothelial damage by the virus with cytokine production, release of cytoplasmic vWF, tissue-type plasminogen activator and urokinase plasminogen activator. Plasmin activation may partially explain the syndrome resulting elevated D-Dimer levels [ 79 , 81 ] .

Despite administration of anticoagulant prophylaxis, reported rates of VTE are high -25% to 69% in critically ill patients in the ICU with COVID-19 and 7% in general medical floor patients [8] [9] [10] [11] [12] . The reported incidence of pulmonary embolism (PE) varies from 2.8% to as high as 21% to 23% [ 8 , 9 , 12 ] . A prospective cohort study of 12 consecutive autopsies performed in COVID-19 deaths revealed bilateral DVT in 7 of 12 with 4 of those patients also demonstrating massive PE as the cause of death [1] . A second autopsy study of 11 randomly selected patients reported segmental and/or subsegmental thrombosis of the pulmonary arteries in all patients despite 10 of the 11 receiving VTE prophylaxis [82] . Thrombosis was not the originally suspected cause of death in any of the patients. Asymptomatic deep vein thrombosis (DVT) was reported to be as high as 85% in critically ill patients and 46% of hospitalized medical patients who underwent ultrasound screening [11] . The high rate of VTE, especially PE, is consistent with what has been observed in critically ill patients with severe acute respiratory or pneumonia associated with other respiratory viruses such as H1N1 [ 10 , 83 ] . A recent systematic review and meta-analysis of 86 studies reporting VTE risk in hospitalized patients with COVID-19 reported a VTE prevalence of 7.9% in non-ICU patients and 22.7% in ICU patients with a PE prevalence of 3.5% and 13.7%, respectively [84] . However, at the time of this writing, no data are available on the incidence of VTE in non-hospitalized patients with less severe COVID-19 disease.

While the data on VTE are quite robust, evidence is evolving on the risk of arterial thromboembolism with COVID-19 infection. The frequency of acute ischemic stroke and myocardial infarction appears to be much smaller than the risk of VTE, although not inconsequential [ 85 , 86 ] . At this time, there are no guidelines supporting the use of aspirin for prophylaxis of arterial thromboembolism in patients with COVID-19 [87] although studies with antiplatelets are planned or ongoing (clinicaltrials.gov NCT04365309).

In addition to its anticoagulant properties, heparins have a long-established history of anti-inflammatory activity. Heparins have demonstrated reductions in IL-6 and IL-8, as well as a reduction in human pulmonary microvascular endothelial cell damage secondary to lipopolysaccharide induced nuclear factor κB signaling involved in sepsis [81] . Heparin inhibits neutrophil chemotaxis, eosinophil migration, blockade of extracellular histones, and one of the early key steps in sepsis, adhesion of leukocytes to endothelium [88] .

There is emerging evidence of heparin's role in reducing infectivity of SARS-COV2. Compared to SARS-CoV1, SARS-CoV2 demonstrates potential binding domains with glycosoaminoglycans, like heparan sulfate located on the surface of almost all mammalian cells, in addition to ACE2. Being a sulfated polysaccharide, heparin has high binding affinity to the spike protein (S-protein) of SARS-CoV2 in vitro. Therefore, heparins have the potential to serve as a decoy to prevent the virus from binding to heparan sulfate coreceptors in host tissues [89] . Recently both unfractionated heparin (UFH) and low-molecular-weight heparins (LMWH) have been found to bind to and destabilize the receptor binding domain of SARS-Cov2 spike protein and directly inhibit spike protein binding to the ACE2 receptor at therapeutic concentrations [90] . Because heparins have poor oral bioavailability [91] , UFH is being investigated both as a nebulized treatment in mechanically ventilated patients with COVID-19 and as a prophylactic intranasal spray (clinicaltrials.gov NCT04490239 & NCT04397510).

Numerous governmental and professional associations have published guidance for screening, prevention and treatment of VTE in hospitalized patients with COVID-19 ( Table 1 [92] [93] [94] [95] ). Typically, patients hospitalized with medical illness would be risk-stratified using tools like the PADUA or IMPROVE risk assessment model, and those estimated at higher risk for VTE and low risk for bleeding would receive prophylaxis with standard dose subcutaneous UFH or LMWH according to published guidelines. Because observational studies have shown high risk for VTE in hospitalized patients with COVID-19, most of the available guidelines suggest that all patients who are not at high risk for bleeding, receive an anticoagulant for VTE prophylaxis. Routine screening for lower extremity VTE with Doppler ultrasound, is not recommended. Clinicians should use a low threshold such as worsening oxygenation and rapidly rising D-Dimer, for ordering an appropriate available diagnostic study and initiate therapeutic anticoagulation, however, if PE suspected.

