Subclavian Artery Continues as Axillary Artery After Passing a First Rib

Introduction

The axillary artery is the principal arterial supply of the upper limb, commencing as a continuation of the subclavian artery as it emerges from underneath the first rib to enter the axilla. It provides key landmarks for understanding the position of other important structures in the region, in particular the brachial plexus. The axillary artery gives off six branches before terminating at the lower border of teres major by becoming the brachial artery.

Structure and Function

The axillary artery commences from the subclavian artery at the outer border of the first rib and terminates when it becomes the brachial artery at the outer border of the teres major muscle. It is enclosed in the axillary sheath, a continuation of fascia from the prevertebral fascia of the neck, and is anatomically divided into three parts by the pectoralis minor muscle, providing a useful way to conceptualize its course. The first part (proximal to pectoralis minor) has one branch, the second part (beneath pectoralis minor) has two branches, and the third part (distal to pectoralis minor) has three branches.[1] The axillary vein runs medially to the artery along its whole course.

A study of 59 cadavers found the average length of the axillary artery to be 11.22 cm, with each section occurring at roughly equal intervals along its course.[2] The surface marking of the axillary artery in the upper limb can be traced by drawing a line between the midclavicular point to the groove behind the coracobrachialis.

The first part of the axillary artery is defined as being between its commencement distal to the lateral border of the first rib and proximal to the pectoralis minor. It lies on top of the serratus anterior muscle. It has one branch called the superior thoracic artery, which supplies the pectoralis minor and pectoralis major muscles.[3]

The second part of the axillary artery lies directly underneath the pectoralis minor. The first branch of the second part of the artery is the thoracoacromial trunk, which pierces the clavipectoral fascia and divides into four terminal arteries: the acromial artery, pectoral arteries, clavicular artery, and deltoid artery. The second branch of the second part of the axillary artery is the lateral thoracic artery, which follows the lower border of the pectoralis minor and provides blood to the pectoralis and serratus anterior muscles, as well as the mammary gland.[4]

The third part of the axillary artery lies distal to the lower border of the pectoralis minor muscle, anterior to the subscapularis muscle, and teres major muscle.[5] It has three branches that can vary in order: the subscapular trunk, the anterior humeral circumflex artery, and the posterior humeral circumflex artery.[6] The subscapular trunk is typically the largest branch of the axillary artery, running down the posterior wall of the axilla.[7]

It gives rise to the scapular circumflex artery and the thoracodorsal artery. The scapular circumflex artery runs through the triangular space and provides blood to the scapular region. The scapular circumflex artery eventually anastomoses with the suprascapular artery, thus connecting and providing a collateral route between the first and third parts of the axillary artery. The thoracodorsal artery supplies the latissimus dorsi muscle. The two remaining branches of the third part of the axillary artery, the anterior and posterior humeral circumflex artery, supply the head of the humerus and shoulder joint. These arteries anastomose with one another. The anterior humeral circumflex is smaller and runs anterior to the humerus. The posterior humeral circumflex is larger and runs through the quadrilateral space and then posterior to the humerus, accompanied by the axillary nerve and supplying the deltoid muscle.[6]

The axillary artery terminates at the lower border of teres minor muscle, becoming the brachial artery which supplies the distal upper limb.

Embryology

The embryological development of the arterial system of the upper limb is highly complex. It appears to commence with the development at day 29 to 31 of a capillary plexus from the dorsal aorta (Carnegie stage 12). This proceeds during Carnegie stage 13 to develop with the limb and selectively enlarges to give rise to the greater and lesser components of the relevant vasculature. The subclavian and axillary arterial vasculature has generally matured from the capillary plexus by Carnegie stage 15.[8]

Nerves

The brachial plexus is intimately related to the vascular structures of the axilla, as the divisions emerge from beneath the clavicle to form the medial, lateral, and posterior cords around the axillary artery.[9] The first part of the axillary artery has the lateral and posterior cords superolaterally and the medial cord posteriorly[10]. Furthermore, a loop from the medial cord and lateral cord, which joins the medial and lateral pectoral nerves, is situated anteriorly. The second part of the artery is related to the medial, lateral, and posterior cords in the way their names indicate. In general, the subsequent branches from each cord remain in this relation to the third part of the axillary artery.[11][12]

Physiologic Variants

Upper limb arterial variants have been reported as occurring in around 25% of the general population.[13] Specific to the axillary artery, it has been reported that 5 to 10% of people have a bifid axillary artery, which bifurcates in the axilla, and each branch becomes the radial and ulnar artery respectively.[14]

