4.3. Biomarkers of Inflammation
4.3.1. GDF15
GDF15 is a member of the TGF-beta superfamily whose expression is increased in response to biomechanical myocardial stress, inflammation, or ischemia/hypoxia
[63]. GDF15 is involved in the regulation of energy homeostasis, thermogenesis, and eating behavior
[64]. Yet, GDF15 also exerts anti-inflammatory and anti-proliferative properties, although the underlying molecular mechanisms are still unclear
[65]. Elevated GDF15 levels were found in patients with any phenotypes of chronic HF, stroke, AF, and T2DM
[66][67][68][69][70]. In the general population, GDF-15 did not show a positive association with the prevalence of AF and the risk of AF occurrence
[71]. The suitability of GDF15 for predicting bleeding and/or atrial thrombosis during anticoagulant therapy remains questionable
[72]. Clinical evidence for the discriminative value of GDF15 for AF relapse or sinus rhythm maintenance is extremely limited. There is one small study that prospectively included 82 patients with persistent AF
[73]. Although log10 serum GDF-15 levels correlated positively with the CHA2DS2-VASc score, there was no close association between GDF-15 levels and sinus rhythm maintenance in patients after successful electric cardioversion
[73]. Thus, a discriminative potency of GDF15 for the prediction of clinical efficacy of electrical cardioversion among patients with nonvalvular/valvular AF is not completely understood and requires scrutiny in large clinical studies.
4.3.2. hs-CRP
High-sensitivity C-reactive protein (hs-CRP) is a classic biomarker of inflammation and is a component of the inflammatory profile observed in AF patients. Elevated hs-CRP levels were found in patients with all forms of nonvalvular/valvular AF, regardless of etiology and concomitant comorbidities
[74][75]. hs-CRP predicted new-onset AF both in the general population as well as in patients with established cardiovascular or metabolic diseases, such as HF, acute myocardial infarction, T2DM, and metabolic syndrome
[76][77][78]. Among patients with AF complicated by systemic thromboembolism, the levels of hs-CRP correlated positively with the CHA2DS2-VASc score
[79].
Loricchio ML et al. (2007)
[80] investigated plausible predictors for a 1-year risk of AF recurrence after electrical cardioversion. In a Cox regression analysis, the authors found that age, gender, hypertension, T2DM, LVEF, left atrial diameter, use of various antiarrhythmic and antihypertensive (including angiotensin-converting enzyme inhibitors or angiotensin II antagonists) drugs, and statins were not associated with relapsing AF. On the contrary, a low quartile of hs-CRP levels was found to be a strong predictor for this outcome
[80]. Lombardi F. et al. (2008)
[81] did not find any changes in hs-CRP levels after cardioversion in patients with persistent AF and preserved LVEF, regardless of the post-procedural underlying rhythm. However, NT-proBNP levels decreased significantly in patients who maintained sinus rhythm but not in those who had AF. Yet, baseline hs-CRP levels, but not echocardiographic features of atrial dysfunction and initial NT-proBNP levels, predicted recurrences of AF after cardioversion in patients without pre-existing left ventricular dysfunction
[81]. Barassi A et al. (2012)
[82] and Korantzopoulos P et al. (2008)
[83] confirmed that in patients with persistent AF and preserved LVEF, elevated hs-CRP levels independently predicted subacute AF recurrence rate, whereas NT-proBNP concentrations were not associated with arrhythmic outcome but corresponded to the alterations of cardiac hemodynamics secondary to the presence of AF.
Overall, there is ample strong evidence that elevated preprocedural hs-CRP levels may provide independent predictive information for both successful electrical cardioversion of AF and maintenance of sinus rhythm after conversion
[84][85]. The meta-analysis by Liu et al.
[86], which included six prospective observational studies (n = 366 patients), showed that peripheral blood CRP levels were higher in patients with failed electric cardioversion than in those with successful restoration of sinus rhythm. In another meta-analysis by Yo CH et al. (2014)
[87], a cut-off value of 1.9 mg/L hs-CRP predicted long-term AF recurrence (77% sensitivity, 65% specificity), and more than 3 mg/L predicted short-term AF relapse (73% sensitivity, 71% specificity). Thus, the measurement of CRP levels before the procedure may provide additional prognostic information about the success of sinus rhythm maintenance.
