Dynamic intracoronary thrombosis does not cause significant downstream platelet embolization

Objective: A mural intracoronary thrombus is a potential source of platelet emboli that may obstruct downstream microvessels, but this phenomenon has not bran characterized. The present study aimed to assess the magnitude of myocardial platelet accumulation downstream of a mural intracoronary thromb us and its modification by a concomitant transient coronary occlusion (OC) or by treatment with aspirin. Methods: The myocardial content of Tc-99m-lab elled platelets was analyzed in 26 pigs submitted to intimal injury of the left anterior descending coronary artery (LAD) followed by no intervention (n=6), 25-min OC (n=6), or 48-min OC preceded (n=8) or not (n=6) by intrave nous administration of 250 mg aspirin. Results: After 2 h, 24 animals had h ad 12+/-1 cyclic flow reductions (CFRs) reflecting dynamic LAD thrombosis. Myocardial platelet content in the inferior region was similar among groups . Platelet content in the LAD region was not significantly different to tha t in the inferior region (129+/-19%, P=NS) in the no intervention group, bu t was increased following OC (172+/-20 and 312+/-71% after 25- and 48-min O C, respectively, P<0.05). Pre-treatment with aspirin lessened the number of CFRs but did not reduce platelet accumulation in LAD myocardium (483+/-148 %). Myocardial platelet accumulation was not associated with the magnitude of platelet deposition in the LAD nor with the number of CFRs, but was corr elated with myeloperoxidase activity (r=0.91, P<0.001) and with infarct siz e (r=0.52, P=0.05). Histological analysis frequently showed sparse platelet s or small platelet or leukoplatelet aggregates in small vessels, brit arte riolar emboli were rare. Tn none of seven additional experiments coronary a ngiography showed obstructions of arterial branches during CFRs. Conclusion : The magnitude of platelet embolization from a mural intracoronary thrombu s into downstream myocardium is small despite the presence of repetitive CF Rs. (C) 2000 Elsevier Science B.V. All rights reserved.