Coronary Artery Bypass Grafting

Coronary artery bypass grafting (CABG) is performed for patients with coronary artery disease (CAD) to improve quality of life and reduce cardiac-related mortality. CAD is the leading cause of mortality in the United States ^[10] and the developed world, ^[11] and 16.5 million US adults (age ≥20 years) are affected by this disease annually. ^[10]  It alone accounts for 530,989 deaths each year in the United States, and the long-term manifestations of CAD with left ventricular dysfunction and heart failure are projected to affect over 8 million people aged at least 18 years by 2030. ^[10]

CABG was introduced in the 1960s with the aim of offering symptomatic relief, improved quality of life, and increased life expectancy to patients with CAD. ^{[12, 13]} By the 1970s, CABG was found to increase survival rates in patients with multivessel disease and left main disease when compared with medical therapy. ^[14]

The relatively new paradigm for treatment of CAD calls for a heart team approach that involves the cardiologist and the cardiac surgeon evaluating the coronary angiogram together and offering the best possible option to the patient to achieve coronary revascularization, whether it be placement of a percutaneous coronary stent or CABG. It is recommended that Individuals under consideration for CABG undergo Society of Thoracic Surgeons (STS) risk stratification. ^[1]  Evaluation of CAD complexity in patients with multivessel CAD using tools such as the SYNTAX (Synergy Between PCI With TAXUS and Cardiac Surgery) score may be useful in guiding revacularization. ^[1]

At present, the typical patient for CABG is older, is more likely to have undergone previous percutaneous coronary intervention (PCI), and has significantly more comorbidities. Despite these adverse risk factors, CABG continues to be one of the most important surgical procedures in the history of modern medicine and probably has prolonged more lives and provided more significant symptomatic relief than any other major operation in medicine. Newer, less-invasive options, advancement in anesthetic and intensive care unit (ICU) management, and technological advances are pushing the boundaries of this procedure to new heights.

Historical information

Alexis Carrel received the Nobel prize in physiology and medicine for his work in 1912. His understanding of the association between angina pectoris and coronary artery stenosis allowed him to anastomose a carotid artery segment to the left coronary artery from the descending thoracic aorta in canine model. ^[15]

In the late 1940s, the famous Canadian surgeon Arthur Vineberg implanted the left internal thoracic (mammary) artery, directly into the myocardium of the anterior left ventricle in patients with severe angina pectoralis. ^{[16, 17, 18]} Surprisingly, this procedure produced significant symptomatic relief in a few patients. ^[19]

In 1962, Sabiston, at Duke University, performed the first planned saphenous vein bypass operation for coronary revascularization. ^[20]  In 1964, Kolessov used the left internal thoracic (mammary) artery to bypass the left anterior descending artery without cardiopulmonary bypass, ^[21] and, in 1973, Carpentier introduced the use of radial artery grafts as conduits for CABG. ^{[22, 23]}

In the 1970s and early 1980s, CABG flourished as the sole therapy for CAD.  With the advent, introduction, and widespread adoption of percutaneous coronary artery stenting in the 1980s and 1990s there was a decline in the number of CABG operations performed. However, several multicenter studies comparing CABG with current stent therapy have clearly demonstrated the superiority of CABG, especially when certain patient characteristics such as diabetes, multivessel CAD and ischemic cardiomyopathy are taken into account.

Coronary artery bypass grafting (CABG) is performed for both symptomatic and prognostic reasons. Indications for CABG have been classified by the American College of Cardiology (ACC) and the American Heart Association (AHA) according to the level of evidence supporting the usefulness and efficacy of the procedure ^{[2, 3]} :

• Class I: Conditions for which there is evidence and/or general agreement that a given procedure or treatment is useful and effective

• Class II: Conditions for which there is conflicting evidence and/or a divergence of opinion about the usefulness or efficacy of a procedure or treatment

• Class IIa: Weight of evidence or opinion is in favor of usefulness or efficacy

• Class IIb: Usefulness or efficacy is less well established by evidence or opinion

• Class III: Conditions for which there is evidence and/or general agreement that the procedure/treatment is not useful or effective, and in some cases it may be harmful

Indications for CABG as detailed by the ACC and the AHA are listed in Table 1 below.

