Coronary Artery Bypass Grafting

Synonyms: CABG; Bypass grafting; Triple bypass; Quadruple bypass

This is one of two procedures for coronary artery disease, the other being percutaneous transluminal coronary angioplasty (PTCA). The patient will already have had a clinical diagnosis of ischaemic heart disease, confirmed by coronary angiography to delineate the obstruction. If PTCA is feasible it is usually performed at the time of coronary angiography. CABG may be performed as a primary procedure, after failed PTCA or as a repeat procedure.

Three major randomised controlled trials compared CABG with medical therapy. They are the:

1. Coronary Artery Surgery Study (CASS)
2. The Veterans Administration (VA) Co-operative Study
3. The European Coronary Surgery Study (ECSS)

A meta-analysis of these 3 studies showed that in high and medium risk patients there was definite benefit from CABG but not in low risk patients.¹ The risk reduction was greater in patients with left main artery disease than in those with disease in three vessels or one or two vessels. The benefits of CABG were most pronounced in patients in the highest risk categories. SIGN states that benefits of surgery tend to be lost if surgery is delayed. The NHS is taking great steps to reduce time from first referral for exercise testing to surgery with considerable political pressure. The NSF sets targets for waiting times but these are dependent upon the degree of urgency. It is necessary to stratify urgency as deaths are often in the early stages of waiting.²

• Short, discrete, non-calcified lesions on straight, unbranched arteries are often amenable to PTCA but longer, calcified lesions in tortuous vessels, or involving major bifurcations, and chronic occlusions are more likely to require CABG.
• PTCA is the treatment of choice for single or two vessel disease unless it includes the proximal left anterior descending (LAD) artery. It can also be used as first choice if a previous CABG shows re-stenosis.
• CABG is the procedure of choice for left main stem (LMS) disease with >50% stenosis, two vessel disease that includes the LAD and triple vessel disease.³
• CABG also has advantages over PTCA if there is left ventricular dysfunction or a strongly positive exercise test.

Around 28,000 CABG operations are performed in the UK each year. There is considerable geographical variation in terms of numbers of operations and referral rates between primary care centres.

• The operation is usually performed through a mid-sternal incision.
• Veins may be harvested from the saphenous vein in the legs but arteries are also transplanted into place and these give better long term outcomes.
• The most commonly used arteries are the internal thoracic (internal mammary) but the gastro-epiploic, inferior epigastric or radial arteries are also used.
• Over 75% of patients have at least 3 grafts and at least one is usually an artery.
• Ten years after CABG 83% of internal thoracic artery grafts are patent but only 41% of saphenous vein grafts.⁴
• Those who receive only saphenous vein grafts have 1.6 times the death rate over 10 years compared with those who receive at least one internal thoracic artery.
• It is possible to use a smaller incision in a technique called MIDCAB (minimally invasive direct coronary artery bypass) with a more lateral approach. Minimal access surgery is where a small incision is made directly over the artery to be bypassed. "Port access surgery" is where a series of small cuts are made in the chest. These new techniques are still being evaluated and are not yet available routinely.
• Most surgeons prefer cardioplegia with cardiopulmonary bypass.
• Sometimes surgeons operate on a beating heart, performing delicate anastomoses on a moving target (usually the case for MIDCAB and Port Access).⁵
• Operating without cardioplegia is said to reduce the risk of complications following surgery but this is disputed.⁶ The results of long-term survival rates are promising (median 23 years for surgery involving the left anterior descending coronary artery).⁷
• As with so many procedures, the evidence is that results are best when operators perform a significant number per year and in a unit with a reasonably high work load. The NSF suggests that each surgeon should perform at least 50 operations a year and that a unit should perform at least 400 cases a year by at least 3 trained surgeons. SIGN suggest a much higher figure of 250 operations year by a trained surgeon.

