Possible explanation for failures during infraclavicular block: an anatomical observation on Thiel's embalmed cadavers

Abstract

Editor—Ultrasound-guided infraclavicular block (US-ICB) yields a higher success rate and improved safety for analgesia of the upper limb compared with nerve-stimulation-guided injection. However, even with adequate US-ICB injection, block failure can occur, and no mechanism explaining such failures has yet been proposed. We investigated the possible causes of block failure after posterior (6 o'clock) injection relative to the axillary artery during bilateral US-ICB in 20 cadavers preserved according to Thiel's embalming method. Before US-ICB was performed, the infraclavicular region was scanned with the probe. The probe was oriented with a 30°–40° angle relative to the clavicle to obtain a transverse view of the axillary vessels. The needle was then inserted cranially relative to the probe, and advanced to reach the region immediately posterior to the axillary artery (6 o'clock point). Then, 0.5 ml kg−1 of a local anaesthetic solution (LAS) composed of lidocaine 0.5% with methylene blue (MB) 0.02% (MB 1%, 2 ml per 100 ml of lidocaine) was injected into this site for each block with a 20 ml syringe, under ultrasound guidance, to achieve a U-shaped spread. A professional anatomist performed fine dissection of each brachial plexus injected and recorded which nerve structures of the brachial plexus were coloured by the MB solution. The primary endpoint was the failure of US-ICB. A procedure was considered successful if sufficient nerve structures were coloured by the MB solution to indicate that the target structures had actually been reached by the LAS. A total of 40 US-ICB were performed, and in 33, all nerves and cords of the brachial plexus were circumferentially coloured by MB, giving an ‘anatomical' success rate of 82.5%. In three cases, only one or two elements of the brachial plexus were coloured, and the combination was considered insufficient to have constituted a successful block. In four additional cases, there was no coloration, despite adequate spread of the LAS around the axillary artery. In these four cases, most of the staining solution was found in the thoracoscapular space, limited laterally by the subscapularis muscle and medially by the serratus anterior muscle (Fig. 1). In these four cases, block failure was likely due to the spread of the solution into this space. We propose that this could be an anatomical explanation for the failure of certain ICB, whether ultrasound-guided or not. The infraclavicular region is divided into a superficial and deep compartment (Fig. 1). The deep compartment contains the large subclavian vessels and the three cords of the brachial plexus, and has a proximal, a distal, and a dorsomedial extension. The distal extension is in the deep axillary space covered by the deep axillary fascia. In the deep axillary space, the cords split into the large nerves to innervate the upper limb. Cords and nerves can be separated from each other by variably developed dense connective tissue layers. The dorsomedial continuation of the deep infraclavicular compartment and of the deep axillary space is the thoracoscapular gliding space, which is filled with soft connective tissue to provide the movements of the scapula along the thorax. This space reaches its limit at the medial border of the scapula, with the insertion of the serratus anterior muscle. In four failed blocks observed in our study, the entire volume of injected LAS leaked into this little-known anatomical space, away from the structures targeted by the injection. We believe that this could provide an anatomical mechanism of action to explain unsuccessful US-ICB. Sonosite® (Villebon sur Yvette, France) laboratory kindly provided the equipment used for the study.