In addition to established evidence-driven therapeutic guidelines, novel translational approaches to cardiac inflammatory processes are needed to improve outcome and ameliorate associated complications in these patients.
4.1. Myocardial oedema and myocarditis
Patients with myocardial inflammation, myocarditis and MO have an increased risk of suffering potentially lethal arrhythmia and sudden cardiac death independent of myocardial dysfunction [
398,
399,
400,
401]. Both in ischaemic as well as in non-ischaemic cardiomyopathies, positive LGE is associated with a stark increase in incidence of arrhythmia [
293]. This association is especially pronounced in patients with reduced left ventricular ejection fraction ≤ 30% (HFrEF) [
293]. The cumulative evidence suggests a close correlation of LGE and T1-mapping in CMR with myocardial fibrosis, inflammation and oedema [
207,
208,
399,
402,
403], which has also been validated by endocardial voltage mapping and histopathologic analysis [
208,
404]. As inflammation, oedema and the subsequent disturbance of cardiac conduction constitutes a substrate for serious arrhythmia [
293,
401,
405,
406,
407], anti-inflammatory treatment strategies may help to alleviate the burden of arrhythmia in addition to guideline directed medical (GDM) therapy (e.g. anticoagulation, antiarrhythmics) . Upon persistence of AF or bradycardia/VT electrical CV or transient/permanent device-therapy may be required [
408,
409].
In addition to (critical) arrhythmia, myocarditis may also promote heart failure and cardiogenic shock [
214,
410,
411]. In these patients, GDM HF treatment is recommended [
214]. In the acute phase characterized by prominent congestion and fluid overload, diuretic treatment with loop diuretics is essential to remove excess water [
214]. In severe cases of cardiogenic shock, inotropes and vasopressors, as well as mechanical circulatory support (MCS) such as veno-arterial extra-corporal membrane oxygenation (VA-ECMO) has to be considered [
412,
413].
Another frequent factor aggravating the inflammatory processes is anemia, which accounts for worse clinical outcome in hospitalized COVID-19 patients [
414,
415]. Anemia leads to enhanced platelet reactivity [
416,
417] and by contribution to prothrombotic processes and ischaemia its impact on myocardial oedema formation may be assumed. Therefore, prevention of anaemia should be a cornerstone in primary prevention.
In order to improve cardiac function and alleviate excessive inflammation in acute myocarditis, immuno-modulatory anti-cytokine therapy utilizing targeted biologics may prove viable depending on aetiology [
418,
419,
420,
421]. However, it should be noted that - though there were no safety concerns- IL-1ß inhibition by canakinumab did not show clinical improvement in the Canakinumab in COVID-19 Cardiac Injury trial at day 14 [
422]. This was a randomized controlled trial comparing canakinumab (at a dose of 600mg or 300mg) to placebo in 45 patients with myocardial injury [
422]. Future studies regarding dosing regimens and longer follow up periods are required for canakinumab and other immunotherapies [
422,
423,
424].
In the context of auto-immune mediated myocarditis, (e.g., giant cell myocarditis or systemic autoimmune diseases including sarcoidosis, systemic lupus erythematosus (SLE), thyrotoxicosis, or granulomatosis with polyangiitis) corticosteroids, azathioprine, and cyclosporine are utilized [
418,
421]. However, evidence for these treatment strategies are often weak and recommendations are solely based on expert consensus and pathophysiologic considerations [
418,
425].
In a retrospective cohort of biopsy-proven virus-negative chronic inflammatory cardiomyopathy, immunosuppression on top of heart failure therapy resulted in improved survival compared to standard heart failure treatment alone [
426]. In patients with ventricular arrhythmia, there are some signals that immunosuppression may alleviate arrythmia burden, but evidence remains tenuous [
401,
409,
427].
Experimental and clinical evidence suggests a decisive role for inflammatory cytokines in regulation of cardiac remodelling, as biomarkers of inflammation are elevated in heart failure patients, even more so during acute cardiac decompensation [
428,
429,
430]. In heart failure associated with disease other than myocarditis, immunomodulation has failed to demonstrate significant positive effects on mortality. [
431].
The targeted biologic immunomodulators etanercept and infliximab, both inhibiting TNF-α, have been tested in HF with reduced ejection fraction (HFrEF), but provided no clinical benefit [
432,
433]. Left ventricular ejection fraction and left ventricular end-diastolic diameter were improved in a meta-analysis of 19 randomized controlled trials of 1341 patients, therefore indicating a potential role for immunomodulation in HFrEF [
431]. In heart failure with preserved ejection fraction (HFpEF), a relevant role for inflammation in the pathogenesis is also assumed, but effective translational therapeutic approaches remain to be explored [
428,
429,
434].
As previously mentioned, CMR is the gold-standard for the assessment of myocardial inflammation, oedema and fibrosis [
207]. Hence, CMR may also be utilized to guide immunomodulatory therapy in chronic myocarditis [
418]. Especially in the context of systemic inflammatory disease, a combination of CMR with functional imaging including fluorodeoxyglucose positron emission tomography could improve diagnosis, management and tapering regimes of immunosuppressive agents [
418,
435,
436,
437,
438].
