CONDUIT ANATOMY
INTERNAL THORACIC (MAMMARY) ARTERY
ANATOMY
ORIGIN
- Thoracic Part of the subclavian artery opposite the thyrocervical trunck and medial to the scalenus anterior muscle
- Descends on the pleural cupule to cross the medial third of the clavicle and the first rib cartilage to enter the thorax
- Crossed anteriorly by the brachiocephalic vein before reaching the first rib (risk of injury)
- IMA may arise from the cervical part of the subclavian artery where it can be identified at the lateral edge of the scalenous anterior muscle. It then transverse the first rib to enter the thorax in the first intercostal space
- Normal type 45% separate branches for IMA, vertebral, costocervical and thyrocervical trucks
- Variations with any or all with different branches sharing a common origin
- It is important to ligate the transverse cervical and the suprascapular arteries near their origin if they branch off the IMA to prevent any coronary steal phenomenon
- Joins the artery shortly after its origin from the lateral aspect
- After a short segment the nerve runs medial and ventral to the artery behind the subclavian vein
Origin of the Right Internal Thoracic (Mammary) Artery (IMA)
COURSE
TERMINATION
- Descends vertical to its branching point
- Covered by the intercostal cartilage and the internal intercostal muscle
- Runs in the connective tissue of the endothoracic fascia on the parietal pleura to the third rib level
- Below this level is runs on the transversus thoracis
- The Conjoined veins runs lateral to the IMA in the first and second intercostal space
- The vein crosses IMA behind of the third rib and runs medial to it
TERMINATION
- Most commonly at the sixth intercostal space lateral aspect
- 90% divided into two branches and in 10% into three terminal divisions
RELATION WITH THE STERNUM
- Runs approximately 13-14mm lateral from the sternal edge with no significant difference between the right and the left internal mammary arteries; closer to the edge in the first intercostal space with a noticeable increase at the level of the sixth intercostal space
Superior (External) View of the Para-sternal area
showing the relation between the IMA, sternum and chest wall layers
Internal and External Intercostal muscles are divided at different points
to reveal the internal mammary vessels
showing the relation between the IMA, sternum and chest wall layers
Internal and External Intercostal muscles are divided at different points
to reveal the internal mammary vessels
BRANCHES
- The IMA supplies the chest wall, abdominal wall, diaphragm and pericardium
Branches of the Internal Mammary Artery
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Thymic Artery:
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The Xiphoid branch
- a small bilateral branch
- 10% branches directly from IMA and it may be considered to be a third terminal branch
- Rarely a branch of the superior epigastric artery.
- Passes in front of xiphoid
- Anastomosis with the opposite artery
- It can be the source of the unpleasant bleeding that can occur after sternotomy
- It is suggest to identify the artery and ligate it before sternotomy
Terminal Branches of IMA
The sternal branches
- Distributed in each intercostal space
- The main source of the sternal blood supply.
- In infant their origin corresponds to the ossification centres of the sternum.
- They form an arcade-pattern blood supply
Sternal Branches
The Pericardiacophrenic branch
- Arises from the initial part of the internal thoracic artery
- Accompanies the phrenic nerve
- Runs toward the diaphragm
- Anastomoses with the musculophrenic and inferior phrenic arteries
Anterior intercostal branches
- Arise in the uppermost five intercostal spaces
- Initially run off in the intercostal space within the internal intercostal muscle
- It branches off to form the anterior intercostal and supracostal arteries
- Initially, both branches run between the pleura and the intercostal muscles
- Later on they travel in the subcostal space
- Anastomose with the posterior intercostal arteries.
- Before entering the subcostal sulcus, a small component branches off as the supracostal artery
Anterior Intercostal Branche
The mediastinal branches
- Small arteries
- Supply the retrosternal fatty tissue, mediastinal lymph nodes, and the muscles of the chest wall
Perforating branches
- Originate directly, either from the internal thoracic artery or from the sternal branches.
- Contribute to the blood supply of the sternum, skin, pectoral muscles, and breast.
- They are connected to the vascular system of the lateral thoracic artery
Perforating Branches
Lateral Thoracic Artery
- A rare branch that occurs with an incidence of only 11%
- Mostly found in male specimens
- Arises from the initial part of the IMA
- It runs on the internal chest wall in the medial axillary line
- Mostly terminates at the sixth or fifth intercostal space
- It communicates with the anterior and posterior intercostal arteries
- The lateral end branch
- Initially runs in the sternocostal recess
- Terminates by piercing the diaphragm at the height of the sixth to seventh ribs, where it ramifies
- The medial terminal branch
- Runs thought the sternocostal triangle (fibrous structure between the sternal and costal part of the diaphragm
- Enters the sheath of the rectus abdominis
Angiogram of the Internal Mammary Artery
CLINICAL SIGNIFICANCE
- Vessels supplying the sternum are prone to damage during IMA harvest and during sternal closure
- The use of sternal wires may occlude the sternal and perforating branches, which are the functional units of the sternal blood supply
- These arteries may be damaged mechanically, especially if the cerclage wires are positioned around the bone in the intercostal space.
- The damage may also occur by the sternal wires by interrupting the vertical flow of the arcade anastomosis in each interspace
- These arcades play an important role in sustaining viability, particularly in those segments wherein the adjoining interspaces have lost their blood supply.
- The logical recommendation is to place the wires in front of the sternocostal joint, as close as possible to the sternal edge.