This open-label, randomized, controlled, non-inferiority trial was approved by the 900TH Hospital of the Joint Logistics Support Force Institutional Review Board (2,014,046) and conducted in accordance with the Helsinki Declaration. This study follows the comprehensive standard guidelines for test reports. Written informed consent of all patients was obtained. This study was registered in ChiCTR.org.cn (ChiCTR2100051784).

This study was conducted from October 2020 to March 2022 at the 900TH Hospital of the Joint Logistics Support Force, one of the largest cancer centers in southeastern China. The sample size was estimated using PASS software, because there were not enough relevant references in the literature at the time. As advised, the noninferiority was established at 1/5 of the conventional group mean [10, 11]. The cumulative AUCs of PACU-24 h and PACU-48 h at the movement of the ten attempts of conventional procedures were 48.20 ± 16.20 and 86.40 ± 42.26. Based on this, we determined that the non-inferiority values of the average cumulative AUC of PACU-24 h and PACU-48 h at movement were 9.5 and 14 points. Based on that, a total of 100 patients were calculated (1:1 ratio) to be needed (50 in each group) for the primary analysis at rest, with a significance level of one side α = 0.025, a power of 1 − β = 0.85, and 25% contingency factor. Similarly, a total of 60 patients were calculated (1:1 ratio) to be needed (30 in each group) for the primary analysis at movement.

Inclusion criteria are as follows: (1) the diagnosis of the gastric malignant tumor was confirmed by CT, MRI, gastroscopy, and pathological diagnosis; (2) preoperative evaluation of lymph node metastasis without organ and distant metastasis and in line with laparoscopic surgery; (3) American Society of Anesthesiologists (ASA) grade I/III; (4) age 18–75 years old.

Exclusion criteria are as follows: (1) unable to cooperate with the experiment due to language communication disorders or severe cognitive impairment; (2) allergic to drug addicts or experimental-related drugs; (3) other surgical history.

Patients were randomly divided into the C-STAPB group or conventional group at a ratio of 1:1. Each patient was randomly grouped by a third-party professional medical staff through a computer-generated random number. The odd random number was the conventional group, and the even random number was the C-STAPB group. For ethical reasons, patients and doctors who took care of the patients since intraoperatively and postoperatively did not blind the patient allocation group.

In the C-STAPB group, after the abdominal incision was closed, the transverse abdominal plane block catheter was established on the lateral side of the bilateral meniscus of the surgical incision. Using ultrasonic positioning, the trocar was inserted from the tail end to the head end into the gap between the internal oblique muscle and the transverse abdominal muscle. 5 ml 0.9% NS was injected to expand the space, and then the trocar was pushed about 0.5–1.0 cm to ensure stability and reliability. Then, the hard needle was withdrawn, the catheter was fixed locally with a transparent paste, and the length of catheter exposure was recorded. After confirming the appropriate position of the catheter, the first dose was injected (40 mL 0.2% ropivacaine, 20 mL on each side). Ultrasound was used to observe the liquid diffusion in the injection space. Repeat the same procedure on the opposite side. In the C-STAPB group, the drug was administered once every 12 h after surgery through a fixed catheter until 48 h after surgery in the post-anesthesia care unit (PACU) (20 ml per side, 5 postoperative injections). In the conventional group, patients received a conventional opioid PCIA pump (2.5 μg/kg sufentanil and 10 mg tropisetron, diluted to 100 mL with 0.9% NS). Considering the strict bias control, all intravenous pumps were locked to administer only the basic dose (2 mL/h), of which patient-controlled additional use was disabled (Fig. 1).

Trial intervention diagram. A The intervention of the C-STAPB group. B The intervention of the conventional group. (a) Bilateral transverse abdominal muscle plane catheterization. (b) Transversus abdominis plane block under ultrasound (red arrow for local anesthetic, orange arrow for the needle, blue arrow for the external oblique, yellow arrow for the internal oblique, green arrow for the transversus abdominis muscle). Abbreviations: C-STAPB, continuous subcostal transversus abdominis plane block

All patients underwent general anesthesia by the same experienced anesthesiologist and the same anesthesia regimen was used. All surgeries are performed by experienced gastrointestinal surgeons. Postoperative pain numeric rating scale (NRS, score: 0–10, the higher the score, the more severe the pain) was used to assess the pain in the first 4 days after surgery. The decision to provide rescue analgesia was made by the surgeon and was only considered when the patient requests and the NRS ≥ 4. Flurbiprofen (50 mg) was the only non-steroidal anti-inflammatory drug (NSAID) used to rescue analgesia. The C-STAPB catheter and PCIA pump were removed 48 h after the operation.

The primary outcomes included the cumulative area under the curve (AUC) of the pain NRS score at 24 and 48 h after surgery both at rest and during movement. Pain score at rest and movement at 24 and 48 h was assessed using the numeric rating scale from 0 to 10, According to the NRS score and postoperative time, the overall AUC of pain score at rest and during movement at 48 h after surgery was compared. The time points were set at PACU (0 h), 24 h, and 48 h after leaving PACU. The calculation formula of AUC is as follows: AUC = ∑ (NRSi + NRSj) * (tj − ti)/2, when NRSi and NRSj were the NRS scores corresponding to two adjacent observation time points tj and ti (j > i), respectively.

The secondary outcomes included the proportion of patients requiring rescue analgesia, postoperative complications, bowel recovery functions, time to first defecation, bowel movement and daily food intake, recovery by time to start ambulation, length of hospital stay, STAPB-related events, and surgical complications. All patients took 40 ml meglumine diatrizoate orally on the second day after the operation, and the time of the radiography agent (meglumine diatrizoate) reaching the ileocecal junction was recorded.

All statistical analyses were performed using SPSS 11.0 software. Continuous variables are expressed as mean (SD) or median [interquartile range (IQR)] and compared using Student’s t test or the Mann–Whitney U test, as appropriate. Categorical variables are expressed as absolute frequencies (percentages) and compared using Pearson’s χ2 test or Fisher’s exact test, as appropriate. Student’s t test was used to analyze the NRS scores of the two groups at each time point. Repeated measurement analysis of variance was used for statistical analysis of measurement data that were repeated for the same individual and conformed to the normal distribution. For the primary outcomes, the difference with the 95% confidence interval [CI] is calculated to estimate the non-inferiority. And the non-inferiority values of the average cumulative AUC of PACU-24 h and PACU-48 h were 9.5 and 14 points. P < 0.05 was considered statistically significant.