Post-operative pain is a frequent complication after abdominal surgery and can have a significant impact on early recovery [1,2]. Poorly managed pain can also reduce the ability to cough, mobilise, take a deep breath and be able to take part in physiotherapy and put off pulmonary complications, delayed recovery, prolonged hospitalisation and higher pain medication needs [3]. Systemic analgesics are often given to relieve post-operative pain, but may have side effects including sedation, constipation, nausea, vomiting, and respiratory depression, especially opioids. It is for this reason that regional analgesic techniques have been receiving more and more attention as one of the components used in multimodal approaches for abdominal procedure-induced pain control [4,5].

The transversus abdominis plane (TAP) block is one of the most popular ultrasound-guided regional anesthetic techniques, which is used to achieve analgesia by injecting local anesthetic into the fascial plane between the internal oblique and the transversus abdominis muscles [6]. TAP block may help to prevent transmission of somatic pain coming from the abdominal wall by blocking anterior rami of thorolumbar spinal nerves and can potentially limit the use of systemic analgesics in the early postoperative period. But its efficacy is dependent on the surgical incision, block approach and amount of sensory coverage [7,8].

Another abdominal wall block for analgesia is the posterior rectus sheath block (PRSB). This method involves injection of local anesthetic behind the rectus abdominis muscle and in front of the posterior rectus sheath, thus blocking nerves to the front of the abdomen [9,10]. PRSB can be used for specific analgesia for midline abdominal incisions, and has been more recently explored as an alternative abdominal wall block [11]. The distribution of local anesthetic in the body could differ between PRSB and TAP block, which may lead to variations in the effectiveness of the post-operative analgesics [12].

It is, therefore, necessary to compare these techniques to assess their effectiveness at controlling pain after abdominal surgery. Standardized pain scores are a practical tool for assessing post-operative pain to enable comparison of the effectiveness of the different types of regional anesthesia.

Research Objective

To compare the mean post-operative pain in patients receiving posterior rectus sheath block versus transversus abdominis plane block undergoing abdominal surgeries.

Study Design The aim of this study was to compare the post-operative pain after ultrasound-guided posterior rectus sheath block (PRSB) versus ultrasound-guided transversus abdominis plane (TAP) block in abdominal surgery performed under general anesthesia. Study Setting and Duration This study was done at Department of Anaesthesiology, National Hospital & Medical Centre Lahore. The length of the study was 6 months from 21 May 2021 to 22 November 2021. Sample Size The total number of patients was 60 and each treatment group had 30 patients. The sample size was determined with a 95% confidence level and 80% statistical power, assuming the difference in the pain scores between patients receiving TAP block and rectus sheath block, reported in a previous study. The size of the samples was deemed sufficient to show an existable difference in post-operative pain for the two groups. Sampling Technique A non-probability consecutive sampling scheme was employed to recruit the patients. All patients with abdominal surgery who meet the inclusion criteria during the study period were consecutively recruited, until the desired study size was reached. Patient Selection Patients of either gender, aged 20–70 years, belonging to American Society of Anesthesiologists (ASA) physical status class I or II, and scheduled to undergo abdominal surgery under general anesthesia were eligible for inclusion. Patients with a documented allergy to any drug used in the study, as determined from their medical records, were excluded. Patients with end-stage hepatic or renal disease were also excluded. Hepatic dysfunction was considered when alanine aminotransferase (ALT) or aspartate aminotransferase (AST) levels were more than twice the normal limit, while significant renal dysfunction was defined as a serum creatinine level greater than 2 mg/dL. Ethical Considerations and Data Collection Ethical approval was obtained from the local ethical review committee of National Hospital & Medical Centre, Lahore after approval of the study synopsis. All the participants had provided written informed consent prior to enrollment. Participants were briefed on the process of the study, and personal and clinical data from the participants in the study were kept confidential throughout the study. Demographic & clinical data such as age, gender, height, weight, body mass index (BMI), ASA class, surgery type & surgery duration was collected on a proforma designed beforehand. Randomization and Intervention The children were divided into two equal groups after enrolment by a simple lottery. Group A had a pre-incisional ultrasound-guided posterior bilateral rectus sheath block. Group B was given an ultrasound-guided TAP block. 20 mL 0.25% bupivacaine was used for each block technique for the respective block technique assigned. At the end of surgery, 40 mL of 0.25% bupivacaine was infiltrated locally in the surgical wound in patients in Group B. The patients were induced and maintained under general anesthesia as per the standard protocol in the institution. Assessment of Post-Operative Pain After surgery, the patients were observed in the post-operative period to assess the pain level. The pain intensity was measured by the visual analogue scale (VAS) with a fixed operational definition after surgery. Measurements of pain were taken at 6 hours’ post-surgery. The VAS score at 6 hours was used as the main outcome variable for comparison between the posterior rectus sheath block and TAP block groups. Statistical Analysis Data was entered and analysed using Statistical Package for the Social Sciences (SPSS) version 24.0. The quantitative variables such as age, weight, height, BMI, length of surgery, and VAS scores for pain after surgery were reported as mean ± SD. Qualitative variables such as gender, ASA class, type of surgery were shown as frequency and percentages. Independent-samples t-test was used to compare the baseline characteristics and mean post-operative pain scores for the two groups. Stratification was used to control for potential effect modifiers such as age, gender, BMI, ASA class, duration of surgery, type of surgery. Within relevant strata, an independent-samples t-test was used to compare the mean pain VAS scores between the two groups after stratification. Values of p≤0.05 were considered to be statistically significant.

