Interventions and blinding
flushing
Patients randomized into the treatment arm received 50 mg intravenous tramadol and 10 mg intravenous metoclopramide. In contrast, patients in the control arm were administered with 50 mg intravenous tramadol and a matching placebo of intravenous metoclopramide, consisting of 10 ml of normal saline solution (NSS). The treatment was prepared and administered by ED nurses who were not involved in the outcome assessment. Patients, attending physicians, data collectors, and outcome assessors were all blinded to the study assignments.For both arms, an intravenous tramadol (50 mg/1 mL) was diluted with 19 mL of NSS, summed up to a total volume of 20 mL, and was slowly pushed within 2 min. After intravenous tramadol was delivered, a 10 mL of NSS was used for flushing the infusion line. Then, either an intravenous metoclopramide (10 mg/2 mL) diluted with 8 mL of NSS or a matching placebo containing 10 mL of NSS were administered within the next minute for the treatment and control arm, respectively.