Background and Objectives In continuous femoral nerve blocks, the various perineural positions of the tip of a conventional catheter and their clinical implication are not completely known. We used stimulating catheters to explore the relationship of catheter tip to nerve.

Methods American Society of Anesthesiologists physical status I-II patients scheduled for total knee arthroplasty were administered effective single-shot blocks of the obturator and sciatic nerves using ropivacaine 0.5% 10 and 20 mL, respectively. Continuous femoral blocks were performed using stimulating catheters, which were advanced blindly. Neurostimulation via catheter was performed but was masked from the investigators' sight. Before general anesthesia was induced for surgery, 5 mL of ropivacaine 0.2% was administered through the femoral nerve catheter, followed by infusion (5 mL/hr). Visual analog scale (VAS) score at rest was recorded on admission to the postanesthesia care unit. Characteristics of neurostimulation via the catheters were analyzed.

Results Thirty-three patients were studied. Motor responses to stimulation via the catheters were obtained at 1 mA or less in 55% of patients and at greater than 1 mA in 45%. Various motor responses involved twitches of the pectineus (12%), sartorius (18%), or quadriceps (48%) muscles, or no muscular twitch up to 5 mA (22%). Catheter-induced motor response at 1 mA or less was associated with median values of VAS scores lower than those at greater than 1 mA (0 vs 30 mm; P = 0.008).

Conclusion The effectiveness of a continuous femoral block depends on neurostimulation characteristics, which likely correspond to various possible catheter tip positions. Conventional catheters provide no information on this issue. These results suggest that better VAS scores are attainable by placing catheters with neurostimulation guidance.