1. Department of Pulmonary, Allergy and Critical Care Medicine, Kyung Hee University College of Medicine, Kyung Hee University Hospital at Gangdong, Seoul, South Korea
2. Division of Pulmonary and Critical Care Medicine, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea
3. Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea