 Immediate re-opening of the acutely occluded infarct-related bloodvessel via primary percutaneous coronary intervention PCI is the treatment of choice to limit ischemic injury in the setting of ST-elevation ACS STE-ACS. However, the sudden re-initiation of blood flow achieved with primary PCI can give rise to further damage, so-called reperfusion injury. Ischemia and reperfusion associated myocardial injury IR-injury involves a wide range of pathological processes. Vascular leakage, activation of cell death programs, transcriptional reprogramming, no reflow phenomenon and innate and adaptive immune activation all contribute to tissue damage, thereby determining the infarct size. The effect of oxygen treatment on these pathological processes, on the extent of IR-injury and the final infarct size in STE-ACS patients has not previously been studied. 