Electrical impedance to detect accidental nerve puncture during ultrasound-guided peripheral nerve blocks

The objective of this study was to assess if an increase in electrical impedance was associated with intraneural (sub-epineural) needle tip placement.

Two electrical impedance measurements were carried out in each of 140 peripheral nerve blocks. The first measurement was performed at a distance of 0.5-1 cm from the nerve trunk (reference value), and the second measurement was performed close to the nerve, either immediately before local anesthetic injection if no nerve puncture was suspected, or immediately before repositioning the needle if nerve puncture was suspected. Nerve puncture was suspected if any one of the following indications was present: pain or paresthesia; motor responses with a minimal stimulating current < 0.4 mA; needle tip observed inside the nerve using ultrasound; nerve swelling after injection of local anesthetic. Electrical impedance variations were compared between the no puncture and the suspected puncture groups.

Nerve puncture was suspected in 21 cases. The median variation [quartiles] of electrical impedance was +6.6% [-20; 36%] in the suspected puncture group (n = 21) and -10.0% [-28; 0%] in the no puncture group (n = 119) (P = 0.02). Absolute values of electrical impedance close to the nerve were greater in the suspected puncture group (15.5 kΩ [12.0; 18.0 kΩ]) vs the no puncture group (12.0 kΩ [8.9; 15.1% kΩ]) (P = 0.013). A receiver operating characteristic (ROC) curve was constructed, and the optimal cut-off for impedance was +4.3%.

A > 4.3% increase in electrical impedance may indicate accidental nerve puncture during peripheral nerve block.