Purpose: Electrical impedance increases following test injections of non-conducting solutions around nerves; however, this increase should diminish rapidly with intravascular needle placement, wherein the systemic circulation will dissipate the solution. For this observational study, we hypothesized that the impedance increases significantly at the perineural space after an injection of 5% dextrose in water (D5W), but that it does not increase correspondingly at the intravascular location
Methods: After Ethics Research Board approval, electrical impedance was measured by a nerve stimulator displaying resistance, Stimuplex HNS 12, before and during (30 sec) an injection of D5W 3 mL: 1) during intravenous cannula placement using an insulated stimulating needle sheathed in its plastic cannula, MultiSet NanoLine with 18G needle; and 2) during needle placement (Pajunk 22G insulated) for an ultrasound-guided supraclavicular block in patients undergoing hand surgery. The impedance changes at each location were analyzed and compared.
Results: Data were collected from 16 patients. Baseline impedance was lower intravascularly (mean 16.5 +/- standard deviation 7.2 kOmega) compared with perineurally (23.5 +/- 8.3 kOmega) (P = 0.037). Peak impedance after intravascular D5W injection was 20.1 +/- 6.8 kOmega, which was not a significant change (P = 0.15). In contrast, peak impedance after perineural D5W injection was 58.6 +/- 29.1 kOmega, an increase of 35.1 +/- 26.4 kOmega (155 +/- 117%), and then it reached a plateau of 36.7 +/- 19.6 kOmega. The increase in impedance was significantly greater at the perineural location (P < 0.0001).
Conclusion: The absence of a significant increase in impedance upon injection of D5W prior to injection of local anesthetic may provide useful information to warn of intravascular injection.