Abstract: AIM: To study the effect of intra-microcatheter diltiazem on no-reflow phenomenon during percutaneous coronary intervention(PCI) for acute myocardial infarction(AMI).METHODS: Forty one patients with no-reflow phenomenon during PCI were included in this study.Patients were divided into guiding catheter group(diltiazem 2 mg injection from guiding catheter to target coronary,n=20) and microcatheter group(diltiazem 2 mg injection from microcatheter to far-end target lesions,n=21).Coronary angiography was rechecked 10 min after injection.Thrombolysis in myocardial infarction(TIMI) and TIMI myocardial perfusion grade(TMPG) of target coronary and 1-week MACE in hospital were assessed.RESULTS: No-reflow phenomenon was relieved in both groups.Patients treated with diltiazem from microcatheter were significantly higher than those treated with guiding catheter in the last contrastographic picture TIMI-3 flow degree(20 cases vs.eight cases,P<0.05).The microcatheter group had a higher rate of TMPG-3 in the last contrastographic picture after drug administration(20 vs.seven cases,P<0.05) and statistical difference was observed in 1-week MACE in hospital between groups(one case vs.six cases,P<0.05).CONCLUSION: Diltiazem injection from microcatheter achieves a better effect than injection from guiding catheter on no-reflow phenomenon during PCI for AMI patients.