Concentration of local anesthetics is important in nerve blocks

Article information

Acute Crit Care. 2025;40(1):150-151

Publication date (electronic) : 2025 February 28

doi : https://doi.org/10.4266/acc.002544

Raghuraman M. Sethuraman^,1

Department of Anesthesiology, Sree Balaji Medical College and Hospital, Chennai, India

Corresponding author: Raghuraman M Sethuraman Department of Anesthesiology, Sree Balaji Medical College and Hospital, Biher, 7 Works Rd, New Colony, Chromepet, Chennai 600044, India Tel: +91-63-791-41854 Fax: +91-44-429-11000 E-mail: drraghuram70@gmail.com

Received 2024 June 11; Revised 2024 October 8; Accepted 2025 February 18.

To the Editor:

I read with great interest the research article comparing the quality of different local anesthetics in femoral nerve block (FNB) in positioning for central neuraxial block (CNB) and wish to comment on that article [1]. Seth et al. [1] used 15 ml of 0.25% bupivacaine, 0.5% ropivacaine, or 1.5% lignocaine in FNB. However, the concentrations were not standardized for potency. The same volume of ropivacaine or bupivacaine in the same concentration (0.5%) was required to produce an effective FNB in 50% of cases [2], although ropivacaine is about 40% less potent than bupivacaine. Considering the same volume used in all groups in the study [1], a concentration of 0.375% of ropivacaine should have been chosen for comparison with 0.25% bupivacaine. Therefore, the conclusions of the study that “0.5% ropivacaine is a favorable choice for FNB due to early onset, ability to yield a good quality block [1],” are misleading. The results might have been different if the local anesthetics at ideal concentrations (0.25% bupivacaine versus 0.375% ropivacaine) were compared.

Furthermore, the concentration of lidocaine chosen was not ideal for comparison with the concentrations of ropivacaine or bupivacaine used in that study [1]. Because of its quicker onset of action, lidocaine (either alone or in combination with other long-acting local anesthetics) is most commonly preferred in such a situation where the “time required for patient positioning” is the primary concern. Furthermore, the choice of local anesthetic did not influence the reduction of pain scores as per a meta-analysis of studies using one among the local anesthetics namely, lidocaine, bupivacaine, or ropivacaine [3]. Notably, lidocaine was used in the majority of the studies included in that meta-analysis, and none of the studies compared the local anesthetics directly [3]. Thus, the concept of the study [1] is questionable. Thus, the novelty of the study is questionable. It is well-known that ropivacaine has a better safety profile than bupivacaine. Nevertheless, bupivacaine can be considered, especially for ultrasound-guided blocks as there is a slim chance of intravascular injection of local anesthetics, and confirmation of negative aspiration of blood completely avoids it. In addition, bupivacaine is more economical than ropivacaine.

Additionally, I respectfully disagree with the results. The time to achieve visual analog scale <4 and grade 1 or 2 sensory block were lowest, intermediate, and highest in the lidocaine, ropivacaine, and bupivacaine groups, respectively. However, the P-values were <0.001 for all these parameters. In addition, it is not clear whether the blocks were performed by the same person or persons with similar training. Also, the mean performance time for CNB (not specified whether spinal or epidural anesthesia) did not vary much between the groups (9.9±1.3, 8.8±0.8, 8.3±0.7). Here again, the P-values of 0.002 and <0.001 are questionable. While the P-value of 0.038 might be correct, it should have been for ropivacaine versus lidocaine and not for bupivacaine versus lidocaine. Above all, this parameter is not important here and might vary based on the expertise of the anesthesiologist and a stroke of luck as CNB (spinal or epidural) was probably performed using the landmark approach. In summary, the conclusions of that study [1] should be reconsidered carefully based on the inappropriate concentrations of local anesthetics chosen and the incorrect/questionable results.