Background-A simple, reproducible invasive method for assessing the coronary microcirculation is lacking. A novel index of microcirculatory resistance (IMR) has been shown in animals to correlate with true microvascular resistance and, unlike coronary flow reserve (CFR), to be independent of the epicardial artery. We sought to compare the reproducibility and hemodynamic dependence of IMR with CFR in humans. Methods and Results-Using a pressure-temperature sensor-tipped coronary wire,

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... ion-derived CFR and IMR were measured, along with fractional flow reserve (FFR), in 15 coronary arteries (15 patients) under the following hemodynamic conditions: (1) twice at baseline; (2) during right ventricular pacing at 110 bpm; (3) during intravenous infusion of nitroprusside; and (4) during intravenous dobutamine infusion. Mean CFR did not change during baseline measurements or during nitroprusside infusion but decreased during pacing (from 3.1Ϯ1.1 at baseline to 2.3Ϯ1.2 during pacing, PϽ0.05) and during dobutamine infusion (from 3.0Ϯ1.0 to 1.7Ϯ0.6 with dobutamine, PϽ0.0001). By comparison, mean values for IMR and FFR remained similar throughout all hemodynamic conditions. The mean coefficient of variation between 2 baseline measurements was significantly lower for IMR (6.9Ϯ6.5%) and FFR (1.6Ϯ1.6%) than for CFR (18.6Ϯ9.6%; PϽ0.01). Mean correlation between baseline measurements and each hemodynamic intervention was superior for IMR (rϭ0.90Ϯ0.05) and FFR (rϭ0.86Ϯ0.12) compared with CFR (rϭ0.70Ϯ0.05; PϽ0.05). Conclusions-Compared with CFR, IMR provides a more reproducible assessment of the microcirculation, which is independent of hemodynamic perturbations. Simultaneous measurement of FFR and IMR may provide a comprehensive and specific assessment of coronary physiology at both epicardial and microvascular levels, respectively.