1 Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Chungbuk National University Hospital, Cheongju, Republic of Korea.
2 Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Daejeon St. Mary's Hospital, The Catholic University of Korea, Daejeon, Republic of Korea.
3 Department of Internal Medicine, Chungbuk National University College of Medicine, Cheongju, Republic of Korea.