Coronary artery bypass surgery is a major heart operation that is carried out in order to bypass coronary arteries that have become narrowed or blocked (usually as a result of atherosclerosis). The procedure involves using additional blood vessels (such as a mammary artery or a vein from the leg) to improve blood flow to the heart muscle.
A coronary artery bypass is performed if symptoms of coronary artery disease have not been relieved by drugs, or if balloon angioplasty (a surgical procedure used to widen blocked arteries) is inappropriate or has failed.
Before surgery, sites of blockage are identified using an imaging procedure called angiography. Usually, a heart–lung machine is needed to maintain the circulation during the operation, although sometimes minimally invasive surgery may be used to bypass the artery, thereby avoiding the need to stop the heart.
The long-term outlook is good following a coronary artery bypass. However, the grafted vessels may also eventually become blocked by atherosclerosis.
Coronary artery bypass surgery in detail - technical
Patients with left main-stem stenosis and three-vessel disease merit surgery for prognosis, regardless of symptom severity, especially if ventricular function is impaired. Coronary artery bypass is also an effective therapy for angina pectoris that is not controlled with medical treatment.
Despite a worsening risk profile in the population undergoing coronary artery bypass, operative mortality remains low. Ten years after operation, about 80% of patients are still alive and two-thirds are free of angina. Contemporary results are likely to be better than this due to increasing use of arteries instead of veins as bypass grafts, and improvements in secondary prevention.
The evidence shows that coronary artery bypass gives patients with coronary artery disease longer survival than percutaneous intervention, even with the use of drug-eluting stents.
The coronary artery bypass operation is arguably the most studied operation in the history of surgery. It is also one of the most successful.
The development of coronary angiography and cardiopulmonary bypass in the 1960s allowed a direct surgical approach to coronary artery disease. Before this various attempts to improve myocardial blood supply had been ineffective and the coronary artery bypass operation rapidly gained favour. Between 1972 and 1984, three prospective randomized trials established its superiority to medical treatment. Between 1977 and 2003 the number of operations performed in the United Kingdom increased 10-fold to 25 000 per year.
Coronary artery bypass is principally indicated for relief of angina pectoris or to improve the prognosis of a patient with coronary artery disease (Table 1). In some cases both indications apply. For any patient the decision to proceed requires an assessment that the benefits outweigh the risk, although those at highest operative risk often have the most to gain.
|Table 1 Indications for coronary artery bypass surgery|
|Coronary anatomy||Reduced left ventricular function||Extensive myocardial ischaemia on noninvasive testing||Limiting angina||Class of evidence||Level of evidence|
|Left main-stem stenosis||I||A|
|Left main-stem equivalent||I||A|
|Proximal left-anterior descending stenosis||Yes||I||B|
|Proximal left anterior descending stenosis||Yes||I||B|
|One- or two-vessel disease||Yes||I||B|
Meta-analysis of the three major and four smaller prospective randomized trials conducted in the late 1970s and early 1980s shows a survival advantage for coronary artery bypass surgery compared to medical treatment alone. This improvement in survival is greatest in those at highest risk, risk being defined by the extent of coronary artery disease. Mortality reduction with surgery is 68% at 5 years in left main stem stenosis and 42% with three-vessel disease. Subsequent registry data has enabled benefit to be defined according to ventricular function, extent of ischaemic myocardium, and severity of angina as well as coronary anatomy.
In asymptomatic patients or those with nonlimiting angina, operation is advised for those in whom it will improve prognosis. This encompasses patients with left main stem stenosis (flow-limiting stenosis of the left coronary artery before division into left anterior descending and circumflex arteries) or equivalent (flow-limiting stenosis of proximal left anterior descending and proximal circumflex arteries) and three-vessel disease (stenosis of all three coronary arteries).
In the presence of limiting angina despite adequate medical treatment, coronary artery bypass is indicated provided the coronary arteries are suitable for grafting and the operative risk is not prohibitive.
Non-ST-elevation myocardial infarction (non-STEMI)
After non-STEMI the indications for coronary bypass are as indicated in Table 1. The timing of surgery is critical: operative risk is increased two- to three fold in the first week and gradually declines over the next month, but delaying operation to ameliorate this risk has to be balanced against the risk of an adverse event during the period of waiting.
ST-elevation myocardial infarction (STEMI)
In contemporary practice coronary artery bypass has little place in the management of acute STEMI. After the acute phase the caveats associated with non-STEMI apply.
The coronary artery bypass operation
Access to the heart is provided by a median sternotomy incision. The left internal mammary artery is mobilized and divided distally. Proximally it is left connected to the left subclavian artery. If the right internal mammary artery is used, this may be left attached to the right subclavian artery or detached and used as a free graft. Enough long saphenous vein is mobilized from the leg. The radial artery is mobilized from the nondominant arm if required.
Cannulae are inserted to allow cardiopulmonary bypass and the bypass machine is started. The ascending aorta is clamped and the heart arrested in diastole by infusing cardioplegia solution with a high potassium concentration into the aorta on the cardiac side of the cross clamp. It may also be administered retrogradely through the coronary sinus.
