1. Department of Anesthesiology, Qingdao Municipal Hospital, Qingdao, Shandong 266011, P.R. China
2. Department of Anesthesiology, Qingdao Central Hospital, Qingdao, Shandong 266000, P.R. China
3. Department of Anesthesiology, Weifang Medical University, Weifang, Shandong 261053, P.R. China