M yo ca rd ia l I sc he m ia a nd In fa rc tio n Single center registries have shown that distal protection with the Medtronic (PercuSurge) GuardWire balloon occlusion and aspiration system is capable of retrieving embolic particulate debris in a large percentage of patients undergoing primary PCI. Whether this translates into improved reperfusion success and enhanced outcomes has not been determined. Methods. In the EMERALD trial, 500 pts within 6 hrs onset of chest pain with >2 mm ST elevation in 2 or more contiguous leads, or LBBB, undergoing primary or rescue PCI are being prospectively randomized at 40 sites to angioplasty with vs. without distal protection with the 0.028” GuardWire Plus system. The primary endpoints include STR 30 mins post procedure (measured by 24 hour continuous ECG monitoring) and infarct size assessed by tc-99m-sestamibi imaging at day 5-14. The study is powered to show an improvement in complete STR from 50% with control to 65% with distal protection (a 30% relative increase), and a reduction in mean infarct size from 13.1% of the left ventricle to 8.8%. (a 33% relative decrease). Secondary efficacy endpoints include achievement of normal (grade 3) myocardial blush by core lab analysis, and the composite clinical rate of death, new onset sustained hypotension or severe heart failure, and readmission for left ventricular failure within 30 days. Results. To date, more than 415 pts have been randomized; enrollment will be complete by November 2003. Conclusions. A large-scale, prospective, randomized multicenter trial has been performed to: 1) Examine whether prevention of distal embolization in patients undergoing primary PCI for AMI improves indices of myocardial reperfusion and resolution of ongoing injury, reduces infarct size and improves clinical outcomes, and 2) Evaluate the safety, feasibility and efficacy of the GuardWire balloon occlusion and aspiration system for this application. The principal results will be reported for the first time in March 2004.