1. Department of Pulmonology, Southwest Hospital, Third Military Medical University (Army Medical University), Chongqing 400038, P.R. China
2. Department of Respiratory Medicine, The First Hospital of Shijiazhuang, Shijiazhuang, Hebei 050011, P.R. China
3. Department of Respiratory Medicine, North China University of Science and Technology of Clinical Medicine, Tangshan, Hebei 063000, P.R. China
4. Department of Nanlou Pulmonology and National Clinical Research Center of Geriatrics Disease, Chinese People's Liberation Army General Hospital, Beijing 100853, P.R. China