Furthermore, there is a well-recognized physiologic variation in the branching pattern from the axillary artery. This typically manifests as a common origin for the lateral thoracic (LTA) and subscapular arteries (SSA), anterior (ACHA) and posterior (PCHA) circumflex humeral arteries, or thoracoacromial (TAA) and lateral thoracic arteries (LTA).[2]

Surgical Considerations

The axillary artery can be accessed surgically by an infraclavicular or deltopectoral incision in the upper arm.[15]. The pectoralis major and pectoralis minor muscles must be mobilized to access the neurovascular bundle in which the axillary artery forms a part, and careful protection of the cords of the brachial plexus to which it is intimately associated must be ensured. Depending on the specific pathology (aneurysm, blunt trauma, gunshot trauma), injury to the axillary artery may be repaired with autologous venous grafting or synthetic grafting with, for example, polytetrafluoroethylene.[15][16]

Clamping the axillary artery can be accomplished without harming the arm. Surgical procedures concerning the axillary artery require delicacy due to the proximity of the brachial plexus. In cardiac surgery, the right axillary artery is frequently used as an arterial cannulation site, especially in the repair of aortic aneurysm or dissection.[17][18][19]

Clinical Significance

Injuries to the axillary artery are rare but can cause limb loss and most commonly arise from penetrating or blunt trauma to the shoulder. Examples include but are not limited to anterior shoulder dislocation, reduction of anterior shoulder dislocation, and humoral neck fracture.[20][21] These mechanisms of injury may cause axillary artery rupture and subsequent thrombosis. Patients may present with pain, pallor, coldness, absent or delayed capillary refill, numbness or tingling, or lack of radial pulse to the injured limb. However, the collateral blood supply in the shoulder may mask some of these symptoms. Thus, the presentation of axillary artery injury is variable, so a high level of suspicion is essential.

Axillary Artery Aneurysm

Axillary artery aneurysms are one of the injuries that can occur due to penetrating or blunt trauma. However, they may develop from atherosclerotic processes over long periods, but this is exceedingly rare.[22][23] They often are associated with neurological complications due to the close proximity of the axillary artery and the brachial plexus. Aneurysms may arise from the improper use of crutches. Presentation of these patients may include neurological deficits of the upper limb, ischemic fingers, or occlusion of the radial and ulnar arteries due to embolization from an aneurysm. Axillary aneurysms are confirmed by imaging. CT angiography is the most useful diagnostic imaging technique and is currently the gold standard. An alternative imaging modality includes magnetic resonance angiography when intravenous contrast is contraindicated.

Ultrasound is also very useful for detection and diagnosis. An aneurysmectomy and grafting with a saphenous vein is commonly the surgical procedure employed to reverse these complications. Another treatment option for patients is an endovascular aneurysm repair.[24]

Arterial Positional Compression

Axillary artery thrombosis and aneurysm may also be seen, although very rarely, in healthy athletes that perform frequent overhead throwing motions such as pitching a baseball or hitting a volleyball. In the throwing motion, the anterior displacement of the humerus places compression on the third portion of the axillary artery. Intermittent axillary artery compression may cause chronic changes in downstream vascular sufficiency. This could lead to subsequent thrombosis and aneurysms. Medical staff dealing with these athletes should be vigilant for symptoms of ischemic pain, cold intolerance, numbness, or arm fatigue immediately following practice or competition. They should note that symptoms will present later in the disease process because only significant embolization or decreased blood flow causes perceptible changes in temperature or sensation.

CT angiogram and magnetic resonance angiogram can be used to diagnose or screen athletes at risk of symptomatic arterial positional compression. A further example of this is quadrilateral space compression syndrome, a space bordered by the humerus, teres major, long head of triceps, and teres minor, which contains the posterior circumflex humeral artery and axillary nerve, which are vulnerable here to compression.[25]

Review Questions

Figure

Aorta, Right Common Carotid Artery, Right Vertebral Artery, Right Subclavian Artery, Brachiocephalic Artery, Ascending Aorta, Left Coronary Artery, Right Coronary Artery, Left Common Carotid Artery, Left Vertebral Artery, Left Subclavian Artery, Left (more...)

Figure

Axillary artery. Image courtesy S Bhimji MD

Figure

Fig 1: Schematic diagram of brachial plexus Fig 2: Ultrasound image of the brachial Plexus at the level of axillary artery. Contributed and Created by Muhammad Salman Janjua, with Permission from Lucas Brio

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Source: https://www.ncbi.nlm.nih.gov/books/NBK482174/

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