Background: Approximately half of the patients presenting with ST-segment–elevation myocardial infarction (STEMI) have multivessel disease. The physiology of the nonculprit artery has not been thoroughly studied to date. We sought to characterize the coronary physiology of the nonculprit artery in the early phase after STEMI and determine the real prevalence of microvascular and endothelial dysfunction. Methods and Results: Patients with STEMI and another coronary artery lesion in a different territory were prospectively included in an observational single-center study. The protocol took place after revascularization of the culprit artery and comprised 3 phases: first, epicardial endothelial functional assessment using intracoronary acetylcholine; second, epicardial severity quantification based on fractional flow reserve, and nonendothelial microvascular function with coronary flow reserve and the index of microvascular resistance; third, endothelium-dependent microvascular function assessment based on the endothelial coronary flow reserve. Eighty-four patients were included. Mean age was 62±10 years, and 86.9% were men. Only 6 subjects had a nonpathological study: macrovascular endothelial dysfunction was present in 60% of the patients; fractional flow reserve ≤0.8, coronary flow reserve <2, and index of microvascular resistance >25 were evident in 34%, 37%, and 28% of the subjects respectively; and microvascular endothelial dysfunction (endothelial coronary flow reserve <1.5) was observed in 44%. In hospital-mortality was 0%, and no major complications occurred. At 6-month follow-up, there were no events related to the nonculprit artery. Conclusions: Microvascular and endothelial dysfunction in the nonculprit artery territory in patients with STEMI are very common. In 93% of the patients, we found functional abnormalities. Acetylcholine administration in the early phase post-STEMI in patients with multivessel disease is safe.

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