Correspondence to: Dr. Shi Yiwei, Department of Respiratory Medicine, First Hospital, Shanxi Medical University, Taiyuan, Shanxi 030001, China (Fax: 86-351-4961994. Email: chinasyw225@ yahoo.com.cn) Correspondence to: Dr. Du Yongcheng, Department of Respiratory Medicine, Shanxi Medical University Affiliated People's Hospital of Shanxi Province, Taiyuan, Shanxi 030012, China (Email: hxkdyc@ yahoo.com.cn)