Association Between Body Mass Index and Acute Postoperative Pain Following Laparoscopic Cholecystectomy: A Retrospective Clinical Study

Summary

This retrospective study shows that higher body mass index is independently associated with greater acute postoperative pain intensity and increased opioid requirements after laparoscopic cholecystectomy, supporting BMI-informed perioperative pain assessment and individualized analgesic planning.

Abstract

Obesity has been increasingly recognized as a factor associated with altered pain perception and postoperative analgesic requirements; however, its relationship with acute postoperative pain following laparoscopic cholecystectomy remains incompletely defined. This retrospective observational study investigated the association between body mass index (BMI) and acute postoperative pain severity in adult patients undergoing elective laparoscopic cholecystectomy. Ninety-six patients were categorized into obese and non-obese groups according to BMI criteria for Chinese adults. Demographic characteristics, perioperative variables, postoperative numeric rating scale (NRS) pain scores at multiple time points, and opioid consumption converted to oral morphine equivalents (OME) were collected from medical records. Multivariable logistic regression was used to evaluate the association between BMI and moderate-to-severe postoperative pain, defined as a 24 h mean NRS score ≥4. Compared with non-obese patients, obese patients had higher NRS scores at 6 h, 12 h, and 24 h after surgery, higher first NRS scores in the post-anesthesia care unit, and greater postoperative opioid consumption. In the adjusted regression model, continuous BMI was independently associated with moderate-to-severe postoperative pain after adjustment for age, sex, American Society of Anesthesiologists classification, and intraoperative opioid consumption. These findings suggest that elevated BMI is associated with greater acute postoperative pain severity and analgesic requirements after laparoscopic cholecystectomy. BMI-informed perioperative risk assessment may help guide individualized postoperative pain management in this patient population.

Introduction

Laparoscopic cholecystectomy (LC) is one of the most widely performed minimally invasive abdominal procedures and is generally associated with less tissue trauma and faster recovery than open surgery. Nevertheless, clinically relevant acute postoperative pain may still occur after LC, and insufficient analgesia can delay mobilization, increase opioid exposure, and reduce patient satisfaction. Current postoperative pain management increasingly emphasizes procedure-specific assessment, multimodal analgesia, and opioid stewardship rather than uniform opioid-based regimens for all patients^1,2.

Obesity is common among surgical patients and may influence perioperative pain and analgesic requirements through several mechanisms, including chronic low-grade inflammation, altered respiratory mechanics, increased abdominal wall thickness, and changes in opioid pharmacokinetics or pharmacodynamics^3,4. These features are particularly relevant after LC, in which trocar-site pain, visceral pain related to pneumoperitoneum, and shoulder or diaphragmatic discomfort may coexist during the early postoperative period. However, interpretation of early postoperative pain after LC remains challenging because pain outcomes may be influenced by analgesic technique, anesthetic management, postoperative rescue analgesia, and the timing and method of pain assessment^1,5,6.

For an international readership, it is important to clarify that this study used the obesity threshold recommended for Chinese adults. According to the Chinese adult body weight classification standard, obesity is defined as BMI ≥28.0 kg/m², which is lower than the conventional World Health Organization threshold of BMI ≥30.0 kg/m² commonly used in Western populations. This distinction reflects population-specific differences in body composition and obesity-related metabolic risk. Therefore, using BMI ≥28.0 kg/m² is clinically appropriate for a Chinese surgical cohort and may improve the applicability of perioperative risk stratification in this population⁷.

Previous studies have evaluated postoperative pain after LC and have examined analgesic techniques such as non-steroidal anti-inflammatory drugs (NSAIDs), local anesthetic infiltration, regional blocks, and opioid-sparing strategies^1,6,8. However, fewer studies have specifically quantified the independent association between BMI and early postoperative pain while accounting for perioperative covariates and standardized postoperative analgesic criteria. In addition, obese patients may require more cautious postoperative opioid use because of increased risks related to airway obstruction, hypoventilation, and opioid-induced respiratory depression⁹. These considerations support the need for BMI-informed pain assessment and individualized analgesic planning, particularly in patients with more severe obesity or higher opioid-related respiratory risk^1,8,9.

