Abstract Body: Introduction. Myocardial infarction (MI) leads to cardiac myocyte death and the formation of a fibrotic scar in the left ventricular myocardium (LV). LV free wall (LVFW) remodeling in MI is known to significantly alter the biomechanical response of the LV and, in turn, LV function. While changes in the elastic behavior of the LVFW in MI have been reported, changes in the viscoelastic behavior of the LVFW remain understudied. Our objective in this study was to determine the changes in the stress relaxation behavior of the LVFW in MI.

Methods. MI was induced in rats via the ligation of the LAD artery, and hearts were harvested for three groups: sham, 2-week (2wk), and 4wk post-MI (n=3 for each group). Prior to sacrifice, echocardiography was performed to estimate ejection fraction (EF). LVFW specimens were harvested and subjected to viscoelastic stress relaxation tests. The specimens were stretched to 15% along the circumferential and longitudinal directions and allowed to relax. The stress was normalized against the peak (initial) stress for each specimen for proper comparison across the groups.

Results. The EF at 2wk and 4wk specimens was reduced compared to the sham specimens (Fig. 1A). The 4wk post-MI specimens showed slower stress relaxation compared to that in the sham and 2wk specimens in both directions (Figs. 1B, C). Considering a 2-minute stress relaxation span, the normalized stress in the 4wk specimens was significantly higher than the sham specimens in both directions (Fig. 1D).

Conclusions. Our results suggest that the viscoelastic relaxation rate of the LVFW diminishes at late MI timepoints. A larger and more mature scar in the LVFW is expected to contribute to slower relaxation at late MI. Future studies can determine the effect of LVFW viscoelastic remodeling on LV diastolic dysfunction and impaired relaxation in MI.