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

Abstract

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. 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. 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. 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.