1 Department of Respiratory Diseases, KU Leuven University Hospitals Leuven Campus Gasthuisberg, Leuven, Flanders, Belgium alexandros.kalkanis@uzleuven.be.
2 Intensive Care Unit, 251 Air Force General Hospital, Athens, Attica, Greece.
3 Department of Respiratory Diseases, KU Leuven University Hospitals Leuven Campus Gasthuisberg, Leuven, Flanders, Belgium.
4 Department of Mathematics, Hellenic Open University, Patras, Western Greece, Greece.