Postoperative analgesic effectiveness of erector spinae plane block vs. rectus sheath block: a reply

Abstract

We wish to respond to recent correspondence [1, 2] regarding our article [3].

We believe that the risk of unblinding in our study is minimal, despite different puncture sites, as the interventions were performed on anaesthetised patients and postoperative care providers were not present during administration.

While we used standard measures like postoperative pain score, total analgesic consumption and time to first rescue analgesic administration, we recognise that peri-operative care is focusing increasingly on patient comfort and satisfaction [3]. The multidimensional nature of pain underscores the need for more comprehensive tools, such as the Quality of Recovery (QoR-40) scale, in conjunction with patient satisfaction measures. In our study, we were limited in our ability to capture endpoints beyond the 24-h postoperative period due to resource constraints. We showed that patients allocated to the erector spinae plane block experienced significantly reduced postoperative pain scores compared with those allocated to the rectus sheath block group. This reduction exceeded the minimal clinically important differences (MCID) threshold (absolute reduction ≥ 1 point on the numerical rating score) [3] at nearly all postoperative time intervals, except immediately on arrival in the PACU. While the reduction in morphine milligram equivalents in our study was statistically significant, it did not meet the MCID threshold. This discrepancy may be attributed to the rescue analgesic protocol employed (based on pain assessment) and resource constraints. Moreover, evidence suggests that MCID values can vary across surgical procedures and patient populations, as both are influenced by study settings and population characteristics [4].

To address the comment that our sample size calculation should have been conducted separately for the three primary endpoints, we derived sample size estimates from a previous study that compared bilateral ultrasound-guided erector spinae plane blocks with rectus sheath blocks [5]: total postoperative analgesic consumption required a sample size of 32 patients per group; postoperative pain score required 31 patients per group; and time to first rescue analgesic administration required 34 patients per group. The power analysis was conducted with a target of 90% power (1-β) and a type 1 error rate (α) of 0.05 which is in line with published advice [6]. However, as we acknowledged in our limitations, the study may have been underpowered to detect differences in postoperative nausea and vomiting outcomes, despite the significant opioid-sparing effect of the erector spinae plane block. Using the example by Kwon et al. for sample size was questioned [5]. It is understood that determining sample size in clinical research is inherently complex and lacks a universally accepted approach, which can lead to disagreements [6]. While conducting pilot research within the same study setting can provide a more targeted and realistic estimation, this method requires additional resources.

Despite its superior analgesic efficacy, administering the erector spinae plane block for midline abdominal surgery postoperatively in anaesthetised patients poses challenges in terms of the need for patient repositioning [7]. The erector spinae plane block may be better suited for procedures where the patient is already positioned favourably for this type of block [8]. For abdominal surgeries, a more pragmatic approach may involve administering the erector spinae plane block in conscious patients before the induction of anaesthesia to mitigate the risks associated with patient repositioning. In our study, the durations of anaesthesia and surgery were comparable. There was no indication of a prolonged non-surgical anaesthesia time for the erector spinae plane block. Any small difference observed may be due to variations in the time between the start of anaesthesia and the incision, which was not assessed separately in our study.

Regarding the generalisability and conclusions, it is important to recognise inherent limitations common to all investigations, particularly in applying results from the study sample to a broader target population and across different populations. While randomised controlled trials are considered the gold standard in study design, their applicability may be constrained when the trial population differs significantly from the intended broader population. Logistical and financial constraints limited the population diversity of our study. Caution is necessary when applying our results to broader populations or diverse healthcare settings in low- and middle-income countries.