Sample size calculation
In advance, we conducted a preliminary experimental study with 10 patients in each group. The results indicated that the mean onset time of sensory block was 19.8 ± 2.27 min in warming group and 17.8 ± 1.89 min in room temperature group. Drawing on these statistics, a minimum of 24 patients were included in each group to detect a difference of onset time for 2 min between both groups with a 2-tailed α error = 0.05, and power 90%. Taking into account potential block failures and patient dropouts, 60 patients were enrolled.