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.

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). There are many defects in the current measurement technology, such as non-timely, long measurement time, high price, potential risk of gadolinium contrast agent.

The index of microcirculatory resistance (IMR) is a readily available, wire-based approach for assessing microvascular dysfunction immediately post-stenting for STEMI. The parameter has been used to characterize MVO, was associated with infarct size, and subsequent cardiac death and heart failure. However, the application of IMR in clinical practice remains limited, primarily due to the requirement of a pressure-temperature sensor wire and hyperaemic agents, which add complexity and time.

The development of functional coronary angiography has enabled the estimation of IMR based on angiography alone. The coronary angiography-derived index of microvascular resistance (caIMR) has shown good diagnostic accuracy compared with wire-based invasive IMR. This simple alternative index showed an association with extent of MVO and was of prognostic importance. 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.

Inclusion Criteria:

• Aged between 18 and 80, regardless of gender;
• Initially diagnosed as STEMI and underwent primary PCI;
• Voluntarily participated in this trial and signed the informed consent form.

Exclusion Criteria:

• Coronary angiography shows that PCI is not suitable;
• Past history of coronary artery bypass grafting (CABG);
• Past history of heart failure;
• Past history of myocardial infarction;
• Primary or secondary cardiomyopathy or heart valve disease;
• Patients with renal insufficiency (eGFR<30ml/min (1.73m^2)) or patients undergoing dialysis;
• Cardiogenic shock
• Persistent left or right coronary ostium lesion after target vessel PCI;
• Immediate effect of target vessel PCI is unsatisfactory [such as thrombolysis in myocardial infarction (TIMI) flow < grade 2, dissection that restricted blood flow, side branch occlusion (diameter>1.5mm), exist distal embolism or angiographically visible thrombus];
• Target vessel exist coronary fistula or myocardial bridge (lumen constriction >50%);
• The target vessel cannot clearly expose the lesion in two positions with an included angle ≥ 30 degrees;
• Severe systemic infection
• Patients with malignant wasting disease, with an estimated survival of less than 1 year;
• The subject is participating in another clinical researches, and the primary endpoint has not been reached;
• The investigator believes that the subject has other conditions that are not suitable for clinical trials.

Aged between 18 and 80, regardless of gender. Initially diagnosed as STEMI and underwent primary PCI.