Table 1. ACC/AHA Indications for Coronary Artery Bypass Grafting ^{[2, 3]} (Open Table in a new window)

 

Alexander and Smith in the New England Journal of Medicine noted the following indications for CABG are associated with a survival benefit over medical therapy, with or without percutaneous coronary intervention (PCI) include ^[24] :

• Acute ST-segment elevation myocardial infarction (STEMI)

• CAD other than acute STEMI

• Coronary anatomy not amenable to PCI

• Mechanical complications, such as ventricular septal defect, rupture of the free ventricular wall, or papillary-muscle rupture with severe mitral regurgitation

• Left main disease of 50% stenosis or greater, and intermediate or high complexity for PCI (Synergy Between PCI with TAXUS and Cardiac Surgery [SYNTAX] score ≥33)

• Three-vessel disease of 70% stenosis or greater, involving the LAD and intermediate or high complexity for PCI (SYNTAX score ≥23)

Other indications for CABG include the following:

• Disabling angina (Class I)

• Ongoing ischemia in the setting of a non-ST segment elevation myocardial infarction (NSTEMI) that is unresponsive to medical therapy (Class I)

• Poor left ventricular function but with viable, nonfunctioning myocardium above the anatomic defect that can be revascularized

• Clinically significant CAD of 70% stenosis or greater, in 1 or more vessel(s), and refractory angina despite medical therapy and PCI ^[24]

• Clinically significant CAD of 70% stenosis or greater, in 1 or more vessel(s), in survivors of sudden cardiac arrest presumed to be related to ischemic ventricular arrhythmia ^[24]

• Clinically significant CAD of 50% stenosis or greater, in 1 or more vessel(s), in patients undergoing cardiac surgery for other indications (eg, valve replacement or aortic surgery) ^[24]

CABG may be performed as an emergency procedure in the context of a STEMI in cases where it has not been possible to perform PCI or where this procedure has failed and there is persistent pain and ischemia threatening a significant area of the myocardium despite medical therapy.

Other indications for CABG in the setting of STEMI are ventricular septal defect related to MI, papillary muscle rupture, free wall rupture, ventricular pseudoaneurysm, life-threatening ventricular arrhythmias, and cardiogenic shock. 

Factors that increase the survival benefit of CABG include the following ^[24] :

• Left ventricular ejection fraction of 45% or less

• Diabetes mellitus

• Ischemic mitral regurgitation

• PCI failure, with or without acute MI (AMI)

Indications for CABG when PCI is noninferior to CABG and when PCI or CABG is preferred over medical therapy include the following ^[24] :

• Left main disease of 50% stenosis or greater, and low-to-intermediate complexity for PCI (SYNTAX score ≤32)

• Three-vessel disease of 70% stenosis or greater, and low complexity for PCI (SYNTAX score ≤22)

• Two-vessel disease of 70% stenosis or greater, involving the LAD and low complexity for PCI (SYNTAX score ≤22)

Factors that increase the benefit of PCI over CABG include the following ^[24] :

• Elevated mortality risk with CABG

• Elevated stroke risk

• Extreme frailty

• Prior CABG

• Acute STEMI at presentation

Table 2 below shows the recommendations for treatment of patients with acute heart failure in the setting of AMI.

Table 2. Treatment Recommendations for Patients with Acute Heart Failure in Setting of Acute Myocardial Infarction (Open Table in a new window)

 

Special recommendations in patients with comorbidities are presented in the tables below.

