In delayed stent group, stent implantation will be performed in individual basis when coronary hemodynamic data indicates that coronary autoregulation was recovered (when initial hyperaemic flow response was subsided and baseline microvascular resistance was increased). Recovery of autoregulatory function will be determined by identification of stabilisation of baseline coronary flow velocity and baseline microvascular resistance in infarct related artery after balloon angioplasty. Stabilization will be decided by comparing basal coronary flow velocity and microvascular resistance values measured in non-infarct related coronary artery immediately before PPCI with the values that was being measured in re-opened infarct related artery. When baseline flow velocity and resistance values being recorded in IRA got close to the values measured in non-IRA, IRA will be stented. Intracoronary pressure and flow data will continuously be monitored until the end of 1 hour follow-up period.