This single-center retrospective observational study analyzed adult patients who underwent elective LC. The study aimed to evaluate whether BMI was independently associated with moderate-to-severe acute postoperative pain, defined as a 24 h mean NRS score ≥4, after adjustment for selected preoperative and intraoperative factors. Compared with previous approaches that focus on single pain assessments or unadjusted group comparisons, this study integrated serial postoperative pain scores, postoperative opioid consumption expressed as oral morphine equivalents (OME), standardized rescue analgesia criteria, and multivariable adjustment for perioperative covariates. The findings may provide practical evidence for BMI-informed postoperative pain assessment and individualized analgesic management after LC.

Protocol

This retrospective observational study was approved by the Ethics Committee of Anji County Hospital of Traditional Chinese Medicine (approval No. 2025-13). The requirement for written informed consent was waived by the Ethics Committee because this study used de-identified retrospective clinical data and involved no additional patient intervention. This study was reported in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guideline, and the completed STROBE checklist is provided as Supplementary File 1. The reagents, equipment, and software used are listed in the Table of Materials.

1. Study population and patient screening

Adult patients who underwent elective laparoscopic cholecystectomy at Anji County Hospital of Traditional Chinese Medicine between January 2025 and September 2025 were retrospectively screened using the hospital electronic medical record (EMR) system and anesthesia information system. The screening process is summarized in Supplementary Figure 1. A total of 132 potentially eligible patients were initially screened, and 36 were excluded according to the predefined eligibility criteria or because of incomplete key data. Finally, 96 patients were included in the final analysis, including 50 patients in the non-obese group and 46 patients in the obese group.

Patients were eligible if they met all of the following criteria: age 18–65 years; first-time elective laparoscopic cholecystectomy; American Society of Anesthesiologists (ASA) physical status I-II; general anesthesia with tracheal intubation; and complete records of height, body weight, perioperative variables, postoperative pain scores, and postoperative analgesic administration. Patients were excluded if they had a history of chronic pain or long-term use of analgesics, sedatives, antidepressants, or anxiolytic drugs; severe cardiac, pulmonary, hepatic, or renal dysfunction, or ASA physical status ≥III; a history of opioid or alcohol abuse; conversion from laparoscopic to open surgery; combined neuraxial anesthesia or regional nerve block; or missing or unclear key data, including pain scores, perioperative medication records, or opioid dosage information.

2. BMI grouping

Preoperative height and body weight were extracted from the EMR system. BMI was calculated as body weight in kilograms divided by height in meters squared. Patients were classified according to the Chinese adult body weight classification standard, in which obesity is defined as BMI ≥28.0 kg/m² and non-obesity as BMI <28.0 kg/m². BMI was used both as a categorical variable for intergroup comparisons and as a continuous variable in the multivariable logistic regression model.

3. Outcome measures and definitions

The primary outcome was moderate-to-severe postoperative pain, defined as a 24 h mean NRS score ≥4. The 24 h mean NRS score was calculated as the average of the NRS scores recorded at 6 h, 12 h, and 24 h after surgery. Secondary outcomes included the first NRS score in the post-anesthesia care unit (PACU), NRS scores at 6 h, 12 h, and 24 h after surgery, postoperative opioid consumption converted to oral morphine equivalents (OME), operative duration, anesthesia duration, time to first ambulation, length of hospital stay, and postoperative nausea and vomiting (PONV) within 24 h after surgery.

Operative duration was defined as the interval from skin incision to completion of skin closure. Anesthesia duration was defined as the interval from initiation of anesthetic induction to completion of tracheal extubation. Time to first ambulation was extracted from nursing documentation. Length of hospital stay was calculated from admission and discharge records in the EMR system.

4. NRS pain assessment

Postoperative pain intensity was assessed using the 11-point NRS, ranging from 0 to 10. A score of 0 indicated no pain, 1–3 indicated mild pain, 4–6 indicated moderate pain, and 7–10 indicated severe pain. Pain was assessed at rest using the standardized prompt: “Please rate your current pain at rest on a scale from 0 to 10, where 0 means no pain, and 10 means the most severe pain imaginable.” Only resting NRS scores were used for the present analysis.

NRS scores were recorded by trained PACU or ward nursing staff according to the institutional postoperative pain assessment routine. The first PACU NRS score was defined as the first valid pain score obtained after the patient had regained consciousness, was clinically stable, and was able to provide a self-reported pain score. Subsequent NRS scores were recorded at 6 h, 12 h, and 24 h after surgery. The first PACU NRS score was usually assessed within 30 min after PACU arrival. Because this was a retrospective study based on routine clinical care, nurses were not specifically blinded to BMI status; however, pain scores were collected before study grouping and statistical analysis. If duplicate NRS assessments were available at the same time point, the score recorded closest to the scheduled time point was used.

