Randomization of participants
All included patients were enrolled and randomized into one of the two groups in a 1:1 ratio, either prophylactic metoclopramide plus intravenous tramadol (treatment arm) or placebo plus intravenous tramadol (control arm), using a computerized blocked randomization generated by an independent research assistant. The randomization sequences were also stratified based on age groups and sex. Allocation concealment was performed using sequentially-numbered opaque sealed envelopes (SNOSE). The sealed envelopes were opened just after the patients were enrolled into the study and all baseline data were collected. The study interventions were provided immediately after randomization.