Background Resistive reserve ratio (RRR) is a novel index that expresses the ratio between basal and hyperemic microcirculatory resistance. We sought to compare the performance of RRR, coronary flow reserve (CFR) and index of microcirculatory resistance (IMR) in predicting the extent of infarct size (IS) after ST-elevation myocardial infarction. Methods Thermodilution parameters were measured after primary percutaneous coronary intervention (PPCI) in 45 patients. In 30 (67%) cases pre-stenting measurements were also performed to assess the effect of PPCI on myocardial reperfusion, defined by CFR. Cardiovascular magnetic resonance (CMR) was performed at 48-h to assess area-at-risk (AAR), microvascular obstruction (MVO) and IS. CMR was repeated at 6 months in 39/45 patients. Results RRR (AUCRRR = 0.85, CI: 0.710.99) performed better compared to CFR (AUCCFR = 0.67, CI: 0.480.86) and IMR (AUCIMR = 0.70, CI: 0.520.88) in predicting IS% at 6-months. Patients with impaired RRR showed larger acute-IS% (27.4 [14.542.5] vs 15.4 [8.326], p = 0.018), MVO% (3.44 [05.97] vs 0 [00.89], p = 0.026), AAR% (43  vs 34 , p = 0.03) and 6-months-IS% (22.7 [10.235] vs 8.8 [6.912.3], p = 0.006), higher rate of adverse remodeling (22.2% vs 0%, p = 0.04) and lower myocardial salvage index (34% [22.859.2] vs 53.2% [37.771], p = 0.032) compared with other patients. Furthermore, RRR but not IMR or CFR resulted independently associated with 6-months-IS%. CFR (1.48 +/- 0.87 vs 1.47 +/- 0.61, p = 0.94) did not improve after PPCI in patients with impaired RRR, whereas it improved significantly in other patients (CFR: 1.37 +/- 0.43 vs 1.93 +/- 0.49, p = 0.018). Conclusions Patients with post-PPCI impaired RRR were more likely to have suboptimal myocardial reperfusion and larger IS at follow-up. RRR may offer incremental prognostic value compared with other thermodilution-derived indices.
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