Electrical Impedance to Distinguish Intraneural from Extraneural Needle Placement in Porcine Nerves during Direct Exposure and Ultrasound Guidance

Background:Intraneural injection during peripheral nerve blockade can cause neurologic injury. Current approaches to prevent or detect intraneural injection lack reliability and consistency, or only signal intraneural injection upon the event. A change in electrical impedance (EI) could be indicative of intraneural needle placement before injection. Methods:After animal care committee approval, eight pigs were anesthetized and kept spontaneously breathing. In four pigs (part 1), the sciatic nerves were exposed bilaterally for direct needle placement; in a further four pigs (part 2), the tissue was kept intact for ultrasound-guided needle placement. An insulated needle (Sprotte 24 gauge; Pajunk GmbH Medizintechnologie, Geisingen, Germany), attached to a nerve stimulator displaying EI (Braun Stimuplex HNS 12; B. Braun Medical, Bethlehem, PA), was placed extraneurally and then advanced to puncture the nerve sheath. Five punctures within approximately a 1-cm length of each nerve were performed. For each Part, overall EI at each compartment and EI after individual punctures were compared using a general linear model, with post hoc analysis using the Duncan multiple range test. Results:The EI was lower extraneurally compared with intraneurally during open dissection (12.1 ± 1.8 vs. 23.2 ± 4.4 k&OHgr;; P < 0.0001; n = 8) and when using ultrasound guidance (10.8 ± 2.9 vs. 18.2 ± 6.1 k&OHgr;; P < 0.0001; n = 7 nerves were visualized adequately). The EI difference was maintained despite performing five sequential punctures. Conclusions:With further study, EI could prove to be a quantifiable warning signal to alert clinicians to intraneural needle placement, preventing local anesthetic injection and subsequent nerve injury.