Patients with an impaired blood supply of the heart routinely receive oxygen in order to improve or preserve the oxygen supply of the heart muscle in acute cardiac care. In recent studies a new innovative MRI-technique that can detect changes in oxygen supply of the heart was able to show that the administration of oxygen or fast breathing can decrease the blood supply of the arteries supplying the heart muscle with oxygen. Thus, the administration of oxygen may paradoxically impair the oxygen supply of the heart muscle. In this study the investigators want to investigate, whether the administration of exogenous oxygen via a mask alone and in combination with fast breathing leads to a decrease in oxygen supply in regions with already impaired blood supply by a narrowing of a coronary artery of the heart.