**2. Effects of the SARS-CoV-2 virus on the cardiovascular system**

The COVID-19 pandemic greatly challenged clinicians, both due to the sheer number of patients, but also because of the lack of therapeutic consensus and incomplete understanding of disease pathogenesis. Most fatal cases of COVID-19 relate to a severe atypical pneumonia, accompanied by a sudden systemic deterioration, despite therapeutic intervention in the hospital setting.

The infection with the SARS-CoV-2 virus primarily affects the respiratory structures, but the involvement of the cardiovascular system is also frequent. Cardiovascular complications in addition to respiratory disease may develop in all phases of COVID-19, which can start with the dramatic picture of acute heart failure (ACF), acute coronary syndrome (ACS), pulmonary venous thromboembolism (VTE), or even sudden cardiac death, as shown in **Figure 1**. The pathophysiological mechanisms underlying these disproportionate effects of the SARS-CoV-2 infection on patients with cardiovascular comorbidities, however, remain incompletely understood [7]. Thromboembolic events, usually accompanied by violent, pulmonary, and/or systemic complications, have been described from early on, since the beginning of the pandemic, with infectious inflammatory response patterns rapidly shifting into a typical systemic inflammatory response syndrome (SIRS) or ARDS, which could potentially induce multi-organ failure (MOF) and, subsequently, death. As we enter the third year of the pandemic, COVID-19 pathophysiology is slowly unraveling as we begin to better comprehend the complex interplay between the direct cytotoxic effects of SARS-CoV-2 on pneumocytes and endothelial cells, the emerging local and systemic inflammatory response, and the ways in which these responses interact with hemostatic homeostasis, a mechanism which has been deemed as central and, at least to this extent, unprecedented [8].

#### **2.1 Cardiac tissue damage**

COVID-19 was initially considered to be solely a respiratory disease, yet clinical outcomes quickly revealed that, undeniably, this infection implies multi-organ involvement. Perhaps most notably, the heart has been shown to represents a target organ for SARS-CoV-2-related pathogenesis, with a high prevalence of cardiac injury following COVID-19, often diagnosed only through biomarker evaluation. Beyond subclinical myocardial damage, SARS-CoV-2 infection may also cause more aggressive, clinically apparent modifications, such as myocarditis, accompanied by a subsequent diastolic dysfunction or severe reduction of left ventricle ejection fraction, not to mention the fact that heart failure may represent a short−/long-term consequence of COVID-19-related inflammatory cardiomyopathy, with dramatic consequences regarding prognosis [9].

Regarding myocardial damage in COVID-19, although the full pathophysiology is still incompletely understood, multiple mechanisms are most likely incriminated (see **Figure 2**), which, globally, can be divided into two main groups: direct, specific modifications, related to the cytopathic effects of SARS-CoV-2 infection, and indirect, general modifications, commonly seen in other severe infections, as well [10].

### *2.1.1 Direct cytopathic myocardial injury*

The aforementioned ACE2, a type I transmembrane protein, highly expressed in different organs (heart, lungs, gut, and kidneys), mediates SARS-CoV-2 entry into

**Figure 1.** *Main COVID-19-associated cardiovascular complications and underlying pathophysiological mechanisms.*

the host cells, with different clinical implications, depending on the targeted organ, and represents the key molecular entity involved in the direct cytopathic effects of SARS-CoV-2 infection within the cardiac tissue. After entering the host cell through the host ACE2 receptor, SARS-CoV-2 utilizes the host's RNA-dependent RNA polymerase to replicate its own structural proteins, which are then assembled, and the newly formed virions are released from the infected cells, perpetuating the viral life cycle. Theoretically, as a consequence of this process, infected cells may become damaged/destroyed [11].

This idea is supported by a recent autopsy study, analyzing cardiac tissue from 39 consecutive patients who died as a consequence of COVID-19, which found viral genome in the myocardial tissue, yet in situ hybridization showed that the most likely localization of SARS-CoV-2 not to be in the cardiomyocytes, but rather in interstitial cells or macrophages invading the myocardial tissue [12]. Even so, in engineered heart tissue models of COVID-19 myocardial pathology, SARS-CoV-2 demonstrated

*Impairment of the Cardiovascular System during SARS-CoV-2 Infection DOI: http://dx.doi.org/10.5772/intechopen.103964*

#### **Figure 2.**

*Pathophysiology of COVID-19-related myocardial injury [15, 16].*

the ability to directly infect cardiomyocytes through ACE2, resulting in contractile deficits, cytokine production, sarcomere disassembly, and cell death [9].