5. Perioperative anesthesia and monitoring

All patients underwent standardized general anesthesia with tracheal intubation (following institutionally approved protocols). After entering the operating room, routine monitoring was established, including electrocardiography, noninvasive blood pressure, pulse oxygen saturation, end-tidal carbon dioxide, and body temperature monitoring. When available, depth of anesthesia was monitored using bispectral index monitoring, with the anesthetic depth adjusted according to routine institutional practice.

Anesthesia was induced intravenously with propofol at 1.5β2.5 mg/kg, sufentanil at 0.2–0.4 µg/kg or an equivalent opioid dose, and a nondepolarizing neuromuscular blocking agent such as rocuronium at 0.6–0.9 mg/kg or cisatracurium at 0.15–0.2 mg/kg. Tracheal intubation was performed after adequate loss of consciousness and neuromuscular relaxation. Anesthesia was maintained with inhalational anesthetics and/or intravenous anesthetics according to the attending anesthesiologist’s routine practice, with opioids administered as needed to maintain hemodynamic stability and adequate analgesia. Ventilation was adjusted to maintain end-tidal carbon dioxide within the clinically acceptable range.

Intraoperative opioid administration was extracted from anesthesia records, including opioid name, route of administration, dose, and administration time. Operative duration, anesthesia duration, and intraoperative monitoring variables were obtained from the anesthesia information system. No neuraxial anesthesia or regional nerve block was used in either group, thereby reducing heterogeneity related to regional analgesic techniques. Body temperature was monitored throughout anesthesia, and warming measures were applied when clinically indicated to maintain perioperative normothermia. The consistency of key perioperative anesthesia and analgesia-related measures between groups is summarized in Supplementary Table 1.

6. Postoperative analgesic protocol and safety monitoring

The same postoperative pain assessment schedule and rescue analgesia criteria were applied to both obese and non-obese patients. Postoperative pain was routinely assessed using the NRS. Rescue analgesia was administered when NRS ≥4 according to the institutional postoperative analgesic protocol. The indication for rescue analgesia was identical between groups. Rescue analgesia consisted of intravenous opioid supplementation and/or non-steroidal anti-inflammatory drugs (NSAIDs), according to clinical judgment, patient condition, and contraindications. All postoperative analgesic exposure within the first 24 h after surgery was extracted from medical orders and nursing administration records, including drug name, dose, route, administration time, and frequency.

For patients receiving postoperative opioid rescue analgesia, respiratory safety monitoring was performed in the PACU and ward according to routine postoperative nursing practice. Monitoring included level of consciousness, respiratory rate, pulse oxygen saturation, and clinical signs of respiratory depression, including excessive sedation, hypoventilation, airway obstruction, or oxygen desaturation. In obese patients, particular attention was paid to respiratory status because obesity may increase susceptibility to opioid-related hypoventilation and airway obstruction. When clinically indicated, supplemental oxygen, intensified observation, or physician reassessment was provided according to institutional postoperative care procedures.

7. PONV assessment

Postoperative nausea and vomiting (PONV) within 24 h after surgery was extracted from nursing records and medical orders. PONV was defined as any documented nausea, retching, or vomiting episode, or the administration of rescue antiemetic medication within the first 24 h after surgery. The incidence of PONV was compared between the obese and non-obese groups.

8. Calculation of Postoperative Opioid Consumption (OME)

Postoperative opioid consumption was extracted from electronic medical orders and nursing administration records within the first 24 h after surgery. For each opioid administration, the drug name, route, dose, and administration time were recorded. To allow comparison across different opioid agents and routes, all opioid doses were converted to OME using the formula: OME (mg) = administered dose × conversion factor.

Dose units were standardized before conversion. Morphine, oxycodone, and tramadol doses were expressed in milligrams, whereas fentanyl and sufentanil doses were expressed in micrograms. The conversion factors used in this study are listed in Supplementary Table 2 and were adapted from published OME conversion literature. The total 24 h postoperative OME was calculated by summing all converted opioid doses administered during the first 24 h after surgery. Intraoperative opioid exposure was also converted to OME using the same conversion approach and included as a covariate in the multivariable regression model.