Group A had a mean age of 35.33 ± 12.33 years compared with 39.73 ± 13.28 years in Group B (p=0.189), shown in table 1. Females predominated in both groups, comprising 80.0% in Group A and 53.3% in Group B (p=0.028). Group B had significantly greater mean height (167.80 ± 7.83 vs. 161.73 ± 6.52 cm; p=0.002), while mean weight was comparable (66.33 ± 11.01 vs. 67.80 ± 8.17 kg; p=0.560). BMI was significantly higher in Group A (26.04 ± 3.80 vs. 23.76 ± 3.20 kg/m²; p=0.015). Duration of surgery was similar between groups (1.85 ± 0.63 vs. 2.07 ± 0.50 hours; p=0.147), whereas VAS pain was significantly higher in Group A (4.73 ± 1.01 vs. 3.57 ± 1.13; p<0.001).

Table 1. Demographic and Anthropometric Characteristics of Study Participants

The most frequent procedure was C-section in Group A (17, 56.7%), whereas Group B had a more varied distribution, with cholecystectomy and laparotomy each accounting for 6 (20.0%) cases (table 2). Appendectomy was performed in 3 (10.0%) patients in Group A and 5 (16.7%) in Group B. Nephrectomy occurred in 1 (3.3%) and 5 (16.7%) patients, while ovarian cystectomy occurred in 3 (10.0%) and 4 (13.3%), respectively.

Table 2. Comparison of Type of Surgery between Study Groups

ASA class II was more common in Group A, occurring in 23 (76.7%) patients compared with 14 (46.7%) in Group B (table 3). Conversely, ASA class I was observed in 7 (23.3%) patients in Group A and 16 (53.3%) in Group B. The difference in ASA classification between groups was statistically significant (p=0.017).

Table 3. Comparison of ASA Class Between Study Groups

Patients in Group B consistently reported lower VAS pain scores across most stratified categories (table 4). For age ≤40 years, VAS was 3.29 ± 1.16 in Group B versus 4.61 ± 0.99 in Group A (p<0.001), while for age >40 years it was 3.92 ± 1.04 versus 5.14 ± 1.07 (p=0.030). Among females, VAS was significantly lower in Group B (3.75 ± 1.00 vs. 4.76 ± 0.74; p=0.001), whereas the difference among males was not significant (3.35 ± 1.28 vs. 4.67 ± 1.86; p=0.159). Significant differences were also observed for BMI ≤25 kg/m² (3.22 ± 1.11 vs. 4.73 ± 1.35; p=0.003), ASA I (3.18 ± 1.05 vs. 5.28 ± 1.60; p=0.001), laparotomy (3.33 ± 1.03 vs. 6.00 ± 1.00; p=0.008), ovarian cystectomy (3.50 ± 1.00 vs. 5.67 ± 0.57; p=0.021), and surgeries lasting ≤1.5 hours (2.50 ± 1.29 vs. 4.33 ± 0.71; p=0.006) or >1.5 hours (3.73 ± 1.04 vs. 4.90 ± 1.09; p<0.001).

Table 4: Stratified Analysis of Postoperative Pain Scores (VAS) Between Posterior Rectus Sheath Block and TAP Block Groups

