Minimum volume of local anaesthetic required to surround each of the constituent nerves of the axillary brachial plexus, using ultrasound guidance: a pilot study

Abstract

Background: The minimum effective volume of local anaesthetic needed to provide effective analgesia of the four main branches of the axillary brachial plexus is unknown. This study was performed to determine the minimum volume of local anaesthetic required to surround the nerves of the axillary brachial plexus and document onset and duration of sensory and motor effects.

Methods: We enrolled 19 ASA I-II patients undergoing hand or forearm surgery. The four nerves of the axillary plexus were identified with ultrasound guidance. Lidocaine 1.5% with epinephrine 1:200 000 was loaded into a syringe driver. A 22 G needle was inserted in the long axis to each nerve and injection commenced using the bolus function (600 ml h(-1)). The needle was repositioned until the nerve was completely surrounded. The bolus dose in millilitres displayed on the syringe driver was recorded. This was repeated for each nerve. The degree of sensory and motor block was recorded as secondary outcomes.

Results: The mean (95% CI) volume to surround each nerve was: radial 3.42 (2.84-3.99) ml, median 2.75 (2.31-3.19) ml, ulnar 2.58 (2.14-3.03) ml, and musculocutaneous 2.30 (1.96-2.64) ml. The mean (95% CI) onset time for complete sensory block was: radial 22.5 (13.5-31.5) min, median 26.8 (18.5-35.0) min, ulnar 26.6 (17.8-35.4) min, and musculocutaneous 15.8 (7.45-24.2) min. The mean (95% CI) last recorded time with complete block was: radial 137.1 (105.6-168.7) min, median 144.7 (123.4-166.0) min, ulnar 183.2 (158.1-208.2) min, and musculocutaneous 158.3 (131.8-184.9) min. Seven patients required additional local anaesthetic infiltration and two required i.v. analgesia. No patient required conversion to general anaesthesia for surgery.

Conclusions: We found that it is possible to surround each nerve of the axillary brachial plexus with 2-4 ml of local anaesthetic. We speculate that increasing this volume would produce blocks of quicker onset and longer duration while still using smaller volumes than previously thought.