Guidelines from the National Institutes of Health (NIH), Anticoagulation Forum (AC Forum), International Society of Thrombosis and Haemostasis (ISTH), and Global COVID-19 Thrombosis Collaborative Group, suggest that an elevated D-dimer may be used to stratify which patients could potentially receive higher than standard dose anticoagulant prophylaxis but acknowledge that data are lacking [92] [93] [94] . However, the CHEST guidelines specifically state that a D-dimer not to be used to guide the intensity of anticoagulation [95] .

Heparins have proven effective for both prophylaxis and treatment of VTE in patients hospitalized with acute illness. Because of the high risk of thromboembolic complications and heparin's unique pharmacology demonstrating a reduction in both inflammation and coagulation markers in COVID-19, heparins are be- ing studied for additional clinical endpoints of reduction in hospitalizations, progression to non-invasive mechanical ventilation, as well as mortality. Some, but not all, recently reported retrospective cohort studies, suggest an in-hospital mortality benefit for both UFH and LMWH administered at both prophylaxis and therapeutic treatment doses, especially in patients with more severe COVID-19 as evidenced by elevated D-dimer or need for mechanical ventilation [ 76 , 96-103 ] (Summarized in Table 2 ).

In the few studies that have reported the frequency of major bleeding, results are mixed with two studies reporting no difference in major bleeding between therapeutic anticoagulation and prophylactic anticoagulation and one study reporting an increased risk of major bleeding [ 96 , 98 , 99 , 101 ] . Because the data reported in these studies are limited with some lacking details regarding heparin type, dose, the reason treatment versus prophylaxis dosing was selected, as well as the potential for confounding, randomized prospective trials are needed. As discussed below, many hospital guidelines and practice guidelines have already been modified to include therapeutic anticoagulation options as a result of these early reports.

There Table 3 ). Trial endpoints include progression to ventilation, ICU admission, ventilated/ICU days, and major bleeding, in addition to reduction in both symptomatic venous and arterial thrombosis. All-cause mortality is either part of a composite primary endpoint or a secondary endpoint in the larger multicenter studies. Most comparative studies are evaluating either intermediate fixed doses or therapeutic treatment doses of LMWH compared to standard prophylaxis doses in hospitalized patients. While most studies prefer administration of LMWH, UFH is also being permitted in patients with significant renal dysfunction where dosing of LMWH is less well-studied and drug accumulation is a concern. Some are requiring evidence of coagulopathy including elevated D-dimer values at least 2 to 3 times the upper limit of the normal range. Two studies, OVID and ETHIC, are enrolling never hospitalized serologically positive patients and testing whether administration of standard or weight-based prophylaxis dose enoxaparin for 14 to 21 days prevents hospitalization (clinicaltrials.gov NCT04400799, NCT04492254). There are additional ongoing single center studies not included in Table 3 .

When selecting an agent for prophylaxis some, but not all guidelines listed in Table 1 recommend LMWH over UFH primarily to decrease the number of injections and risk of healthcare provider exposure. UFH is recommended over LMWH for patients with severe renal insufficiency. A direct oral anticoagulant (DOAC) is not recommended for VTE prophylaxis secondary to the potential for drug interactions and longer half-lives which may make hemostasis following surgery or invasive procedures difficult to manage. Most guidelines recommend standard doses of anticoagulant prophylaxis for non-critically ill hospitalized patients with consideration of intermediate doses for obese patients and critically ill patients. Two guidelines, those from the AC Forum and ISTH, give an option for using therapeutic anticoagulation in the highest risk patients [ 92 , 94 ] . Fondaparinux is recommended in pa-tients with a history of or suspected heparin-induced thrombocytopenia. If an anticoagulant is contraindicated, prophylaxis with intermittent pneumatic compression (IPC) is recommended. There are mixed recommendations regarding the addition of IPC to anticoagulant prophylaxis for high-risk patients [ 92 , 94 ] .

