150 patients are expected to be included. They all must have a mechanical heart valve in the aortic position implanted between 3 months and 4,5 years prior to the investigation. The patients will be divided into tree main groups, which correspond to the type of valve they have acquired - ATS (ATS Medical Inc, Minneapolis, MN, USA), Medtronic-Hall™ (Medtronic, Minneapolis, MN, USA) and St. Jude Medical Standard (St. Jude Medical Inc, St. Paul, MN, USA). With the purpose of recruiting a greater number of patients than those available in the geographic region of Denmark, we have established an agreement with The Heart Center in Bad Oeynhausen, Germany, who will contribute to the study with all 150 patients.