The mean VAS pain score after surgery was significantly lower in the TAP block group compared with the posterior rectus sheath block (PRSB) group (3.57 ± 1.13 vs. 4.73 ± 1.01, respectively; p<0.001). This discovery is consistent with the analgesic efficacy of TAP block for abdominal surgery. In the present study, the clinically important early analgesic effect of TAP block was confirmed by a large randomized controlled trial study, which included 51 RCTs and revealed a reduction of postoperative pain by 1.4 points at 6 hours compared to placebo (p<0.001) [13]. Ages of both groups were generally similar with mean age of 35.33 ± 12.33 years in Group A and 39.73 ± 13.28 years in Group B (p=0.189). However, significant differences were observed in gender distribution (p=0.028), height (161.73 ± 6.52 vs. 167.80 ± 7.83 cm; p=0.002) and BMI (26.04 ± 3.80 vs. 23.76 ± 3.20 kg/m²; p=0.015). These differences should be taken into account when interpreting the main results, but the subsequent stratified analysis showed that the lower VAS scores with TAP block were stable across several patient subgroups. Variability in the reported effectiveness of abdominal wall blocks has been highlighted in previous literature and may be due to variations in patient characteristics, surgical procedure and block technique [14]. The operative duration was comparable between groups, (1.85 ± 0.63 vs. 2.07 ± 0.50 hours; p=0.147), suggesting that the main difference in the pain score was not due to the operative duration. Importantly, TAP block remained to be significantly associated with lower pain scores after stratification by surgery duration. For procedures lasting ≤1.5 hours, VAS was 2.50 ± 1.29 in Group B compared with 4.33 ± 0.71 in Group A (p=0.006), while for procedures >1.5 hours, the corresponding values were 3.73 ± 1.04 and 4.90 ± 1.09 (p<0.001). These results are in line with the previous meta-analysis that reported that TAP block offers significant early postoperative pain relief, especially within the first 6–12 hours after abdominal surgery [15]. The advantage of TAP block was also shown in various demographical and clinical subgroups. In patients aged ≤40 years, VAS was 3.29 ± 1.16 in Group B versus 4.61 ± 0.99 in Group A (p<0.001), while in those aged >40 years it was 3.92 ± 1.04 versus 5.14 ± 1.07 (p=0.030). Among females, pain scores were significantly lower with TAP block (3.75 ± 1.00 vs. 4.76 ± 0.74; p=0.001), whereas the difference among males was not significant (3.35 ± 1.28 vs. 4.67 ± 1.86; p=0.159). Similarly, patients with BMI ≤25 kg/m² had lower VAS scores with TAP block (3.22 ± 1.11 vs. 4.73 ± 1.35; p=0.003). Our findings corroborate data from other abdominal and gynecological procedures demonstrating the analgesic effect of TAP block but can differ in magnitude between surgical populations [13]. The ASA-stratified results demonstrated that the pain scores were significantly lower with TAP block in ASA I patients (3.18 ± 1.05 vs. 5.28 ± 1.60; p=0.001), but not in ASA II patients (4.00 ± 1.11 vs. 4.56 ± 0.73; p=0.069). By surgical procedure, significant differences were observed for laparotomy, where VAS was 3.33 ± 1.03 in Group B versus 6.00 ± 1.00 in Group A (p=0.008), and ovarian cystectomy, where scores were 3.50 ± 1.00 versus 5.67 ± 0.57 (p=0.021). These results are clinically plausible since TAP block has been shown to be effective in other abdominal surgical procedures such as those in the gynecological and urological fields. A meta-analysis of laparoscopic surgical procedures revealed less use of post-operative analgesics and lower pain scores at 2 hours and 6 hours after TAP block [16]. Overall, the present study shows that there was a clear early analgesia advantage of the TAP block (absolute difference of 1.16 VAS points (3.57 ± 1.13 vs. 4.73 ± 1.01; p<0.001). The results of the study should however be considered in light of the available evidence on PRSB. For a 2024 systematic review and meta-analysis of 20 RCTs of patients who underwent rectus sheath block (RSH) (N = 1,421), RSH significantly reduced pain during the initial 0-2 hours after surgery as well as during the 10-12 hours after surgery, but the effectiveness depended on the comparator and clinical setting [17]. Therefore, the results of the present study that showed lower pain scores with TAP block than with PRSB do not mean that PRSB is ineffective, but rather that the PRSB protocol may be inadequate for providing analgesia under the conditions of the surgical procedure and the PRSB protocol utilized in this trial. Further supporting evidence is the observation of decreased opioid use and postoperative pain following TAP block in a wide variety of abdominal surgeries [18]. STUDY STRENGTHS AND LIMITATIONS The strength of this study is the randomized controlled design allowing a direct comparison of the use of posterior rectus sheath versus transversus abdominis plane block for postoperative analgesia after abdominal surgery. The ultrasound guided regional anesthesia and the standardized assessment of pain with the visual analogue scale also contributed to the methodological uniformity of the study. Stratified analysis based on key demographic, clinical, and operative variables also offered additional insight on the consistency of the analgesic effect in each of these subgroups. But there are some drawbacks to be taken into account. This study was performed at a single center in a relatively small patient population and may not be representative of other centers or surgical populations. Variability in the incision type and the degree of pain experienced after the procedure was added by the inclusion of various types of abdominal procedures. In addition, the baseline difference in gender, height, BMI, and ASA classification among the groups might have played a role in the outcomes of the overall pain assessment after stratification. Pain was only assessed at one time following surgery, so the duration of analgesia and changes in pain over time were not evaluated. Other clinically relevant parameters (e.g., opioid use, rescue analgesic use, block related complications, patient satisfaction, functional recovery, hospital stay) were not assessed. When interpreting the observed superiority of TAP block, these limitations should be taken into account.

To summarize, in a randomized controlled study, ultrasound-guided transversus abdominis plane block was shown to be superior to posterior rectus sheath block for early postoperative pain following abdominal surgery. TAP block was correlated with reduced postoperative pain and was found to have an analgesic effect that was persistent in various significant demographic and perioperative subgroups. These results indicate that TAP block might be more useful when multimodal analgesia is used for postoperative pain management in a heterogeneous population for abdominal surgery. However, the selection of the regional anesthetic technique should be considered in relation to the surgical incision, type of pain that is likely to occur, patient factors and the skills of the anesthesiologist. Future studies with larger, multicenter randomized trials that standardize surgical patient populations and assess longer-term outcomes of pain, opioid use, recovery parameters, and adverse events are warranted to further define clinical utility and comparative effectiveness of TAP block and posterior rectus sheath block.