Ultrasound-guided tranversus abdominis plane block for herniorrhaphy in children

Editor, We read with great interest the prospective dose-finding study by Chrystelle et al.1 using the up-and-down sequential method of analysis. The study showed that ultrasound-guided tranversus abdominis plane block with 0.2 ml kg−1 of 0.2% levobupivacaine provided successful peroperative analgesia in children who underwent herniorrhaphy. We would like to point out some issues that might strengthen the conclusions of this study and that may perhaps be beneficial for future studies. First, the effective dose of levobupivacaine for a tranversus abdominis plane block was obtained by using Dixon's up-and-down sequential allocation technique, and the ED50 and ED95 were obtained by bootstrap and assuming a logistic dose–response relationship. The assigned doses of up-and-down method experiments are not independent variables, as each dose is dependent on the response to the previous dose. Although the assumption of dose–response curve symmetry of certain mathematical models (e.g. normal or logistic distribution) is typically made, this assumption is unverifiable, and estimators may have a large bias. Valero and Sundberg2 specifically re-examined and criticised the use of the up-and-down sequential method. Furthermore, studies by Benhamou et al.3 and Polley et al.4 showed that the confidence intervals of bootstrap are wide. As standard probit or logit regression is likely to produce biased estimators and considering the lack of familiarity, we consider that it is better to use the isotonic regression estimator with confidence intervals derived by bootstrapping. Second, in their study, the transversus abdominis plane block was performed by placing the ultrasonic probe transversely on the vertical mid-axillary line between the iliac crest and the 12th rib, and the needle was introduced anteriorly and advanced in-plane. The ilioinguinal and iliohypogastric nerves are branches of the primary ventral ramus of L1, which stems from the lumbar plexus and immediately receives a branch from the 12th spinal nerve.5 We are uncertain about the efficacy of the local anaesthetic injection at the above-mentioned level, as the ilioinguinal and iliohypogastric nerves are difficult to locate at this level, and this insertion site may rely on the fan-like injection and the spread of the local anaesthetic inferiorly for an adequate block. Meanwhile, the sevoflurane that was used during the procedure may have concealed the insufficiency of the nerve block. To improve the success rate of ilioinguinal-iliohypogastric nerve blocks in paediatric patients, we suggest that small volumes of the local anaesthetic should be placed cephalad to the iliac crest at the anterior axillary line. Acknowledgements relating to this article Assistance with the letter: none. Financial support and sponsorship: none. Conflicts of interest: none. Comment from the editor: Dr Chrystelle and colleagues did not respond to the Journal's invitation to reply to this letter.