1. Department of Epidemiology and Biostatistics, School of Public Health, Medical College of Soochow University , Suzhou, 215123 , P. R. China
2. Section of Pulmonary and Critical Care Medicine, Medical Service, Providence VA Medical Center, Providence, RI 02908, USA
3. Division of Pulmonary, Critical Care, and Sleep Medicine, Alpert Medical School of Brown University, Providence, RI 02903, USA