Right ventricular diastolic area, left ventricular diastolic area, and Eccentricity Index. Assessment of coronary blood flow in the right main coronary artery and the left anterior descendant branch of the left main coronary artery (Note: coronary artery blood flow measurements proved technically difficult and time-consuming and were thus removed from the study protocol). The same measurements will be repeated at 120, 180, and 240 min after HFO initiation, and at 60 min following return to CMV. At the same time points, we will determine gas exchange, and hemodynamics including cardiac output with PICCO plus. Lastly, respiratory mechanics will be assessed with rapid end-expiratory/end-inspiratory airway occlusion during CMV.