The trial received approval on December 16, 2022, from the affiliated Lianyungang Hospital of Xuzhou Medical University Ethics Committee (Ethical Application Reference: KY-20221123002-01). On May 11, 2023, the Chinese Clinical Trial Registry has it listed (ChiCTR2300071313). The study's protocol complied with CONSORT recommendations. Written informed consent was signed by every participant.

The inclusion criteria of this study: 18–65 years old, ASA grade I–III, BMI 28–55 kg/m2, elective laparoscopic sleeve gastrectomy patients. Patients were excluded if they had known severe cardiac, liver and renal dysfunctions, history of mental illness, alcoholism, drug abuse, and chronic pain, history of gastrointestinal surgery, allergy to drugs used in the surgery, and other conditions that the investigators consider to be inappropriate to participate in this trial. Patients with severe perioperative complications and postoperative follow-up data loss were also excluded from this trial.

Using a randomization process created by a statistician, all of the included patients were divided into two groups at a 1:1 ratio: light anesthesia group (BIS 50) and deep anesthesia group (BIS 35). The grouping information was enclosed in opaque envelopes, which could only be disclosed when performing anesthesia induction. Both patients and follow-up investigators were blinded to the grouping.

Standardized monitoring processes were initiated on arrival in the operation room. Anesthesia was induced with intravenous 1 μg/kg dexmedetomidine, 2–3 mg/kg propofol, 0.15–0.25 mg/kg cisatracurium, and 0.3–0.5 μg/kg sufentanil. The anesthesia depth needed to be changed to the objective value within ten minutes after the skin incision in accordance with the grouping. Pursuing BIS objectives at the price of patient safety was not allowed. Propofol and remifentanil infusion rates were modified during the procedure in accordance with the BIS target established in the sealed envelope.

Total intravenous anesthesia was used during the surgery. In order to prevent vomiting, all patients received dexamethasone 10 mg and palonosetron hydrochloride 0.25 mg intravenously. In addition, in order to reduce the awakening pain, 40 mg parecoxib sodium was given half an hour before the end of the surgery. Each patient received local infiltration anesthesia with 0.75% ropivacaine before the skin closure. Patients were ventilated with pure oxygen to restore spontaneous respiration and the endotracheal tube was extubated in accordance with the indication of extubation.

All patients were equipped with patient-controlled intravenous analgesia (PCIA) pump using 2 µg/kg sufentanil and 0.5 mg palonosetron hydrochloride in 100 mL saline. The analgesia pump's parameters were set to 2 mL per hour as the background infusion. The mean arterial pressure (MAP), heart rate (HR), peripheral oxygen saturation (SpO2), and BIS were recorded at 5-min intervals during the maintenance period. At the same time, the intraoperative drug use, the anesthesia conditions, and operative complications were recorded during the surgery. All the recordings were put back into the sealed envelope after the surgical procedure.

The primary outcome of the trial was the degree of pain, which was evaluated by visual analogue scale (VAS) at 0, 12, 24, 48, and 72 h when returned to the ward. Secondary outcomes include the extra use of analgesics, patient satisfaction, Quality of Recovery-15 (QoR-15) score, and postoperative nausea and vomiting (PONV). In the initial three days after surgery, the patients were followed up in the ward. If the patient's VAS score is > 4, additional parecoxib sodium 40 mg will be given for analgesia.

A 10-point rating system was used to gauge the pain's intensity, with 0 indicating no pain, and 10 indicating the maximal pain. The PONV grade was used to record and evaluate postoperative nausea and vomiting, with grade I indicating no nausea and vomiting, and IV indicating severe nausea (feel nausea and vomiting stomach contents). Overall satisfaction was assessed using a 0–10 scale (where 0 represents extremely dissatisfied and 10 extremely satisfied) within 72 h.

It should be noted that the dose calculation methods of all drugs used in this trial refer to the Guidelines of Association of Anesthetists of Great Britain and Ireland Society for Obesity and Bariatric Anesthesia. Sufentanil, remifentanil, cisatracurium (maintenance dose), propofol (maintenance dose) and intraoperative infusion volume were calculated by lean body weight. Propofol (load dose), cisatracurium (load dose), dexmetomidine, and sufentanil (analgesia pump) were calculated by corrected body weight, and tidal volume was calculated according to ideal body weight.

According to the published data, 72 patients in a t-test with a two-sided alpha of 5% and 90% power were required. Taking into account the 1:1 ratio, we projected a 20% dropout rate. Ultimately, 90 patients were included in the study.

The statistical software SPSS 25.0 (IBM, New York, USA) was used to process the data. Regularly distributed data were shown as means; irregularly distributed data were shown as medians; categorical data were shown as frequencies. Additionally, for categorical data, a risk ratio with a 95% confidence interval was displayed. Independent two-sample t-tests and Pearson's χ2 tests were used to compare the normally distributed data and categorical data, respectively. The Mann-Whitney U-test was used for continuous variables with a non-normal distribution. P values on both sides of less than 0.05 were considered to be statistically significant.