Ischemia-Reperfusion Dynamics in Acute Myocardial Infarction Experimental Swine Model: New Insights from Quantitative CMR

Aims To reveal the pattern and dynamics of myocardial edema induced by myocardial infarction (MI) during ischemia and subsequent reperfusion, that remain largely unknown, as are the factors that contribute to reperfusion injury. To propose a time-resolved dynamic myocardial tissue characterization by quantitative CMR, with T1&T2 mapping and Pixel-wise standardized analysis to circumvent inter-animal differences and subjective ROI positioning. Methods and Results We measured T1&T2 relaxation times at baseline, during a 40-minutes transient coronary occlusion, and after reperfusion in an open-chest swine MI model (n=20; 2 shams) using MRI. Myocardial function, early and late gadolinium enhancement were also assessed. Pixel-wise standardized analysis was used to compare the image contents at each pixel across individuals and time points. A significant increase in cardiac T1&T2 times in the ischemic regions occurred during ischemia compared to baseline (mean ΔT1=118.8ms i.e. +11.1%, ΔT2=5.6ms i.e. +11.3%; p<0.05). A global significant and marked increase in T1&T2 times further appeared immediately after reperfusion (mean ΔT1=256.8ms i.e. +23.3% mean ΔT2=11.9ms i.e. +23.6%, p<0.001). This increase was associated with myocardial wall thickness changes, with regional and global dysfunction in the ischemic myocardium. Three different reperfusion patterns were differentiated by the pixel-wise T1 signal analysis: effective reperfusion with microvascular obstruction (MVO), effective reperfusion without MVO and absence of effective reperfusion. We found no correlations between baseline, per-ischemia, and post-reperfusion native T1&T2 times when effective reperfusion occurred. Conclusions Objective quantification of tissue response by pixel-wise analysis demonstrated rapid and significant changes in myocardial water content status post-reperfusion, with three different early-reperfusion patterns observed, suggesting distinct reperfusion mechanisms. The water content after reperfusion does not reflect its state before and it does not provide insight into the final tissue status observed within 3 hours after recanalization.