Blinding and randomization
Patients were randomly assigned to receive butorphanol (Group I) or normal saline (Group II) in a 1:1 ratio based on computer-generated stratified randomization numbers. The random numbers were sealed in separate opaque envelopes until the analysis was complete. Patients in Group I received 10 μg/kg butorphanol intravenously 3 min before the intravenous injection of propofol. The patients assigned to Group II received an identical volume of normal saline at the same infusion rate.The investigators did not participate in other processes while preparing the research solution. A designated staff prepared the solution, enclosed in a dark syringe (volume: 5 ml) labeled "study solution," The two solutions seemed identical. Randomization was blinded to the participants, chief anesthesiologists, and outcome assessors.