Venous and arterial thrombosis in COVID-19: An updated narrative review

Hospitalized patients with coronavirus disease 2019 (COVID-19), particularly those admitted to the intensive care unit (ICU) are at high risk of morbidity and mortality. Several observational studies have described hemostatic derangements and thrombotic complications in patients with COVID-19. The aim of this review article is to summarize the current evidence on pathologic findings, pathophysiology, coagulation and hemostatic abnormalities, D-dimer’s role in prognostication epidemiology and risk factors of thrombotic complications, and the role of prophylactic and therapeutic anticoagulation in patients with COVID-19. While existing evidence is limited in quality, COVID-19 appears to increase micro-and macro-vascular thrombosis rates in hospitalized and critically ill patients, which may contribute to the burden of disease. D-dimer can be used for risk stratification of hospitalized patients, but its role to guide anticoagulation therapy remains unclear. Evidence of higher quality is needed to address the role of therapeutic anticoagulation or high-intensity venous thromboembolism prophylaxis in COVID-19 patients. Take-home points • The prevalence of venous thromboembolism (VTE) in hospitalized COVID-19 patients is high, therefore, clinicians should have a high index of suspicion. • The pathophysiology of thrombosis is likely related to a combination of SARS-CoV-2 direct endothelial injury and dysregulated inflammation causing coagulation activation. • The current evidence on the value of D-dimer guided therapy is limited. • The rate of VTE post-hospital discharge is very low, supporting the safety of current discharge practice without VTE prophylaxis in most patients. • The role of higher-intensity VTE prophylaxis or therapeutic anticoagulation in critically ill COVID-19 patients without documented or suspected VTE remains uncertain. • Therapeutic anticoagulation in hospitalized non-critically ill patients with COVID-19 may improve outcomes but more research is warranted.

Rapid recognition of the relationship between COVID-19 and thrombosis, and the paucity of high-quality evidence resulted in considerable variation in clinical practice [7]. Both intensified thromboprophylaxis and full-dose therapeutic anticoagulation have been used in patients with COVID-19, despite unclear data on the impact upon patient outcomes and safety [8][9][10].
In this review, we summarize the current evidence on the pathologic findings, pathophysiology, derangements in coagulation, the role of D-dimer in prognostication, epidemiology and risk factors of thrombosis, and the role of prophylactic and therapeutic anticoagulation in patients with COVID-19.

Data sources
We searched MEDLINE for studies of thrombosis in patients with COVID-19, published between December 1st, 2019 and October 30, 2021, with no language or study design restrictions. In addition, we reviewed reference lists of identified reports and searched medRxiv.org for preprint articles. The results of the review was summarized qualitatively.

Pathologic findings
Post-mortem findings in COVID-19 describe pathologic microand macro-vascular thromboses, possibly due to an inflammatory thrombotic state.

Microvascular thrombosis
Multiple post-mortem studies compared the histopathological features in deceased COVID-19 patients to deceased non-COVID-19 viral pneumonia patients and uninfected controls. While most viralassociated ARDS manifest diffuse alveolar damage (DAD), COVID-19 has distinctive angiocentric features, including severe endothelial injury associated with intracellular virus and cell membrane disruption, and widespread vascular thrombosis with microangiopathy and alveolar capillary occlusion [4,11,12].
Other findings include small pulmonary vessel thromboses and alveolar capillary fibrin microthrombi, up to nine times more common among patients with COVID-19 than those with influenza, including both damaged and preserved lung parenchyma [4][5][6]13]. Interestingly, microvascular thrombosis may be present in less severe disease. A case series of five patients with COVID-19 lacking DAD features also reported significant deposits of terminal complement components in the pulmonary microvasculature, further supporting a prominent vascular process [14]. Fox et al. described post-mortem findings in four patients, the small pulmonary vessels showed platelets and fibrin causing aggregation of inflammatory cells with entrapment of neutrophils and small thrombi formation [15].
Thrombotic phenomena were not limited to the pulmonary vasculature; thromboembolic events were also found in the kidneys, with 30% of the examined samples showing thrombi in the glomerular capillaries [5]. A recent systematic review summarized the above histopathological findings classified according to the system involved [12].

