Patients with acute myocardial ischemia receive oxygen as a standard measure of care to maintain or improve the myocardial oxygen supply. In 1975 Neil et al. were able to show that higher oxygen concentration reduces myocardial blood flow. However, since the oxygen concentration in the blood was significantly increased it was/is assumed that the higher oxygen supply outweighs the decrease in perfusion. Oxygenation-Sensitive (OS) Cardiovascular Magnetic Resonance (CMR) is a newer technology that uses the paramagnetic deoxyhemoglobin in the blood as an endogenous contrast. An drop in tissue oxygenation (drop in oxyhemoglobin) results in an increase in deoxyhemoglobin, which results in an drop in SI in OS-images. Vice versa an increase in tissue oxygenation results in an increase in OS-SI. Therefore, OS-CMR can assess myocardial tissue oxygenation changes non-invasively, free of contrast and free of radiation. While other diagnostic modalities only give information on myocardial oxygenation with surrogate parameters that may indicate ischemia OS-CMR gives local functional information of the myocardial oxygen supply. Studies have been performed using OS-CMR for the detection of coronary artery disease. More recently breathing maneuvers that change systemic blood gases have been suggested as a vasoactive stimulus to detect coronary artery disease and measure changes to the myocardial oxygen supply. Guensch et al. were able to show that hypercapnia and the combination of hypercapnia and hypoxia lead to a increase of myocardial oxygenation that can only be explained by an increase blood flow. However, hypocapnia resulted in a decrease in myocardial oxygenation in healthy volunteers. In a porcine model the investigators were able to show that the increase in systemic oxygen supply resulted in a decrease in myocardial blood flow in healthy and swine with an acute coronary artery stenosis, but a decrease in myocardial oxygenation was only observed in the animals with a coronary artery stenosis (Abstract Guensch et al: Administration of exogenous oxygen may worsen myocardial ischemia, ESC 2014, accepted). The impact of hyperoxia as well as hypocapnia and hypercapnia in humans with a chronic coronary artery disease is poorly investigated. As the administration of oxygen is part of the treating guidelines in acute myocardial ischemia and in certain situations (e.g. anesthesia) higher oxygen concentrations are maintained for safety reasons, it is important to further investigate the role of higher oxygen concentrations in coronary artery disease. Further the investigators want to shed light on transient states of hypocapnia and hypercapnia induced by voluntary breathing. Because OS-CMR is a safe diagnostic tool (no radiation, no contrast, no pharmacologic vasodilator required) and can give insight on the oxygenation changes of the heart is it ideal to test the hypothesis. Therefore the investigators want to invite patients with a known coronary stenosis (confirmed by previous coronary angiography) scheduled for a percutaneous coronary intervention (staged PCI) or coronary artery bypass surgery to participate in this study prior to the intervention and compare the MRI results to healthy volunteers. All participants will perform hyperventilation with a consecutive breath-hold as well as inhale oxygen for 3-5 minutes while the breathing maneuver is being repeated. For the patients the results will then be compared to the findings of coronary angiography (QCA, reduction in lumen-diameter of the vessel).