A randomized comparison of long-and short-axis imaging for in-plane ultrasound-guided femoral perineural catheter insertion.

Objectives-Continuous femoral nerve blocks provide effective analgesia after knee arthroplasty, and infusion effects depend on reliable catheter location. Ultrasound-guided perineural catheter insertion using a short-axis in-plane technique has been validated, but the optimal catheter location relative to target nerve and placement orientation remain unknown. We hypothesized that a long-axis in-plane technique for femoral perineural catheter insertion results in faster onset of sensory anesthesia compared to a short-axis in-plane technique. Methods-Preoperatively, patients receiving an ultrasound-guided nonstimulating femoral perineural catheter for knee surgery were randomly assigned to either the long-axis in-plane or short-axis in-plane technique. A local anesthetic was administered via the catheter after successful insertion. The primary outcome was the time to achieve complete sensory anesthesia. Secondary outcomes included the procedural time, the onset time of the motor block, pain and muscle weakness reported on postoperative day 1, and procedure-related complications. Results-The short-axis group (n = 23) took a median (10th-90th percentiles) of 9.0 (6.0-20.4) minutes compared to 6.0 (3.0-14.4) minutes for the long-axis group (n = 23; P = .044) to achieve complete sensory anesthesia. Short-axis procedures took 5.0 (4.0-7.8) minutes to perform compared to 9.0 (7.0-14.8) minutes for long-axis procedures (P <.001). In the short-axis group, 19 of 23 (83%) achieved a complete motor block within the testing period compared to 18 of 23 (78%) in the long-axis group (P = .813); short-axis procedures took 12.0 (6.0-15.0) minutes versus 15.0 (5.1-27.9) minutes for long-axis procedures (P = .048). There were no statistically significant differences in other secondary outcomes. Conclusions-Long-axis in-plane femoral perineural catheters result in a slightly faster onset of sensory anesthesia, but placement takes longer to perform without other clinical advantages.