BackgroundAtrial fibrillation (AF) is one of the widest spread forms of arrhythmia, which is associated with the increased mortality and thromboembolic complications. To date, the involvement of renin‐angiotensin‐aldosterone system and immunomediators of inflammation into the mechanisms of development and maintenance of isolated AF is not clear. Specificity of their changes with respect to the latent myocarditis at AF is not proved.MethodsIn 96 patients with persistent isolated atrial fibrillation (IsAF), scheduled for radiofrequency ablation and endomyocardial biopsy (EMB), and in 20 healthy volunteers (HVT), levels of plasma tumor necrosis factor‐α (TNF‐α), interleukin (IL)‐1β, IL‐6, IL‐8, IL‐10, fatty acid‐binding protein (FABP), neopterin, C‐reactive protein (CRP) were determined by ELISA, level of aldosterone and the renin activity were determined by radioimmunoassay. Results were compared between the study and HVT groups and related to the EMB data.ResultsEndomyocardial biopsy revealed lymphocytic myocarditis in 29%, immunohistochemical signs of viruses' persistence in the myocardium—in 43.8% of patient. We formed 4 subgroups: «myocarditis», «fibrosis», «virus positive», «virus negative». In the group «myocarditis», level of IL‐6 was significantly higher than in group «fibrosis» (P < .01). ROC analysis showed its sensitivity 75%, specificity 75% (AUC = 0.759, Cutoff Value > 1.6 pg/mL, P < .01). In the group «virus positive», level of neopterin was significantly higher than in group «virus negative» (P < .01), with sensitivity 51%, specificity 84% (AUC = 0.675, Cutoff Value > 13.2 nmol/L, P < .01).ConclusionLevels of plasma IL‐6 and neopterin may serve as a marker of latent viral myocarditis in IsAF.