Only one guideline, the Global COVID-19 Thrombosis Collaborative Group, addresses non-hospitalized patients with COVID-19, recommending increased mobility with a consideration for anticoagulant prophylaxis for those with limited mobility, history of VTE or active malignancy [93] . Routine post-hospital discharge prophylaxis is not recommended by the NIH or CHEST [95] . However, the NIH, American Society of Hematology (ASH), AC Forum, Global COVID-19 Thrombosis Collaborative Group and ISTH suggest individualized patient decision making based on continued VTE risk, such as those who were hospitalized in the ICU [ 92 , 93 ] . When indicated, the guidelines recommend either enoxaparin or rivaroxaban for a duration of 14 to 45 days post-discharge.

For treatment of VTE in hospitalized patients with COVID-19 most of guidelines suggest parenteral anticoagulation with switch to a DOAC (assuming no drug interactions) as the patient transitions to the outpatient setting [92] [93] [94] . When using UFH in therapeutic doses, the guidelines suggest monitoring anti-Xa levels rather than an aPTT as prolonged aPTT with elevated levels of factor VIII and positive lupus anticoagulants is common [104] .

Given the data reporting a higher incidence of VTE in patients hospitalized with COVID-19, especially with severe disease being treated in the ICU and a lack of randomized controlled trials evaluating intermediate-and therapeutic treatment LMWH and UFH, several health systems have updated their institutional recommendations to recommend higher dosing. For example, Brigham and Women's Hospital recommends standard treatment with enoxaparin 40 mg subcutaneous daily (UFH 50 0 0 units subcutaneous three times daily for CrCl < 30 mL/min) for VTE prophylaxis in non-ICU hospitalized patients with CrCl ≥30 mL/min weighing less than 120 kg. For patients in ICU and post-ICU, higher doses for these patients are recommendedenoxaparin 40 mg subcutaneous twice daily (UFH 7500 units subcutaneous three times daily) ( https://covidprotocols.herokuapp. com/pdf/Covid-19%20Drugs&Treatment%20Guide%20092020.1.pdf ). In contrast, Yale New Haven Health System recommends prophylaxis stratified by D-Dimer with standard prophylaxis doses (plus low-dose aspirin 81 mg/day unless contraindicated) for patients with COVID-19 and a D-Dimer less than 5 mg/L and intermediate dose prophylaxis anticoagulation (enoxaparin 0.5 mg/kg subcutaneous twice daily) for patients with either D-dimer ≥5 mg/L or receiving convalescent plasma (in addition to aspirin 81 

COVID-19 pandemic has dramatically increased the risk of venous and arterial thromboembolic events in many patients. Indeed, the link between COVID-19 and coagulopathy is attracting attention from both clinicians and basic scientists. At the molecular/cellular levels, numerous signaling pathways due to dysregulated RAAS may contribute to the observed coagulopathy in COVID-19. On the other hand, excessive innate immune response to SARS-CoV2 for which there is no prior acquired immunity mediates various pathways that may lead to thrombosis. These pathways may offer new opportunities for the development of innovative therapies to treat COVID-19 induced coagulopathy. Currently clinicians are facing challenges with selecting the most effective prophylactic anticoagulant strategies. Future research is moving forward to identify pathophysiologic mechanisms, biomarkers and appropriate anticoagulation dosing strategies to improve patient outcomes.

Currently we recommend that clinicians enroll patients in clinical trials where available. Because data are lacking on the safety of therapeutic versus prophylactic anticoagulation and current guidelines give options, an individualized approach is recommended balancing benefit versus bleeding risk. Studies evaluating the role of anticoagulation in all phases of care of patients with COVID-19 are underway including out-of-hospital ambulatory, hospitalized, and post-hospital patients. Results of these ongoing studies will help shape our future practice.

The authors declare no conflict of interest.

We the undersigned declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere.

We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

We confirm that all authors are responsible for the content and have read and approved the manuscript; and that the manuscript conforms to the Uniform Requirements for Manuscripts Submitted to Biomedical Journals published in Annals in Internal Medicine

We understand that the Corresponding Author is the sole contact for the Editorial process. He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.

Signed by all authors as follows: Mohammad A.M Ali Sarah A. Spinler

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.tcm.2020.12.004 .

