Microvascular Resistance Reserve to Assess Microvascular Dysfunction in ANOCA Patients

Microvascular resistance reserve (MRR) is a new index to assess coronary microvascular (dys)function, which can be easily measured invasively using continuous thermodilution. In contrast to coronary flow reserve (CFR), MRR is independent of epicardial coronary disease and hemodynamic variations. Its measurement is accurate, reproducible, and operator independent.The aim of this study was to establish the range of normal values for MRR and to determine an optimal cutoff point.In this exploratory study in 214 patients with angina and no obstructive coronary artery disease, after excluding significant epicardial disease, all physiological parameters, such as fractional flow reserve, index of microvascular resistance, CFR, absolute blood flow, absolute microvascular resistance, and MRR, were measured. On the basis of concordant positive or concordant negative results of index of microvascular resistance and CFR, subgroups of patients were defined with high probability of either normal (n = 122) or abnormal (n = 24) microcirculatory function, and MRR was studied in these groups.Mean MRR in the "normal" group was 3.4 compared with a mean MRR of 1.9 in the "abnormal" group; these values were significantly different between the groups. MRR >2.7 ruled out coronary microvascular dysfunction (CMD) with a certainty of 96%, whereas MRR <2.1 indicated the presence of CMD with a similar high certainty of 96%.MRR is a suitable index to distinguish the presence or absence of CMD in patients with angina and no obstructive coronary artery disease. The present data indicate that an MRR of 2.7 virtually excludes the presence of CMD, while an MRR value <2.1 confirms its presence.