In addition, there are data illustrating that hs-CRP levels measured shortly after electrical cardioversion may be a powerful biomarker for assessing the risk of relapsing AF in the long-term. In particular, Celebi OO et al. (2011)
[88] reported that hs-CRP levels measured before and 2 days after electrical cardioversion predicted the 1-year risk of AF relapse. Whether postprocedural hs-CRP provides more information to predict the event than preprocedural hs-CRP is still unclear. However, elevated levels of hs-CRP predicted new-onset AF in the general population and among patients with known cardiovascular diseases, while their role as a marker of sustainable sinus rhythm control places under question.
4.4. Myokines and Adipocytokines
Several interdependent canonic signaling pathways, such as the renin-angiotensin-aldosterone system; TGF-beta pathway, inflammatory chemokines, and cytokines lead to cardiac fibrosis through modulation of oxidative stress and inflammation. However, the direct mechanical stretch may act as a modulator of extracellular matrix remodeling by attenuating the expression of matrix metalloproteinases and their inhibitors. Recently, another signaling pathway has been identified that induces atrial fibrosis via the secretion of adipokines from epicardial, perivascular, and adipose tissue white adipocytes. In addition, recent studies have shown that myokines derived from cardiac and skeletal muscle myocytes may act as adaptive regulators of extracellular matrix remodeling and can attenuate fibrosis
[89][90]. Depending on their origin, adipokines and myokines may modulate myofibroblast capabilities, regulate myocyte energy homeostasis and protect against inflammation and fibrosis
[91][92]. However, some pro-fibrotic adipokines and myokines can switch a generation of reactive oxygen species to pro-inflammatory and pro-fibrotic stimuli, stimulate myofibroblast differentiation through JAK/STAT3 and JNK/c-Jun signaling, interfere with myocyte electrophysiology, and promote fibrosis in the myocardium
[93][94][95]. Numerous previous studies have shown that resistin, apelin, and adiponectin are adipokines associated with several known risk factors for AF and risk of AF
[96][97][98][99]. A recent meta-analysis of 34 studies (total number of patients = 31,479) showed that some adipokines, mainly adiponectin, apelin, and resistin, were associated with the risk of AF in the pooled univariate data, whereas the associations were not apparent after multivariate adjustment
[100]. However, there is limited evidence of the relation between adipokine and myokine signatures and the risk of AF-related outcomes after electric cardioversion.
4.5. Biomarkers of Oxidative Stress and Endothelial Dysfunction
4.5.1. Cell-Free Circulating DNA
Cell-free circulating DNA (cfcDNA) circulates in two main pools: circular and single-stranded molecules belonging to mitochondrial-derived and nuclear-derived subpopulations, reflecting patterns of DNA methylation and a variety of neutrophil extracellular traps (NETosis)
[101][102]. The cfcDNA are determined in subdetectable concentrations under certain physiological conditions, such as physical exercise, whereas increased circulating levels of these fragments are strongly associated with cardiovascular, autoimmune, rheumatic diseases, infections, and malignancy
[103][104][105][106][107]. The main causes of cfcDNA production are mitochondrial dysfunction and inflammation, which are powerful drivers of numerous diseases and conditions, including AF
[108].
Wiersma M. et al. (2020)
[109] reported that levels of cell-free circulating mitochondrial DNA (cfc-mtDNA) were significantly increased in patients with paroxysmal AF undergoing AF treatment, especially in men and in patients with AF recurrence after electrical cardioversion or pulmonary vein isolation. In contrast, cfc-mtDNA levels gradually decreased in patients with persistent AF and long-standing persistent AF. Nevertheless, the authors suggested that cfc-mtDNA levels might be associated with the stage of AF and the risk of AF recurrence after treatment, especially in men. Gender differences in descriptive values of cfc-mtDNA for AF recurrence remain poorly understood but could be related to different comorbidities in both subpopulations. However, another study found no significant changes in mtDNA copy number in the peripheral blood of AF patients of different sex and age
[110]. Perhaps, cfcDNA may be included in the multiple biomarker models with the aim of improving their predictive potency in AF patients with low levels of NT-proBNP or in AF patients with malignancy who are treated with chemotherapy.
