Characterization of Atrial and Ventricular Structural Remodeling in a Porcine Model of Atrial Fibrillation Induced by Atrial Tachypacing

Abstract

Background: Atrial fibrillation (AF) is characterized by electrical and structural remodeling. Irregular and/or fast atrio-ventricular (AV) conduction during AF can result in AV dyssynchrony, tachymyopathy, pressure and volume overload with subsequent dilatation, valve regurgitation, and ventricular dysfunction with progression to heart failure. Objective: To gain further insight into the myocardial pathophysiological changes induced by right atrial tachypacing (A-TP) in a large animal model. Methods: A total of 28 Landrace pigs were randomized as 14 into AF-induced A-TP group and 14 pigs to control group. AF pigs were tachypaced for 43 ± 4 days until in sustained AF. Functional remodeling was investigated by echocardiography (after cardioversion to sinus rhythm). Structural remodeling was quantified by histological preparations with picrosirius red and immunohistochemical stainings. Results: A-TP resulted in decreased left ventricular ejection fraction (LVEF) accompanied by increased end-diastolic and end-systolic left atrium (LA) volume and area. In addition, A-TP was associated with mitral valve (MV) regurgitation, diastolic dysfunction and increased atrial and ventricular fibrotic extracellular matrix (ECM). Conclusions: A-TP induced AF with concomitant LV systolic and diastolic dysfunction, increased LA volume and area, and atrial and ventricular fibrosis.

Keywords: atrial fibrillation; echocardiography; left ventricular dysfunction; pig model; remodeling.

Figure 1

Flowchart of the pigs randomized into A-TP and SHAM group with included and excluded animals within the single procedures of the study.

Figure 2

Timeline of the study with indication of the actions taken at each time point. SHAM pigs have a comparable flowchart, except for A-TP and development of atrial fibrillation. A-TP, atrial tachypacing; AF, atrial fibrillation; DC, direct current.

Figure 3

Comparison between echocardiographic techniques in the substernal window. (A) Standard trans-thoracic echocardiography (TTE) and (B) Newly introduced trans-diaphragm echocardiography (TDE). Images of apical 4-chamber view of the porcine heart are slightly rotated but quality improved with TDE technique. (C) Approach to TDE creating a surgical window under the xiphoid process and a preparation toward the diaphragm (top image) allows placing the probe directly on the diaphragm (bottom).

Figure 4

Picrosirius red staining for collagen in the left atrial tissue of pigs. Scale bars indicate 500 μm. Picrosirius red stains the collagen in red and the cardiomyocytes in yellow (A,B). The trained program Intellesis Zen 2.3 recognizes the fibrotic tissue in red, the background in blue and the cardiomyocytes in yellow (C,D).

Figure 5

Immunofluorescence staining for extracellular matrix (ECM) in the left atrial tissue of pigs. Original magnification is 400 x. Scale bars indicate 500 μm. The triple immunofluorescence stained the WGA-expressing tissue in red (A,F), the vimentin-expressing cells (fibroblasts) in blue (B,G) and the Isolectin B4 (GSI-B4)-sensitive capillaries in green (C,H). On the merged ROIs (D,I) the Intellesis software recognized ECM (red), background (grey), fibroblasts (blue), capillaries (green) and the overlapping of capillaries and fibroblasts (purple) (E,J).

Figure 6

Apical 4-chamber view of left ventricular end-diastolic (left) and systolic (right) frames at follow-up of a SHAM pig (A) compared to an A-TP pig (B) obtained with the newly introduced TDE technique. Higher volumes and rounder shape of both left ventricle and atrium can be appreciated in the frame image from an A-TP pig in comparison with a control animal.

Figure 7

Heart-to-body weight ratio for the whole heart and for the individual chambers. The total heart-to-body weight ratio was significantly higher for the A-TP (n = 11) compared to the SHAM (n = 12) pigs (5.9 ± 0.32 vs. 4.2 ± 0.15). This was also the case for the left atrium (LA), right atrium (RA), left ventricle (LV), and right ventricle (RV) where the heart-to-body weight ratio on average was 2.3-, 2.2-, 1.4, and 1.3-fold higher in the A-TP than their SHAM counterparts, respectively.

Figure 8

The area of ECM (A), cardiomyocytes (B), fibroblasts (C), and capillaries (D) as percentage of the total tissue analyzed for each cardiac chamber. In three cases one outlier was identified and removed from the analysis—these have been marked with # in the graphs. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

Figure 9

The relative area of fibrotic tissue as percentage of the total tissue area analyzed for each cardiac chamber. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.