Introduction

The rising global prevalence of obesity has led to a significant increase in metabolic and bariatric surgery (MBS), with laparoscopic sleeve gastrectomy (LSG) being one of the most commonly performed procedures.1 Effective postoperative pain management is crucial for enhancing recovery, as it facilitates early mobilization and shortens hospital stay.2,3 Regional anesthesia techniques, including paravertebral block (PVB), play a key role in multimodal analgesia by reducing reliance on systemic opioids.4–6 PVB provides somatic and sympathetic blockade to the abdominal wall and has shown promise in improving pain outcomes after bariatric surgery.7–11

The timing of regional anesthesia—particularly whether administered before or after surgical incision—relates to the concept of preemptive analgesia, which aims to prevent central sensitization by blocking nociceptive signals prior to surgical stimulus.12,13 While preoperative PVB aligns with this model, evidence supporting its superiority over postoperative PVB remains limited, especially in the MBS population. Current practice is often guided by logistical factors—such as operating room schedules, the time window available for performing the block before surgical incision, and patient flow through the post-anesthesia care unit (PACU)—rather than by robust comparative evidence specifically addressing block timing.

Therefore, the objective of this study was to compare preoperative versus postoperative PVB in patients undergoing LSG. The primary outcome was opioid consumption within the first 24 hours after surgery. We hypothesized that preoperative PVB would be associated with lower 24-hour opioid consumption compared with the postoperative approach.

Methods Study Design

This single-center retrospective cohort study was approved by the Medical Ethics Committee of Zhengzhou Central Hospital Affiliated to Zhengzhou University (Ethics approval number: ZXYY2025170). Due to the retrospective nature, the Ethics Committee granted a waiver for informed consent. The study adhered to the principles of the Declaration of Helsinki and the STROBE reporting guidelines.

Study Population

The study included adult patients who underwent primary elective LSG and received bilateral T8-level PVB at our hospital between January 2025 and October 2025. Inclusion criteria were: age ≥18 years, American Society of Anesthesiologists (ASA) physical status classification II–III, undergoing primary elective LSG, and receiving ultrasound-guided bilateral T8-level PVB. Exclusion criteria included: preoperative long-term use of analgesic medications (defined as continuous use exceeding 3 months), missing data, conversion from laparoscopy to open surgery intraoperatively, and postoperative admission to the intensive care unit.

Grouping and Intervention

Based on the timestamp of PVB administration in the anesthesia record, patients were divided into two groups: the Pre group (PVB performed after entering the operating room but before general anesthesia induction) and the Post group (PVB performed after surgery completion, upon the patient’s arrival in the post-anesthesia care unit). All PVBs were performed by experienced anesthesiologists using a standardized ultrasound-guided technique. With the patient in the lateral decubitus position, a low-frequency convex probe was used to identify the T8 transverse process and parietal pleura. The in-plane technique was used to advance the needle into the paravertebral space, with position confirmed by saline hydrodissection. For each side at T8, 20–25 mL of 0.25% ropivacaine was injected, totaling 40–50 mL bilaterally.

Perioperative Management and Analgesia Protocol

Anesthesia was induced with sufentanil (0.4–0.5 μg/kg), propofol (1.5–2.5 mg/kg), and rocuronium (0.6–1 mg/kg). Mechanical ventilation commenced after endotracheal intubation. Anesthesia was maintained with intravenous infusion of propofol (4–8 mg/kg/h), remifentanil (0.1–0.2 μg/kg/min), and inhaled sevoflurane (1–2%), with intermittent boluses of rocuronium. The intraoperative goal was to maintain blood pressure within ±20% of baseline.

Prophylaxis for postoperative nausea and vomiting followed a risk-stratified strategy based on the Apfel score. All patients received intravenous sufentanil 10 μg and flurbiprofen axetil 100 mg approximately 30 minutes before the end of surgery. At the end of surgery, sugammadex 2 mg/kg was administered intravenously to antagonize residual neuromuscular blockade. The tracheal tube was removed after the patient regained consciousness and could follow commands, and the patient was transferred to the PACU. In the PACU, if the numeric rating scale (NRS) pain score was ≥4, patients were instructed to press the patient-controlled intravenous analgesia (PCIA) button for a rescue bolus. If pain persisted, intravenous sufentanil 5 μg was administered as rescue analgesia.