Furthermore, ACE2 must not be viewed as a mere bystander in the pathophysiology of COVID-19 myocardial injury, seeing as, besides being the host cell receptor of SARS-CoV-2, ACE2 is an enzyme involved in the renin-angiotensin-aldosterone system (RAAS). Specifically, ACE2 cleaves angiotensin II, a very potent vasoconstrictor, into angiotensin 1–7, which manifests vasodilator and anti-inflammatory effects. ACE2 also demonstrates a weak affinity for angiotensin I (or proangiotensin, formed by the action of renin on angiotensinogen), competitively limiting angiotensin II synthesis by ACE. Angiotensin I is converted by ACE2 into the nonapeptide angiotensin 1–9, which will manifest vasodilator effects through subsequent angiotensin type 2 (AT2) receptor stimulation. Therefore, ACE2 can counteract the undesirable effects of angiotensin II, demonstrating vasodilator, antioxidant, and anti-fibrotic effects [13]. In the context of SARS-CoV-2 infection, after S protein binding is complete, the virus attaches ACE2 through membrane fusion and invagination, causing a downregulation of ACE2 enzymatic activity [13]. Additionally, ACE2 also demonstrates immunomodulatory properties, both directly, via its interactions with macrophages, and indirectly, as it reduces expression of angiotensin II, which stimulates inflammation [14]. Thus, ACE2 downregulation in the context of SARS-CoV-2 infection may increase angiotensin II levels, favoring AT1 receptor activity, with a subsequent vasoconstriction, fibrotic, proliferative, and pro-inflammatory effects [10].

#### *2.1.2 Indirect mechanisms of myocardial injury*

As is the case with all severe respiratory infections, COVID-19 has a general deleterious effect on the cardiovascular system, with fever and sympathetic activation causing tachycardia and implicitly increasing myocardial oxygen consumption [9, 10], while prolonged bed rest and systemic inflammation will favor coagulation disorders, as supported by clinical findings – both venous and atypical arterial thromboembolic events have been documented in COVID-19 patients (see subchapter 3.4. Thromboembolic events and bleeding risk). Hypoxemia, another hallmark of COVID-19, will determine enhanced oxidative stress and increased production of reactive oxygen species, with subsequent intracellular acidosis, mitochondrial damage, and cell death [7, 9].

Moreover, another series of indirect mechanisms for COVID-19-related myocardial damage appears as a result of the abnormal inflammatory response which may be elicited by SARS-CoV-2 infection (i.e. a pro-inflammatory surge, the so-called "cytokine storm," which may occur as early as 1 week after the initial exposure and infection) [15].

Indeed, individual immune response is the cardinal element behind SARS-CoV-2 infection progression. Upon viral genome expulsion into the host cytosol, SARS-CoV-2 viral replication begins, with aberrant RNA sequences, byproducts of replication, being, in turn, detected by intracellular receptors, which activate the cellular antiviral response, involving enhanced leukocyte chemotaxis and transcriptional induction of type I and III interferons (IFN-I/-III), followed by under-regulation of IFN-stimulated genes [16]. Lung cell damage incurred during replication will also activate the local immune response, resulting in monocyte/macrophage recruitment [16], while chemokines will induce specific leukocyte subset recruitment and coordination [16]. Circulating immune cell relocation in the pulmonary tissue will determine additional cytokine/chemokine production, while also creating multiple imbalances in immune cell populations – increased leukocyte count and neutrophillymphocyte ratio, with decreased lymphocytes (especially T cells [17]), thus setting the scene for immune response dysregulation [3].

