Poster H14 - Fibrosis Analysis and Cardiac Remodeling in a Canine Model of Mitral Regurgitation- Manuela Biselli

Friday, October 25, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Manuela Biselli

Student
University of Miami
Miami, Florida, United States

Primary Investigator(s)

CZ

Christian Zemlin

Associate Professor of Surgery, Division of Cardiothoracic Surgery
Washington University in St. Louis School of Medicine, United States

Co-Author(s)

JY

Jack Yi
SP

Samantha Procasky

Introduction: Mitral Regurgitation (MR) affects up to 2 million people in the United States, standing as the most common valvular disease in the country. Atrial fibrillation (AF) affects more than half of patients with long-standing mitral valve disease1,2.; however, the cause of AF development in MR patients remains an area of active clinical debate. MR is defined as a chronic volume overload state which triggers a cascade of physiological adaptations over time—eccentric hypertrophy of the left ventricle (LV), increased LV chamber size and stroke volume, and left atrial (LA) enlargement and thinning of the wall1,3,4. These structural changes promote the formation of reentrant pathways, where abnormal electrical circuits can develop within the atrial tissue, disrupting the normal rhythm and leading to AF. Understanding these reentrant circuits is crucial as they are a primary mechanism underlying many forms of atrial arrhythmias. Our goal in this study was to create a minimally invasive, reproducible animal model of MR to evaluate how the remodeling of atrial and ventricular structures leads to reentrant circuit pathways in atrial tissue responsible for atrial arrhythmias.

Materials and

Methods: Adult purpose-bred mongrels were chosen for MR induction due to the close similarity between canine and human cardiac anatomy and physiology. The chronic study consisted of four stages: baseline CMR imaging, survival surgery, clinical surveillance, and terminal surgery(McGilvray et al., 2023)5. MR was created via chordae avulsion of the mitral valve under echocardiographic guidance during survival surgery, followed by terminal surgery at 4 weeks.

After terminal surgery, canine hearts were collected and samples were taken from the posterior atrial wall (LAPW), left ventricular free wall (LVFW), mitral annulus (MA), and papillary muscle (PM). The cardiac tissue samples were fixed in 4% PFA or 10% Formalin for 3 days, transferred to cassettes, and embedded in 70% ethanol for another 3 days. The samples were sectioned and stained with Masson’s Trichrome (MT), Hematoxylin and Eosin (HE), and WGA/DAPI, and examined under a microscope (Figure 1). MT scans were analyzed using Matlab to quantify fibrosis. The algorithm quantifies MT-stained images by counting blue pixels (fibrosis) and red pixels (extracellular matrix and cytoplasm), excluding white space (pixels greater than 75% of 255), and producing a black-and-white image where white represents collagen, thus determining the fibrosis percentage for each image (Figure 2). An unpaired t test was used to evaluate differences between control and 4-week groups. Statistical significance was considered to be less than 0.05.

Results, Conclusions, and Discussions: Analysis of fibrosis levels in the left ventricular free wall (LVFW), papillary muscle (PM), mitral annulus (MA), and left atrial posterior wall (LAPW) revealed differences in fibrosis between control and 4-week post-MR induction groups. Fibrosis at LVFW was 10.4% for control and 9.63% for 4-week canine. At PM, fibrosis was 8.33% for control and 5.40% for 4-week. Both locations had p< 0.001 and N=1. The data did not show significance at LAPW (10.4% vs. 9.63%) and at MA (22.7% vs. 27.7%).

The decreased fibrosis levels in LAPW, especially LVFW and PM, in the 4-week dog align with Stewart Jr et al. (2003)6 and Dillon et al. (2012)7, who observed reduced fibrosis in the initial weeks post-MR induction in. During the early phases of remodeling there is a rapid collagen loss, along with high chymase activity and matrix metalloproteinase (MMP) activation (Dillon et al., 2012)7. Chymase and MMPs reduce fibrosis by breaking down collagen and other extracellular matrix components, leading to structural changes in the heart. This degradation is part of the heart's response to increased wall stress and volume overload due to mitral regurgitation, resulting in a transient phase of initial collagen loss. Based on histology (Figure 1), we can qualitatively visualize the structural remodeling occurring between cell distance, elongation, and cell size.

The exception was at the mitral annulus, where fibrosis was higher in the 4-week canine. This change could be due to the control dog having induced ATAs, while not sustained. Further evaluation with data from additional 4-week and control canines is needed.

The study demonstrated that fibrosis levels in atrial and ventricular regions are comparable between control and 4-week post-MR induction groups, with slightly higher fibrosis at most locations in the control canine.These findings emphasize the model's potential for studying early cardiac remodeling and the progression of fibrosis. Further analysis, including additional data from 6-month survivals, as well as more 4-week and control canines, will enhance our understanding of long-term MR-induced changes in cardiac structure and their role in atrial fibrillation.

Acknowledgements (Optional):