Coronary Artery Bypass Grafting (CABG)

Coronary Arteries

Coronary arteries are usually visible on the surface of the heart and supply the heart muscle (‘myocardium’) with blood. The two coronary arteries originate from the beginning of the aorta. The left coronary artery supplies the left ventricle and the septum of the heart with blood and divides early into two main branches: the left anterior descending artery (LAD) and the circumflex artery. The LAD supplies blood to the front and side of the left ventricle and the septum while the circumflex artery covers the side and back wall of the left ventricle. The right coronary artery distributes blood to the right ventricle, parts of the left ventricle, the septum and the electrical conduction system of the heart. Both coronary arteries give rise to several branches which vary significantly in size and importance. 

Coronary Artery Disease

Coronary arteries develop narrowings (‘stenoses’) through thickening of their wall. The diagnosis of coronary artery disease is usually confirmed by coronary angiogram. The degree and importance of a coronary artery stenosis can be measured by various investigations.

A stenosis is regarded as important is it causes more than 50% reduction in the diameter of the artery as seen on the coronary angiogram. There are some more ‘functional’ imaging techniques for coronary artery disease such as stress echocardiography, cardiac magnetic resonance imaging (MRI) and measuring fractional flow reserve (FFR). FFR measures pressure differences across coronary artery stenoses. 

Significant coronary stenoses result in impaired blood flow, and therefore reduced supply of oxygen, to the myocardium. Impaired blood supply is also called ischaemia. Symptoms of myocardial ischaemia include angina and shortness of breath. Angina is often experienced as tightness or pain in the chest which may travel into the jaw or left arm. These symptoms tend to worsen during physical exertion. Sudden occlusion of a coronary artery causes severe ischaemia resulting in myocardial infarction. This can cause death of a part of the myocardium. A myocardial infarction is often experienced as sudden crushing pain in the chest. Occasionally, myocardial infarctions occur without symptoms (‘silent’). Silent myocardial infarctions are more common in diabetes.

PCI is a non-surgical procedure performed by an interventional cardiologist. It involves visualising the coronary arteries with an angiogram, inflating the stenoses with a balloon and inserting a stent to keep the stenotic area open. PCI is sometimes performed during acute myocardial infarction. It is also effective in patients with angina.  

CABG is performed to treat angina, to prevent heart failure and to reduce the risk of a myocardial infarction that can cause death. CABG is a surgical procedure that uses arteries and veins of the patient as grafts to by-pass stenoses in the coronary arteries. The most commonly used grafts are the left internal mammary artery (LIMA) and the great saphenous vein (GSV).

The LIMA runs on the inside of the chest wall, on the left side of the sternum, and is most commonly used to bypass stenoses of the LAD. The GSV is located on the inside of the leg. A minimally invasive technique, using two small incisions, can be used to remove the GSV.  

Alternative types of grafts include the right internal mammary artery (RIMA) and the radial artery. While the superiority of the LIMA as a graft is not disputed, there is less convincing evidence of the advantages the RIMA and the radial artery offer. It is likely, however, that arteries are more durable than veins provided they are used to treat coronary arteries with tight stenoses. The terms single, double, triple or quadruple bypass refer to the number of connections (’anastomoses’) the surgeons performs between the grafts and the coronary arteries.

CABG or PCI? – Clinicians and scientist have carried out a very large amount of work aiming to decide which patients benefit from PCI and which should be undergoing CABG. The most commonly mentioned clinical trial in this field is SYNTAX . Furthermore, there are published international guidelines aiming to assist our decision making in treating coronary artery disease. Generally speaking, patients with complex coronary artery disease that involves the LAD artery have better long term outcomes with CABG. Patient with diabetes gain additional benefits from this type of surgery. Each patient should, of course, be assessed as an individual before the best treatment is decided. Numerous parameters, such as age and fitness, will be taken into consideration.

Techniques of CABG

Most surgeons perform CABG by connecting the patient’s circulation to a cardio-pulmonary bypass (CPB) machine. This is known as ‘on-CPB’ or ‘on-pump’ CABG. The CPB machine takes over the function of the heart and lungs for the duration of the main operation. It pumps blood and uses a gas exchange device to add oxygen and remove CO2 from the blood.

During an on-pump operation, the heart is stopped, through the infusion of a solution high in potassium (K+), and the surgeon performs the anastomoses in a still and bloodless field. Once the grafts have been sewn, the heart is allowed to start beating again before the CPB is discontinued and the operation is completed.

Heparin is administered during the procedure to prevent formation of clots in the CPB machine. The effects of heparin are reversed by protamine at the end of the operation. ‘On-pump’ is the commonest technique for performing CABG worldwide as it creates very good conditions for the formation of graft connections. Downsides include the need to manipulate and insert cannulae into the aorta. Another downside is bleeding after the operation due to damage of blood cells.

CABG can also be performed without using CPB. This technique is known as ‘beating heart’ or ‘off-pump’ coronary surgery. During ‘off-pump’ CABG the heart continues to beat and the lungs continue to provide oxygenation. Areas of the coronary arteries that receive a bypass graft are kept relatively still with a mechanical stabiliser while the surgeon performs the connections.

The main advantage of the off-pump technique is that it avoids excessive manipulation of the aorta. It is also associated with less bleeding that on-pump CABG. It is, however, a technically demanding technique that requires specific training by the surgeon and the team. There is evidence that off-pump is advantageous in high risk patient when performed by an experienced surgeon.

Arteries vs Veins

Arteries are vessels that carry blood away from the heart and towards organs and tissues. Arteries are subjected to blood pressure that is much higher than in veins. The left internal mammary artery (LIMA) is the commonest type of graft used in CABG surgery. The LIMA is associated with excellent durability when used in CABG as over 90% of LIMA grafts remain patent at 10 years and beyond following surgery. Other arterial grafts include the RIMA and the radial artery.

Veins return blood towards the heart at low pressure. The great saphenous vein (GSV) is readily accessible on the inside of the leg and has been used extensively as a graft in cardiac surgery. While the GSV is a very good choice, there is some evidence that the RIMA and possibly the radial artery are likely to last longer when they are used to graft the left coronary artery.

Preferred technique for CABG

Mr Asimakopoulos carries out over 90% of CABG off-pump as he trained extensively in the technique and believes in its advantages. He has performed over 1,300 off-pump CABG procedures with excellent results. The survival rate of his patients after CABG is 99%, including high risk and emergency operations.

He would not hesitate, of course, to use CPB if this was the best option for patients that provide particular challenges such as a failing heart.

He also favours the use of more than one arterial graft for stenoses of the left coronary artery and  often uses both the LIMA and the RIMA to perform two or more anastomoses.