1. Department of Internal Medicine, Seoul National University College of Medicine, Seoul, Korea.
2. Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Veterans Health Service Medical Center, Seoul, Korea.
3. Division of Pulmonary, Allergy, and Critical Care Medicine, Department of Internal Medicine, Kang Dong Sacred Hospital, Seoul, Korea.
4. Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Serim General Hospital, Incheon, Korea.
5. Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Seoul Metropolitan Government-Seoul National University Boramae Medical Center, Seoul, Korea.
6. Department of Internal Medicine, Healthcare Research Institute, Healthcare System Gangnam Center, Seoul National University Hospital, Seoul, Korea.