Yang Liu1, Bo Wang2. 1. Department of anesthesia, Shandong Provincial Hospital Affiliated to Shandong University, No 324 Jingwuweiqi Road, Jinan 250021, Shandong, People's Republic of China; Cheeloo College of Medicine, Shandong University, No 44 Wenhuaxi Road, Jinan 250012, Shandong, People's Republic of China. 2. Department of anesthesia, Shandong Provincial Hospital Affiliated to Shandong University, No 324 Jingwuweiqi Road, Jinan 250021, Shandong, People's Republic of China; Department of anesthesia, Shandong Provincial Hospital Affiliated to Shandong First Medical University, No 324 Jingwuweiqi Road, Jinan 250021, Shandong, People's Republic of China. Electronic address: wangb1688@sina.com.