The "axillary tunnel": an anatomic reappraisal of the limits and dynamics of spread during brachial plexus blockade.

Background: Various anatomic factors have been described as affecting the distribution of a solution injected around the brachial plexus. Using computerized axial tomography dye studies, we introduce a new concept.

Methods: Ten patients with brachial plexus catheters sited using the bent needle supraclavicular technique were studied. After the catheter tip was located, 20 mL 50% diluted Omnipaque dye was injected through the catheter. The limits of spread of dye and patterns of dye distribution were described and quantified.

Results: The brachial plexus is contained within, and closely surrounded by, rigid muscular and bony boundaries, which effectively create a tunnel. Tunnel unit volumes are small (5.21-9.5 cm3), differing significantly from the volume of dye injected (P < 0.001), so spread must occur along the tunnel. Tunnel dimensions vary, with potential points of resistance at the apex of the axilla and in the subcoracoid region. Catheters placed for shoulder surgery, with tips located inferomedial to the medial edge of the coracoid process, were associated with 90% retrograde flow (95% C.I. = 83-97). Catheters placed for more distal surgical procedures, with tips located inferolateral to the medial edge of the coracoid process, were associated with equally antero- and retrograde flow.

Conclusions: We conclude that the brachial plexus is contained within a rigid-walled tunnel of variable dimensions, which we call the "axillary tunnel." The scapula/subscapularis complex, related to the subcoracoid point of resistance, may account for the differing patterns of dye distribution observed.