In December 2019, Wuhan in China experienced a Coronavirus Disease 2019 (COVID-19) outbreak, leading to a global pandemic. This highly infectious virus, the severe acute respiratory syndrome coronavirus (SARS-CoV-2), primarily invades the respiratory system, causing severe pneumonia that can progress to acute respiratory distress syndrome (1). By February 2023, the World Health Organization (WHO) had reported approximately 756 million confirmed cases of COVID-19 globally, along with a staggering 6.8 million cumulative deaths attributable to the virus (2).

The COVID-19 virus contains five structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N). The distinctive crown-like morphology of the coronavirus results from the spike protein, which also facilitates host cell entry and consists of S1 and S2 subunits (3).

SARS-CoV-2, part of the Beta-CoV genus and the Nidovirales order, belongs to the Coronaviridae family. Notably, two previous beta-CoV virus outbreaks—SARS-CoV and Middle East Respiratory Syndrome (MERS) viruses—hit the world in 2002 and 2012, respectively (3).

SARS-CoV-2 has shown higher virulence than SARS-CoV, which is attributed to two key acquired mutations. The receptor binding domain (RBD), a highly variable segment in the SARS-CoV and SARS-CoV-2 genomes, uses six central amino acid residues for adequate recognition of the host angiotensin-converting enzyme 2 (ACE2) receptor. In SARS-CoV-2, mutations in five of the six primary amino acid residues in the RBD allow for a greater binding affinity to the ACE2 receptor. Moreover, an insertion mutation at the S1/S2 fusion site introduces a furin cleavage site, enhancing cellular entry and viral tissue tropism due to S-priming with furin and other proteases (4, 5).

SARS-CoV-2 has led to a significant cardiovascular burden in the infected population, with harmful cardiovascular sequelae persisting even after the acute phase and in asymptomatic patients. This review focuses on the pathophysiology underlying the long-term cardiovascular burden associated with COVID-19 and explores potential new therapeutic targets.

Host neuropilin-1 (NRP-1) binding to the NRP-1 binding domain in RBD precedes ACE2 recognition. This action facilitates host ACE2 recognition by the RBD of COVID-19, triggering a conformational change that allows the virus to become closer to the host cells (6).

SARS-CoV-2 can enter cells through one of two pathways: membrane fusion or endocytosis. In membrane fusion, a host protease known as transmembrane protease serine 2 (TMPRSS2) cleaves and primes the spike protein. Subsequently, the fusion peptide within the S2 subunit is inserted into the host cell membrane. The two heptad repeat domains in the S2 subunit, HR1, and HR2, interact, drawing the viral and host membranes together and enabling membrane fusion. This process allows the injection of the viral genome into the host cell. In the endocytosis pathway, the virus-receptor complex is endocytosed into the endosome, where the S protein is cleaved by cathepsin enzymes (7–9).

SARS-CoV-2 exhibits a broad tissue tropism, as its receptor is expressed in various tissues, including the lung, heart, small intestine, oral mucosa, and testis. These tissues have been found to harbor SARS-CoV-2 RNA (10–13). Viral entry triggers multiple immune responses. Toll-like receptor 3 (TLR-3) recognizes the cytosolic viral genome and activates nuclear factor kappa B (NF-κB), which translocates into the nucleus and stimulates the transcription of interferon-gamma and other inflammatory cytokines. Additionally, the NOD-like receptor (NLRP) gene is activated, leading to the production of NLRP3 proteins. NLRP3 initiates the formation of inflammasomes, which induce cell death through pyroptosis and promote inflammation by stimulating the secretion of inflammatory cytokines (14, 15). This immune response activates a wide range of immune cells, including dendritic cells (DCs), T-cells, and B-cells. DCs carry the viral antigen with major histocompatibility complex class II (MHC-II) receptors and migrate to lymph nodes to stimulate further T and B cell responses. T-cells and B-cells are then activated to eliminate the virus through direct cell killing and antibody secretion, respectively (14, 16).