In fact, the relationship between SARS-CoV-2 infection and extensive activation of inflammation signaling pathways has been well documented, representing the main immunopathological mechanism through which severe forms occur, in susceptible individuals. During the acute phase of the infection, a disproportionate response occurs between T helper cell populations (types 1 and 2), characterized by high circulating levels of interleukin (IL)-1β, IL-1RA, IL-2, IL-6, IL-7, IL-8, IL-9 IL-10, interferon gamma-induced protein-10 (CXCL10), monocyte chemoattractant protein-1 (CCL2), macrophage inflammatory protein 1α (CCL3) and 1β (CCL4), granulocyte colony-stimulating factor, vascular endothelial growth factor (VEGF), and tumor necrosis factor (TNF) α [16, 18, 19], which mediate widespread lung inflammation, in an attempt to eradicate the pathogen [3]. The resulting hyperinflammatory status, as well as the individual excessive levels of certain circulating cytokine species, have been independently associated with an unfavorable evolution and increased mortality [20]. This hyper-inflammatory state seems, at least intuitively, to be pivotal in the development of cardiac injury, seeing as positive correlations have been established between the increase in inflammatory markers and myocardial damage in COVID-19 [21, 22]. Indeed, this idea is additionally supported by previous studies, in other septic conditions, evidencing that the release of

pro-inflammatory cytokines such as TNFα and IL-1β, were responsible for myocardial cells depression through modulation of calcium channel activity and nitric oxide production [23].

It may also be the case that the cytokine storm following SARS-CoV-2 infection determines the AHF, recurrently seen in severe COVID-19, as the inflammatory activation and oxidative stress background are similarly expressed generally in heart failure, predisposing to a more severe clinical course [24].

Lastly, the aforementioned marked inflammatory changes will also take place in the endothelium, as shown in postmortem histological studies, evidencing lymphocytic endotheliitis with apoptotic bodies and viral inclusion in multiple organs [7, 25]. Endotheliitis can lead to disseminated intravascular coagulation, with small or large vessels thrombosis and infarction, and will determine significant new vessel growth through a mechanism of intussusceptive angiogenesis [25].

#### **2.2 Coagulation disturbances**

After becoming infected, roughly 20% of COVID-19 patients will be incapable of controlling/halting viral replication through their initial immune response, which may be aberrant/insufficient or overwhelmed by a high initial viral load, or both [26]. This subgroup of patients will thus progress to a more severe disease phenotype, with aggravating symptomatology secondary to uncontrolled viral replication, leading to host pneumocyte and endothelial cell apoptosis, which in turn will activate platelets, induce procoagulant factor expression (fibrinogen, factors V, VII, VIII, X, and von Willebrand), and increase inflammatory response, as the body tries and fails to keep the infection localized to the lungs [27]. This sequence of host responses will additionally damage the pulmonary parenchyma (through further destruction of pneumocytes, microangiopathy, and inflammatory microthrombi), causing even more severe symptoms and hindering oxygenation, thus imposing the need for an additional oxygen supply. Even so, at this point, a relative balance between procoagulant and anticoagulant (but also pro-inflammatory/anti-inflammatory) factors is still maintained. In only approximately 5% of symptomatic patients, the pro-inflammatory processes involved in the immune response to SARS-CoV-2 infection will derail into the so-called "cytokine storm," which will fuel pro-inflammatory and pro-coagulatory processes even further, resulting in systemic endotheliitis and capillary leakage, cellular dysfunction, organ dysfunction (including ARDS), and overt activation of the (systemic) coagulation cascade resulting in the need for critical organ support [28]. In fact, SARS-CoV-2 infection may trigger endothelial dysfunction not only through the direct cytopathic effect of invasion on vascular endothelial cells but also through indirect mechanisms, such as hypoxia and the induced inflammatory response [27]. Moreover, some patients have also manifested antiphospholipid antibodies [28].

Therefore, all factors of the classic Virchow triad are influenced during the course of COVID-19, and they contribute synergically to the risk of thromboembolic events: hemodynamic changes (increased blood viscosity due to elevated fibrinogen, but also venous stasis due to hospitalization and disease-related immobilization); hypercoagulability (due to an overwhelming inflammatory state, occurring early after infection); and endothelial injury/dysfunction (ACE2 receptor expression on endothelial cells allows viral entry and cytopathic effects – endotheliitis) [3].
