The coronary arteries begin just centimeters from the beginning of the aorta, at the aortic root. There are two main arteries, the right coronary and the left. As these encircle the heart, they branch off into smaller and smaller arteries and arterioles which bring oxygenated blood to the heart muscle. Blockages or narrowing in these small diameter vessels create areas of injury called ischemia (is-key-me-ya). To prevent permanent damage to the heart muscle (myocardium), adequate blood flow must be reestablished. Coronary artery bypass grafts (CABGs) are a frequent treatment method for reestablishing blood flow to the myocardium.
Commonly referred to as “open heart” surgery, bypass grafting is actually performed on the surface of the heart. A suitably sized vessel is harvested from its normal place and reattached in such a way that blood can be rerouted around the blocked or narrow part of the coronary artery. The vessels used are either veins or arteries.
By far the most common vessel used for bypass surgery is the greater saphenous vein, found in the leg. Previously harvested as a free graft by making an incision starting at the ankle and extending it to the groin, either in one long incision or multiple smaller ones, “endoscopic” vein harvesting is now a popular and less traumatic method of obtaining the vein graft. In the endoscopic technique, a small incision is made near the knee and a thin, long lens is inserted into the leg, allowing the vein to be visualized and the branches identified, cut, and cauterized with no further incisions needed. A smaller stab incision may be made at the groin in order to completely tie off the top part of the saphenous vein. The vein has tiny valves in it, which must be removed or the vein must be reversed in orientation to prevent the valves from obstructing the blood flow. Once the vein is prepared, it is sewn (grafted) onto the target coronary artery. The vein can be used several times, either by cutting into smaller pieces or by sequencing or wrapping the vein along several bypass areas the entire length. Smaller pieces require that each one be attached to the aorta, while sequencing requires one attachment to the aorta, but there are several “ports” made along its length.
Another vessel that has been used with even better results is the internal mammary artery. The left internal mammary artery (LIMA) is uniquely suited to grafting the left anterior descending (LAD) artery, which supplies the left ventricle, the chamber of the heart which pushes the blood through the body circulation, making it one of the hardest working muscles in the body. The LIMA branches off at the top of the inside of the chest (picture under the collarbone) and travels down the inside of the chest behind the breast area. The farther end (distal), closer to the diaphragm, is carefully dissected from the chest wall, and then dissected back up the chest wall to where it originates. The distal end is cut, but the originating end is left intact. This preserves the blood flow in the LIMA while making it mobile enough to reach to the area of the LAD. The cut end is grafted onto the artery just past the area of blockage, thereby completing the graft.
Bypass surgery is routinely done by making an incision in the middle of the chest and cutting the sternum in half. Called a median sternotomy incision, it has recently begun to be replaced by minimally invasive techniques which spare the sternum and the complications that can accompany a sternotomy. The usual approach for minimally invasive bypass surgery is a left mini-thoracotomy, through which the LIMA is harvested and grafted to the LAD through a two- or three-inch incision in between the ribs on the left side. This is a consideration if there is disease in only the LAD, since the right side of the heart is not accessible, nor is the aorta.
The minimally invasive techniques have built on the advances of off-bypass surgery. Coronary bypass surgery has historically been done by using the cardiopulmonary bypass machine (or heart-lung machine), which involves placing tubes into the great veins and aorta which are connected to the bypass machine which does the oxygenating and pumping normally done by the heart and lungs. (Some clarification here: coronary bypass is rerouting blood around a blocked artery. Cardiopulmonary bypass is mechanical oxygenating and pumping of the body’s circulating volume of blood.) Cardiopulmonary bypass has benefits and drawbacks, and certain types of heart surgery cannot be done without using it. However, coronary artery bypass surgery, since it is on the surface of the heart, can be done without its use. Great techniques have been developed to perform bypass grafts using specially designed stabilizing devices which allow the heart to beat but keep a small area relatively motion-free. These techniques are being refined and adapted to allow smaller and smaller incisions.