The excessive release of inflammatory cytokines often leads to a cytokine storm, a lethal state of hyperactive immune response associated with systemic inflammation. Persistent systemic inflammation can cause vasculitis, increasing the risk of blood clot formation, hypercoagulability, ischemia, and cell death. Additionally, acute respiratory distress syndrome results in poor perfusion, hypoxia, multiorgan failure, and death (14, 16).

Although COVID-19 was initially considered primarily a respiratory illness, it is now evident that it causes a systemic infection, damaging multiple organs during and after the infection. This damage may arise from a direct viral infection or the detrimental effects of systemic inflammation (17).

Cardiovascular injury is common among SARS-CoV-2-infected patients, with damage to the myocytes varying from initial injury with elevated troponins to eventual heart failure, indicated by increased levels of the N-terminal-prohormone brain natriuretic peptide BNP (NT-proBNP) (18–20). Among the cardiovascular complications linked to COVID-19 are arrhythmia, myocarditis, acute coronary syndrome, myocardial infarction, and venous thrombosis embolisms, which are detectable through methods such as echocardiography, MRI, electrocardiogram (ECG), coronary angiography, and cardiac autopsies in COVID-19 patients (20–22).

Cardiac involvement in COVID-19 patients predisposes to a higher mortality rate compared to COVID-19 patients without such complications. Higher troponin levels in COVID-19 patients with pre-existing cardiovascular diseases have been associated with doubling the mortality rate (18, 23). Children may also suffer from cardiac dysfunction and coronary abnormalities, such as Kawasaki disease (24).

Although several mechanisms have been proposed, the precise pathophysiological pathway by which SARS-CoV-2 causes cardiac damage has not been entirely established. One such pathway involves SARS-CoV-2-induced hypoxemia secondary to pulmonary dysfunction and respiratory distress syndrome. This results in poor organ perfusion, leading to hypoxic injury in cardiac cells and contributing to myocardial infarction. Another related mechanism involves microvascular injury and thrombosis in the lung microvasculature, which leads to right ventricle heart failure (7, 25).

Systemic inflammation and cytokine storms play a vital role in microvascular injury, endothelial cell activation, venous thrombosis, and the hypercoagulative state associated with COVID-19 (26). This concurs with findings from heart autopsies of COVID-19 patients, where venous thrombosis and immune cell infiltration by CD3+ and CD8+ cytotoxic lymphocytes, CD68+ macrophages, and CD45RO memory cells were identified (27–29). Overexpression of cytokines can cause myocarditis and myocardial injury (27, 30), and cardiac biomarkers have been found to correlate with the levels of inflammatory cytokines (26, 31). Moreover, patients with predisposing cardiovascular risk factors are more susceptible to microvascular injury and endothelial dysfunction since SARS-CoV2 associated inflammation exacerbates the pre-existing endothelial damage aggravating thrombosis which leads to an increased risk of acute coronary syndrome and myocardial injury (32).

Direct virus entry is another proposed mechanism of COVID-19-associated cardiac damage. Inflammation and oxidative stress have also been shown to increase the entry of the SARS-CoV-2 virus into cardiac cells and promote cellular apoptosis (33). Linder et al. documented the presence of SARS-CoV-2 RNA in 61.5% of myocardial autopsies of patients who died of COVID-19, with the viral load exceeding 1,000 copies per μg RNA in 41% of the tissues (27). On the other hand, another study denied the existence of viral RNA and recorded very few RNA copies (34, 35). Similarly, Moustafa et al. found that traces of SARS-CoV-2 RNA are present in peripheral blood mononuclear cells of COVID-19 patients, although in significantly lower quantities compared to bronchoalveolar lavage specimens, suggesting limited viral RNA in the blood (36). Han et al. suggested that COVID-19 could directly infect the heart's pacemaker, causing arrhythmia in hamster models and human embryonic stem cell (hESC)-derived SAN-like pacemaker cells, where the viral RNA was detected inside the pacemaker cells and was capable of inducing ferroptosis (37). In Drosophila and mouse models, non-structural protein 6 (NSP6) of SARS-CoV-2 interacted with host proteins in the heart, potentiating glycolysis, disrupting mitochondrial function, and increasing ROS formation (38).