Postoperative analgesia was provided via PCIA with the following formulation: hydromorphone 10 mg and flurbiprofen axetil 300 mg, diluted with normal saline to a total volume of 100 mL. The pump was set with a background infusion rate of 0.5 mL/h, a bolus dose of 3 mL, a lockout interval of 5 minutes, and a maximum hourly limit of 10 mL. During the ward stay, patients regularly received ketorolac tromethamine 30 mg every 12 hours and drotaverine 40 mg every 24 hours. If the NRS remained ≥4 after a PCIA press, intravenous propacetamol 2 g was administered as rescue analgesia.

Data Collection and Outcome Measures

Data were extracted from the hospital’s electronic medical records and anesthesia clinical information system. Collected variables included: sex, age, height, weight, body mass index (BMI), ASA classification (II/III), comorbidities (hypertension [HTN]/diabetes mellitus [DM]/obstructive sleep apnea [OSA]), duration of surgery, timing of PVB, and outcome measures. The primary outcome was the total opioid consumption (sufentanil and hydromorphone) within the first 24 postoperative hours, converted to oral morphine milligram equivalents (MME). The 24‑hour cumulative consumption was obtained directly from the electronic interrogation records of the PCIA pumps. Conversion factors based on prior studies were: 1 μg sufentanil = 2 MME, 1 mg hydromorphone = 17.5 MME.14,15 Secondary outcomes included: time to first rescue analgesia, proportion of patients requiring rescue analgesia within 24 hours (ward), incidence of postoperative nausea and vomiting (PONV) within 24 hours, time to first ambulation, and length of hospital stay.

Statistical Analysis

Statistical analysis was performed using R software (version 4.5.1). Continuous variables are presented as mean ± standard deviation or median (interquartile range) based on normality tests, and compared between groups using the t-test or Mann–Whitney U-test. Categorical variables are presented as frequency (percentage) and compared using the chi-square test or Fisher’s exact test. To control for confounding factors, propensity score matching (PSM) was employed. The propensity score was calculated using the variables sex, age, BMI, ASA classification, comorbidities, and duration of surgery. Height and weight were excluded from the propensity score model to avoid collinearity with BMI, and the same exclusion applied to the multiple linear regression models used in the sensitivity analysis. A 1:3 nearest-neighbor matching with a caliper width of 0.2 was performed. Post-matching balance was assessed, with a standardized mean difference (SMD) <0.1 considered acceptable Primary and secondary outcomes were compared in the matched cohort. The secondary outcomes were considered exploratory in nature; therefore, no adjustment for multiple comparisons was applied. To verify the robustness of the primary outcome results, a sensitivity analysis was performed in the matched cohort using two multiple linear regression models: Model 1 included only the grouping variable (unadjusted); Model 2 additionally adjusted for all covariates used in the propensity score model, including age, BMI, sex, ASA classification, comorbidities (HTN, DM, and OSA), and duration of surgery. All tests were two-sided, and a P-value <0.05 was considered statistically significant.

Results

Between January and October 2025, 403 patients who underwent LSG were initially considered. After applying the inclusion and exclusion criteria, 366 patients were enrolled and categorized into the Pre group (n = 299) or the Post group (n = 67) based on the timing of the block. Three of these patients were excluded from the final analysis because the 24‑hour cumulative opioid consumption data were not available from the PCIA pump records. Before PSM, patients in the Pre group were younger (SMD = 0.176), taller (SMD = 0.200), had a lower prevalence of HTN (SMD = 0.286), and had a higher prevalence of DM (SMD = 0.162). After matching, all characteristics were well-balanced between groups, with SMDs < 0.1 (Table 1). The detailed patient screening process is shown in Figure 1. A kernel density distribution plot of propensity scores before and after matching is provided as Supplementary Figure 1.

Table 1 Patient Characteristics Before and After PSM

Figure 1 Patient screening and enrollment flowchart.

Primary Outcome

The primary outcome, total opioid consumption within the first 24 postoperative hours converted to morphine milligram equivalents, did not differ significantly between the Pre and Post groups (Table 2).