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SARS-CoV-2 reverse genetics reveals a variable infection gradient in the respiratory tract

Expression of the SARS-CoV-2 cell receptor gene ACE2 in a wide variety of human tissues

A novel angiotensin-converting enzyme-related carboxypeptidase (ACE2) converts angiotensin I to angiotensin 1-9

ACE2: from vasopeptidase to SARS virus receptor

The pivotal link between ACE2 deficiency and SARS-CoV-2 infection

Structural basis of receptor recognition by SARS-CoV-2

The variety of cardiovascular presentations of COVID-19

Regulation of ACE2 in cardiac myocytes and fibroblasts

Human intestine luminal ACE2 and amino acid transporter expression increased by ACE-inhibitors

Continuing versus suspending angiotensin-converting enzyme inhibitors and angiotensin receptor blockers: impact on adverse outcomes in hospitalized patients with JID: TCM

severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-the BRACE CORONA trial

Highlights from studies in cardiovascular disease prevention presented at the digital 2020 European society of cardiology congress: prevention is alive and well

Cardiac involvement in a patient with coronavirus disease 2019 (COVID-19)

Angiotensin II regulates the expression of plasminogen activator inhibitor-1 in cultured endothelial cells. A potential link between the renin-angiotensin system and thrombosis

Inflammation and coagulation

Angiotensin-converting enzyme II in the heart and the kidney

Angiotensin II cell signaling: Physiological and pathological effects in the cardiovascular system

The ACE2/Angiotensin-(1-7)/MAS axis of the renin-angiotensin system: Focus on angiotensin

Endothelial cell infection and endotheliitis in COVID-19

Vitamin C as a possible therapy for COVID-19. In: Infect Chemother, 52; 2020

Angiotensin II and the endothelium: Diverse signals and effects

Endothelial cell control of thrombosis

Degradation and detoxification of azo dyes with recombinant ligninolytic enzymes from aspergillus sp. with secretory overexpression in pichia pastoris

The dialogue between endothelial cells and monocytes/macrophages in vascular syndromes

Von willebrand's disease

Endothelial dysfunction and vascular disease -a 30th anniversary update

Expression of factor VIII by murine liver sinusoidal endothelial cells

A conditional knockout mouse model reveals endothelial cells as the principal and possibly exclusive source of plasma factor VIII

Murine coagulation factor VIII is synthesized in endothelial cells

Pulmonary vascular endothelialitis, thrombosis, and angiogenesis in covid-19

Clinical characteristics of 113 deceased patients with coronavirus disease 2019: retrospective study

The significance of subendothelial von willebrand factor

Endotheliopathy in COVID-19-associated coagulopathy: evidence from a single-centre, cross-sectional study

Infanta leonor thrombosis research group. Incidence of pulmonary embolism in non-critically ill COVID-19 patients. predicting factors for a challenging diagnosis

Deep vein thrombosis in non-critically ill patients with coronavirus disease 2019 pneumonia: deep vein thrombosis in non-intensive care unit patients

Potent thrombolytic effect of N-acetylcysteine on arterial thrombi

Immune response in COVID-19: addressing a pharmacological challenge by targeting pathways triggered by SARS-CoV-2

Complement activation contributes to severe acute respiratory syndrome coronavirus pathogenesis. mBio

Blockade of the C5a-C5aR axis alleviates lung damage in hDPP4-transgenic mice infected with MERS-CoV

Atypical haemolytic uraemic syndrome: a case report of a rare cause of reversible cardiomyopathy

Distinct contributions of complement factors to platelet activation and fibrin formation in venous thrombus development. In: Blood, 129

New aspects in thrombotic research: complement induced switch in mast cells from a profibrinolytic to a prothrombotic phenotype

A novel C5a receptor-tissue factor cross-talk in neutrophils links innate immunity to coagulation pathways

Highly pathogenic coronavirus N protein aggravates lung injury by MASP-2-mediated complement over-activation. In: medRxiv preprint; 2020

Neutrophil extracellular traps kill bacteria

Neutrophil extracellular traps in the second decade

Extracellular DNA traps promote thrombosis

Extracellular histones promote thrombin generation through platelet-dependent mechanisms: Involvement of platelet TLR2 and TLR4. In: Blood, 118

