key: cord-0908314-8wk1w0z9 authors: Meizoso, Jonathan P.; Moore, Hunter B.; Moore, Ernest E. title: Fibrinolysis Shutdown in COVID-19: Clinical Manifestations, Molecular Mechanisms, and Therapeutic Implications date: 2021-03-22 journal: J Am Coll Surg DOI: 10.1016/j.jamcollsurg.2021.02.019 sha: 6b50b0e7ab4bb9f3f8c162abc01154a8d5b17485 doc_id: 908314 cord_uid: 8wk1w0z9 The COVID-19 pandemic has introduced a global public health threat unparalleled in our history. The most severe cases are marked by the acute respiratory distress syndrome attributed to microvascular thrombosis. Hypercoagulability, resulting in a profoundly prothrombotic state, is a distinct feature of COVID-19 and is accentuated by a high incidence of fibrinolysis shutdown. The aims of this review are to describe the manifestations of fibrinolysis shutdown in COVID-19 and its associated outcomes, review the molecular mechanisms of dysregulated fibrinolysis associated with COVID-19, and discuss potential implications and therapeutic targets for patients with severe COVID-19. amplitude at 10 The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was discovered in a cluster of patients with pneumonia of unknown origin in December 2019 in Wuhan, China (1) . The SARS-CoV-2 virus was subsequently identified as the causative pathogen for coronavirus disease 2019 (COVID-19), a clinical syndrome characterized initially by fever, cough, and progression to the acute respiratory distress syndrome (ARDS) (2) . Severe disease will develop in 5 -16% of patients who require a prolonged intensive care unit (ICU) stay (3, 4) and 50 -70% of those will require mechanical ventilation (4, 5) . The overall mortality for COVID-19 is 1 -5%, however this incidence increases to 22 -64% in patients who progress to ARDS (4-6). and has rapidly become the largest public health emergency in modern times (7) . As of February 18, 2021, there are 110 million confirmed cases and 2.5 million confirmed deaths from COVID-19 worldwide (8) . Shortly after the start of the COVID-19 pandemic, it became increasingly clear that this disease was associated with a frequent and oftentimes severe coagulopathy that was augmented in nonsurvivors of the illness. In fact, in the first large and comprehensive evaluation of coagulation function in COVID-19 patients, Tang and colleagues showed that 71% of non-survivors exhibited disseminated intravascular coagulation (DIC) defined by International Society on Thrombosis and Haemostasis (ISTH) standards (9, 10) . However, it has also become apparent that these patients exhibit a unique hypercoagulable phenotype of DIC with a propensity toward thrombosis rather than a bleeding diathesis (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) . It is now apparent that patients with COVID-19 rarely progress to a state of overt DIC as defined by the ISTH (22 J o u r n a l P r e -p r o o f Therefore, our objectives are to describe the manifestations of fibrinolysis shutdown in COVID-19 and its associated outcomes, review the molecular mechanisms of dysregulated fibrinolysis associated with COVID-19, and discuss potential implications and therapeutic targets for patients with severe COVID-19. Thrombotic complications are now recognized as leading causes of morbidity and mortality in COVID-19. These include VTE, ischemic stroke, myocardial infarction, acute limb ischemia, and other macro-and microthrombotic complications including multiple organ failure. In fact, between 30 -80% of ICU patients with COVID-19 experience a thrombotic complication at some point during their disease course (44) (45) (46) (47) . VTE is the most common thrombotic complication in COVID-19 patients, occurring in 15 -85% of patients (12) . A meta-analysis of six studies including 678 patients identified five risk factors for the development of VTE in COVID-19 patients (13) . These include age, ICU admission, leukocytosis, lymphopenia, and elevated D-dimer. D-dimer, a marker of fibrinolysis and fibrin deposition, is consistently elevated in patients with severe COVID-19 and is a predictor of poor outcomes (9, 47, 48) . Although D-dimer has been associated with fibrinolysis, D-dimers are a biomarker of clot formation and, in the COVID-19 patient, may represent unbridled clot formation (49) (50) (51) . In fact, D-dimer levels in the first 7 days of disease and the rate of change of D-dimer levels have been shown to reliably predict VTE (52) . Other thrombotic complications, including stroke and cerebral venous sinus thrombosis (14, 15) , myocardial infarction (16) (17) (18) , acute limb ischemia 7 (19), acute kidney injury (20) , and ischemic colitis (21) , have been reported in patients with COVID-19. Mechanistically, the markedly elevated levels of D-dimer in patients with fibrinolysis shutdown may represent local thrombosis in the microvasculature (e.g., pulmonary, renal) that are not consistently captured on whole blood assays. Inconsistencies in timing of sample measurements and within-patient variability in coagulation profiles make interpretation of these data even more complex. underscores the necessity of thoroughly studying an intervention for benefit as well as potential risks even during a pandemic as treatments that may seem intuitive can in fact be harmful. Furthermore, these diametrically conflicting results emphasize the necessity for monitoring the coagulation status of hospitalized COVID-19 patients. Bleeding complications in patients with fibrinolysis shutdown have been well-described in other populations (63) . Trauma patients in fibrinolysis shutdown with elevated D-dimers have been reported to require more transfusions than patients with low D-dimers (64) (65) (66) . As with traumaand sepsis-induced coagulopathy, the coagulopathy caused by COVID-19 is likely multifactorial, has primary and secondary components, and is affected by timing of infection and resuscitation (67) . While the particular mechanisms behind COVID-19-associated coagulopathy are currently poorly understood, they likely involve many of the same features common to other welldescribed coagulopathies. A potential mechanism that has not received as much attention in COVID-19 could be related to hyperfibrinogenemia from an