The primary aim is to confirm the efficacy of a new non-invasive method for measuring the oxygen uptake during general anesthesia of patients undergoing surgery under cardiopulmonary bypass. The method has already been successfully tested in the laboratory under controlled conditions. In the patient group chosen, oxygen uptake is simultaneously calculated, for comparison purposes, from monitoring measurements made routinely as part of normal patient care in these cases, using a pulmonary artery catheter - an invasive technique from which severe complications occasionally arise, including death.