Macrovascular thrombosis
Autopsy studies also describe the presence of thrombi in large vascular beds, most frequently pulmonary vasculature. In one postmortem series (n = 12 patients), previously undiagnosed DVT was found in 58% of patients in whom venous thromboembolism (VTE) was not suspected, with PE identified as the direct cause of death in four patients [6]. Similarly, a case-series of 80 post-mortem COVID-19 cases identified DVT in about 40% of them.

Mechanisms of thrombosis
Researchers proposed several mechanisms for COVID-19 associated thrombosis related to SARS-CoV-2′s affinity for the Angiotensin-Converting Enzyme-2 (ACE-2) receptors, which are extensively expressed in the vascular endothelium leading to direct endothelial damage and exocytosis ( Fig. 1) [12,13,16,17,15]. Endothelial tissue damage induced by SARS-CoV-2 results in hyperinflammatory response, apoptosis, tissue factor exposure and release (von Willebrand factor, interlukin-6, and P-selectin), coagulation cascade activation along with leukocytes and platelets recruitment followed by aggregation, which in turn causes microvascular thrombosis [15,18]. In addition, the virus will also trigger innate immunity resulting in mononuclear phagocyte activation, which upregulates tissue factors, pro-inflammatory cytokine release, and complement activation contributing to cytokine storms [19]. More importantly, neutrophils were found to play a major role in microthrombi formation through enhanced extracellular traps (NETs) release resulting in further tissue damage, release of platelets-derived factor, thrombin generation and fibrin deposition, inhibition of fibrinolysis, and platelets-neutrophils aggregation, resulting in immune-thrombosis [19][20][21][22]. Initially this process is limited to the injured lungs, but the hemostatic dysregulation can spread to other vascular beds leading to micro and macro-vascular thrombosis [16].

D-dimer and thrombotic complications
About 43-60% of patients with COVID-19 present with elevated D-dimer levels (Table 1) [1,23,24]. Higher D-dimer levels are noted in patients with VTE compared to those without [25]. Multivariable regression analysis showed an association between D-dimer level > 1570 ng/mL and increased risk of asymptomatic DVT among noncritically ill patients (adjusted odds ratio [OR] 9.1, 95% confidence interval [CI] 1.1-70.1) [26]. A retrospective study of 429 hospitalized COVID-19 patients found a strong association between D-dimer > 2500 ng/mL on presentation to the hospital and thrombotic complications during hospitalization (adjusted OR 6.79, 95% CI 2.39-19.30) [27]. Another retrospective study (n = 3334) found an incremental dose-response between D-dimer level and VTE, where a D-dimer level > 10,000 ng/mL was associated with an adjusted hazard ratio (HR) of 32.63 (95% 17.2-61.89) for VTE [28]. It is also important to highlight that elevated D-dimer levels can be associated with higher odds of bleeding, particularly when thrombocytopenia is present [27]. High-quality diagnostic accuracy studies are lacking; therefore, D-dimer level should be interpreted carefully, considering the clinical context.
Although D-dimer appears to be prognostically relevant in COVID-19 outcomes, no study prospectively validated the use of Ddimer to guide therapy. In addition, variations in test assays and cutoff values make it challenging to pool and generalize these data in the clinical setting [41].

Prothrombin time (PT) and activated partial thrombin time (aPTT)
PT and aPTT, but not thrombin time, are usually prolonged in patients with COVID-19 compared to healthy controls [42]. However, the proportion of patients with abnormal results varies [36,30,43]. The association between coagulation parameters and clinical outcomes is unclear. A case series of 35 patients with COVID-19 showed that over 90% of patients with an increased aPTT demonstrated the presence of lupus anticoagulant and factor XII deficiency, but the VTE rates were low (1/35 patients, 2.85% had confirmed PE) [44]. On the other hand, several studies have linked coagulation times to VTE. Prolonged aPTT was associated with a higher incidence of VTE, while PT was higher in ICU than non-ICU cases [33,45]. Another study showed an association between thromboembolic events and prolonged PT and aPTT (aHR 4.1, 95% CI 1.9-9.1) [46]. Tang et al. compared COVID-19 survivors and non-survivors, and reported a longer PT in the latter group, but not aPTT [30]. Conversely, a retrospective study of 1859 patients with COVID-19 reported a 4% relative increase in death with each 1-second increase in aPTT (HR 1.04, 95% CI 1.02-1.05, per 1-second increase) [36]. The ability of PT on admission to discriminate between survivors and non-survivors is poor, with an area under the receiver operating curve (ROC) curve of 0.64 [24].

