COVID-19, cardiovascular diseases and cardiac troponins

There has been strong evidence of myocardial injury in coronavirus disease 2019 (COVID-19) patients with significantly elevated serum cardiac troponin (cTn). While the exact mechanism of injury is unclear, possible suggested pathological mechanisms of injury are discussed. These include increased susceptibility of the myocardium and endothelium to viral invasion, underlying hyperinflammatory state and subsequent cytokine storm, a hypercoagulable and prothrombotic state, and indirect myocardial injury due to hypoxemia. As a result of these pathological mechanisms in COVID-19 patients, cTn may be elevated largely due to myocarditis, microangiopathy or myocardial infarction. The utility of cTn as a biomarker for measuring myocardial injury in these patients and assessing its ability as a prognostic factor for clinical outcome is also discussed.

into a receptor-binding domain and a membrane-fusion domain following cleavage by host cell proteases [37][38][39]. Because ACE2 is a target for SARS-CoV-2, a natural possibility is that cells with the greatest expression of ACE2 are the most vulnerable to infection. Thus, in the human body, the areas that are among the most at risk are the lungs, heart, vasculature endothelium, gastrointestinal tract epithelium and kidneys. ACE2 is also involved in the catalysis of Angiotensin (Ang) II to Ang 1-7. Ang 1-7 are involved in protecting the cardiovascular system by opposing the vasoconstrictive, pro-inflammatory, proproliferative and profibrotic actions of Ang II [40]. Therefore, the myocardium and endothelium are more susceptible to SARS-CoV-2 due to ACE2 expression, and by the virus binding to ACE2, physiologic level of ACE2 is decreased, reducing the production of Ang 1-7, ultimately decreasing the protective effect on the cardiovascular system.
In a combination of the underlying chemokine storm affecting the entire body, SARS-CoV-2 preference for the myocardium and endothelium, and the inhibitory effects on ACE2 and Ang 1-7, myocardial injury can occur via multiple mechanisms. The hyperinflammation and cytokine storm cause an excessive proliferation of T cells and monocytes, which can result in myocyte stress, injury and apoptosis, as well as subsequent fibroblast activation and extracellular matrix remodeling, ultimately resulting in diffuse myocardial and endothelial injury [41,42]. The underlying hyperinflammatory state is also a hypercoagulable state which can cause the development of coronary microvasculature thrombosis [20,41,43]. This high stress state can also cause any pre-existing coronary plaques to rupture and form additional thrombi. There is also evidence of direct viral invasion of both the myocardium and endothelium, which have been noted on biopsy and autopsy results [42,44]. Another mechanism of injury is systemic secondary damage to the myocardium and the cardiovascular system, which can be from respiratory failure and hypoxemia due to COVID pneumonia or ARDS or a diffuse hypercoagulable state resulting in decreased perfusion of the myocardium.
Through the various mechanisms of myocardial damage discussed, elevated cTn in COVID-19 patients has been noted to occur largely via three primary pathologies: myocarditis, microangiopathy and myocardial infarction (MI). Myocarditis can occur either by a direct viral invasion and damage to the cardiomyocytes resulting in a T-cell response and subsequent inflammation of the myocardium or by an indirect inflammation due to the cytokine storm response to the infection [40,45,46]. On biopsy, high viral load or diffuse inflammatory monocular infiltrates may be seen in the myocardium. Microangiopathy can similarly occur due to the direct viral invasion of the endothelium via ACE2 found on the endothelium resulting in vasculature injury. It can also occur indirectly via the hypercoagulable and prothrombotic state in these patients resulting in endothelial dysfunction and ultimately myocardial damage due to perfusion defects, vessel hyperpermeability and vasospasms [47]. Perhaps the currently most elucidated cardiovascular complication of COVID-19 is MI. Past studies of patients during the influenza seasons have similarly shown that patients with severe influenza symptoms had a significantly increased risk of MI due to the hyperinflammatory state as a result of the virus, which resulted in proliferation of proinflammatory cytokines, macrophage infiltration of the arterial walls, and subsequent prothrombotic state resulting in myocardial injury and ultimately MI [48,49]. Similar to the pathogenesis of myocarditis and microangiopathy, MI in COVID-19 patients can occur either due to direct or indirect damage to the myocardium [40,45]. Direct damage can occur via direct viral invasion and indirect hyperinflammatory state, like what has been seen in influenza patients, resulting in plaque instability and rupture with subsequent thrombosis. Indirect damage and subsequent MI are largely due to hypoxemia due to the underlying lung pathology; however, there is evidence that fever, tachycardia, or endocrine dysregulation, all of which is prominent in COVID-19 patients, can also result in hypoxemia. Several of the key studies cited here that may be important further consider are listed Table 1.
Briefly mentioned above, there has been evidence of COVID-19-associated myocardial damage presenting similarly to what was previously seen in patients with influenza-associated myocardial disease. Due to the widerange of clinical complications of COVID-19 contributing to the mortality rates, along with the many underlying comorbidities these patients may have, it is very difficult to accurately compare the mortality rates specifically due to CVD secondary to influenza and COVID-19. However, knowledge from past influenza seasons can serve as a key model in myocardial damage seen in COVID-19 patients by considering whether there are notable similarities to past viral pandemics or the annual influenza. Past studies of influenza have shown a significant correlation between seasonal influenza incidence and cardiovascular mortality, mainly MI and ischemic heart disease [48,51,52]. Study of the mechanism of myocardial damage in these patients showed transient endothelial dysfunction, which in turn destabilized vulnerable atherosclerotic plaques, increasing the risk of acute MI [48]. There was also evidence of an overactivation of inflammatory and coagulation pathways due to the overexpression of inflammatory cytokines, infiltration of arterial and myocardial walls by macrophages, and indirect damage to vascular endothelium due to a hypercoagulable state. A study of the pandemic (H1N1) 2009 virus infection similarly showed a strong association between acute myocardial dysfunction and elevated cTns [53]. Biopsies of the myocardium of patients with the H1N1 viral infection also showed microscopic foci of lymphoid aggregates with myofibril necrosis, comparable to the diffuse inflammatory monocular infiltrates that could be seen in the myocardial biopsies of COVID-19 patients. It is thus important to reassess whether the current findings of myocardial damage and subsequent complications are in fact significantly different from that of past pandemics. More importantly, however, this similarity provides insight into both how the current management of COVID-19 patients does not significantly vary from the treatment of critically ill patients in previous pandemics, and the overall success in being able to manage and treat patients with COVID-19 and CVD complications.