9. Data extraction and quality control

Demographic characteristics, BMI, ASA physical status, operative duration, anesthesia duration, intraoperative opioid consumption, postoperative opioid consumption, postoperative NSAID use, PONV, time to first ambulation, length of hospital stay, and postoperative NRS scores were extracted from the EMR system, anesthesia information system, and nursing records. A standardized data extraction form was used before statistical analysis.

To improve data reliability, extracted data were checked against the original electronic records. Implausible or inconsistent values, including BMI, NRS scores, opioid doses, operative duration, and anesthesia duration, were rechecked using the original source records. Records were considered incomplete if any key variable required for primary outcome definition or multivariable regression analysis was missing or unclear. Incomplete records were excluded from the final analysis rather than imputed. Nine records with missing or unclear key variables were excluded rather than imputed. After exclusion, no missing values remained for variables included in the primary outcome definition or multivariable regression analysis.

10. Statistical analysis

Statistical analyses were performed using SPSS software, version 26.0. Continuous variables were assessed for normality using the Kolmogorov-Smirnov test and for homogeneity of variance using Levene’s test. Normally distributed continuous variables were presented as mean ± standard deviation and compared using the independent-samples t-test. Non-normally distributed continuous variables were presented as median [interquartile range] and compared using the Mann-Whitney U test. Categorical variables were presented as n (%) and compared using the chi-square test or Fisher’s exact test, as appropriate.

A multivariable logistic regression model was constructed to evaluate the association between continuous BMI and moderate-to-severe postoperative pain. The dependent variable was moderate-to-severe postoperative pain, defined as a 24 h mean NRS score ≥4. The independent variables included BMI, age, sex, ASA physical status, and intraoperative opioid consumption. NRS-related variables were not included as covariates to avoid circular definition bias. The regression model was fitted using the enter method. Regression coefficients, standard errors, Wald χ² values, odds ratios (ORs), 95% confidence intervals (CIs), and P values were reported.

Multicollinearity among regression covariates was assessed using variance inflation factors (VIFs), with VIF values <5 considered to indicate no substantial multicollinearity. Model discrimination was evaluated using the C-statistic, equivalent to the area under the receiver operating characteristic curve. Model calibration was evaluated using the Hosmer-Lemeshow goodness-of-fit test. Because sex may be clinically associated with postoperative pain, an exploratory sex × BMI interaction term was tested to assess whether the association between BMI and moderate-to-severe postoperative pain differed by sex.

Because this was a single-center retrospective study with a limited sample size, a post-hoc power analysis was performed for the primary continuous pain outcome using the observed between-group difference in the 24 h mean NRS score. The analysis was based on a two-sided independent-samples t-test, α = 0.05, n = 50 in the non-obese group, n = 46 in the obese group, and the observed group means and standard deviations. The estimated post-hoc power was >0.99. However, because the multivariable logistic regression model included 32 moderate-to-severe postoperative pain events and five covariates, corresponding to approximately 6.4 events per predictor variable, the regression analysis was interpreted cautiously as exploratory.

The number needed to harm (NNH) was calculated using the absolute risk increase in moderate-to-severe postoperative pain between the obese and non-obese groups. The formula was NNH = 1 / (risk in the obese group - risk in the non-obese group), and the result was rounded up to the nearest whole number. All statistical tests were two-sided, and P < 0.05 was considered statistically significant.

Results

Patient screening and baseline characteristics

A total of 132 patients who underwent elective laparoscopic cholecystectomy during the study period were initially screened and assessed for eligibility. Among them, 36 patients were excluded, including 6 patients aged <18 or >65 years, 5 patients with ASA physical status ≥III, 7 patients with chronic pain or long-term use of analgesics, sedatives, antidepressants, or anxiolytics, 2 patients with a history of opioid or alcoh...

Discussion

This retrospective observational study found that BMI was independently associated with acute postoperative pain severity after laparoscopic cholecystectomy. Compared with non-obese patients, obese patients had higher resting NRS scores at 6 h, 12 h, and 24 h after surgery, a higher 24 h mean NRS score, and greater postoperative opioid consumption expressed as OME.