Other coagulation parameters
Compared to healthy controls, patients with COVID-19 have higher fibrinogen, fibrin degradation products (FDP) and antithrombin levels [47]. A prospective study of 150 ICU patients with COVID-19 showed markedly increased fibrinogen levels, factor VIII and von Willebrand factor antigen, while factor V and antithrombin levels were normal [43]. FDP levels appear to be associated with increased disease severity and mortality [30,34]. Several studies used global coagulation tests to assess the pathophysiology of COVID-19 associated coagulopathy. In a case series of 24 critically ill patients, thromboelastography showed increased fibrin generation, greater clot strength and reduced fibrinolysis, suggesting that COVID-19 associated coagulopathy is inconsistent with disseminated intravascular coagulopathy but rather hypercoagulability with a severe inflammatory state [48]. Another study of 22 patients with critical COVID-19 in whom rotational thromboelastography showed shorter clot formation time and increased maximum clot firmness, suggests a state of hypercoagulability [49].
Arterial thrombosis is less common than VTE irrespective of disease severity, though reported rates vary widely. Some studies demonstrated a low incidence of acute coronary syndrome (0.0-2.1% in ICU) and 1.0% in general ward, limb ischemia (0.7% in ICU) and mesenteric artery ischemia (0.7% in ICU) [43,51,54]. However, a large retrospective study reported a much higher incidence of myocardial infarction (MI) and stroke, especially in ICU patients when compared to non-ICU patients (13.9% vs. 7.3% and 3.7% vs. 0.9%, respectively) [28]. This may be related to variations in event definitions and detection: myocardial injury appears to be common (i.e. increased cardiac troponin level) ranging between 7.2% and 36%, though actual rates of coronary artery thrombosis are unclear [32,55].
After MI, ischemic stroke is the second most frequently reported arterial thrombotic event (incidence 0.9-3.7% for non-ICU and ICU patients, respectively) [28]. A case series of patients < 50 years old from the Extracorporeal Life Support Organization (ELSO) registry reported a higher rates of mechanical complications in COVID-19 patients on ECMO support in comparison to historic 2019 rates [56].

Risk factors for thrombosis in COVID-19 patients
Few studies described thrombosis risk factors in COVID-19 patients. A prospective observational study of 3334 COVID-19 patients showed an increased risk of thrombosis with a D-dimer level > 10,000 ng/mL (HR 7.09, 95% CI 4. 69 [61]. However, It is unclear whether D-dimer in COVID-19 patients is merely a marker of current thrombosis, a predictor of a future thrombotic event, or both.
Another retrospective observational study (n = 4389) assessed association between the use of prophylactic or therapeutic anticoagulation and mortality, intubation and bleeding [66]. Authors found an association between therapeutic or prophylactic anticoagulation use and lower in-hospital mortality and intubation [66]. This observation came at the expense of a higher risk of major bleeding in patients using therapeutic anticoagulation (3%) when compared to prophylactic (1.7%) or no anticoagulation (1.9%).
Several studies attempted to evaluate the effects of anticoagulants according to risk stratification tools. When stratified by D- dimer level > 3000 ng/mL, COVID-19 patients receiving VTE prophylaxis had lower 28-day mortality than those without (32% vs. 52%, P = 0.017) [64]. Viscoelastic coagulation tests have also been used to risk-stratify [76]. In one case series, patients were given an intensified prophylactic dose (nadroparin 6000 IU twice a day or 8000 IU twice a day in patients with BMI > 35 kg/m 2 ), along with clopidogrel based on viscoelastic testing. None of the patients developed VTE; however, 7 patients died due to refractory hypoxemia and multiorgan failure. Given the competing risks of bleeding and thrombosis, clinical trials are needed to investigate the potential benefit of such intensified thromboprophylaxis in risk-stratified groups.
These observational studies come with inherit risk of bias due to confounding, survival bias. Therefore, their results should be interpreted with caution especially if the results contradict the results of high quality RCTs that were not available at the time of their publication.

