Background: Atrial fibrillation (AF) is associated with structural remodeling of atrial tissue. Myocyte hypertrophy and endomysial fibrosis form a barrier for cell-to-cell conduction. Established models for histological analysis do not sufficiently exploit possibilities for correlation of different hallmarks of structural remodeling.
Objective: We present a validated, imageJ-based tool for automated analysis of markers of structural remodeling including overall and endomysial fibrosis, spatial patterns of endomysial fibrosis, fibroblasts density, myocyte hypertrophy, capillary density and size.
Methods: Human left (n=14) or right (n=8) atrial appendages were obtained from 22 patients in 5 centers. Patients were stratified by cardiovascular risk profile (CVRP- vs CVRP+) for structural remodeling. CVRP+ patients were diagnosed with persistent AF and heart failure. CVRP- patients had no history of AF or heart failure. Patients were matched for age (+/-5y), sex and sampling location. Structural features were visualized applying a triple stain including WGA, CD31 and vimentin. Inter-myocyte distance was quantified as a measure for endomysial fibrosis. For each myocyte, the degree of dissociation from its neighbors was calculated, reflecting the amount of surrounding fibrosis. Clusters of myocytes linked by enhanced endomysial fibrosis were visualized and quantified.
Results: Myocytes were more hypertrophic (p<0.01) in CVRP+ patients (15.0 +/- 0.5 μm) than in CVRP- patients (14.5 +/- 0.5 μm). Fibroblast density was higher (p<0.01) in the CVRP+ group (44.2 +/- 3.45%) compared to the CVRP- group (34.8 +/- 3.26%). More endomysial fibrosis (p<0.01) was observed in CVRP+ patients (9.13 +/- 0.36μm) than in CVRP- patients (7.38 +/- 0.36 μm). Extreme endomysial fibrosis clustered more (p=0.01) in CVRP+ patients (14.34 +/- 2.21%) than in CVRP- patients (7.15 +/- 2.15%). Myocytes were more dissociated (p<0.001) in CVRP+ patients (28.3 +/- 1.96%) than in CVRP- patients (16.1 +/- 2.72%).
Conclusion: Several features of structural remodeling were observed in CVRP+ patients. Myocyte hypertrophy, increased and clustered endomysial fibrosis and dissociation of cardiomyocytes from their neighbors could be important contributors to a pro-arrhythmic substrate.