coronary artery bypass

coronary artery bypass is a surgical procedure for treating patients with obstructions in their coronary arteries. The procedure overcomes the effects of the obstructions, while not directly influencing their underlying causes. The operation was first used in substantial numbers in the late 1960s; it has now become one of the most commonly undertaken surgical operations.

The muscular pumping chambers of the heart require a copious blood supply, which is conducted by the left and right coronary arteries and their branches. These lie on or near the surface of the heart, giving rise to small vessels which penetrate into the muscle to supply the nutrient capillary networks surrounding individual heart muscle cells.

In many populations, particularly where cigarette smoking, diets rich in saturated fats, low exercise activity, diabetes, and high blood pressure are prevalent, the coronary arteries are frequently affected by the process known as atherosclerosis. This is a degenerative disease of arteries characterized by development of fatty accumulations within the inner portion of the arterial wall, separated from the blood by a fibrous layer or plaque. These lesions predominantly affect the coronary arteries on the surface of the heart, near their origins from the aorta, sparing arteries further downstream (thus fortuitously making bypass surgery feasible).

Atherosclerotic lesions may gradually expand, narrowing the coronary artery, over many years, ultimately restricting blood flow to the heart muscle. This prevents an increase in blood supply to the heart muscle when it is most needed (e.g. during exercise or emotion), and results in an unpleasant, constricting, central chest pain known as angina.

Another possibility is rupture of the fibrous plaque with release of the fatty material and clotting of the blood within the coronary artery, abruptly interrupting the blood supply to a portion of the heart muscle. This is manifested clinically as a heart attack, which may be fatal, may leave part of the heart muscle permanently scarred, or may be followed by near-complete recovery, particularly if the clot breaks up rapidly (spontaneously or in response to thrombolytic drugs).

The coronary arteries, and any obstructions, can be demonstrated by the procedure of coronary angiography. The passage of a radio-opaque fluid is recorded by cine-radiography, following its injection into the coronary arteries via long catheters introduced through a conveniently placed peripheral artery (e.g. the femoral artery in the groin).

Coronary artery bypass is usually used for treating angina, particularly in those found to have obstructions in multiple coronary arteries, or obstructions at strategic sites. Bypass surgery may be appropriate even when angina is not present, if coronary angiographic study after a ‘coronary event’ (usually a heart attack) shows severe coronary disease. Coronary artery bypass is only one of the possible treatment options, which include drug therapy and cardiological interventions such as angioplasty (inflation of a small balloon at the site of obstruction) and stenting (placing a tubular mesh at the expanded site to help keep it open). Surgery is usually reserved for patients with more extensively diseased coronary arteries where angioplasty may be less successful or impractical.

In principle, the operation consists of placing a conduit to conduct blood to the coronary artery beyond the obstruction. The heart is exposed by dividing the sternal bone vertically, and a heart–lung machine is used to maintain the general body circulation while the heart is immobilized to allow work on the coronary arteries. The superficial veins of the legs provide a good source of bypass conduit. The internal mammary artery, which lies to the side of the sternum, can be used as a particularly useful conduit for bypassing the anterior descending coronary artery branch. Other arteries, such as the radial artery from the forearm, may also be used as conduits. The coronary artery is opened for a few millimetres at a suitable site beyond the obstruction and the end of the conduit is sewn to the opened artery. The other end of the conduit is sewn to an opening made in the aorta a few centimetres downstream from the natural openings of the coronary arteries, except in the case of the internal mammary artery, when its origin is left undisturbed. Usually several conduits are required, commonly three or four arteries being bypassed. The coronary arteries are typically 1–3 mm in diameter, and magnification is used to enhance surgical precision. If there is extensive coronary obstruction it may be difficult to find a suitable site for insertion of a bypass conduit. In this case it is often possible to open into the diseased artery and extract the atheromatous occlusion from a sufficiently long segment to allow a bypass graft to be inserted (the procedure known as endarterectomy).

There is a mortality rate for this operation, usually about 1–3%, and stroke, bleeding, impaired cardiac function, and chest infections are among the early complications which may occur, depending on age and physical condition.

Bypass of all severe obstructions restores normal blood supply to the heart muscle on exercise. Angina is abolished and normal life can usually be resumed. Over subsequent years there is a gradual return of angina, such that up to 50% of patients have some recurrent symptoms by 10 years after surgery, some even requiring further surgery. Recurrent angina is due either to progression of disease in the coronary arteries to affect new areas, or blocking of conduits (particularly vein conduits) by a process similar to atherosclerosis.

Coronary artery bypass can ameliorate the consequences of subsequent plaque rupture in a bypassed artery. This has been shown to improve the survival prospects of those patients who have multiple major coronary branch obstructions, particularly when there is damage to the heart muscle from previous heart attack.

In attempts to simplify the operation, efforts have been made to undertake coronary bypass without using the heart–lung machine, while the heart is beating, and video-assisted minimal access techniques are being developed. These methods are presently feasible only in a small proportion of patients, usually with limited disease.

David Weatherall

Bibliography

Millner, R. and and Treasure, T. (1995). Explaining cardiac surgery: patient assessment and care. BMJ Publishing Group, London.