To the Editor,


In a single-center, prospective randomized controlled trial with the small sample including 62 elderly patients who underwent elective laparoscopic colorectal surgery, Shen et al. [1] compared postoperative analgesic efficacy of ultrasound-guided erector spinae plane block (ESPB) and oblique subcostal oblique subcostal transverse abdominis plane block (TAPB) and showed that the ESPB was superior to the oblique subcostal TAPB in decreasing postoperative pain level and total opioid consumption. Given the facts that the use of multimodal analgesia protocol including a nerve block can improve postoperative pain control with decreased analgesic consumption and is being emphasized in current practice of Enhanced Recovery After Surgery protocols [2], this study has potential implications. However, there are several issues in this study that need further clarification. We hope to get the responses from the authors.

First, the primary outcome was visual analogue scale (VAS) pain scores at resting and active states during the first 24 postoperative hours. The sample size was calculated on the basis of their pilot study, in which mean VAS pain scores over the first 24 postoperative hours were 2.61 ± 1.64 in the ESPB group and 3.96 ± 1.78 in the TAPB group. However, Shen et al.’ study did not provide the mean VAS pain scores over the first 24 postoperative hours. Furthermore, the authors did not clearly describe whether mean VAS pain scores for sample size calculation were resting or active pain scores. Just like this study has shown, it is believed that postoperative pain is more intense during movement than at resting state in patients undergoing abdominal surgery. As the between-group difference in means of postoperative resting or active pain scores is an important determinant of the minimal sample size required in a randomized controlled trial [3], we argue that clarification of this issue would improve the transparency of this study design.

Second, in the results section, the authors described that the ESPB compared to the TAPB decreased VAS pain scores at both resting and active states during the first 24 postoperative hours. In Fig. 2 of Shen et al.’s article [1], however, the statistical details about between-group comparisons of resting and active VAS pain scores at each observed time point were not provided. According to Fig. 2 of Shen et al.’s article [1], we noted that beside the time point of 0.5 h after surgery, the between-group differences of mean resting and active VAS pain scores at each observed time point were less than 1, with large standard deviations. We are very interested in knowing at which time points the between-group differences of resting and active VAS pain scores reach statistical significance and minimal clinically important difference for acute postoperative pain control (i.e., 1.5 on a 0–10 VAS) [4].

Third, both ESPB and oblique subcostal TAPB were performed before general anesthesia, which was induced with sufentanil and propofol, and maintained using 1.5–2% sevoflurane and remifentanil 0.1–0.2 μg/kg/min. However, the authors did not compare intraoperative opioid consumption in the two groups, unlike previous studies assessing perioperative analgesic efficacy of nerve blocks in patients undergoing abdominal surgery [5,6,7]. Because of this design limitation, an important issue that this study cannot answer is whether the ESPB provides a more intraoperative opioid sparing than the oblique subcostal TAPB does.

Finally, in this study, the ESPB significantly decreased sufentanil consumption of patient-controlled intravenous analgesia during the first 24 postoperative hours. However, the between-group difference of mean sufentanil consumption in the first postoperative 24 h was 12 μg. In a randomized controlled trial comparing opioid consumption for postoperative pain control, it is generally required that total opioid consumption should be converted into milligram morphine equivalent (MME) for between-group comparison and the recommended minimal clinically important difference of MME is an absolute reduction of 10 mg intravenously administered morphine in the 24 h [4]. As the conversion factor of equivalent dose for intravenously administered sufentanil and morphine is 1:1000 [8], intravenously administered sufentanil 12 μg is equivalent to 12 mg intravenously administered morphine. That is, opioid sparing for postoperative pain control with the ESPB compared to the oblique subcostal TAPB exceeds the recommended minimal clinically important difference of MME. This is vital evidence to support their conclusion.