STUDY OBJECTIVE: To assess the effect of a 33% coronary stenosis on myocardial blood flow during normal sinus rhythm and CPR. DESIGN: Prospective, before and after cardiac arrest and CPR; before and after creation of a 33% stenosis. SETTING: The University of Arizona Resuscitation Research Laboratory. SUBJECTS: Ten domestic closed-chest swine with patent coronary stenoses. INTERVENTIONS: A Teflon cylinder was placed in the mid-left anterior descending coronary artery to create a 33% stenosis. Myocardial blood flow was measured with colored microspheres both proximal and distal to the stenosis during normal sinus rhythm and during CPR. MEASUREMENTS AND MAIN RESULTS: During normal sinus rhythm, the stenosis did not alter the amount of myocardial blood flow distribution or quantity. Proximal to the stenosis the endocardial/epicardial flow ratio was 1.49 +/- 0.33, and distal to the stenosis it was 1.50 +/- 0.50. Likewise, during normal sinus rhythm, blood flow proximal and distal to the stenosis did not differ for either the epicardium (79 +/- 9 versus 66 +/- 13 mL/min/100 g) or the endocardium (111 +/- 27 versus 83 +/- 19 mL/min/100 g). However, the distribution of myocardial blood flow was markedly altered during CPR. The resultant endocardial/epicardial flow ratios were significantly less than during normal sinus rhythm, 0.49 +/- 0.11 (three minutes of CPR) and 0.74 +/- 0.07 (eight minutes of CPR) proximal to the stenosis and 0.39 +/- 0.15 (three minutes of CPR) and 0.49 +/- 0.14 (eight minutes of CPR) distal to the stenosis (P less than .05 versus normal sinus rhythm). In the presence of a 33% mid-left anterior descending coronary artery stenosis, endocardial blood flow at eight minutes of CPR was significantly lower distal to the stenosis compared with proximal to the stenosis (23 +/- 7 mL/min/100 g versus 74 +/- 18 mL/min/100 g; P less than .02). CONCLUSION: Minimal coronary lesions that do not diminish myocardial perfusion during normal physiologic conditions appear to significantly decrease subendocardial blood flow during cardiac arrest and CPR.