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Background: Abnormal Doppler-derived coronary flow reserve (CFR) during dipyridamole infusion in patients (pts) with idiopathic dilated cardiomyopathy (IDC) identifies a subgroup at higher risk of developing progressive LV deterioration and heart failure (HF). Chronic biventricular stimulation (BiV) may improve symptoms, LV function and outcome in HF pts. Acute CFR modifications after BiV remain unsettled.Aim: to assess in IDC pts whether BiV may determine acute CFR changes in the LAD territory.Methods: Fifteen IDC patients -NYHA Class III, basal LBBB (QRS >150 msec), LVEF < 35% - underwent high-dose dipyridamole (0.84 mg/Kg over 6') stress-echo with Doppler evaluation of left anterior descending artery within 3 days from BiV implantation. CFR was calculated as the ratio between hyperemic and resting coronary diastolic peak velocities under RV pacing alone and after switching to BiV, in the same setting and "beat to beat" at peak of dipyridamole infusion.Results: CFR was significantly higher under Biv than RV pacing, 2.03 ± .48 vs 1.67 ± .45 (p< 0.05) Comparable systolic and diastolic flow velocities were observed in basal conditions under RV and BiV (16.8 ± 5.4 cm/sec vs 16.9 ± 4.8 cm/sec, 30 ± 9 cm/sec vs 29.6 ± 8 cm/sec, respectively).Conclusions: In ICD pts Biv acutely improves CFR as compared to RV alone. The improvement of CFR is likely mirrored by improvement in microcirculatory function which on its turn, may be related to remodeling and recovery of left ventricular function in pts under chronic treatment.