Longitudinal incisions (2–3 mm) are made in the coronary arteries distal to the stenosis and the bypass grafts are sutured to them. The left anterior descending coronary artery is grafted with the left internal mammary artery. If a free right internal mammary artery graft or radial artery graft is used, the proximal anastomosis is made either to the aorta or to the side of the left internal mammary artery. The proximal ends of vein grafts are attached to the ascending aorta. Removal of the cross clamp reperfuses the heart, washing out the cardioplegia solution, and cardiac contraction restarts spontaneously. When adequate cardiac contraction is achieved the patient is separated from cardiopulmonary bypass.
The operation may also be performed without the use of cardio-pulmonary bypass. A special retractor is used to stabilize the heart and provide a still operative field. At present there are no conclusive data showing the superiority for the coronary artery bypass operation performed with or without cardiopulmonary bypass.
With time, the risk profile for patients undergoing coronary artery bypass has steadily increased. This has been contributed to by an ageing population, extension of the indications for the operation, and increasing treatment of low risk patients with percutaneous coronary intervention (PCI). However, despite a worsening risk profile the operative mortality for isolated coronary artery bypass has steadily declined and was 1.5% in the United Kingdom in 2008. Patients less than 60 years of age without significant comorbidity have an operative mortality of 0.4%. Operative mortality is adversely influenced by advancing age, reduced left ventricular function, urgency of the operation, female sex, and more extensive coronary artery disease. Repeat operation trebles operative risk.
Permanent neurological injury occurs in around 3% of patients. Increasing age, previous stroke, carotid artery disease, and ascending aortic atheroma are risk factors for the development of this complication. Up to 30% of postoperative strokes are associated with carotid artery disease. Preoperative carotid endarterectomy may be indicated in patients with severe carotid artery disease, especially if symptomatic.
Local and cardiac complications
Sternal wound infection
Dehiscence of the sternal wound with mediastinitis occurs in around 1% of patients and is predisposed to by diabetes mellitus and obesity. For this reason use of both internal mammary arteries is avoided in obese diabetics. Treatment generally requires surgical revision of the wound and may require the use of muscle flaps.
With use of the internal mammary artery grafts pleural effusions are common, but these are generally small and self-limiting. If large enough to cause shortness of breath they should be aspirated.
Between 25% and 50% of patients develop a pericardial effusion following operation, but in only a few per cent is this large enough to cause cardiac tamponade. Percutaneous drainage may be possible with echocardiographic control: if not surgical drainage is required. Pericardial effusions generally develop within a few days of surgery but may present some weeks later.
Postcardiotomy syndrome is characterized by fever and anterior chest pain. A pericardial friction rub, lymphocytosis, and persistent pericardial and pleural effusions are often associated. Nonsteroidal anti-inflammatory drugs are generally effective. Steroids are second line treatment.
Around 30% of patients suffer postoperative atrial fibrillation, which can be reduced by peri- and postoperative β-blockade. If it is persistent, patients should be anticoagulated. Amiodarone usually restores sinus rhythm and should be continued for 6 weeks. Electrical cardioversion is required if amiodarone is unsuccessful.
In a heterogeneous group of patients 10-year survival is around 80%. Half of late deaths are from cardiac causes. The use of a pedicled left internal mammary artery to the left anterior descending coronary artery, in place of a saphenous vein graft, gives a 10% survival advantage at 10 years which persists for at least another 5 years. Using both left and right internal mammary arteries further improves outcome.
Two-thirds of patients are free from angina 10 years after operation, although contemporary results are expected to be better due to increased use of arterial grafts and improved secondary prevention. Relief of angina after coronary artery bypass largely depends upon continued bypass graft patency, although progression of native coronary artery disease and incomplete revascularization play a part.
The presence of a left internal mammary artery to the left anterior descending coronary artery reduces late myocardial infarction and reoperation, but its impact on recurrent angina is less certain. Use of both internal mammary arteries extends the benefits of a single mammary artery; angina-free survival is improved and reoperation reduced.
A strategy of total arterial revascularization has not been shown to be superior to two mammary arteries and supplementary vein grafts. It may be useful for selected younger patients and those in whom there is no venous conduit. The radial artery is widely used as an arterial conduit and there is increasing data to suggest that it has improved patency compared to saphenous vein.
General measures such as smoking cessation and weight loss are beneficial. All patients should be offered cardiac rehabilitation, which improves postoperative exercise tolerance. Specific attention should be paid to antiplatelet therapy and cholesterol reduction, which improve graft patency as well as their known role in secondary prevention for cardiovascular disease.
Aspirin reduces vein graft thrombosis from 23% to 13% in the first postoperative year. Doses of between 75 and 300 mg per day are effective, with the first given early after operation and certainly within 48 h. Clopidogrel should be used if aspirin is not tolerated. The superiority of a combination of aspirin and clopidogrel is not established. Warfarin is not indicated to improve graft patency.
Aggressive reduction of LDL cholesterol to less than 2.5 mmol/litre with a statin reduces saphenous vein graft disease progression and the need for further revascularisation by 30%.
Coronary artery bypass and percutaneous intervention
Randomized studies comparing coronary artery bypass with balloon angioplasty typically show reintervention rates of 50% in the angioplasty group compared to 10% in the surgical arms. The use of bare metal stents halves reintervention after angioplasty. Procedural mortality is similar in both groups, but for patients with diabetes survival at 7 years is 76% with surgery and 56% with PCI. Patients with extensive coronary artery disease have a survival advantage with coronary artery bypass. Even with the use of drug-eluting stents, coronary artery bypass remains the standard of care for patients with left main stem stenosis and three-vessel disease.