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OBJECTIVES: The aim of this study was to determine the effect of supplemental oxygen in patients with myocardial infarction (MI) on the composite of all-cause death, rehospitalization with MI, or heart failure related to baseline oxygen saturation. A secondary objective was to investigate outcomes in patients developing hypoxemia. BACKGROUND: In the DETO2X-AMI (Determination of the Role of Oxygen in Suspected Acute Myocardial Infarction) trial, 6,629 normoxemic patients with suspected MI were randomized to oxygen at 6 l/min for 6 to 12 hours or ambient air. METHODS: The study population of 5,010 patients with confirmed MI was divided by baseline oxygen saturation into a low-normal (90% to 94%) and a high-normal (95% to 100%) cohort. Outcomes are reported within 1 year. To increase power, all follow-up time (between 1 and 4 years) was included post hoc, and interaction analyses were performed with oxygen saturation as a continuous covariate. RESULTS: The composite endpoint of all-cause death, rehospitalization with MI, or heart failure occurred significantly more often in patients in the low-normal cohort (17.3%) compared with those in the high-normal cohort (9.5%) (p < 0.001), and most often in patients developing hypoxemia (23.6%). Oxygen therapy compared with ambient air was not associated with improved outcomes regardless of baseline oxygen saturation (interaction p values: composite endpoint, p = 0.79; all-cause death, p = 0.33; rehospitalization with MI, p = 0.86; hospitalization for heart failure, p = 0.35). CONCLUSIONS: Irrespective of oxygen saturation at baseline, we found no clinically relevant beneficial effect of routine oxygen therapy in normoxemic patients with MI regarding cardiovascular outcomes. Low-normal baseline oxygen saturation or development of hypoxemia was identified as an independent marker of poor prognosis. (An Efficacy and Outcome Study of Supplemental Oxygen Treatment in Patients With Suspected Myocardial Infarction; NCT01787110).