In the adjusted logistic regression model, each 1 kg/m² increase in BMI was associated with higher odds of moderate-to-...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Anesthesia information system	Anji County Hospital of Traditional Chinese Medicine	Institutional anesthesia information system	Used to extract anesthesia duration, operative duration, intraoperative opioid administration, intraoperative monitoring records, and anesthesia-related timestamps.
Anesthesia workstation	Dräger	Fabius plus XL	Representative anesthesia workstation used for delivery and maintenance of general anesthesia with tracheal intubation.
Bispectral index monitor	Medtronic	BIS Vista Monitor 186-1046; BIS Quatro sensor 186-0106	Used when available for depth-of-anesthesia monitoring and adjustment of anesthetic depth according to institutional practice.
Cisatracurium besylate injection	Jiangsu Hengrui Pharmaceuticals Co., Ltd.	China NMPA approval No. H20060869	Nondepolarizing neuromuscular blocking agent used to facilitate tracheal intubation when selected by the attending anesthesiologist.
Electronic medical record (EMR) system	Anji County Hospital of Traditional Chinese Medicine	Institutional EMR/HIS system	Used to extract demographic data, height, body weight, BMI, ASA physical status, admission/discharge records, length of hospital stay, medication orders, and other clinical variables.
Endotracheal tube	Medtronic / Shiley	Shiley cuffed oral/nasal tracheal tube, adult sizes	Representative airway device used for tracheal intubation during general anesthesia.
Fentanyl citrate injection	Jiangsu Nhwa Pharmaceutical Co., Ltd.	China NMPA approval No. H20113508; 2 mL:0.1 mg	Opioid analgesic used for perioperative analgesia and included in OME conversion when administered.
Flurbiprofen axetil injection	Beijing Tide Pharmaceutical Co., Ltd.	China NMPA approval No. H20041508; 5 mL:50 mg	Non-steroidal anti-inflammatory drug used as adjunctive postoperative analgesia when clinically indicated and not contraindicated.
Laryngoscope or video laryngoscope	Verathon	GlideScope video laryngoscope system	Representative device used to facilitate tracheal intubation after induction of general anesthesia.
Microsoft Excel	Microsoft Corp.	Microsoft Excel 2021 / Microsoft 365	Used for data organization, supplementary table preparation, and calculation verification before statistical analysis.
Multiparameter patient monitor	Mindray	BeneVision N15	Representative monitor used for ECG, noninvasive blood pressure, pulse oxygen saturation, end-tidal CO2, and body temperature monitoring.
Numerical Rating Scale (NRS)	Clinical assessment standard tool	11-point NRS, 0-10	Pain intensity scale used for resting postoperative pain assessment in PACU and at 6 h, 12 h, and 24 h after surgery.
Nursing documentation system	Anji County Hospital of Traditional Chinese Medicine	Institutional nursing documentation system	Used to extract postoperative NRS scores, PACU and ward nursing records, PONV documentation, time to first ambulation, and medication administration records.
Ondansetron hydrochloride injection	Qilu Pharmaceutical Co., Ltd.	Ondansetron hydrochloride injection; 2 mL:4 mg or 4 mL:8 mg	Antiemetic medication used for prevention or treatment of postoperative nausea and vomiting when clinically indicated.
Patient-controlled analgesia/infusion pump	ICU Medical / Smiths Medical	CADD-Solis VIP 2120	Representative infusion system used when patient-controlled or programmed opioid analgesia was clinically indicated.
Propofol injection	Fresenius Kabi	Propofol 1% MCT/LCT; China NMPA approval No. HJ20150657	Intravenous anesthetic used for induction and/or maintenance of general anesthesia.
Remifentanil hydrochloride for injection	Yichang Humanwell Pharmaceutical Co., Ltd.	China NMPA approval No. H20030199; 2 mg/vial	Short-acting opioid used for intraoperative analgesia or anesthesia maintenance according to institutional practice.
Rocuronium bromide injection	Guangdong Jiabo Pharmaceutical Co., Ltd.	China NMPA approval No. H20183109; 5 mL:50 mg	Nondepolarizing neuromuscular blocking agent used to facilitate tracheal intubation when selected by the attending anesthesiologist.
SPSS Statistics	IBM Corp.	Version 26.0	Software used for statistical analyses, including normality testing, homogeneity of variance testing, group comparisons, multivariable logistic regression, and model diagnostics.
Sufentanil citrate injection	Yichang Humanwell Pharmaceutical Co., Ltd.	China NMPA-approved sufentanil citrate injection; 1 mL:50 μg	Opioid analgesic used for perioperative analgesia and included in OME conversion when administered.