1. Department of Cardiac Surgery, University of Heidelberg , Heidelberg, Germany
2. Department of Cardiac Surgery, University of Halle , Halle (Saale), Germany
3. Department of Cardiac Surgery, University of Hamburg , Hamburg, Germany
4. Department of Cardiac and Vascular Surgery, Heart Center Rotenburg , Rotenburg, Germany
5. Department of Cardiac Surgery, University of Jena , Jena, Germany