Neutrophils and neutrophil extracellular traps drive necroinflammation in COVID-19

Neutrophil extracellular traps in COVID-19

Complement and tissue factor-enriched neutrophil extracellular traps are key drivers in COVID-19 immunothrombosis

Mitogen-activated protein kinases in innate immunity

Signal transduction in SARS-CoV-infected cells

Suppression of coronavirus replication by inhibition of the MEK signaling pathway

Angiotensin II up--regulates angiotensin I-converting enzyme (ACE), but down-regulates ACE2 via the AT1-ERK/p38 MAP kinase pathway

Molecular insights into SARS COV-2 interaction with cardiovascular disease: role of RAAS and MAPK signaling

The p38 mitogen-activated protein kinase (MAPK) pathway mediates induction of the tissue factor gene in monocytes stimulated with human monoclonal anti-beta2Glycoprotein I antibodies

Anticoagulant treatment is associated with decreased mortality in severe coronavirus disease 2019 patients with coagulopathy

COVID-19: review on latest available drugs and therapies against SARS-CoV-2. coagulation and inflammation cross-talking

Abnormal coagulation parameters are associated with poor prognosis in patients with novel coronavirus pneumonia

COVID-19 coagulopathy vs disseminated intravascular coagulation

Elevated interleukin-6 and severe COVID-19: A meta-analysis

Different signaling pathways involved in the anti-inflammatory effects of unfractionated heparin on lipopolysaccharide-stimulated human endothelial cells

Pulmonary arterial thrombosis in COVID-19 with fatal outcome: results from a prospective, single-center, clinicopathologic case series

Empirical systemic anticoagulation is associated with decreased venous thromboembolism in critically ill influenza A H1N1 acute respiratory distress syndrome patients

Risk of venous thromboembolism in patients with COVID-19: a systematic review and meta-analysis

EXPRESS: stroke in COVID-19: a systematic review and meta-analysis. Int J Stroke 2020 Epub ahead of print

Registry of arterial and venous thromboembolic complications in patients with COVID-19

Is acetylsalicylic acid a safe and potentially useful choice for adult patients with COVID-19. In: Drugs, 80; 2020

The role of heparin in sepsis: much more than just an anticoagulant

Sulfated polysaccharides effectively inhibit SARS-CoV-2 in vitro

Unfractionated heparin inhibits live wild-type SARS-CoV-2 cell infectivity at therapeutically relevant concentrations

The quest for non-invasive delivery of bioactive macromolecules: a focus on heparins

Thromboembolism and anticoagulant therapy during the COVID-19 pandemic: Interim clinical guidance from the anticoagulation forum

Global COVID-19 thrombosis collaborative group, endorsed by the ISTH, NATF, ESVM, and the IUA, supported by the ESC working group on pulmonary circulation and right ventricular function. COVID-19 and thrombotic or thromboembolic disease: Implications for prevention, antithrombotic therapy, and follow-up: JACC state-of-the-art review

Subcommittee on perioperative, critical care thrombosis, haemostasis of the scientific, standardization committee of the international society on thrombosis and haemostasis. scientific and standardization committee communication: clinical guidance on the diagnosis, prevention, and treatment of venous thromboembolism in hospitalized patients with COVID-19

Prevention, diagnosis, and treatment of VTE in patients with coronavirus disease 2019: CHEST guideline and expert panel report

Association of treatment dose anticoagulation with in-hospital survival among hospitalized patients with COVID-19

The association between treatment with heparin and survival in patients with covid-19

Anticoagulation, bleeding, mortality, and pathology in hospitalized patients with COVID-19

Association of anticoagulation dose and survival in hospitalized COVID-19 patients: a retrospective propensity score-weighted analysis

Thromboprophylaxis with enoxaparin is associated with a lower death rate in patients hospitalized with SARS-CoV-2 infection. A cohort study

Therapeutic anticoagulation delays death in COVID-19 patients: crosssectional analysis of a prospective cohort

Humanitas COVID-19 task force. The role of anti-hypertensive treatment, comorbidities and early introduction of LMWH in the setting of COVID-19: a retrospective, observational study in northern italy

Intensity of anticoagulation and survival in patients hospitalized with COVID-19 pneumonia

Lupus anticoagulant and abnormal coagulation tests in patients with covid-19