The IMR was assessed shortly after primary PCI by using thermodilution-derived method. After successful interventional reperfusion, intracoronary nitroglycerin (100~200μg) was administered and coronary pressure guidewire (Radi Pressure Wire 5, Radi Medical Systems, Uppsala, Sweden) was calibrated outside the patient. The guidewire was equalized at the distal tip of guiding catheter, and then advanced towards the distal third of infarct related artery (IRA). Three times of saline injection (3~5 ml) were administered to IRA and baseline mean transit time was assessed. Microcirculatory hyperemia was induced by using an adenosine infusion (140 μg/kg.min) administered via peripheral venous line. During maximal hyperemia, hyperemic mean transit time (Tmn) was measured using same method as prior separate saline injections. Mean aortic (Pa) and distal coronary pressures (Pd) was measured during hyperemia. The IMR was calculated the formula as follows: IMR = Pd x hyperemic Tmn. Fractional flow reserve (FFR) was calculated by ratio of Pd to Pa during maximal hyperemic status. Coronary flow reserve (CFR) was derived from dividing the resting Tmn by hyperemic Tmn.