Background: In acute myocardial infarction(AMI), distal protection device may protect microvascular integrity during primary angioplasty. Preserved microvascular integrities and favorable coronary phasic flow patterns are known as prognostic factors for LV functional improvement and clinical prognosis in AMI. We assessed the effect of distal protection device on protecting microvascular integrity of myocardium with coronary flow study. Methods and Results: Forty-three consecutive patients with AMI(55 짹15, 34 males), who received parimary stenting within 24 hours after onset, were studied. Averaged peak velocity(APV) and diastolic deceleration time(DDT) were measured after stenting with a Doppler wire at baseline(b) and intracoronary adenosine (24-48 關g) induced hyperemia(h). Coronary flow reserve(CFR), microvascular resistance index(MVRI) were calculated. The patients were divided into two groups; with (Group 1, n=19) and without (group 2, n=24) PurcuSurge GuardWire system. There were no significant differences in pre and post-stenting angiographic findings between two groups. There was no difference in CFR between two Groups. However, hAPV was significantly higher and hMVRI was significantly lower in Group 1 than Group 2. DDT was significantly longer in Group 1 than Group 2 at baseline and hyperemia(table).Conclusion: These data showed more favorable coronary flow patterns in patients with PurcuSurge GuardWire system. Distal protection during primary stenting in AMI may effectively prevent distal embolization and protect microvascular integrity of myocardium.