1. Department of Respiratory and Critical Care Medicine, Yantaishan Hospital, Yantai, Shandong 264001, P.R. China
2. Department of Respiratory, Yantai Beihai Hospital, Yantai, Shandong 265701, P.R. China
3. Department of Emergency, Tai'an Central Hospital, Tai'an, Shandong 271000, P.R. China