Troponin as a prognostic factor
As the pathogenesis of cTn elevation due to myocardial injury in COVID-19 is better understood, it is important to consider the potential benefit of measuring cTn in COVID-19 patients. As discussed, pre-existing cardiovascular health is a powerful predictor of disease severity following SARS-CoV-2 infection and more specifically, the presence of myocardial injury is associated with an increased risk of complications and death in COVID-19 patients [50,54]. Therefore, it is necessary to consider the most effective methods of quantifying and representing the level of myocardial injury in patients. Pro-inflammatory markers such as N-terminal-pro brain natriuretic peptide, elevated cTn, and elevated high-sensitivity CRP are known markers of myocardial injury, and recent international guidelines for the diagnosis of myocardial damage and acute MI emphasize the value of measuring high-sensitivity cardiac troponin T (hs-cTnT) and cardiac troponin I (hs-cTnI) [55][56][57]. Within the context of COVID-19, while cTnT levels have shown promise as a prognostic factor for severity of illness, cTnI has been the more commonly reported metric for virus-related cardiac injury in COVID-19 patients [58,59]. Many studies have shown evidence of significantly elevated hs-cTnI in COVID-19 patients. A study of 700 COVID-19 patients at the University of Pennsylvania hospital reported elevated troponin concentrations for ICU patients upon admission [60]. In a separate study of 113 patients in Italy, patients were stratified into different groups depending on the serum level of hs-cTnI, and it was noted that patients in the group with the highest level of hs-cTnI had a longer hospital stay (median 37 days) and a greater need for ICU admission [61]. Additionally, in a study completed in Illinois, 673 patients with elevated troponin had significantly increased odds of critical illness (OR 3.65; 95% CI 2.03-6.57) [62]. Not only is cTnI an independent predictor of disease severity and ICU admission, but it is also, unsurprisingly, associated with increased patient mortality [63][64][65]. For example, in a study of 1919 patients in Wuhan, patients with acute cardiac injury, defined as serum hs-cTnI above the 99th percentile upper reference limit, had an odds ratio of 80.07 for in-hospital mortality, and nonsurvivors had significantly higher levels of hs-cTnI (nearly tenfold) than survivors [66]. The relevance of cTnI is further highlighted when considering that after adjusting for relevant clinical factors, even small amounts of myocardial injury were associated with a significantly increased risk of patient mortality, and that elevated levels of cTnI remain an independent predictor of death regardless of other elevated acute phase proteins and inflammatory markers in patients with CVD [67]. It is also important to note the prognostic value of cTnI in comparison to other inflammatory biomarkers such as CRP, D-dimer and lactate dehydrogenase. In a recent study comparing the short-term prognostic values of the 10 Review above mentioned biomarkers, increased levels of all these inflammatory markers were associated with increased short-term mortality and increased risk of all-cause death in COVID-19 patients [68]. However, as a predictor for 30-day all-cause death, cTnI was a significantly better predictor compared with CRP, lactate dehydrogenase and D-dimer and additionally, cTnI as low as 21 ng/l was able to provide excellent prediction capacity. In summary, cTnI is a biomarker highly useful for measuring myocardial injury and is considered a gold standard for the early diagnosis of cardiac complications such as MI, microangiopathy or myocarditis with great clinical relevance [55,69]. Furthermore, as pre-existing cardiovascular health is a good predictive factor for risk and severity following SARS-CoV-2 infection, and serum cTnI additionally is an independent predictor of COVID-19 disease severity and mortality, it is critical to consider the role of cTnI in COVID-19 risk stratification [64,70]. In studies of patients with cardiac injury, defined as cTnI levels above the 99th percentile, cardiac injury was significantly related to a lower survival rate, further emphasizing the relationship between myocardial health and COVID-19 severity [71,72]. Naturally, on patient admissions, one can measure a variety of biomarkers such as cTnT, N-terminal-pro brain natriuretic peptide, alanine transaminase, D-dimer or CRP, as determiners of inflammatory status; however, cTnI has been reported with the greatest prognostic value for patient risk and is thus the most promising agent for the hopeful identification of patients who would develop the most severe systemic inflammatory response to SARS-CoV-2 infection and subsequent cardiac complications, which can be fatal.