Table 2 Outcomes Between the Pre and Post Groups After PSM

Secondary Outcomes

No statistically significant differences were observed in any of the secondary outcomes. The time to first request for rescue analgesia, the proportion of patients requiring rescue analgesia within 24 hours, the incidence of PONV, the time to first ambulation, and the length of hospital stay were all comparable between the two groups (Table 2).

Sensitivity Analysis

A sensitivity analysis for the primary outcome was performed using multiple linear regression, including an unadjusted model and one adjusted model. The analysis confirmed the robustness of the primary finding, showing no significant association between the timing of paravertebral block (preoperative vs postoperative) and 24-hour MME consumption across all models (Table 3).

Table 3 Sensitivity Analysis for the Primary Outcome Using Multiple Linear Regression in the Matched Cohort

Discussion

This retrospective study explored whether there was an association between the timing of a regional analgesic intervention—administered preemptively versus postoperatively—and opioid consumption following LSG. The primary finding is that a preoperative PVB was not associated with reduced 24-hour postoperative opioid consumption compared to an identical block performed after surgery. Furthermore, no significant differences were observed across a range of secondary recovery outcomes, including rescue analgesia requirements, PONV, and mobilization times. These results do not support the hypothesized superiority of preemptive analgesia via PVB in this specific surgical context.

The absence of a demonstrable benefit for preoperative PVB may be explained by the following factors. First, the comprehensive, multimodal analgesic protocol used perioperatively for all patients—including intraoperative analgesia, scheduled non-opioid medications, and PCIA with a basal infusion—may have created a high analgesic baseline. This potent background regimen could have attenuated any incremental benefit from altering the timing of a single component (the PVB), making a statistically significant difference difficult to detect. Second, the surgical stimulus in LSG, while significant, may not induce a level of central sensitization that is robustly preventable by a single-level bilateral PVB, or the block’s prolonged duration of action may render its initiation time less critical within the first 24 hours.

While the concept of preemptive analgesia is well-established, clinical studies investigating its application in regional anesthesia (as opposed to systemic medication) are relatively scarce. Our findings contribute to this ongoing discussion by showing that, within a stringent multimodal analgesic protocol for LSG, the timing of the PVB was not associated with early postoperative outcomes. This result aligns with the broader observation that the putative advantage of preemptive regional anesthesia has been difficult to consistently demonstrate in modern perioperative care, especially when evaluated against a background of potent multimodal analgesia.12,16 An apparent contradiction exists between our results and some earlier studies which reported benefits for preoperative blocks.17–19 This discrepancy may be explained by key methodological differences, including the specific block technique performed, the analgesic regimen used for comparison, the definition of “preemptive” timing, and the patient population. Most notably, although a few studies have examined the timing of regional blocks in other surgical settings—such as PVB for laparoscopic cholecystectomy and brachial plexus blockade for upper extremity surgery—high-quality evidence specifically evaluating the timing of PVB in MBS has been scarce.17,18 Therefore, our study provides novel and directly relevant data for this surgical population, indicating that logistical flexibility in performing the PVB is feasible without compromising analgesic efficacy.

From a practical standpoint, the implications are straightforward: for institutions implementing a robust multimodal protocol for LSG, the decision to perform a PVB before or after surgery might be guided by operational workflow and anesthetic preference rather than an expectation of superior pain outcomes. This flexibility could potentially enhance operating room efficiency. Theoretically, these results highlight the importance of evaluating individual analgesic interventions within the context of the entire perioperative regimen, where synergistic or ceiling effects may exist. However, given the retrospective design and the fact that higher‑level evidence in other surgical settings has suggested a benefit of preoperative blocks,12,16,17 these practical considerations should be interpreted as hypothesis‑generating rather than as definitive clinical recommendations, and they warrant confirmation in future prospective studies.

