Anatomy of the infraclavicular brachial plexus (Fig. 21.7a) and perineural structures changes in a progressive and predictable fashion from the apex in the anatomic axilla (brachial plexus divisions and cords) at the first rib through to the base of the axilla (brachial plexus cords and branches). Probe placement from medial to lateral demonstrates the changes in anatomy and is relevant to performance of the infraclavicular block. The ultrasound probe is placed close to the sagittal plane and may be slightly rotated on its axis with the caudal edge more medial than the cephalad edge to obtain the best transverse view of the pulsatile axillary artery (AA) (Fig. 21.7b). The AA is more superficial medially, and the axillary vein is identified caudad and medial to the AA. The axillary vein may be collapsible or seen to vary in size with respiration. A significant amount of hypoechoic fat surrounds the axillary contents. At the medial aspect of the clavicle, the pectoralis major and pectoralis minor form two distinct muscular layers superficial to the artery. The rib and pleura can be identified as a hyperechoic structure deep to the AA with anechoic air deeper still. All three cords of the plexus are typically cephalad to the artery as is their position crossing the first rib. These brachial plexus elements (cords) are seen as hyperechoic structures with a fibrillar pattern (Fig. 21.7c).
At the midpoint of the clavicle, but still medial to the coracoid process, the lung is positioned deeper than the infraclavicular portion of the brachial plexus. The medial cord begins to pass posterior to the AA as it twists around it to eventually rest medial to the AA further distally. The posterior cord moves closer to the posterolateral edge of the AA and will ultimately rest posterior to the artery. The lateral cord is readily identifiable using ultrasound as the most superficial hyperechoic structure lateral and superficial to the AA. The medial and posterior cords sometimes can be more difficult to distinguish as they are positioned at an
Figure 21.7b Photograph demonstrating ultrasound probe and needle orientation for an infraclavic-ular brachial plexus block.
increased depth, and shadowing artifact from the AA often occurs. Bigeleisen et al. (2006) demonstrated that abduction of the upper arm brings the cords of the brachial plexus into a more superficial position in the infraclavicular region facilitating ultrasound visualization.
At the lateral aspect of the clavicle directly inferior to the coracoid process, the lung is not usually seen, the pectoralis minor may not be seen, and the axillary vessels and cords
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Figure 21.7c Ultrasound anatomy of the infraclavicular (cords) brachial plexus.
of the plexus are deeper than at the other two sites previously described (interscalene and supraclavicular). The description of the anatomy of the cords in relation to the AA at this point is the most common configuration, but the infraclavicular plexus may have significant variability that may be identified by an ultrasound exam. The depth of the plexus structures and the AA lying anterior to two of the cords may make it difficult to determine the position of the posterior and medial cords (lateral cord remains relatively easy to visualize because of the superficial location). Imaging of the neurological structures in the parasagittal plane is easier to obtain by moving laterally. Moving further laterally from midline reduces the chance of pneumothorax, but the cords lie further apart and therefore there is a greater chance of slow onset or failure to anesthetize the medial cord.
The infraclavicular approach to the brachial plexus has increased in popularity since its initial description in 1981. To achieve a successful infraclavicular block with the nerve stimulator technique, a distal motor end point is required and to seek such a twitch sometimes can be time consuming, and the multiple needle passes required can be painful and increase the risk of complications. The use of ultrasound for infraclavicular block has significant potential to reduce complications compared to other infraclavicular approaches to the brachial plexus. Blockade of all three cords of the brachial plexus may often be achieved with a single injection, and the risk of pneumothorax is considerably less than with blind supraclavicular approaches (parasthesia or nerve stimulation techniques). Fixation of a continuous infraclav-icular catheter on the anterior chest wall is the most stable and comfortable for the patient compared to other brachial plexus catheter sites.
Secondary to the depth of the brachial plexus at the coracoid level (4-6 cm), it is often advantageous to use a lower frequency probe (5-10 MHz) of either the linear or the curved array type. The needle is inserted in-line with the US probe and advanced until the needle tip lies at the 9 o'clock position (superolateral) to the AA. The lateral cord is often identified and can be stimulated in this position. The posterior and medial cords lie deeper and often posterior (posterior and medial) or posterior and inferior (medial cord) to the AA at this point. Currently the best method of performing ultrasound-guided infraclavicular block is with the infraclavicular neurovascular structures in a cross-sectional view and the PNB needle
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