This is a double-blind, multicenter, randomized, placebo-controlled clinical trial. It is planned to enroll patients admitted with anterior ST-segment elevation myocardial infarction (STEMI) within 6h of symptom onset and undergo primary percutaneous coronary intervention (pPCI). Patients who meet the inclusion criteria and without exclusion criteria were randomized 1:1 into the dexmedetomidine (DEX) group or the placebo (saline) group after signing the informed consent. In the DEX group, intravenous injection of DEX was started immediately after enrollment, covering the entire PCI operation, and the administration was stopped at the end of the pPCI. The administration of saline was the same as those in the DEX group. The primary endpoint was the myocardial infarct size (MIS) as assessed by cardiac magnetic resonance imaging (CMR) at 5±2 days post-STEMI. Based on a superiority design and assuming an 20.0% relative infarct size reduction (from 26.0% to 20.8% with a SD of 13.0%), 250 patients are required to be enrolled, accounting for 20% drop-out (α= 0.05 and power= 80%).

Inclusion Criteria: the enrolled subjects must meet all of the following criteria:

• Aged 18-75 years old (inclusive);
• Diagnosed with anterior STEMI within 6h of symptom onset: (1) ischemic chest discomfort; (2) electrocardiogram (ECG) with ST elevation ≥0.2 mV in 2 or more contiguous precordial leads (one of which should be V2, V3, or V4);
• Sign the informed consent form.

Exclusion Criteria: subjects who meet any one of the following criteria are excluded from the study:

• Ventricular fibrillation, cardiogenic shock, Killip III-IV grade;
• Sinus bradycardia (heart rate sustained <60 beats/min), PR interval> 240ms or II-III degree atrioventricular block;
• Continuous systolic blood pressure <120mmHg;
• Severe breathing difficulties, aterial blood oxygen saturation <92%;
• Thrombolytic therapy has been performed before the first medical contact in the hospital;
• Consciousness disorder or past cerebrovascular disease;
• Previous history of myocardial infarction or PCI/CABG treatment;
• Known severe liver and kidney dysfunction;
• Known allergy to dexmedetomidine;
• CMR contraindications: such as claustrophobia, pacemaker or ICD implantation;
• Pregnant or lactating women;
• Malignant tumor or expected survival time <1 year;
• Any condition which in the opinion of the investigator would make it unsafe or unsuitable for the patient to participate in this study (eg, poor compliance, inability of the patient to comply with study procedures and/or follow up);
• Participate in other randomized controlled studies at the same time.