After viral entry, COVID-19 patients are susceptible to hypertension due to the downregulation of ACE2 receptors in host cells. The ACE2 receptor plays a critical role in the renin-angiotensin-aldosterone (RAAS) system, with its downregulation leading to the upregulation of angiotensin 2, a potent vasoconstrictor (10, 14, 16).

Recent studies using omics-based analyses have provided a more comprehensive understanding of the pathophysiology underlying COVID-19-associated cardiovascular diseases. In the heart tissues of COVID-19 patients, there was an observed upregulation in the transcriptional level of phospholipase C γ2 (PLCG2) in pericytes, fibroblasts, and cardiomyocytes. In contrast, the Afadin level, encoded by the Adherens Junction Formation Factor (AFDN) gene, was predominantly high in endothelial cells (34). Although the mechanism of PLCG2 in the heart is not thoroughly investigated, PLC-γ is known as a regulator of calcium homeostasis within heart tissues and plays a crucial role in cardiac pathogenesis. Its upregulation has been associated with cardiomyocyte apoptosis and mitochondrial dysfunction in patients with myocardial infarction, while its dysregulation can cause cardiac hypertrophy (39, 40). Furthermore, PLC is proposed as an arrhythmogenic factor, exacerbating cardiac fibrosis (41, 42). Whereas Afadin is essential for maintaining endothelial barrier function, the mechanism by which it is linked to SAS-CoV2-associated CVD is not known. Enrichment analysis revealed upregulation in cell differentiation, cell adhesion with elevated immune response, and apoptosis due to oxidative stress in fibroblasts, cardiac cells, and pericytes, respectively (34).

Meanwhile, endocrine senescence pathways in plasma were significantly upregulated in COVID-19 patients with cardiac complications. High FSTL3 and low ADAMTS13 levels were associated with heart failure in COVID-19 patients. The former is an indirect marker of biological aging induced by the TGF-B pathway, and the latter is an antithrombotic agent (19, 43). Another study by Garg et al. found elevated levels of miR-155, miR-499, miR-208a, miR-21, and miR-126 in the blood of critically ill COVID-19 patients. These miRNAs are associated with inflammation, myocardial function, fibroblasts, and endothelial cells (44).

Lipopolysaccharide (LPS)-binding protein (LBP) was upregulated and strongly associated with NT-pro-BNP in critically ill patients with cardiac involvement. LBP levels increase in response to circulating LPS, a major component of the outer membrane of gram-negative bacteria, suggesting gut leakage as an additional mechanism that could exacerbate heart failure in SAR-CoV-2-infected patients (45–48).

Recently, it became evident that the adverse effects of COVID-19 are not limited to the acute phase but also extend to a condition known as long COVID. According to the National Institute for Health and Care Excellence (NICE), patients with ongoing symptoms of COVID-19 or newly developed ones that last beyond three months are diagnosed with long-term COVID (49).

In the Netherlands, patients were followed for three months post-infection and showed evidence of elevated inflammatory cytokines and endothelial dysfunction. This is in line with the elevated plasma levels of endothelin-1 (ET-1) that correlate with contact activation factors, hence suggesting endothelial activation and the coagulation process (50). In England, hospitalized COVID-19 patients suffered from an increased rate of major adverse cardiovascular events after discharge. People without any prior cardiovascular events developed cardiovascular diseases at a rate three times higher compared to healthy individuals (51). Another study followed patients for four months post-infection, and the rate of various cardiovascular diseases increased with new onsets of hypercoagulability and cardiomyopathy sequelae post-infection (52). Similarly, in the USA, a substantial excess burden of cardiovascular diseases occurred in a graded fashion according to the severity of the acute phase when patients were followed in two studies for six months and a year (17, 53). The commonly detected long-term cardiovascular sequelae were cerebrovascular disorders, dysrhythmia, pericarditis, myocarditis, ischemic heart diseases, hypertension, heart failure, and thrombotic disorders (17, 52, 53).

