Background and Objectives The optimal site for local anesthetic injection during an ultrasound-guided supraclavicular brachial plexus block (BPB) is not known. We tested the hypothesis that local anesthetic injected deep to the “brachial plexus sheath” during supraclavicular BPB would produce faster onset of surgical anesthesia than an injection superficial to the sheath.

Methods After research ethics approval and informed consent, 32 patients undergoing upper-extremity surgery under an ultrasound-guided supraclavicular BPB were randomly assigned to receive 25 mL of a 1:1 mixture of 2% lidocaine with 1:200,000 epinephrine and 0.5% bupivacaine, deep to (subfascial, Gp SF) or superficial to (extrafascial, Gp EF) the brachial plexus sheath. Sensory-motor blockade of the ipsilateral musculocutaneous, median, radial, and ulnar nerves and time to “readiness for surgery” (defined as a sensory and motor block scale of 1 in all the 4 nerves tested) were assessed by a blinded observer, using a 3-point qualitative scale (2 to 0), every 5 minutes for 40 minutes and at 2, 4, 6, 8, 10, 12, and 24 hours after surgery.

Results The time to “readiness for surgery” was significantly shorter (Gp SF: 7 ± 3 minutes vs Gp EF: 20 ± 10 minutes; P < 0.001), and the duration of postoperative analgesia was longer (Gp SF: 9.3 ± 1.4 hours vs Gp EF: 6.1 ± 1.4 hours; P < 0.001) in the subfascial group than in the extrafascial group. There were no complications directly related to the technique or the local anesthetic injection.

Conclusions Injection of local anesthetic deep to the brachial plexus sheath at the supraclavicular fossa, under ultrasound-guidance, results in faster onset of surgical anesthesia and prolonged duration of postoperative analgesia than an injection superficial to the sheath.