Table 3. Specific Treatment Recommendations for Coronary Artery Disease in Patients with Mild to Moderate Chronic Kidney Disease (Open Table in a new window)

  Table 4. Specific Treatment Recommendations for Coronary Artery Disease in Diabetic Patients (Open Table in a new window)

  Table 5. Recommendations for Combining Valve Surgery and Coronary Artery Bypass Grafting (Open Table in a new window)

  Table 6. Carotid Revascularization in Patients Scheduled for Coronary Artery Bypass Grafting (Open Table in a new window)

  Table 7. Management of Patients with Associated Coronary and Peripheral Arterial Disease (Open Table in a new window)

  Table 8. Management of Patients with Renal Artery Stenosis (Open Table in a new window)

  Table 9. Recommendations for Patients with Chronic Heart Failure and Systolic Left Ventricular Dysfunction (Ejection Fraction = 35%), Presenting Predominantly with Angina Symptoms (Open Table in a new window)

  Table 10. Recommendations for Patients with Chronic Heart Failure and Systolic Left Ventricular Dysfunction (Ejection Fraction = 35%), Presenting Predominantly with Heart Failure Symptoms (No or Mild Angina: Canadian Cardiovascular Society 1-2) (Open Table in a new window)

Coronary artery bypass grafting (CABG) carries a risk of morbidity and mortality and is therefore not considered appropriate in asymptomatic patients who are at a low risk of myocardial infarction or death. Patients who will experience little benefit from coronary revascularization are also excluded.

CABG is performed in elderly patients for symptomatic relief. Although advanced age is not a contraindication, CABG should be carefully considered in the elderly, especially those older than 85 years. These patients are also more likely to experience perioperative complications after CABG. A multidisciplinary heart team approach that emphasizes shared decision making in patients with complex coronary artery disease is essential to offer the patient the best chance of a successful revascularization strategy.

Best practices

Either arteries or veins may be used as conduits for coronary artery bypass grafting (CABG). The survival benefits of grafting the left internal thoracic (mammary) artery to the left anterior descending coronary artery was established many years ago in a landmark paper from the Cleveland Clinic. ^[25]  This remains true; in fact, bilateral internal thoracic (mammary) artery grafting, if possible, confers a significant long-term survival benefit. Robust evidence suggests that the use off an additional arterial graft rather than a vein graft is associated with further improvement in late outcomes. ^[26] The greater saphenous vein and, very rarely, the short saphenous vein are the most commonly used vein grafts, whereas the internal thoracic (mammary) artery is the most commonly used artery graft. The radial artery graft was reintroduced into clinical practice in the 1990s and continues to show high patency rates of 80% or higher at 10 years follow-up, especially if the target vessel stenosis was greater than 90%. ^[27]

The disadvantage of saphenous vein grafts is their declining patency with time: 10-20% are occluded 1 year after surgery because of technical errors, thrombosis, and intimal hyperplasia. ^[2] Another 1-2% of vein grafts occlude every year from 1-5 years after surgery, and 4-5% occlude every year from 6-10 years after surgery. Vein graft occlusion that occurs 1 or more years after CABG is due to vein graft atherosclerosis with developing neointimal hyperplasia. At 10 years after surgery, only 50-60% of saphenous vein grafts are patent, and only half of these are free of angiographic atherosclerosis. ^[2] As part of appropriate secondary prevention, patients should receive life-long antiplatelet therapy, most commonly with daily low-dose (81 mg) aspirin.

Unlike saphenous vein grafts, internal thoracic (mammary) artery grafts exhibit stable patency over time. ^[2] At 10 years, more than 90% of internal thoracic (mammary) artery grafts are patent. The left internal thoracic (mammary) artery should be the conduit used when the left anterior coronary artery is bypassed.

Technical recommendations for CABG are presented in Table 11 below.

Table 11. Technical Recommendations for Coronary Artery Bypass Grafting (Open Table in a new window)

Procedure planning

The formation of a multidisciplinary heart team enables a balanced decision-making process (see Table 12 below). ^{[28, 29]} Clinicians should approach the informed consent process as an opportunity to enhance objective decision-making rather than solely as a legal requirement. It is vital to be aware that factors such as sex, race, availability, technical skills, local results, referral patterns, and patient preference may affect the decision-making process independent of clinical findings. ^[28]

Table 12. Multidisciplinary Decision Pathways, Patient Informed Consent, and Timing of Intervention ^[28] (Open Table in a new window)

Additional input from general practitioners, anesthesiologists, geriatricians, and intensivists may be needed.

