1 Lung Health Center, Division of Pulmonary, Allergy, and Critical Care, Heersink School of Medicine, University of Alabama at Birmingham, Birmingham, AL, USA mdransfield@uabmc.edu.
2 Department of Preventive Medicine and Division of Pulmonary and Critical Care Medicine, Department of Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.
3 Clinic of Respiratory Medicine and Faculty of Medicine, University of Freiburg, Freiburg, Germany.