The Internal Mammary Artery
Mark M. Levinson, MD
Hutchinson Hospital, Hutchinson Kansas
The Internal Mammary Artery (IMA) is a blood vessel located on the inside of the chest cavity. It is an artery, not a vein. thus it carries red blood under the same blood pressure as that seen in the aorta or the coronary arteries themselves. There is one IMA on each side of the breastbone (aka sternum). This unique blood vessel runs along the inside edge of the sternum, sending off small branches to the bones, cartilage, and soft tissues of the chest wall. For unclear reasons, the IMA is remarkably resistant to cholesterol buildup. In studies of people who die beyound the age of 90, only 10% will show any atherosclerosis in the IMA vessels, while nearly all such individuals have atherosclerosis in the coronary arteries and other places. The reasons for mammary artery's resistance to atherosclerosis is not known at present.
The IMA is also conveniently located near the most important coronary branch, the left anterior descending (LAD). The surgeon can transfer the lower end of the IMA down to the heart surface to use as a bypass graft to the coronary vessels. As compared to the veins from the lower extremity, the IMA is smaller and more delicate. However, studies have shown that the use of the left internal mammary artery (LIMA) is associated with improved long term results from coronary artery bypass surgery. In most places around the world, surgeons implant the LIMA into the LAD whenever possible. Sometimes the LIMA is too small to use. And in other cases, the vessel is so delicate that just the steps taken to remove if from underneath the ribs will cause harm to the vessel wall, making the IMA useless. However, in about 90% of coronary bypass operations, this vessel is the best conduit available for surgical bypass to the major arteries of the heart.
Kolessov, a Russian surgeon living in Leningrad, was the first person to connect the internal mammary artery (IMA) to the coronary artery for the purposes of relieving angina. His intrepid efforts were mostly ignored by the surgical community for many years. During the early development of coronary bypass surgery, the greater saphenous vein (GSV) from the lower extremity was utilized by most surgeons because it was:
- Quick and easy to harvest,
- Always long enough,
- Stronger tissue and not as delicate at the IMA, and
- Larger in caliber than the IMA, making surgery technically easier.
However, through the ceaseless efforts of some committed surgeons, most notably Dr. George Green of New York, the advantages of using the IMA for CABG surgery became clear. As experience with LIMA grafting grew, Dr. Greens predictions for longevity proved correct. The patients with LIMA grafts enjoyed better long-term results when compared with patients receiving only vein grafts.
In the first illustration below, you can see the LIMA just as the surgeon first sees it when retracting the edge of the sternum. As shown, the LIMA travels parallel to the edge of the sternum and inside of the rib cage. As the overlying tissue is swept away, the LIMA itself can be easily seen.
After dividing and sealing off all the side branches of the LIMA, the entire length of this vessel can be mobilized from underneath the rib cage. At this stage, both ends are still attached, but all of the side branches have been divided.
In preparation for use as a bypass graft, the far end of the LIMA is detached while the near end (at its origin from the artery to the left arm) is left intact. If carefully harvested, the LIMA has adequate length, caliber, and flow characteristics for use as a bypass graft to the LAD vessel. The prepared LIMA is shown below.
The next photograph shows the LIMA in the chest near the heart during the preparations for heart-lung byass.
If you look carefully, you can see the LAD coronary artery on the front surface of the heart. After establishing heart-lung bypass, the heart beat is stopped and a small opening made in the front wall of the LAD with a very fine blade. The tip of the IMA is beveled and sewn to this opening to create a surgical bypass.
For a further details on the surgical treatment of coronary artery disease, visit the page discussing Coronary Artery Bypass Surgery in The Learning Center.
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