1. Department of Medicine, Division of Pulmonary, Allergy, Critical Care, and Sleep Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA
2. Atlanta Veterans Affairs Health Care System, Decatur, GA 30033, USA
3. VA Nebraska Western Iowa Health Care System, Omaha, NE 68105, USA
4. Division of Pulmonary, Critical Care & Sleep, Department of Internal Medicine, University of Nebraska Medical Center, Omaha, NE 68198, USA