Sample size
In a preliminary trial that we conducted, the mean of first postoperative flatus time in patients who accepted additional use of S-ketamine was 2.01 days, the standard deviation (SD) was 0.736. The mean of first postoperative flatus time in patients without S-ketamine was 2.54 days, the SD was 0.694. According to PASS 15 power analysis and sample size software (NCSS, LCC. Kaysville, Utah, USA, ncss.com/software/pass.) using a test for two sample T-tests allowing unequal variance with a randomization ratio of 1:1, sample size was 40 patients per group was calculated to provide 90% power to detect this difference based on a two-tailed significance level of 0.05. Considering a dropout rate of about 20%, we intended to enroll 50 patients in each group.