1. Medica Cor Hospital, 7000 Ruse, Bulgaria
2. Faculty of Public Health and Health Care, Ruse University “Angel Kanchev”, 7017 Ruse, Bulgaria
3. Department of Cardiology, Pulmonology and Endocrinology, Medical Faculty, Medical University of Pleven, 5800 Pleven, Bulgaria
4. Cardiology Department, Alexandrovska University Hospital, 1431 Sofia, Bulgaria
5. Ospedali Riuniti Padova Sud, 35043 Padova, Italy
6. National Medical Institute of Internal Affairs and Administration Ministry, 02-507 Warsaw, Poland