1 Department of Thoracic and Cardiovascular Surgery, West German Heart and Vascular Center Essen, University Hospital of Essen, University Duisburg-Essen, Hufelandstr. 55, 45147 Essen, Germany. Electronic address: konstantin.zhigalov@yahoo.com.
2 Department of Cardiac Surgery, European Medical School Oldenburg-Groningen, Carl von Ossietzky University Oldenburg, Oldenburg, Germany.