Microvascular resistance reserve: diagnostic and prognostic performance in the ILIAS registry

AIMS: The microvascular resistance reserve (MRR) was introduced as a means to characterize the vasodilator reserve capacity of the coronary microcirculation while accounting for the influence of concomitant epicardial disease and the impact of administration of potent vasodilators on aortic pressure. This study aimed to evaluate the diagnostic and prognostic performance of MRR. METHODS AND RESULTS: A total of 1481 patients with stable symptoms and a clinical indication for coronary angiography were included from the global ILIAS Registry. MRR was derived as a function of the coronary flow reserve (CFR) divided by the fractional flow reserve (FFR) and corrected for driving pressure. The median MRR was 2.97 [Q1–Q3: 2.32–3.86] and the overall relationship between MRR and CFR was good [correlation coefficient (R(s)) = 0.88, P < 0.005]. The difference between CFR and MRR increased with decreasing FFR [coefficient of determination (R(2)) = 0.34; Coef.—2.88, 95% confidence interval (CI): -3.05–−2.73; P < 0.005]. MRR was independently associated with major adverse cardiac events (MACE) at 5-year follow-up [hazard ratio (HR) 0.78; 95% CI 0.63–0.95; P = 0.024] and with target vessel failure (TVF) at 5-year follow-up (HR 0.83; 95% CI 0.76–0.97; P = 0.047). The optimal cut-off value of MRR was 3.0. Based on this cut-off value, only abnormal MRR was significantly associated with MACE and TVF at 5-year follow-up in vessels with functionally significant epicardial disease (FFR <0.75). CONCLUSION: MRR seems a robust indicator of the microvascular vasodilator reserve capacity. Moreover, in line with its theoretical background, this study suggests a diagnostic advantage of MRR over other indices of vasodilatory capacity in patients with hemodynamically significant epicardial coronary artery disease.