Long COVID's clinical burden stresses healthcare systems and demands resources (54), emphasizing the urgency to understand the clinical pathology behind the disease and identify novel clinical targets (Figure 1). However, the absolute mechanism underlying long COVID sequelae has yet to be extensively deciphered.

Long-term cardiovascular effects of COVID-19 have been observed in the infected population. However, whether these effects extend to future generations is still unknown (17, 53). Systematic reviews suggest that 20%–30% of neonates born to SARS-CoV-2-positive mothers are infected due to vertical transmission, exhibiting clinical manifestations such as hypotension and tachycardia (99, 100).

Neonates born to positive mothers or preconception-positive parents are still at high risk of developing adverse events, even if they are not infected. Growing evidence shows that parental environmental factors, stress, and infection before or during conception can influence the offspring's phenotype by modulating their epigenetics (101–103). These epigenetic markers can be inherited by the offspring or grandoffspring, suggesting a potential role for the intergenerational and transgenerational inheritance of SARS-CoV-2 adverse events due to parental exposure to SARS-CoV-2 (102).

In the context of paternal contributions, infection and immune activation can alter the sperm epigenome and affect future generations. The sperm transfers its epigenetic information into the embryos, impacting embryonic development (104–106). Moreover, cardiovascular disease inheritance by sperm non-coding RNA was suggested by Wagner et al. (107). There is mounting evidence that SARS-CoV-2 negatively affects sperm, altering the semen proteomics of convalescent patients, which is associated with multiple pathways, including inflammatory cytokines, sperm differentiation, and ROS formation. DNA fragmentation has also been reported in the sperm of convalescent COVID-19 patients (108–110).

Furthermore, maternal immune activation during pregnancy can have adverse effects on the cardiovascular system of the offspring and may potentiate the activation of inflammatory pathways in neonates and ROS formation (102). SARS-CoV-2 can influence mitochondrial damage in peripheral blood cells and cardiac cells, which can potentially affect oocyte mitochondria (59, 62, 63). However, the quality of oocytes in recovered patients has been reported as normal (111). Since maternal mitochondria are transferred to the zygote during fertilization and mitochondrial DNA mutations can predispose individuals to cardiovascular diseases, there may be risks associated with mitochondrial damage due to COVID-19 (112, 113).

SARS-CoV-2's impact extends beyond the acute respiratory distress often spotlighted in medical narratives. Its ability to affect multiple organs and lead to serious cardiovascular complications provides a broader perspective on the virus's toll. As highlighted in our review, while the mechanisms underlying these complications are not fully defined, they likely involve hypoxia, endothelial dysfunction, and an exacerbated immune response. Notably, persistent cardiovascular ramifications have emerged not just in symptomatic COVID-19 survivors but also in those who remained asymptomatic.

With an astounding 756 million confirmed COVID-19 cases worldwide, the repercussions of long COVID, especially its cardiovascular sequelae, present both a challenge and a burden to global healthcare systems. We have reviewed several proposed pathophysiological mechanisms for these complications, including systemic biological aging, gut microbiome disruptions, mitochondrial and direct cardiac damage, sustained systemic inflammation, and epigenomic alterations.

Nevertheless, a significant gap remains in our comprehensive understanding of these mechanisms, with many still hypothesized and awaiting rigorous scientific validation. The urgency of the current situation underscores the need for continued research into the pathophysiology of long COVID. Only by elucidating these pathways can we hope to identify effective therapeutic targets and address the extensive cardiovascular consequences of this pandemic.