Despite the success of restoration of epicardial blood flow by the primary percutaneous coronary intervention (PPCI), approximately a half of patients with ST-segment elevation myocardial infarction (STEMI) have failed myocardial reperfusion, as reflected by microvascular obstruction (MVO) and intramyocardial haemorrhage (IMH). The coronary angiography-derived index of microvascular resistance (caIMR) is a brand new method for assessing coronary microcirculation in ischemia and no obstructive coronary artery disease (INOCA). In this trial, the investigators aim to verify the safety and effectiveness of caIMR in STEMI who underwent primary PCI. The investigators will immediately measure the caIMR of patients who successfully underwent pPCI, and combine the caIMR with the long-term prognosis, to verify the prognostic performance of caIMR.