Decongestion with diuretics and standard HF treatment also relieves MO [
53]. Recent guidelines recommend sodium-glucose cotransporter 2 inhibitors (SGLT-2i) in heart failure patients to reduce the risk for hospitalisation and cardiovascular death [
214,
439,
440]. Though SGLT-2i are suggested to have anti-inflammatory properties and counteract myocardial fibrosis, an improvement of patient survival during hospitalised COVID-19 infection could hitherto not be shown in a metaanalysis including the DARE-19, RECOVERY and ACTIV-4A trial [
29,
441,
442,
443,
444,
445]. Another drug class recommended for heart failure treatment are mineralocorticoid receptor antagonists [
214,
440]. Herein, finerenone is suggested to preserve endothelial glycocalyx and to protect against COVID-19 associated adverse events in patients with type 2 diabetes and chronic kidney disease [
446].
Additional future treatment options could target stimulation of lymphatic water removal. Experimental increase of lymphangiogenesis in a rat model of heart failure post myocardial infarction has been shown to improve restoration of myocardial fluid balance, and reduce cardiac inflammation, fibrosis, and dysfunction [
447]. Also targeting intravascular pressure, colloid osmotic pressure and intravascular permeability could prove useful.
In myocarditis due to viral infection, direct antiviral therapy, interferon, and intravenous immunoglobulins may be considered depending on the viral pathogen [
399]. In chronic viral myocarditis with entero-, or adenovirus, interferon-β treatment increased viral clearance of entero-, adeno, and parvovirus B19 and reduced endothelial damage in parvovirus B19 infection [
340,
399,
448,
449].
Rather than direct viral infection of cardiomyocytes, which has only been demonstrated for enterovirus, (e.g., coxsackievirus), molecular mimicry and subsequent auto-immune reaction is thought to contribute to cardiomyocyte injury [
399,
450,
451,
452]. However, the therapeutic implications of these findings remain a matter of debate as evidence is currently scarce and randomized controlled trials would be required [
399,
425].
Currently, several specific antiviral agents are available [
399]. Pocapavir and pleconaril as well as intravenous immunoglobulin therapy have been explored for neonatal enteroviral myocarditis [
453,
454,
455]. Anti-herpesvirus drugs such as ganciclovir can be used against persistent Epstein–Barr virus, cytomegalovirus or human herpesvirus 6 to reduce viral load [
456]. Antiviral therapy against hepatitis C virus-associated myocarditis consists of established antiviral drugs such as ombitasvir, paritaprevir, ritonavir and dasabuvir [
457]. Influenza positive myocarditis can be treated with the neuraminidase inhibitors peramivir and zanamivir [
458,
459]. Intravenous immunoglobulin therapy is often used in parvovirus B19 infection, with new treatment strategies such as synthetic nucleotide analogues cidofovir and brincidofovir (broad-range antivirals), synthetic coumarin derivates, flavonoid molecules, and hydroxyurea currently being explored [
460]. Another approach consists of targeting autoantibodies via immunoadsorption or aptamers (synthetic oligonucleotides that can bind specific molecules like antibodies) [
461].
Since SARS-CoV-2 induced COVID-19 is associated with cardiovascular injury, besides direct strategies targeting the virus itself, protection of the endothelium and the glycocalyx, as well as prevention of complications from endothelial injury and dysfunction may prove advantageous [
182,
216,
217,
238,
239]. Several approaches to combat COVID-19 infection are currently being explored [
462]. These include targeting the viral entry mechanisms, immune regulation pathways, or the lifecycle of the virus [
462,
463].
At the beginning of the COVID-19 pandemic, drug repurposing was explored, but with limited success [
463]. Chloroquine and hydroxychloroquine prevent viral entry into the cell via the inhibition of glycosylation of host receptor proteins and manipulation of endosomal proteolytic processing [
463]. Furthermore, both agents are also demonstrating ani-inflammatory effects by inhibiting cytokine production by reducing T cell activation [
463,
464]. Camostat mesylate and arbidol also inhibit host cell entry by inhibiting a host serine protease and interacting with the angiotensin converting enzyme 2 receptor and the S protein, respectively [
463]. Lopinavir, darunavir, and remdesivir, agents that interfere with RNA synthesis, were also considered for trial in severe COVID-19 [
463].
As of mid-2023, several direct antiviral therapeutics are available [
462]. These include inhibitors of RNA-dependent RNA polymerase (remdesivir, molnupiravir, JT001), inhibitors of SARS-CoV-2 main protease (nirmatrelvir–ritonavir, ensitrelvir), and agents, which interfere with the interaction of the S protein and the angiotensin converting enzyme 2 receptor (bebtelovimab, regdanvimab, sotrovimab and others), the latter group being discontinued due to resistance of more recent virus strains [
462]. In addition to antiviral agents, in some situations, immunomodulators are also recommended, including glucocorticoids, janus kinase inhibitors, and targeted cytokine antagonists against IL-6 and IL-1β [
462].
