Background The definitive treatment for myocardial ischemia is reperfusion.However,reperfusion injury has the potential to cause additional reversible and irreversible damage to the myocardium.One likely candidate for a cardioprotection is adenosine.The present study aimed at investigating the effect of intravenous adenosine on clinical outcomes in patients with ST-segment elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI).Methods Patients with STEMI within 12 hours from the onset of symptoms were randomized by 1:1:1 ratio to receive either adenosine 50 μg·kg-1·min-1 (low-dose group,n=31),or 70 μgkg-1·min-1 (high-dose group,n=32),or saline 1 ml/min (control group,n=27) for three hours.Drugs were given to the patients immediately after the guide wire crossed the culprit lesion.Recurrence of no-reflow,TIMI flow grade (TFG) and TIMI myocardial perfusion grade (TMPG),and collateral circulation were recorded.The postoperative and preoperative ST segment elevation sum of 18-lead electrocardiogram (ECG) and their ratio (STsum-post/STsum-pre) were recorded,as well as the peak time and peak value of CK-MB enzyme.Serial cardiac echo and myocardial perfusion imaging were performed at 24 hours and 6 months post-stenting.The primary endpoint was left ventricular function,and infarct size.The secondary end-point was the occurrence of cardiac and non-cardiac death,non-fatal myocardial infarction,and heart failure.Results A total of 90 STEMI patients were studied.No-reflow immediately after stent procedure was seen in 11 (35.5％)patients in the control group,significantly different from 6.3％ in the low-dose group or 3.7％ in the high-dose group (both P=0.001).STsum-post/STsum-pre in the low-dose and high-dose groups was significantly different from the control group (low-dose group vs.control group,P=0.003 and high-dose group vs.control group,P=0.001),without a dose-dependent pattern (P=0.238).The peak value of CK-MB enzyme was significantly reduced in the high-dose group compared to the control group (P=0.024).Compared to the left ventricular ejection fraction (LVEF) in control group,LVEF in the low-dose group increased by 5.8％ at 24 hours (P=0.012) and by 10.9％ at 6 months (P=0.007),LVEF in the high-dose group increased by 9.5％ at 24 hours (P=0.001) and by 10.0％ at 6 months (P=0.001),respectively.Significant reduction of infarct size by 24.2％ was detected in the high-dose group vs.low-dose or control groups (P=0.008).There was no significant difference regarding secondary endpoints at 6 months among the treated groups.Cardiac function by NYHA classification in both the low-dose and the high-dose groups was improved significantly (P=0.013,P=0.016).Conclusion Intravenous adenosine administration might significantly reduce the recurrence of no-reflow,with resultant improved left ventricular systolic function.High-dose adenosine was further associated with significant reduction of infarct size.