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Myocardial blood flow under normal physiologic conditions remains relatively unaltered by coronary lesions with diameter stenosis of less than 70%. However, during cardiac arrest and cardiopulmonary resuscitation (CPR), autoregulation of coronary blood flow is compromised. We studied the effect of previously considered 'trivial' coronary stenoses of 10, 25 and 50% on distal myocardial perfusion in 39 swine undergoing CPR for cardiac arrest. Endocardial blood flow distal to the stenosis was significantly less than that proximal in all the different stenoses groups (P < 0.05). A 50% diameter stenosis also compromised epicardial blood flow distal to the stenosis (P < 0.05). This study suggests that any coronary lesion may compromise myocardial perfusion during CPR. Since the vast majority of cardiac arrest victims have coronary artery disease, careful attention to maximizing myocardial blood flow during CPR is needed.