4.2. Endothelial damage and glycocalyx disintegration
In COVID-19, endothelial cell infection leads to dysfunction of the endothelial surface layer and subsequent disturbances of haemostasis, thrombocyte aggregation and MO formation [
25,
26]. While this may also be observed in early disease stages, endothelial damage is thought to be a major contributor to multi-organ failure in severe COVID-19 [
23,
24,
25,
26].
Detailed pathophysiologic insight into the exact processes and deleterious stimuli, which the endothelium is exposed to, both in the context of ischaemia and reperfusion injury as well as viral infection, may inspire several techniques aiming to ameliorate glycocalyx disintegration [
465,
466,
467]. While there are currently no established agents for this indication, various compounds have undergone testing [
60].
The administration of nitric oxide during postischemic reperfusion was demonstrated to reduce vascular leakage and vascular resistance, as well as preserve glycocalyx integrity in guinea pig hearts in vitro [
468]. Hawthorn extract WSS 1442 has the ability to increase coronary flow by boosting nitric oxide release from vascular endothelium [
469], which aligns safeguarding or augmentation of the glycocalyx [
470]. Moreover, WSS 1442 thickens the glycocalyx, which is linked to significantly reduced sodium permeability in vitro [
471].
Hyperbaric oxygen, as a preconditioning stimulus, was shown to provide benefits and protection against ischaemia and reperfusion injury [
466]. The presumed mechanism involves improving endothelial function and oxygenation while reducing local inflammation, vascular permeability, and tissue oedema [
466]. The effect of preconditioning may be appreciated in nuclear magnetic resonance imaging and spectroscopy, where muscle metabolism is positively influenced by preconditioning during reperfusion, with increased production of phosphocreatine and greater oxygen consumption [
472].
Furthermore, various agents resembling glycocalyx components are being explored [
473,
474,
475,
476,
477]. Sulodexide is a natural glycosaminoglycan which regenerates the glycocalyx by boosting glycosaminoglycan synthesis and reducing degradation [
473]. In the setting of type 2 diabetes and chronic venous disease it has been shown to have beneficial effects by regenerating the glycocalyx and combating endothelial dysfunction with anti-inflammatory effects [
478,
479]. Pentosan polysulphate is an oral heparin-like substance without notable anticoagulant properties and currently approved by the US Food and Drug Administration for the treatment of interstitial cystitis [
480]. Research indicates it boosts glycosaminoglycan levels in diabetic mice and reduces glycocalyx breakdown via decreased MMP activity [
60,
474]. Wheat germ agglutinin lectin attaches to heparan sulphate and hyaluronic acid and has been shown to decrease albumin filtration and albuminuria in a rat model of chronic kidney dysfunction [
475]. Rhamnan sulphate is a heparin-like compound [
476] that resulted in an improved glycocalyx and decreased permeability in vitro [
481]. Cationic copolymers were designed to specifically boost endothelial barrier function [
477]. They have been demonstrated to diminish the increase in hydraulic conductivity caused by shear stress and pressure, as well as to decrease capillary filtration in an isolated perfused mouse lung model, suggesting potential utility in the treatment of pulmonary oedema [
477]. As parts of the glycocalyx are often involved in viral entry, this could provide a potential new approach in targeting these components/developing new antibodies targeting these components. For example, interfering with the binding of a virus such as SARS-CoV-2 with heparan sulphate via neutralizing antibodies recovered from COVID-19 patients could be used to combat further infection [
223].
Furthermore, substances that interfere with molecular signalling involved in leukocyte diapedesis and migration are also being explored [
482]. The inhibition of intercellular adhesion molecule (ICAM)-1 to limit neutrophil infiltration prior to reperfusion exhibited protective properties as it demonstrated to reduce neutrophil recruitment and smaller infarct size following ischaemia [
482].
In a murine model, the extracellular matrix protein Secreted Protein Acidic and Rich in Cysteine (SPARC) was shown to play a crucial role in protecting against cardiac inflammation and mortality in cases of viral myocarditis by preserving the integrity and barrier function of the endothelial glycocalyx [
88]. The absence of SPARC lead to increased inflammation, reduced cardiac function, and mortality, but administering recombinant SPARC could reverse these effects, highlighting its potential therapeutic significance in viral myocarditis [
88].
Plasma proteins like albumin can be used to treat conditions such as subarachnoid haemorrhage, shock, and trauma [
483,
484]. In a rodent model of haemorrhagic shock, the degradation of the glycocalyx and subsequent restoration by infusion of plasma was demonstrated in comparison to Ringer lactate [
483]. Plasma-treated rodents showed increased syndecan-1 mRNA expression and reduced lung injury. This restoration aligns with S1P effects, where albumin carries sphingosine-1 phosphate (S1P) that protects against matrix metalloproteinase mediated glycocalyx degradation [
149,
485]. Therefore, plasma proteins like S1P could be crucial for safeguarding the glycocalyx. However, the glycocalyx and its implications in various diseases from atherosclerosis to shock and infection remain to be fully understood. Further research in this field is crucial and may provide new insights and novel treatment options.