Low-volume ultrasound-guided nerve block provides inferior postoperative analgesia compared to a higher-volume landmark technique.

BACKGROUND AND OBJECTIVES: Ultrasound guidance reduces the required local anesthetic volume for successful peripheral nerve blockade, but it is unclear whether this impacts postoperative analgesia. This prospective, randomized, observer-blinded study tested the hypothesis that a low-volume ultrasound-guided ankle block would provide similar analgesia after foot surgery compared with a conventional-volume surface landmark technique.

METHODS: A total of 72 patients presenting for elective foot surgery under general anesthesia were randomized to receive a low-volume ultrasound-guided ankle block (n = 37; ropivacaine 0.5% adjacent the anterior/posterior tibial arteries and short saphenous vein; subcutaneous infiltration around the saphenous and superficial peroneal nerves) or conventional-volume surface landmark guided technique (n = 35; 30 mL of ropivacaine 0.5%). Patients received regular postoperative acetaminophen, diclofenac, and rescue tramadol. Assessment was in the recovery room and at 24 hours for pain and tramadol consumption.

RESULTS: Mean (SD) total local anesthetic volume for the low-volume ultrasound group was 16 (2.1) mL. Block success in the recovery room was similar between groups (low-volume ultrasound 89% versus conventional-volume landmark 80%, P = 0.34; however, during the first 24 hours, numerically rated (0-10) "average pain" (median [10-90th percentiles] = 1 [0-4] versus 0 [0-2], P = 0.01), worst pain at rest (1 [0-6] versus 0 [0-2], P = 0.03), and the proportion of patients requiring rescue tramadol (% [95% confidence interval]: 50 [34-46] versus 20 [10-36], P = 0.01) were higher in the low-volume ultrasound group. Numerically rated numbness, weakness, satisfaction, and procedural time were similar between groups.

CONCLUSIONS: Low-volume ultrasound-guided ankle block is associated with a high block success rate after foot surgery; however, compared with a conventional volume (surface landmark) technique, the reduced local anesthetic volume marginally compromises postoperative analgesia during the first 24 hours.