Hospitals without a surgical cardiac unit or with interventional cardiologists working in an ambulatory setting should refer to standard evidence-based protocols devised in collaboration with expert interventional cardiologists and cardiac surgeons or should seek the opinions of these physicians for complex cases. Consensus on the best revascularization treatment should be documented. Standard protocols that are in accordance with current guidelines may be used to obviate individual case review of each diagnostic angiogram.

Ad hoc PCI is a therapeutic interventional procedure that is performed directly after the diagnostic procedure rather than during a different session. ^[28] Although it is convenient and often cost-effective, ad hoc PCI is not desirable for all cases; some patients may be in categories for which CABG is the most suitable choice. The anatomic criteria and clinical factors that determine whether a patient can or cannot be treated by means of ad-hoc PCI should be defined by institutional protocols designed by the heart team. ^[28]

Complication prevention

Cerebrovascular complications are a major cause of morbidity after CABG. The main causes of these complications are hypoperfusion or embolic events. Accordingly, it is important to maintain adequate mean arterial pressures as a prophylactic measure against hypoperfusion, although there is little that can be done to protect the patient from embolic events.

In patients with multivessel coronary disease, coronary artery bypass grafting (CABG), as compared with percutaneous coronary intervention (PCI), leads to a reduction in long-term mortality and myocardial infarctions (MIs) as well as reductions in repeat revascularizations, regardless of whether patients are diabetic are not, according to a meta-analysis of six randomized clinical trials comprising 6055 patients from the era of arterial grafting and stenting. ^[30]

In a meta-analysis of eight randomized studies that included a total of 3612 adult patients with diabetes and multivessel coronary artery disease (CAD), treatment with CABG significantly reduced the risk of all-cause mortality by 33% at 5 years, as compared with PCI. This relative risk reduction did not differ significantly when patients who underwent CABG were compared with subgroups of patients who received either bare metal stents or drug-eluting stents. ^{[31, 32]}

In a study of 3723 patients with multivessel coronary disease that compared whether the effect on survival from PCI (n = 1097) compared with CABG (n = 5626) is related to the age of the patient, Benedetto et al found that CABG resulted in a significant reduction in late-phase mortality across all age groups compared to PCI. ^[33] At a mean follow-up of 5.5 ± 3.2 years, there were 301 deaths overall (PCI: 208; CABG: 93). Overall survival for the PCI group was 95% at 1 year, 84% at 5 years, and 75% at 8 years compared to 95% at 1 year, 92.4% at 5 years, and 90% at 8 years for the CABG group. ^[33]

In a retrospective (1997-2013), nationwide, population-based Swedish study that evaluated long-term survival, major adverse cardiovascular events, and factors associated with elevated risk in 4086 young adults (≤50 years) undergoing CABG, Dalen et al found better outcomes in younger adults than their older counterparts. ^[34] At a median follow-up of 10.9 years, 490 (12%) patients died, with 96% survival at 5 years, 90% at 10 years, and 82% at 15 years. The survival of patients aged 51 to 70 years and those older than 70 years who underwent CABG during the same period was significantly worse. The primary risk factors for all-cause mortality were chronic kidney disease, reduced left ventricular ejection fraction, peripheral vascular disease, or chronic obstructive pulmonary disease. ^[34]

Results of the Surgical Treatment for Ischemic Heart Failure (STICH) Extension Study (STICHES), which evaluated the long-term, 10-year outcomes of CABG in 1212 patients with ischemic cardiomyopathy and an ejection fraction of 35% or less, concluded that the rates of death from any cause, death from cardiovascular causes, and death from any cause or hospitalization for cardiovascular causes were significantly lower in patients who underwent CABG and received medical therapy than among those who received medical therapy alone. ^[35]  However, more recent evidence from meta-analyses and the International Study of Comparative Health Effectiveness With Medical and Invasive Approaches (ISCHEMIA) trial found no advantage of CABG over medical therapy in stable ischemic heart disease. ^[1]