Conclusion & future perspective
Not only is CVD a significant risk factor for worse clinical outcome in COVID-19 patients, but studies have also suggested the onset of new myocardial injury and cardiac pathologies due to the infection with SARS-CoV-2, as suggested by significantly elevated cTn. As discussed, there are many pathologic mechanisms of injury to the myocardium whether it is due to direct viral invasion into the myocardium and endothelium or damage to the myocardium due to the underlying hyperinflammatory, hypercoagulable and prothrombotic state or indirect damage due to systemic disease, subsequent hypoxemia and poor myocardial perfusion. While new onset MI has been most seen in COVID-19 patients, the risk of myocarditis and microangiopathy remains high and is of serious concern in patients who are already critically ill. As a prognostic factor for these patients, cTn, and specifically cTnI, have significant value in predicting the clinical outcomes in these patients and is an independent predictor of disease severity, ICU admission, and mortality in COVID-19 patients. This is especially important for patients with underlying inflammatory states such as chronic illnesses or end-stage renal disease, as baseline cTnI can already be elevated and these patients are at an increased risk of severe disease.
Thus, moving forward in the care of COVID-19 patients, it is imperative to monitor cTnI levels in hospitalized patients to identify and manage any clinical or subclinical myocardial inflammation prior to the development of major cardiac pathologies such as myocarditis, microangiopathy and MI, and to allow for a more specific and prompter therapy to be undertaken. As noted previously, due to a significant proportion of the population with underlying comorbidities experiencing an increased risk for severe COVID-19 illness, this current review of cTns in COVID-19 patients can provide insight on the vulnerability of select populations to severe illness. The number of vaccinated people has continued to increase nationally, but there is still hesitance among a significant portion of the country, thus challenging the goal of effectively lowering the number of COVID-19 infections both nationally and globally. It would be imperative for future studies to see how the vaccinations or lack thereof impacted these vulnerable populations so that we can better understand the efficacy of the vaccine and perhaps better understand the pathogenesis of COVID-19.

Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
future science group 10.2217/fca-2021-0054 • Cardiovascular diseases (CVD), hypertension, Type 2 diabetes mellitus, cancer, chronic kidney diseases, congestive heart failure, obesity and smoking are associated with significantly increased severity of coronavirus disease 2019 (COVID-19) infection. • Increased levels of cardiac troponin have been correlated with higher rates of CVD complications and mortality in COVID-19 patients. • Risk of severe COVID-19 illness is increased for patients with pre-existing CVD.
• Primary pathological mechanism of injury to myocardium is through direct viral invasion into the myocardium and endothelium or damage to the myocardium due to the underlying hyperinflammatory, hypercoagulable and prothrombotic state. • Secondary pathological mechanism of injury to myocardium is through indirect damage due to systemic disease, subsequent hypoxemia and poor myocardial perfusion. • Cardiac troponins can be an effective biomarker for measuring myocardial injury in COVID-19 patients and can be an independent predictor of COVID-19 disease severity and mortality and has shown greater prognostic value for at risk patients compared with other inflammatory biomarkers.