Invasive minimal Microvascular Resistance Is a New Index to Assess Microcirculatory Function Independent of Obstructive Coronary Artery Disease

BACKGROUND: Coronary microcirculatory dysfunction portends a poor cardiovascular outcome. Invasive assessment of microcirculatory dysfunction by coronary flow reserve (CFR) and hyperemic microvascular resistance (HMR) is affected by coronary artery disease (CAD). In this study we propose minimal microvascular resistance (mMR) as a new measure of microcirculatory dysfunction and aim to determine whether mMR is influenced by CAD. METHODS AND RESULTS: We obtained 482 simultaneous measurements of intracoronary Doppler flow velocity and pressure. The mMR is defined as the ratio between distal coronary pressure and flow velocity during the hyperemic wave‐free period. Measurements were divided into 2 cohorts. Cohort 1 was a paired analysis involving 81 pairs with a vessel with and without CAD to investigate whether HMR, CFR, and mMR are modulated by CAD. CFR was lower, and HMR was higher, in vessels with CAD than in vessels without CAD: 2.12±0.79 versus 2.56±0.63 mm Hg·cm(−1)·s, P<0.001, and 2.61±1.22 versus 2.31±0.89 mm Hg·cm(−1)·s, P=0.04, respectively. mMR was equal in vessels with and without CAD: 1.54±0.77 versus 1.53±0.57 mm Hg·cm(−1)·s, P=0.90. Differences for CFR occurred when FFR was 0.60 to 0.80 or ≤0.60 but not when FFR ≥0.80. For HMR, the difference occurred only when FFR ≤0.60. For mMR, no difference was observed in any FFR stratum. Cohort 2 was used for validation and showed significant relationships for CFR and HMR with FFR: Pearson r=0.488, P<0.001 and −0.159, P=0.03, respectively; mMR had no association with FFR: Pearson r=0.055; P=0.32. CONCLUSIONS: mMR is a novel index to assess microcirculatory dysfunction and is not modified by the presence of obstructive CAD.