In single-center retrospective analysis (2003-2013) of 763 elderly patients (age ≥75 years) with multivessel disease who underwent PCI or CABG within 30 days of the index catherization, CABG was associated with the best overall clinical outcomes. ^[36] However, only 20% of the patients (n = 150) underwent CABG. The best treatment strategy for this population remains to be determined. ^[36]

Similarly, results from analysis of 2007-2014 data from the National Cardiovascular Data Registry Acute Coronary Treatment and Intervention Outcomes Network Registry-Get With The Guidelines that evaluated trends in CABG utilization and in-hospital outcomes showed that CABG was used infrequently in 15,145 patients with ST-segment elevation myocardial infarction (STEMI) during the index hospitalization, with CABG rates declining over time. ^[37]  In addition, there was a wide hospital-level variation in CABG rates in STEMI, and CABG was generally performed within 1-3 days following angiography. In-hospital mortality rates were similar for patients who underwent CABG and those who did not. ^[37]

In a meta-analysis of comparison of 5-year outcomes of PCI with drug-eluting stents versus CABG in 6637 patients with unprotected left main CAD from nine studies over a 14-year period (2003-2016), PCI with drug-eluting stents was associated with equivalent cardiac and all-cause mortality but lower rates of stroke and higher rates of repeat revascularization. ^[38] A trend favoring CABG over PCI for major adverse cardiac and cerebrovascular events did not reach statistical significance.

With regard to quality of life following CABG compared with PCI for multivessel CAD, both interventions provide improvements in the frequency of angina. ^[39] However, at 1 month postprocedure, PCI patients appear to recover faster and have improved short-term health status compared to patients who undergo CABG, whereas at 6 months and longer postprocedure, CABG patients appear to have greater angina relief and improved quality of life relative to those who undergo PCI. ^[39]

Despite a steady increase in the proportion of older and higher risk patients being referred for surgery, major perioperative morbidity and mortality continues to be low, and long-term outcomes are excellent. With an operation that has stood the test of time, future advances in percutaneous coronary interventions (PCIs), molecular therapeutics, and novel surgical approaches must be rigorously compared to the gold standard of coronary artery bypass grafting (CABG).

Current mortality risk prediction models for CABG do not have a standardized approach to defining outcome and predictor variables, and they include problematic issues such as inadequate sample sizes, inappropriate handling of missing data, as well as suboptimal statistical techniques. ^[40] Future risk modelling will need to improve upon these factors to refine the quality of mortality risk prediction.

The surgical robot allows surgeons to remotely manipulate fully articulating videoscopic instruments by way of "master-slave" servos and microprocessor control. The improved video resolution is an advantage, but the added expense and time required as well as difficulty with learning this technique, in addition to the limited applications in CABG surgery, has limited the role of robotic-assisted CABG.

A relatively recent development is hybrid surgical and percutaneous revascularization. In this approach, patients undergo not only minimally invasive CABG, most often with the use of the left internal thoracic (mammary) artery graft to the left anterior descending coronary artery, but also undergo PCI of lesions in the circumflex and right coronary arteries. This strategy provides the benefits of CABG with a lower morbidity and could emerge as the new standard for patients with multivessel coronary artery disease (CAD). ^{[41, 42]}

CABG does not prevent the progression of native CAD, however, both disease progression and vein graft failure can be ameliorated by aggressive secondary prevention with medical therapy. ^[43] The American Heart Association recommends life-long antiplatelet therapy. ^[44]  Daily intake of low-dose (81 mg) aspirin may be preferable to minimize the risk of bleeding. Beta blockers should be used in patients with recent myocardial infarction, left ventricular systolic dysfunction, or in patients with non-revascularized CAD. All patients, regardless of their lipid values should receive life-long high-intensity statin therapy. Diet, exercise, and smoking cessation are well known adjuncts to promote improved cardiovascular health. ^[44]