acute phase response, which J o u r n a l P r e -p r o o f demonstrates a step wise increase in prolonged clotting time with reptilase (68) . Animal viral disease associated with hyperfibrinogenemia has also been associated with mucosal bleeding and respiratory disease (69) . Fibrin formation has a feedback mechanism to reduce thrombin generation (70) . In addition, when thrombin is bound to fibrin its activity is modified (71) . More recently, it has also been proposed that thrombin can be trapped by fibrin networks (72) , and other fibrinogen binding molecules can impair thrombin generation (73) . This interaction is further complicated by the local environment in which cellular factors and the geographic location for thrombin generation can promote bleeding versus clotting (74) . Therefore, it is not surprising that patients can manifest with a mixed phenotype of clotting and bleeding at the same time when the coagulation and fibrinolytic systems have been pushed to extremes. Understanding the local environment driving intracranial bleeding and mucosal bleeding in COVID-19 compared to micro and macrovascular thrombosis is an important future area of research to evaluate. The tightly regulated balance between thrombosis and fibrinolysis is clearly disrupted in COVID-19. This coagulation dysregulation is intimately associated with the host immune response to viral infection. Indeed, recent proteomic work has shown a significant dysregulation in coagulation factor function and increased antifibrinolytic activity as a function of elevated interleukin-6 (IL-6) levels (75) . A comprehensive review of the crosstalk between inflammation and coagulation is outside the scope of this text [see Whyte et al. (37) ]. In this section, we will focus on mechanisms of fibrinolytic dysfunction in COVID-19 (Figure 1) . known that ARDS from other causes is associated with a local hypercoagulable state within the lungs, leading to abnormal fibrin deposition and microthrombi development as an end result (76) (77) (78) (79) . Similar pathologic findings have since been described in deceased patients with COVID-19 and ARDS (80) (81) (82) (83) . This hypercoagulable state is at least in part mediated by tissue injury and inflammation resulting in increased levels of tissue factor production by alveolar macrophages and epithelial cells (84) , leading to thrombin generation and fibrin deposition (37) . Another significant mechanism contributing to hypercoagulability in COVID-19 is endothelialitis (85) (86) (87) (88) (89) . compared to those with sepsis from other causes (95) . A separate study measured thrombin generation potential and anti-Xa levels in 48 ICU patients with COVID-19 on anticoagulation and found thrombin generation potential within the reference range despite elevated anti-Xa levels (33). As the authors point out, this suggests either a hypercoagulable state not affected by heparin therapy or heparin resistance, however they measured antithrombin levels as well and these were within the reference range, indicating that a hypercoagulable state despite anticoagulation was more likely. High median evoked thrombin potential values have been described in COVID-19 patients up to a week after ICU admission (49) . The local hypercoagulable state in the pulmonary alveoli in ARDS is further exacerbated by an impaired fibrinolytic response primarily mediated by overexpression of plasminogen activator inhibitor 1 (PAI-1) from endothelial cells and activated platelets (93, 96, 97) . We believe these mechanisms lead to a state of fibrinolysis shutdown in these patients, which, coupled with increased thrombin generation, lead to poor outcomes including ARDS and other markers of microvascular thrombosis in other patient populations (25-28). While PAI-1 is likely the most potent antifibrinolytic mediator, data from patients with interstitial lung disease has also identified elevated thrombin activatable fibrinolysis inhibitor (TAFI) and protein C inhibitor (also known as PAI-3) levels in the alveolar space (37, 98) . These findings were also noted during the related SARS-CoV epidemic in 2002 (37) . Markedly elevated PAI-1 levels have now been reported in COVID-19 patients as well, with some reports describing levels up to 4-fold higher in COVID-19 patients compared to controls (39, 49) . This elevation in PAI-1 levels in COVID-19 may be exacerbated by elevated levels of circulating angiotensin II. It is now known that the SARS-CoV-2 virus infects via the ACE-2 receptor, resulting in elevated levels of circulating angiotensin II given saturation of its endogenous receptor. These elevated levels of angiotensin II may subsequently increase stimulation of PAI-1 production by endothelial cells (37, 99, 100) . In addition to higher PAI-1 levels, Nougier et al. found markedly elevated circulating tPA and TAFI levels in COVID-19 patients (33). Despite elevated tPA, these patients were hypofibrinolytic, suggesting that the higher levels of PAI-1 and TAFI likely overwhelm the capabilities of tPA, leading to microvascular fibrin deposition. In summary, it is clear that the hypercoagulable state seen in COVID-19 patients is multifactorial and complex. Increased thrombin generation potential mediated by virus-induced tissue injury and expression of tissue factor leads to profound hypercoagulability that is exacerbated by a significant state of fibrinolysis shutdown mediated by overexpression of PAI-1 and TAFI. This overexpression overwhelms the local capabilities of tPA and urokinase despite elevated circulating tPA levels and increased plasmin generation potential (37, 93, 95) . Indeed, the balance between coagulation and fibrinolysis is lost in patients with COVID-19. However, knowledge of these mechanisms allows for the development of treatment strategies targeted at this maladaptive response to potentially improve patient outcomes. As previously stated, there are several potential therapeutic targets within However, there have been several experimental models that also suggest a benefit of fibrinolytic therapy in ARDS (102) . The use of tPA has since been proposed for patients with severe ARDS from COVID-19 (103) . 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