1. Anesthesia Specialty, Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
2. Department of Anesthesiology, Northwest Minzu University, Lanzhou, Gansu 730030, P.R. China
3. Department of Anesthesiology, People's Hospital of Ningxia Hui Autonomous Region, Yinchuan, Ningxia 750002, P.R. China