Several limitations of this study must be acknowledged. First, its retrospective and observational design inherently carries risks of selection bias and unmeasured confounding, despite our use of PSM to balance observed covariates. Related to this, the wide confidence interval for the primary effect estimate (95% CI: −26.03 to 13.80 MME) indicates that our data cannot precisely exclude a minimum clinically important difference (MCID), even though the observed mean difference (−6.12 MME) fell below the 10 mg MCID.20 Given the strong multimodal analgesic background, the true effect size of PVB timing is likely attenuated, which further limits power to detect a difference. We therefore caution against overinterpreting the null finding as evidence of equivalence. Second, the significant imbalance in group sizes before matching reflects real-world practice patterns but may influence the robustness of the matching process. Third, as a single-center study, the findings may not be fully generalizable to other institutions with different surgical techniques, analgesic protocols, or patient populations. Fourth, baseline pain scores were not systematically documented in our electronic medical records for this elective surgical population and therefore could not be included in the propensity score model. Fifth, we did not collect opioid consumption data stratified by finer time intervals (eg, 0–6 h, 6–12 h, 12–24 h), as our electronic records only reliably captured the cumulative 24‑hour consumption; thus, a time‑stratified sensitivity analysis could not be performed. Sixth, the lack of formal sensory confirmation of PVB placement means we cannot exclude the possibility that suboptimal blocks biased our results toward the null. Seventh, we did not systematically record technical difficulty, block performance time, patient positioning-related issues, or block-related complications in a way that would allow reliable comparison between the preoperative and postoperative groups. The conclusion that PVB timing may be guided by logistical considerations therefore rests on the analgesic outcome data rather than on a formal comparison of procedural feasibility between the two timing strategies. Finally, the assessment was limited to the first 24 hours; differences in longer-term outcomes or chronic pain development were not evaluated. We also acknowledge that a Type II error cannot be fully excluded, and a prospective study with adequate power would be necessary to definitively determine whether a clinically meaningful difference exists. Collectively, these limitations preclude a definitive conclusion, and our findings should be interpreted as hypothesis‑generating rather than confirmatory.

Conclusion

In this retrospective cohort, preoperative PVB was not associated with reduced 24‑hour opioid consumption or improved early recovery outcomes compared to postoperative PVB in patients undergoing LSG within a multimodal analgesic protocol. These findings suggest that PVB timing may be guided by logistical considerations without compromising analgesic efficacy. However, the possibility of a Type II error cannot be excluded, and prospective studies with adequate power are needed to confirm these hypothesis‑generating observations.

Data Sharing Statement

Datasets can be obtained from the corresponding author (Qinjun Chu) upon reasonable request, subject to ethical and legal restrictions.

Ethics Approval and Consent to Participate

This study was approved by the Medical Ethics Committee of Zhengzhou Central Hospital Affiliated to Zhengzhou University (Ethics approval number: ZXYY2025170). Due to the retrospective nature, the Ethics Committee granted a waiver for informed consent. All patient data were anonymized prior to analysis, and confidentiality was strictly maintained throughout the study.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Funding

This study was supported by Henan Provincial Medical Science and Technology Research Project (LHGJ20230707).

Disclosure

The authors declare no competing interests.

References

1. Mingrone G, Panunzi S, De Gaetano A, et al. Metabolic surgery versus conventional medical therapy in patients with type 2 diabetes: 10-year follow-up of an open-label, single-centre, randomised controlled trial. Lancet. 2021;397(10271):293–8. doi:10.1016/S0140-6736(20)32649-0

2. Maia L, Chaves I, Medeiros H, Silva GF, Da Silva WA. Impact of opioids on hospital stay and mortality in patients undergoing abdominal surgeries. Anesthesiol Perioper Sci. 2024;2(4). doi:10.1007/s44254-024-00071-5

3. El-Boghdadly K, Levy NA, Fawcett WJ, et al. Peri-operative pain management in adults: a multidisciplinary consensus statement from the Association of Anaesthetists and the British Pain Society. Anaesthesia. 2024;79(11):1220–1236. doi:10.1111/anae.16391

4. Kianian S, Bansal J, Lee C, Zhang K, Bergese SD. Perioperative multimodal analgesia: a review of efficacy and safety of the treatment options. Anesthesiol Perioper Sci. 2024;2(1). doi:10.1007/s44254-023-00043-1