Randomized controlled trials
A multicenter superiority RCT (n = 600) of ICU COVID-19 patients showed no difference in the primary composite outcome of venous or arterial thrombosis, ECMO institution, and 30-day mortality with the use of higher-intensity prophylaxis (enoxaparin 1 mg/kg daily) compared to standard prophylaxis (enoxaparin 40 mg daily) [68]. The pre-specified secondary non-inferiority analysis with respect to major bleeding also failed to meet the non-inferiority criteria (noninferiority margin OR 1.8). It showed a higher bleeding risk in the higher-intensity prophylaxis (OR 1.83, 1-sided 97.5% CI 0.00-5.93.).

Therapeutic anticoagulation
Six retrospective observational studies and six RCTs assessed the use of therapeutic anticoagulation in hospitalized COVID-19 patients with mixed results, (

Observational studies
Several observational studies reported conflicting results with few studies demonstrating either reduced or similar in-hospital mortality rates in patients receiving therapeutic anticoagulation compared to those receiving VTE prophylaxis or no prophylaxis, along with a similar risk of bleeding [8][9][10]66,77]. In contrast, a retrospective study (n = 501) showed higher in-hospital mortality in patients receiving preemptive therapeutic anticoagulation compared to standard VTE prophylaxis (OR 2.3, 95% CI 1-4.9) [69].
The REMAP-CAP, ATTACC, and ACTIV-4a trials randomized ICU patients with COVID-19 to receive either therapeutic anticoagulation or VTE prophylaxis. After the second interim analysis, the recruitment in these trials was terminated due to futility as pre-specified in the protocol [71]. These trials enrolled a total of 1074 patients and were analyzed as one trial using Bayesian statistics. Authors defined severe COVID-19 cases as those requiring ICU admission for respiratory or hemodynamic support. The median D-dimer level was 800 ng/mL, and only 47% of the patients had a D-dimer > twice the upper limit of normal. There was no difference in organ support free days (aOR 0.87, 95% credible interval [CrI] 0.70-1.08), in-hospital survival (aOR 0.88, 95% CrI 0.67-1.16), or major bleeding [71]. Another open-label RCT (n = 615) compared therapeutic versus prophylactic anticoagulation in hospitalized COVID-19 patients with elevated D-dimer. In the therapeutic anticoagulation arm, patients received either 20 mg of rivaroxaban or LMWH/UFH depending on the acuity of illness, followed by rivaroxaban up to 30 days [70]. The use of therapeutic anticoagulation did not reduce the risk of 30-day composite outcome of time to death, hospital length of stay, or the duration of supplemental oxygen use in hospitalized patients but increased the risk of major bleeding compared to VTE prophylaxis (8.4% vs 2.3%, respectively) [70].
These trials showed lower rates of thrombotic events with the use of therapeutic anticoagulation compared to usual care which included both low-dose and higher-intensity prophylaxis (6.4% vs 10.4%), however, this did not translate into reduction in mortality or other important outcomes [71]. However, it is challenging to impact mortality in critically ill patients due to various reasons including the heterogeneous population, variable causes of death even if the intervention seems to be effective in other secondary outcomes [79].