4.5.2. mRNA
MicroRNAs (miRNAs) participate in atrial remodeling and cardiac fibrosis, contributing to the development of AF
[111]. Garcia-Elias A et al. (2021)
[112] established that circulating levels of miR-199a-5p and miR-22-5p, which regulate fibrogenic response in the myocardium, were higher in HFrEF patients with AF than in those without AF
[112]. MiR-21, which corresponds to atrial fibrosis, is associated with the risk of persistent AF in patients with left atrial enlargement
[113]. Interestingly, increased circulating levels of miR-1-3p, which is a myosine gene regulator involved in hypertrophy, myocardial infarction, and cardiac arrhythmogenesis, predicted a high risk of subclinical AF
[114]. MiR423, which downregulates fibrosis-related genes such as collagen I, collagen III, fibronectin, and TGF-beta, may be a pivotal factor in stratifying patients at risk of AF occurrence and persistence
[115]. Moreover, differences in miRNA expression in the atrial myocardium of men and women may mediate a sex-specific association between circulating miRNAs in plasma and AF at the population level
[111]. In addition, there is evidence that epigenetic regulation of NETosis may participate in the development of AF susceptibility. As a matter of fact, miR-146a and miR21 may provide prognostic information in patients with AF
[116][117] due to its direct effects on NETosis. In a study by da Silva AMG (2018)
[118], miR-21, miR-133b, and miR-499, which are directly involved in the downregulation of apoptosis and fibrosis, were found to be directly involved in AF. However, it remains to be determined whether a signature of mi-Rs can be used to predict poor response to AF treatment, including electrical cardioversion. At the same time, Zhou Q et al. (2018)
[118] reported that among 123 miRs affecting cardiac fibrosis, hypertrophy, and inflammation by relation with the SMAD7 and FASLG genes, only miR-21 demonstrated a positive correlation with left atrial low-voltage areas in patients with persistent AF and was associated with post-ablation outcome. Overall, the signature of miRs appears to be a more promising tool for higher AF risk than for outcomes after treatment, although this conjecture needs to be further investigated in the future.
4.5.3. Asymmetric Dimethylarginine
Asymmetric dimethylarginine (ADMA) is a well-known biomarker of endothelial dysfunction that indirectly reflects vascular NO production and exhibits certain predictive information for mortality and morbidity of cardiovascular diseases, including AF
[119][120]. In the population-based Gutenberg Health Study (n = 5000), ADMA levels were correlated with left ventricular hypertrophy and AF prevalence
[121]. An ARISTOTLE (Apixaban for Reduction in Stroke and Other Thromboembolic Events in Atrial Fibrillation) substudy showed that elevated ADMA levels exhibited a weak association with thromboembolic events in AF patients treated with anticoagulants (warfarin or apixaban) for a median of 1.9 years
[122]. The investigators found that tertile groups of ADMA levels were sufficiently associated with death, stroke, and systemic embolism and that incorporating ADMA into CHA2DS2-VASc or HAS-BLED predictive models significantly improved C-indices for those clinical outcomes
[122].
There is strong evidence that acute and persistent episodes of AF seem to show elevated ADMA levels accompanied by increased biomarkers of ischemic myocardial injury like cardiac troponins
[123]. In the animal AF model, ADMA concentrations in peripheral blood returned to normal within 24 h after successful electrical cardioversion
[123]. Along with it, increased circulating levels of ADMA in AF may be reduced by a Mediterranean diet and statin treatment
[124][125]. Thus, being closely associated with thrombus formation and CHADS2/CHA2DS2-VASc score, ADMA is a biomarker for predicting pro-thrombotic risk in AF
[126][127].
There are controversial data for ADMA’s predictive ability regarding AF recurrence after electrical cardioversion. Xia W et al. (208)
[128] reported that elevated ADMA levels were strongly associated with an increased risk of AF relapse within 1 month after electrical cardioversion. On the contrary, Tveit A et al. (2010)
[129] found that the levels of ADMA and the L-arginine/ADMA ratio did not exert predictive ability for sinus rhythm maintenance after electrical cardioversion, while the L-arginine/ADMA ratio remained elevated in patients with sinus rhythm for 6 months compared with patients with AF recurrence. The discriminative potency of ADMA may be strongly related to comorbidities. Indeed, serum ADMA levels were not associated with incident AF in the general population after adjusting for other cardiovascular risk factors
[129]. Overall, the utility of ADMA refines clinical risk stratification in AF regardless of the treatment strategy.