5. Pincus E. Regional anesthesia: an overview. Aorn j. 2019;110(3):263–272. doi:10.1002/aorn.12781

6. Ghandour MAM, Abokhozima A, El-Masry H, et al. Laparoscopic preperitoneal local anesthetic infiltration versus ultrasound guided quadratus lumborum block in laparoscopic bariatric surgery: a randomized clinical trial. Obes Surg. 2026;36(1):29–37. doi:10.1007/s11695-025-08401-7

7. Ardon AE, Lee J, Franco CD, Riutort KT, Greengrass RA. Paravertebral block: anatomy and relevant safety issues. Korean J Anesthesiol. 2020;73(5):394–400. doi:10.4097/kja.20065

8. Slinchenkova K, Lee K, Choudhury S, Sundarapandiyan D, Gritsenko K. A review of the paravertebral block: benefits and complications. Curr Pain Headache Rep. 2023;27(8):203–208. doi:10.1007/s11916-023-01118-1

9. Li L, Wang J, Hu T, Xu J, Huang J. The effect of ultrasound-guided multipoint thoracic paravertebral nerve block in Metabolic and Bariatric Surgery (MBS): a prospective randomized controlled trial. Obes Surg. 2025;35(9):3471–3481. doi:10.1007/s11695-025-08154-3

10. Yang G, Wang P, Yin Y, et al. Erector spinae plane block versus paravertebral block on postoperative quality of recovery in obese patients undergoing laparoscopic sleeve gastrectomy: a randomized controlled trial. PeerJ. 2024;12:e17431. doi:10.7717/peerj.17431

11. Kanawati S, Fawal H, Maaliki H, Naja ZM. Laparoscopic sleeve gastrectomy in five awake obese patients using paravertebral and superficial cervical plexus blockade. Anaesthesia. 2015;70(8):993–995. doi:10.1111/anae.13037

12. Xuan C, Yan W, Wang D, et al. Efficacy of preemptive analgesia treatments for the management of postoperative pain: a network meta-analysis. Br J Anaesth. 2022;129(6):946–958. doi:10.1016/j.bja.2022.08.038

13. Byrne K, Smith C. Preemptive Analgesia: an Unobtainable Goal? J Cardiothorac Vasc Anesth. 2019;33(2):460–461. doi:10.1053/j.jvca.2018.08.008

14. Nielsen S, Degenhardt L, Hoban B, Gisev N. A synthesis of oral morphine equivalents (OME) for opioid utilisation studies. Pharmacoepidemiol Drug Saf. 2016;25(6):733–737. doi:10.1002/pds.3945

15. Almodibeg BA, Adam R, Alevrogianni F, et al. Standardising oral morphine equivalent conversion ratios for research: a modified Delphi consensus survey. Eur J Anaesthesiol. 2026. doi:10.1097/EJA.0000000000002463

16. Ong CK, Lirk P, Seymour RA, Jenkins BJ. The efficacy of preemptive analgesia for acute postoperative pain management: a meta-analysis. Anesth Analg. 2005;100(3):757–773. doi:10.1213/01.ANE.0000144428.98767.0E

17. Holmberg A, Sauter AR, Klaastad Ø, Draegni T, Raeder JC. Pre-operative brachial plexus block compared with an identical block performed at the end of surgery: a prospective, double-blind, randomised clinical trial. Anaesthesia. 2017;72(8):967–977. doi:10.1111/anae.13939

18. Naja ZM, El-Rajab M, Ziade F, Al-Tannir M, Itani T. Preoperative vs postoperative bilateral paravertebral blocks for laparoscopic cholecystectomy: a prospective randomized clinical trial. Pain Pract. 2011;11(6):509–515. doi:10.1111/j.1533-2500.2011.00447.x

19. Liu H, Ding L, He Y, et al. Comparing the analgesic effect of regional nerve block technique in laparoscopic nephrectomy: a systematic review and network meta-analysis. J Clin Anesth. 2025;103:111829. doi:10.1016/j.jclinane.2025.111829

20. Laigaard J, Pedersen C, Rønsbo TN, Mathiesen O, Karlsen APH. Minimal clinically important differences in randomised clinical trials on pain management after total Hip and knee arthroplasty: a systematic review. Br J Anaesth. 2021;126(5):1029–1037. doi:10.1016/j.bja.2021.01.021