Randomized controlled trials in hospitalized non-critically ill patients
The same trials REMAP-CAP, ATTACC addressing the role of therapeutic anticoagulation (LMWH or UFH) compared to the standard of care thromboprophylaxis in hospitalized non-critically ill patients (n = 2219) were stopped after meeting the superiority criteria [72]. The use of therapeutic anticoagulation was associated with a higher rate of organ support-free days at day 21 (aOR 1.27, 95% Crl 1.03-1.58) and higher absolute difference in survival rates to hospital discharge rate without the need for organ support (80.2% vs 76.4%), but it was statistically not significant (95% Crl 0.5-7.2) [72]. In support of this, the HEP-COVID, a blinded multicenter RCT of 257 COVID-19 hospitalized critically ill and non-critically ill patients with D-dimer levels > 4 times ULN or SIC ≥ 4 [73]. Compared with standard or higher-intensity VTE thromboprophylaxis, therapeutic anticoagulation was associated with lower rates of mortality and major thromboembolism among inpatients with COVID-19 with high D-dimer levels (RR 0.46; 95% CI 0.27-0.81, P = 0.004) but not in critically ill patients (RR 0.92, 95% CI 0.62-1.39, P = 0.71) [73]. The RAPID open-label RCT(465 patients), showed a similar results of lower mortality in COVID-19 patients with elevated D-dimer levels receiving therapeutic anticoagulation compared to prophylactic anticoagulation (1.8% vs 7.6%) but no difference in the composite primary outcome of death, invasive and non-invasive mechanical ventilation and ICU admission (OR 0.69, 95% CI 0.43-1.10) [74]. In summary, little evidence supports a benefit of full-dose anticoagulation in critically ill patients with COVID-19 compared to standard VTE prophylaxis. However, there are about 70 ongoing RCTs assessing the role of anticoagulation in different populations (outpatient, hospitalized and critically ill inpatients) with various types of pharmacological agents [80].

Heparin resistance in COVID-19 patients
In a retrospective study, 14 of 69 COVID-19 ICU patients received therapeutic anticoagulation with either unfractionated heparin (UFH) (n = 10) or LMWH (n = 5) [81]. Around 80% of patients on UFH developed heparin resistance, and all patients on LMWH had an unexplained suboptimal anti-factor Xa level. Although intriguing, we need high-quality studies to validate the results before making clinical inferences. Heparin resistance is rarely reported in critically ill patients; however, data from patients undergoing cardiac surgery, which are usually high risk for heparin resistance, reported this phenomenon in around 20% of patients showing evidence of heparin resistance [82].

Clinical Implications
Despite some limitations, the current body of evidence has several clinical implications. First, the high prevalence of VTE in COVID-19 patients should prompt clinicians to use pharmacologic VTE prophylaxis in hospitalized patients without contraindications [83]. Second, although it appears unlikely that intensified VTE prophylaxis or therapeutic anticoagulation improves clinical outcomes in COVID-19 patients without documented VTE, clinicians may have a lower threshold to investigate for VTE, especially in patients with elevated D-dimer (e.g., six times of normal or > 10,000 ng/mL), coronary artery disease, and in those > 65 years of age [26,28,45]. Third, measuring D-dimer in critically ill COVID-19 patients may help clinicians risk-stratify patients for further VTE testing, especially when the degree of hypoxemia is disproportionate to the findings in chest radiographs. Lastly, the low rates of VTE post-hospital discharge suggest the safety of standard discharge practice without VTE prophylaxis.

Knowledge and research gaps
We identified several key research gaps for further study, (Supplementary Table 2). Many of these questions are already being addressed prospectively, and given the rapid development of research into COVID-19, it is anticipated that definitive data which can inform clinical practice is forthcoming.

Conclusions
While existing evidence is limited in quantity and quality, COVID-19 appears to increase micro-and macro-vascular thrombosis rates in critically ill patients, which may contribute to the burden of disease. The clinical impact upon patients without critical illness is less clear. More research is needed to inform clinical practice.

Financial support
No funding or sponsorship was received for this review to be conducted or for the publication of this article.

Declarations
• Ethics approval and consent to participate: not applicable. • Consent for publication: not applicable. • Availability of data and materials: not applicable. • Competing interests: the authors declare that they have no competing interests.
• Funding: no funding or sponsorship was received for this review to be conducted or for the publication of this article.
• Authors' contributions: Zainab Al Duhailib was responsible about critical review of the data, and preparing the first draft of the manuscript, graphs, tables, and editing the final draft of the manuscript. Simon Oczkowski was responsible about conception of the project. All authors were responsible about manuscript editing of the first draft and subsequent drafts, methodology review, critical review of the data.