1. Department of Respiratory, The Affiliated Hospital of Yan'an University, Yan'an, Shaanxi 716000, P.R. China
2. Department of Emergency, Yan'an Hospital of Traditional Chinese Medicine, Yan'an, Shaanxi 716000, P.R. China
3. School of Clinical Medicine, Harbin Medical University, Harbin, Heilongjiang 150081, P.R. China