• This surgical technique is being increasingly used since the 1990's.
• Its benefits include shorter procedure times and reduced complications - although these have not been consistently demonstrated.⁸
• When first used OPCAB was usually only undertaken in those with a single vessel disease - now multivessel OPCAB is feasible.
• The European System for Cardiac Operative Risk Evaluation Formula or EuroSCORE, is a scoring method which can predict early and mid-term mortality for patients undergoing CABG. It may have a role to play in determining patient risk for those undergoing OPCAB.⁹

• CABG significantly improves symptoms of angina and quality of life, exercise capacity and reduces the need for anti-anginal therapy compared with medical treatment.¹⁰
• Over 75% of patients are free of ischaemic events at 5 years and nearly 50% at 10 years.¹¹
• There is less limitation of physical activity.
• At 10 years the benefit of CABG over medical therapy is lost in those who had only saphenous vein grafts but after arterial grafts it remains.
• Survival is improved with the greatest relative benefit going to those with left main stem stenosis of >50%. Patients with angina triple vessel disease or two vessel disease including proximal LAD stenosis also survive longer but those with two vessel disease excluding proximal LAD stenosis or with single vessel disease gain no survival advantage.
• Patients with abnormal left ventricular function or strongly positive exercise tests derive greater absolute survival benefit from coronary artery bypass surgery than from medical therapy.
• Women undergoing CABG have worse outcomes compared to men which is thought to be the result of greater co-morbidity, smaller coronary arteries and older age. However despite this, long-term survival does not differ between genders.¹²

• Significant left main stem stenosis of 50% or more
• Triple vessel disease
• Two vessel disease including a significant proximal LAD stenosis
• Impaired left ventricular function or strongly positive exercise test

The operative mortality rate is around 1 to 3% but varies according to case type.

Both operative mortality and outcome are more likely to be adverse in the following:

• Increasing age
• Smoking
• Being female¹³ (the reason for this is debated but is probably multifactorial as mentioned above)
• Diabetes
• Overweight or obesity¹⁴
• Being short
• Other illness
• LMS disease, poor LV function and multivessel disease
• If the operation was an emergency for unstable angina or shortly after myocardial infarction

• Myocardial infarction occurs in 2% with probably rather more cases of diffuse myocardial damage
• Low output states requiring supportive measures
• Ventricular arrhythmias like broad complex tachycardia
• Stroke occurs in 2% but a third show impairment on psychometric testing. Reducing handling of the aorta may reduce embolism. The problem is that patients needing CABG are at risk of cardiovascular disease in general and it has been suggested that pre-operative investigation of cerebrovascular disease should be undertaken.¹⁵ This includes looking for carotid artery stenosis - the risk of perioperative stroke being higher in those who have symptomatic stenoses.¹⁶
• Damage to other organs is rare in elective surgery, but the risk increases with bypass time and increasing age
• Clopidogrel is much used with PTCA but should be avoided around CABG (stop 7 days before operation). It increases the need for substantial transfusion and also the need for re-operation to secure haemostasis.¹⁷ Aspirin should be stopped 3 to 7 days before surgery as stopping it 2 days or less before operation increases the need for transfusion.¹⁸ It is usually started the day after surgery. During surgery with cardiopulmonary bypass (CPB) heparin is used. Patients who have had CPB seem to have an increased resistance to aspirin compared with those who were not operated on using CPB. This is probably because CPB causes an increased turnover of platelets.¹⁹
• Cognitive decline has been reported in various studies following CABG. However, some of these studies have been criticized on the basis of no consensus on how best to measure preoperative cognition. Most changes in cognition are thought to be mild and reverse within the first few months after surgery.²⁰

Attention must be paid to secondary prevention of IHD. Stopping smoking is essential if it has not been done already.²¹ Long term aspirin is essential as is the use of an ACEI/ARB and beta blockers. Statins should be used to get optimum cholesterol levels. Control of blood pressure and any diabetes should be optimised. Encourage exercise within reason and a healthy diet.