Fibrosis and microvascular density in human atrial wall during atrial fibrillation

Introduction:  Atrial fibrillation (AF) is one of the most common arrhythmias in the clinical practice and it is associated with an increase in mortality risk that is strongly related with old age. Its pathogenesis is still not sufficiently explored. One of the generally recognized factors contributing to the initiation and maintenance of atrial fibrillation is structural remodeling of the myocardium. Structural remodeling is reflected by changes that affect both atrial cardiomyocytes as well as endomysium.

Aim: In this project we focused on morphological and functional changes in endomysium of atrial myocardium. We focused on changes in the collagen volume fraction and microvascular density.

Methods:   We studied the morphological changes of atrial biopsies performed at 46 patients (19 patients with AF, and 27 with sinus rhythm (SR)) undergoing bypass or mitral valve surgery. The atrial samples were fixed with 4% paraformaldehyde and embedded into paraffin. Sections from atrium were histologically examined using routine hematoxylineosin staining. For quantification of collagen volume fraction (CVF), the sections were stained with the picrosirius staining. All morphometrical parameters were obtained using interactive image analysis software (LeicaQWin, Leica Microsystems). CVF was quantified as an area fraction of myocardial tissue section containing collagen fibers labeled with picrosirius staining. Only endomysial collagen fibers were quantified, while perimysial connective tissue was omitted. To detect capillaries and to quantify microvascular density we stained the sections using UEA -lectin and used the above mentioned image analysis software.

Results: We found variable amount of endomysial collagen in myocardial samples from both groups of patients. Morphometrical calculation of CVF revealed the following results. The CVF in the left auricle was 23.8±5.1% in patients with SR and 28.4±6.7% in patients with AF. The CVF in the left atrium was 24.2±3.1% in patients with SR and 29.3±8.8% in patients with AF. The CVF in the right auricle was 27.4±6.1% in patients with SR and 27.2±4.5% in patients with AF. In all atrial samples the average CVF was 27.9±6.2% in patients with AF and 26.0±5.7% in patients with SR. Morphometrical calculation of absolute number of microvessels (MVD, vessels per mm²) revealed the following results. The MVD in the left auricle was 436,7±154,7 in patients with SR and 456,8±160,6 in patients with AF. The MVD in the left atrium was 369,4±167,8 in patients with SR and 379,6±124,4 in patients with AF. The MVD in the right auricle was 321,4±96,3 in patients with SR and 289,2±101,5 in patients with AF. In all atrial samples the average MVD was 353,4±125,3 in patients with AF and 388,7±147,3 in patients with SR.

Conclusion: Our preliminary results document the variable level of fibrosis in atrial myocardium from patients undergoing open heart surgery. In the left atrial samples from patients with AF we observed an increased CVF and MVD.

This work was supported by grants from Ministry of Health (MZO -00023001), by the EU Operational Program Prague – Competitiveness; project „CEVKOON “ (CZ.2.16/3.1.00/22126) and by PRVOUK – P25/LF1/2.