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Background and Aims Accurate monitoring of the needle tip position during a nerve block procedure enables the procedure to be performed effectively and safely. Electrical impedance (EI) values, which indicate the electrical resistance of the needle tip, can be measured by using a nerve stimulator. The EI values vary depending on the water retention of the tissue at the needle tip. We report changes in the EI values in three patients in whom EI values were measured at multiple points during a popliteal sciatic nerve block.

Methods We obtained written case report consent from three adult patients undergoing elective lower extremity surgery. All of the blocks were performed before induction of general anesthesia. EI values were recorded when the block needle tip was within the biceps femoris muscle (#1), just outside the paraneural sheath (#2), inside the paraneural sheath (#3) on the ultrasound monitor, and after a local anesthetic had been administered within the paraneural sheath (#4).

Results The 4-point EI values (kΩ; #1, #2, #3, #4) for the three patients were (8.3, 8.3, 14.3, 5.9), (6.5, 7.3, 10.1, 5.2), and (6.5, 9.0, 12, 3.0) respectively. In all cases, the EI values increased when the needle tip entered from the outside to inside the paraneural sheath, and the EI values significantly decreased after local anesthetic administration. No adverse events occurred.

Conclusions The results suggested that monitoring changes in the EI value during a popliteal sciatic nerve block may be a new indicator of the needle tip location.