The Internal Mammary Artery (IMA), formally known as the Internal Thoracic Artery, is a blood vessel located in the chest. It is recognized as the preferred conduit for Coronary Artery Bypass Grafting (CABG), the standard treatment for severe blockages in the heart’s arteries. The IMA has demonstrated a superior long-term ability to remain open and functional compared to other vessels used. This durability leads to improved survival rates for patients and establishes the IMA as the gold standard for restoring blood flow to the heart muscle.
Location and Original Role
The IMA is a paired artery, with one running down each side of the anterior chest wall, positioned approximately one to two centimeters lateral to the sternum. It originates from the subclavian artery, situated beneath the collarbone. The artery descends along the inside of the rib cage until it reaches the sixth or seventh intercostal space, where it divides into its terminal branches.
The IMA’s primary role is supplying blood to the chest wall structures, including the sternum, intercostal muscles, and anterior rib cage connective tissues. In females, it also contributes to the blood supply of the breasts. Because the chest contains an abundance of other smaller arteries, a network of collateral circulation exists. This network allows the IMA to be harvested for grafting without causing significant long-term harm to the chest wall tissues.
Why It Is the Preferred Bypass Vessel
The IMA is the ideal bypass conduit due to its unique biological structure, which resists the processes causing graft failure in other vessels. It has an exceptional long-term patency rate; studies show over 90% of grafts remain open and free of significant narrowing a decade after the operation. This success rate is significantly higher than that observed in grafts made from the patient’s leg veins, which are prone to re-narrowing due to disease progression.
A significant factor in its durability is the IMA’s inherent resistance to atherosclerosis, the accumulation of fatty plaque. This resistance is attributed to the unique composition of the artery’s inner lining, or endothelium. The artery possesses a non-fenestrated internal elastic lamina, a structural feature that makes it less susceptible to the thickening and stiffening of the vessel wall known as intimal hyperplasia, a primary cause of late failure in vein grafts.
The IMA also displays superior physiological properties. As a muscular artery, its relatively thin middle layer (the media) allows it to produce a high basal rate of nitric oxide. Nitric oxide is a potent chemical that relaxes blood vessel walls, promoting dilation and ensuring optimal blood flow in response to the heart’s changing demand. This ability to regulate its own diameter, coupled with its disease resistance, makes it an enduring conduit for the heart.
How the Graft is Utilized in Surgery
The surgical application of the IMA in CABG involves detaching one end of the artery from its original position and connecting the free end to a coronary artery beyond the point of blockage. The Left Internal Mammary Artery (LIMA) is the most frequently utilized vessel because of its proximity and direct path to the Left Anterior Descending (LAD) coronary artery, which is the most common site for serious blockages. The connection is made to restore flow downstream of the obstruction, effectively bypassing the diseased segment.
The IMA is most commonly used as an in situ or pedicled graft, meaning the artery remains attached to its origin point on the subclavian artery. The surgeon carefully dissects the artery from the chest wall, freeing its entire length so it can be “swung down” to reach the target coronary artery on the heart’s surface. This technique preserves the natural blood flow and pressure provided by the subclavian artery, contributing to the graft’s long-term success.
In some cases, the IMA can be harvested using a technique called skeletonization, where the artery is meticulously dissected free of its surrounding tissue, including fat and lymphatics. This is often done to increase the length of the vessel or to facilitate the use of both the left and right IMAs for multiple bypasses. Less frequently, the IMA is taken entirely as a “free graft,” where it is completely detached from the subclavian artery and reconnected to the aorta or another source of high-pressure blood flow, similar to a vein graft.
Impact on the Donor Site
The harvesting of the IMA, while providing a durable graft for the heart, does create a necessary trade-off for the chest wall’s blood supply. The temporary disruption of blood flow to the sternum and surrounding tissues is the main concern at the donor site. This is particularly noticeable in the inferior portions of the sternum, where the blood supply is already less robust.
The primary risk associated with harvesting the IMA is a slightly increased potential for delayed healing of the sternal wound or, in rare cases, a sternal wound infection. This risk is generally low but is amplified in certain patient groups, such as those with diabetes or obesity, and when both the left and right IMAs are used for grafting. The surrounding tissues, however, quickly compensate for the loss of the artery by activating a network of collateral vessels. These smaller arteries take over the task of supplying blood to the chest wall, ensuring the long-term viability of the tissue.