Please confirm that an ethics committee approval has been applied for or granted: Not relevant

Background and Aims The Peripheral Nerve Blocks (PNB) are becoming a major analgesic technique for the children’s inferior/superior limbs surgery. The objective of this research is to design a formula which will help predict with accuracy the depth of the nerves according to the weight of patients benefitting from PNB.

Methods This prospective and analytical study includes children that will undergo limbs surgery. The PNB were realized with a guided ultra-sound or a neurostimulation. Additionally, the Distance between the Nerve and the Skin (DNS) was measured in all children under study. The data were analyzed by SPSS ‘20’ as well as Stata software for a linear regression.

Results 355 patients were included in this study. The average age was 9,29 ± 4,13 years old and the average weight was 34,7 ± 17 kg. The average DNS was 21,97 ± 10,02 mm. The findings also showed an average correlation R2= 0,48 between the DNS and the children’s weight (P < 0,001). This enabled us to elaborate a formula to predict the length of the needle according to: the weight of the child, the detecting technique and the PNB type realized [DNS (DNP) = 4,33 + 5,48 (technique) + 0,23 (weight) + β (Corresponding to the type of block).

Conclusions DNS measurement can be a good guide for needle placement in order to reduce the risk of nerves complications.