In coronary artery disease (CAD), the arteries that supply blood and oxygen to your heart muscle grow hardened and narrowed. You may try treatments such as lifestyle changes, medicines, and angioplasty, a procedure to open the arteries. If these treatments don't help, you may need coronary artery bypass surgery.
The surgery creates a new path for blood to flow to the heart. The surgeon takes a healthy piece of vein from the leg or artery from the chest or wrist. Then the surgeon attaches it to the coronary artery, just above and below the narrowed area or blockage. This allows blood to bypass (get around) the blockage. Sometimes people need more than one bypass.
The results of the surgery usually are excellent. Many people remain symptom-free for many years. You may need surgery again if blockages form in the grafted arteries or veins or in arteries that weren't blocked before. Lifestyle changes and medicines may help prevent arteries from becoming clogged again.
NIH: National Heart, Lung, and Blood Institute