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Objectives: The aim of this study was to determine the effects of an acute myocardial infarction (AMI) on baseline and hyperemic flow in both culprit and nonculprit arteries., Background: An impaired coronary flow reserve (CFR) after AMI is related to worse outcomes. The individual contribution of resting and hyperemic flow to the reduction of CFR is unknown. Furthermore, it is unclear whether currently used experimental models of AMI resemble the clinical situation with respect to coronary flow parameters., Methods: Intracoronary Doppler flow velocity measurements were obtained in culprit and nonculprit arteries immediately after successfully revascularized ST-segment elevation myocardial infarction (n = 40). Stable patients without obstructive coronary artery disease served as control subjects and were selected by propensity-score matching (n = 40). Similar measurements in an AMI porcine model were taken both before and immediately after 75-min balloon occlusion of the left circumflex artery (n = 11)., Results: In the culprit artery, CFR was 36% lower than in matched control subjects (Δ = -0.9; 1.8 ± 0.9 vs. 2.8 ± 0.7; p < 0.001) with consistent observations in swine (Δ = -0.9; 1.5 ± 0.4 vs. 2.4 ± 0.9 for after and before AMI, respectively; p = 0.04). An increased baseline and a decreased hyperemic flow contributed to the reduction in CFR in both patients (baseline flow: Δ = +5 and hyperemic flow: Δ = -7 cm/s) and swine (baseline flow: Δ = +8 and hyperemic flow: Δ = -6 cm/s). Similar changes were observed in nonculprit arteries (CFR: 2.8 ± 0.7 vs. 2.0 ± 0.7 for STEMI patients and control subjects; p < 0.001). CFR significantly correlated with infarct size as a percentage of the left ventricle in both patients (r = -0.48; p = 0.001) and swine (r = -0.61; p = 0.047)., Conclusions: CFR in both culprit and nonculprit coronary arteries decreases after AMI with contributions from both an increased baseline flow and a decreased hyperemic flow. The decreased CFR after AMI in culprit and nonculprit vessels is not a result of pre-existing microvascular dysfunction, but represents a combination of post-occlusive hyperemia, myocardial necrosis, hemorrhagic microvascular injury, compensatory hyperkinesis, and neurohumoral vasoconstriction., (Copyright © 2016 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.)