Anatomy, Thorax, Brachocephalic (Right Innominate) Arteries

Structure and Function

In normal anatomy, the innominate artery is the most proximal branch of the aortic arch.  It arises to the right of and anteriorly to the left common carotid artery.  The artery is short in length and courses superiorly and posteriorly from the aortic arch until it bifurcates into the right subclavian artery and the right common carotid artery at about the level of the sternoclavicular joint. Along its short course, it typically crosses from left to right anterior to the trachea around the ninth tracheal ring. However, due to anatomic variation, it is not uncommon for it to cross at any level from around the sixth ring to the thirteenth ring.[1] The innominate artery is also in close contact with the thymus, which typically sits atop the artery and separates the artery from the posterior surface of the manubrium.[2] 

After bifurcation, the right subclavian artery goes on to provide the entire blood supply to the right upper extremity as well as collateral circulation of the head via the right vertebral artery. The right common carotid artery bifurcates into the external carotid and internal carotid arteries, which provide a large percentage of the blood supply to the head and neck.

Embryology

The proximal portion of the aortic arch derives from a structure known as the aortic sac, which is chronologically the first portion of the aorta to form and appears as a dilation superior to the truncus arteriosus.  This sac then gives rise to two horns

1. The Left Horn – Combines with the stem of the aortic sac to form the proximal aortic arch.
2. The Right Horn – Gives rise to the innominate artery.

The right horn of the aortic sac (now the innominate artery) fuses with the right-sided third and fourth aortic arches, which give rise to the right common carotid artery and the proximal right subclavian artery respectively.[3][4]

Nerves

The vagus nerve, and its branch the right recurrent laryngeal nerve, course close to the innominate artery.  The vagus nerve runs directly anterior to the bifurcation of the innominate artery, while the right recurrent laryngeal doubles back posterior to the bifurcation and courses anteriorly to the larynx.[4]

Physiologic Variants

The complex embryology of the aortic arch and the great vessels lends itself to numerous natural variations.  Studies show that the normal variant branching pattern of the aortic arch is likely only present in approximately 80% of the population [5][6]. The innominate artery, in particular, lends itself to variations that make it of much clinical interest.  By far, the most common aortic arch branching pattern variant is what is known as a bovine arch.  This variant is when both the right and the left carotid artery (along with the right subclavian artery) share a common trunk, essentially making the innominate artery have three branches instead of the typical two. This variation is found in approximately 14% of the population worldwide, and findings may suggest that there is a higher than average prevalence in those of African descent and lower than the average prevalence in those of Asian descent [5]. Other common variations that involve the innominate artery are:

• Common carotid artery – The innominate artery effectively does not exist with there being a right subclavian followed by a common branch of both carotid arteries.
• Aberrant right subclavian – The right subclavian artery branches from the left side of the aortic arch or descending aorta distal to the other branches. This variation can be with or without a common carotid branch.
• Right-sided arch – aortic arch goes from left to right, giving a mirrored pattern to the normal variation with the innominate artery giving rise to the left subclavian and the left common carotid or an aberrant left subclavian. [5]

 Other less common variations involving the innominate artery are:

• Left and right innominate arteries
• Four branches with no innominate artery
• Double aortic arch
• Left-sided arch with a left-sided innominate artery
• Aberrant right subclavian
• Common trunk of the innominate, left common carotid, and left subclavian.[5]

Surgical Considerations

Due to its prominence in the superior thoracic cavity, the innominate artery should always be taken into consideration when operating inside the thoracic cavity, especially in the superior mediastinum.  Because of the innominate artery’s typical anatomic position directly posterior to the manubrium, full exposure usually requires a median sternotomy.  When operating on or adjacent to the innominate artery, the vagus and right recurrent laryngeal nerves should be identified due to their proximity to the artery.

The innominate artery’s position directly anterior to the trachea leaves it particularly vulnerable to injury when performing a tracheostomy. This potential danger can be from direct puncture of the artery during the procedure, as well as erosion of the vessel from prolonged direct contact with the tubing leading to the formation of a tracheoesophageal fistula. The risk of this happening is much greater the lower the tracheostomy site, which is why the typical suggestion is that it be performed no lower than the third or fourth tracheal ring.[7][8] Great vessel anatomic variations could be problematic as well, especially if they involve a high riding innominate artery, and it may be advantageous to review any chest imaging the patient has before performing the procedure if at all possible.

In the setting of aortic repair procedures, innominate artery cannulation is becoming an increasingly used method of maintaining cerebral perfusion over the historically more widely used method of axillary artery cannulation. There is no difference in postoperative neurologic complications, and innominate artery cannulation poses several advantages such as decreased operative time, no need for an additional incision, reduced risk of brachial plexus injury, and direct visualization of the artery during the entire procedure.[9]

Clinical Significance

Likely the most clinically significant abnormalities involving the numerous physiologic variations of the aortic arch, in general, are vascular rings, which are abnormalities of the aortic arch and great vessels that cause tracheal and/or esophageal compression. A true vascular ring involves the presence of a right-sided or a double aortic arch with a left-sided ligmentum arteriosum and an aberrant or retroesophageal left subclavian artery. These rings can involve several innominate artery variations. As previously discussed, one type of variation that is possible is a right aortic arch with a “mirrored” branching pattern (distal innominate artery gives rise to the left common carotid and left subclavian). This variation occurs in over a third of patients with a right-sided aorta and causes symptoms of tracheoesophageal compression.[10] While not a true vascular ring, it can sometimes be referred to as a vascular ring due to presenting with the same clinical picture.

Innominate artery compression syndrome is another cause of tracheal compression that is not considered a true vascular ring. It is caused by abnormal compression of the anterior aspect of the trachea as the artery crosses it; therefore, it is usually not involved with any abnormal variation of the artery.[10] This condition typically only occurs in infants and children and is very rare in adults. There have been suggestions that this is due to the origin of the innominate artery becoming progressively more rightward with maturation, along with an increase in the space between the artery and the anterior surface of the trachea.  More potential space in the anterior mediastinum due to the thymus becoming smaller with age could be contributing to this phenomenon as well.[11] Severe innominate artery compression syndrome can be fixed surgically with suspicion of the artery to the sternum; however, this condition tends to improve with age with little to no intervention.

As previously discussed, damage to the innominate artery is a major consideration when performing a tracheostomy. One of the major delayed complications of the procedure is the formation of a tracheo-innominate artery fistula and can occur in up to 1% of patients who undergo a tracheostomy.[12][1] These commonly result from pressure necrosis from prolonged direct contact with a tracheostomy pressure cuff, but can also be caused by prolonged steroid or immunosuppressant usage, or infection from the stoma site.[7] This condition is a particularly feared complication as it can lead to massive hemorrhage before it is identified and is the most common delayed complication of tracheostomy, with mortality rates ranging from 75 to 90%.[8][13][1] Most sources in the literature will suggest that hemorrhage will typically present after one-week post-op and prior to 3 weeks but, delayed hemorrhage can occur months to even years after the procedure. Bleeding from the ostomy site, hemoptysis, or signs of hemorrhage in any patient with a history of tracheostomy should raise concern for the presence of a bleeding tracheoesophageal fistula.[7][13][12]    

Other vascular anomalies can affect the innominate artery, such as aneurysm or stenosis, due to atherosclerotic disease. However, these are relatively uncommon when compared to other locations. Supraaortic aneurysms, in general, are quite uncommon, and innominate artery aneurysms comprise only about 3% of these.[14] When they do occur, innominate artery aneurysms can often present with signs of innominate artery compression syndrome and have a very high risk of rupture.