The heart is a mechanical device designed to pump blood around the body. Like any pump that pumps liquid it requires two particular sets of components. Firstly, a series of one-way valves to ensure the blood goes in one direction, and secondly, a fuel supply to provide the energy for pumping. The heart has its own fuel lines, the coronary arteries, which provide oxygen and nutrients for the heart muscle. The heart is, in fact, two pumps. The right side receives blood from the body which it pumps through the lungs, where the blood picks up oxygen, and back to the left side of the heart where the blood now containing new oxygen is then pumped around the whole body. The left side of the heart is stronger because it has to pump blood further.  

The heart is a muscle pump, which contains four one-way valves to ensure blood flows in one direction. These valves may become narrowed (stenotic) or leaky (regurgitant) and one or more may need to be repaired or replaced.

The right side of the heart contains two valves: the tricuspid valve where blood enters the main pumping chamber, the right ventricle, and the pulmonary valve where the blood leaves the right ventricle to go to the lungs. Similarly when blood returns from the lungs it enters the main pumping chamber, the left ventricle, through the mitral valve and is then pumped out of the heart through the aortic valve. All this requires energy, which is delivered to the heart muscle in the form of nutrients and oxygen by the coronary arteries.

The coronary arteries may become narrowed or blocked reducing the blood supply to the heart muscle depriving the heart of oxygen (ischaemia) and giving rise to chest pain (angina). A heart attack (myocardial infarction) occurs when the reduction in oxygen supply is so bad that a portion of heart muscle dies. This may go unnoticed or be so severe that the person dies. Varying degrees of oxygen deprivation may impair the pumping efficiency of the heart. “Ejection fraction” is a term used to describe how well a heart is functioning. A good heart ejects 50-70% of blood from its main pumping chamber with each beat. Decreasing percentages therefore indicate hearts in worse condition.

Surgery of the heart is best conducted on a heart that is still and empty of blood. This is effected by artificially pumping blood around the body with a heart lung machine (cardiopulmonary bypass) and stopping the heart either electrically or with a chemical solution for the duration of the operation.

In a coronary artery bypass graft operation (CABG) a blood vessel is taken from elsewhere in the body, cut to an appropriate length and then one end is plumbed into the diseased coronary artery upstream of the narrowing and the other end downstream of the narrowing, hence the term “bypass graft”. This restores the blood supply to the area of heart normally supplied by the diseased artery. The number of grafts constructed in any operation depends on the number of diseased coronary arteries. These operations may have to be repeated some time later if the disease in the coronary arteries becomes worse, or if the bypass graft itself becomes narrowed. A repeat operation is more difficult to perform and carries a greater risk to the patient than the initial operation.

Bypass grafts can be contsructed from either arteries or veins. Arteries are blood vessels which convey high pressure blood containing oxygen and nutrients away from the heart to the rest of the body. The aorta, the main artery of the body is about an inch in diameter. The coronary arteries, the first branches of the aorta, are about 1-2mm in diameter. Arteries divide into smaller and smaller branches in order to reach all parts of the body. Finally, they become microscopic capillaries.

Once blood has passed through the tissues and organs of the body the small blood vessels join to form larger blood vessels called veins which convey the blood which is now depleted of oxygen and nutrients back to the heart. These vessels are of a different structure to arteries, mainly because the blood after passing through the tiny capillaries in the tissues is at a fraction of the pressure.

On the whole each bypass graft requires about 6-7 inches of additional blood vessel. Such lengths are more easily found among veins than arteries, particularly in the leg where there is one vein, the long saphenous vein, that runs near the surface from ankle to groin and is easily removed. For this reason the long saphenous vein is the most common blood vessel used for constructing bypass grafts.

There are also two arteries in the chest which normally provide blood to the breastbone, the left and right internal mammary arteries. These can be detached, cut at one end, swung down and attached to the diseased coronary artery downstream of the narrowing, but they are not long enough to reach all the coronary arteries. The commonest of these arteries in use is the left internal mammary artery. Because it is an artery, is used to arterial blood pressures, is the same diameter as the coronary arteries and seldom suffers any narrowing itself, it is generally used to bypass the left anterior descending coronary artery which is the most important artery of the heart. For the reasons above it also lasts longer than a vein bypass graft. Veins are most commonly used to bypass the other coronary arteries. However there is a slowly growing trend to use more arteries such as the radial artery in the arm and one of the arteries which normally provide blood to the stomach.

Operations on the heart valves are a little more complicated than coronary bypass opereations because the valves are deep inside the heart and therefore the heart must be opened in order to gain access to the valves.

The two most common valves to be replaced or repaired are the aortic and mitral valves. These have very different structures as seen below.

Each of the valves can be either replaced or repaired. A valve can usually be replaced with another animal valve (bioprosthesis), a human valve (homograft) or most commonly a mechanical valve. Common abbreviations are used for valve replacements: aortic valve, AVR; mitral valve, MVR; tricuspid valve TVR and pulmonary valve PVR.

Despite being simple in concept these are major operations that carry a variable risk to the patient. View outcomes of cardiac surgery in the UK