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Aims: The aim of the study was assessment of the influence of viable myocardium on left ventricular function and clinical improvement in patients (pts) with severe ischemic heart failure referred for coronary artery bypass surgery (CABG).Materials and methods: We have evaluated 85 pts, age 66+/-5, with ischemic heart failure (LV ejection fraction <35%) referred for myocardial viability assessment before CABG. Evaluation of left ventricular function was performed with echocardiography and rest/nitrate myocardial perfusion imaging (MPI) with Tc-99m sestamibi Gated SEPCT before and 10+/-3 months after CABG. Control MPI and echocardiography study were used for assessment of LVEF, LV regional and global function, SPECT myocardial perfusion and MPI wall motion index. MPI images were read using 17-segment analysis, with. Bull's eye quantitative analysis. Postoperative improvement in LVEF >5% was considered significant.Results: Patients with viable myocardium in >4 segments of LV assessed by MPI (n= 46) had improvement in regional and global function and also in LV geometry (shape and size) on both MPI and echocardiography, which was incremental to the improvement in LV function. They demonstrated improvement of LVEF at 12 months after CABG (from 32±5 to 43±7%, P<0.001), with reverse remodelling (LV ESV decreased from 145+/-32 to 89+/-21ml, P<0.001; LV EDV decreased from 254±23 to 188±17 ml, P<0.001), and improved in NYHA class with average one grade. Complete revascularization of viable segments improve regional perfusion and function (WMI index decreased from 3.4+/-0.6 to 2,3+/-0.3, P<0.001), that had contributed for global LV function improvement. Conversely, patients with <4 jeopardized segments on MPI (n=39) failed to improve in LVEF (32±4 vs. 30±5%, NS), and exhibited ongoing remodelling (LV ESV increased from 122+/-21 to 153±19 ml P<0.001; LV EDV increased from 277±21 to 318±25 ml, P<0.001), without improvement in NYHA class in 9 pts (23%). Ongoing remodelling and failed improvement in LVEF after CABG was also find in patients with large non viable areas (>20% of myocardium). Pts with improved LVEF after CABG also had improved quality of life.Conclusion: Revascularization of viable myocardium in ischemic heart failure can improve regional and global LV function, and LV geometry. Improvement in LV geometry contributes to better LV systolic function, which is predictor of clinical improvement after revascularization. Non-invasive assessment with echocardiography and MPI before revascularization can predict postoperative improvement of LV function in high risk patients with severe ischemic heart failure.