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Background: Cardiac resynchronization therapy (CRT) has become a mainstay in heart failure management, but still 30% of patients failed to respond to such therapy. In patients with non-ischemic dilated cardiomyopathy, abnormal coronary flow reserve is independent prognostic marker of bad prognosis.Aim: The aim of this study was to investigate the potential impact of coronary flow reserve (CFR) and myocardial contractile reserve on left ventricular function recovery following CRT implantation.Method: Eighteen patients with heart failure (EF 26±5%) and QRS duration of 159 ±23ms, underwent transthoracic Doppler echocardiography adenosine test to evaluate CFR and dobutamine echocardiography test to assess global contractile reserve (improvement in LVEF), before CRT implantation. Responders were defined by decrease in end-systolic volume (ESV) ≥ 15%, 6 months after CRT.Results: Fourteen patients were responders, whereas 4 were nonresponders. At inclusion these groups did not differ in LVEF, ESV, EDV, QRS duration, 6 min walk test distance and coronary flow velocity at rest.Before CRT implantation, responders, compared with nonresponders, showed a greater increase in coronary flow velocity during hyperemia, and consequently higher CFR: 2,39±0,74 vs. 1,68±0.29, p=0.03. During dobutamine test responders also had higher LVEF, p=0.02, as well as after 6 months: 45±12% vs. nonresponders 28±8%, p=0.013.By univariate analysis, LVEF during dobutamine infusion (p=0.02) and coronary flow velocity during hyperemia (p=0.04) were predictors for improvement of left ventricular function after CRT.Conclusion: Our results showed that prerequisite for positive response to CRT are preserved CFR and microcirculation and a presence of contractile reserve.