Coronary heart disease (CHD) is a leading cause of morbidity and mortality in the United States. According to the American Heart Association statistics, greater than 650,000 deaths were attributed to CHD in 2003. Moreover, treatment costs for CHD represent the largest healthcare expenditure for a single disease in the United States. Acute coronary syndromes (ACSs), which include the clinical entities of unstable angina (UA), nonST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI), account for more than 1.5 million hospital admissions annually in the United States alone. Approximately 1 million of these admissions are classified as UA/NSTEMI and approximately 500,000 are STEMI. Percutaneous coronary intervention (PCI) has revolutionized the management of angina and myocardial infarction (MI), frequently negating the need for coronary bypass surgery and permitting a more rapid return to normal activities. The clinical use of PCI is reflected in the number of patients who undergo this procedure. In the United States alone, 664,000 procedures were performed in 652,000 patients in 2003, representing a 326 percent increase from the number of procedures performed in 1987. Coronary stents and adjunctive pharmacologic agents-including glycoprotein IIb/IIIa receptor inhibitors and thienopyridines-have improved the effect of PCI establishing near normal antegrade blood flow in the vast majority of patients. However, dislodgement of atherothrombotic material from coronary lesions during PCI can result in distal embolization that leads to what is commonly referred to as the "no-reflow phenomenon." This phenomenon, characterized by inadequate flow at the cardiac tissue level despite patent coronary vessels is often defined as (1) a thrombolysis in myocardial infarction (TIMI) flow grade less than or equal to 2 despite vessel patency and the absence of dissection, spasm or distal macroembolus, (2) a myocardial blush grade (MBG) of 0 or 1 (Table 2), or (3) a contrast perfusion defect observed upon myocardial contrast echocardiography. Depending on the exact clinical definition used, the incidence of no-reflow has been found to range from 12 to 39 percent, and may be associated with advanced age, presence of diabetes mellitus, left ventricular systolic dysfunction, longer ischemic times, poor initial TIMI flow grades, and anterior myocardial infarction. Our objective was to perform a Comparative Effectiveness Review examining the benefits to harms associated with using adjunctive devices to remove thrombi or protect against distal embolization in patients with ACS who are undergoing PCI of native vessels. The Key Questions (KQs) examined in this report are: KQ 1. In patients with ACS who are undergoing PCI of native vessels, what are the comparative effects of adjunctive devices from different classes (e.g., thrombus aspiration, mechanical thrombectomy, distal balloon embolic protection, distal filter embolic protection, proximal balloon embolic protection) on intermediate outcomes (e.g., ST-segment resolution, MBG, TIMI-3 flow, ejection fraction, and distal embolization) and final health outcomes (mortality, MACE, health-related quality of life)? KQ 2. In patients with ACS who are undergoing PCI of native vessels, how do the rate and type of adverse events (e.g., coronary dissection, coronary perforation, prolonged procedure time) differ between device types when compared to PCI alone? KQ 3. In patients with ACS who are undergoing PCI of native vessels, which patient characteristics (e.g., gender, age, ethnicity, diabetes, smoker, ejection fraction, primary or rescue PCI, use of glycoprotein IIb/IIIa inhibitors, ischemia time, presence of a thrombus-containing lesion, infarctrelated artery and pre-PCI TIMI flow, use of direct stenting) affect outcomes?