Study of optimum arm position for ultrasound visualisation of the brachial plexus at the axilla

Introduction:
Visualisation and separate blockade of the four primary constituent nerves (radial, median, ulnar, musculocutaneous) increases the success rate of ultrasound-guided brachial plexus block at the axillary level. However, the upper limb is still positioned as if performing the landmark-oriented approach described by Winnie, with the shoulder and elbow at 90o. Thus, we aimed to find the optimum arm position for visualisation of the brachial plexus at the axilla using ultrasound.

Material and methods:
After the Institutional Ethics Committee’s approval, this prospective observational study was conducted on 36 consenting individuals more than 18 years of age. The ultrasound probe was placed on a short axis at the intersection of the pectoralis major muscle and the biceps brachii muscle, with just enough probe pressure to cause light compression of veins. Each arm was placed in three different positions – shoulder at 90º and elbow at 90º, shoulder at 90º and elbow at 0º, and shoulder at 120º and elbow at 90º – in which the nerves were assessed using a six-point visibility scale. The path of each nerve was traced down for confirmation. Distance from the skin to axillary artery, skin to individual nerves, and artery to nerves was measured.

Results:
Visibility scores of the individual nerves and the distances measured in the three positions were comparable (P > 0.05). The skin artery and skin nerve distances were the shortest in the 120/90 position, and the radial nerve was more often located in this position.

Conclusions:
Arm position with 120º shoulder and 90º elbow had favourable results. Further studies will confirm its clinical utility and block success rate.