Assessing electrical bioimpedance across tissue levels in patients undergoing ultrasound-guided axillary brachial plexus block: a prospective observational study

Bioimpedance measures biological tissues’ electrical resistance. Bioimpedance monitoring may improve axillary brachial plexus block by providing real-time feedback alongside ultrasound guidance and nerve stimulation while performing ultrasound-guided regional anesthesia. However, there is a paucity of research examining bioimpedance fluctuations. This study aims to assess bioimpedance changes across various tissue types during an axillary brachial plexus block, characterize these changes, and explore their potential supportive role in tissue identification. This prospective, exploratory observational study included patients undergoing axillary brachial plexus block for upper-limb surgical procedures. Before the procedure, patients received 50 µg of fentanyl and 1 mg of midazolam, with continuous standard monitoring. An anesthesiologist performed a nerve block under ultrasound guidance using nerve stimulation in a sentinel model (0.1 ms, 0.5 mA, 2 Hz) with a nerve stimulator device that continuously displayed bioimpedance. An assistant administered the local anesthetic, while another, blinded to the study, recorded bioimpedance values across subcutaneous tissue, muscle, fascia, and at the proximity of the nerves. A linear mixed model was fitted to assess bioimpedance across tissue types, treating tissue type as a fixed effect and including subject-level random effects to account for within-subject correlation and individual variability. Statistical analysis was conducted using STATA-18. Among 80 participants (average age: 53.1 ± 18.7 years), bioimpedance measures were statistically significantly different across the pre-specified tissue types. The percentage change in bioimpedance from subcutaneous tissue to muscle was 4.18 ± 14.01, to fascia was 10.47 ± 22.62, to the radial nerve was 15.82 ± 15.14, and to the median nerve was 20.81 ± 13.21. An apparent decrease in bioimpedance was observed from the subcutaneous tissue to the median nerve. Bonferroni-corrected pairwise comparisons showed significant ( p < 0.01) differences in this change among various tissue types and nerves. No complications (i.e., intraneural/intravascular injections) were detected. The observed trend demonstrates a consistent reduction in bioimpedance as the needle advances toward nerves. Using bioimpedance monitoring could improve nerve block accuracy by providing an extra parameter for guiding needle placement through tissue layers.