1. Department of Pulmonary and Critical Care Medicine, National Medical Center, Seoul 05464, Republic of Korea
2. Daechung Hospital, Daejeon 35403, Republic of Korea
3. Asan Institute for Life and Science, Asan Medical Center, University of Ulsan College of Medicine, Seoul 05535, Republic of Korea
4. Department of Pulmonary and Critical Care Medicine, and Clinical Research Center for Chronic Obstructive Airway Diseases, Asan Medical Center, University of Ulsan College of Medicine, Seoul 05535, Republic of Korea