The indications for coronary artery revascularization in patients with angina pectoris are often debated. There is general agreement that otherwise healthy patients in the following groups should undergo revascularization: (1) patients with unacceptable symptoms despite medical therapy to its tolerable limits, (2) patients with left main coronary artery stenosis greater than 50% with or without symptoms, (3) patients with three-vessel disease with left ventricular dysfunction (ejection fraction < 50% or previous transmural infarction), (4) patients with unstable angina who after symptom control by medical therapy continue to exhibit ischemia on exercise testing or monitoring, and (5) post-myocardial infarction patients with continuing angina or severe ischemia on noninvasive testing. (See sections on Unstable Angina and Myocardial Infarction.)
In addition, many cardiologists feel that patients with less severe symptoms should be revascularized if they have two-vessel disease associated with underlying left ventricular dysfunction, anatomically critical lesions (> 90% proximal stenoses, especially of the proximal left anterior descending artery), or physiologic evidence of severe ischemia (early positive exercise tests, large exercise-induced thallium scintigraphic defects, or frequent episodes of ischemia on ambulatory monitoring).
This trend toward aggressive intervention has accelerated as a result of the growing use of coronary angioplasty and stenting. While such patients are at increased risk, it has not been proved that their prognosis is better after coronary revascularization by either surgery or angioplasty.
A. Coronary Artery Bypass Grafting (CABG)
CABG can be accomplished with a very low mortality rate (1-3%) in otherwise healthy patients with preserved cardiac function. However, the mortality rate of this procedure rises to 4-8% in older individuals and in patients who have had a prior CABG. Increasingly, younger individuals with focal lesions of one or several vessels are undergoing coronary angioplasty as the initial revascularization procedure.
Grafts employing one or both internal mammary arteries (usually to the left anterior descending artery or its branches) provide the best long-term results in terms of patency and flow. Segments of the saphenous vein (or, less optimally, other veins) or the radial artery interposed between the aorta and the coronary arteries distal to the obstructions are also utilized. One to five distal anastomoses are commonly performed. After successful surgery, symptoms generally abate. The need for antianginal medications diminishes, and left ventricular function may improve.
Minimally invasive surgical techniques utilize different approaches to the heart than standard sternotomy and cardiopulmonary bypass. The surgical approach may involve a limited sternotomy, lateral thoracotomy (MIDCAB), or thoracoscopy (port-access). These approaches may be used in conjunction with standard cardiopulmonary bypass, with peripheral cardiopulmonary bypass, or with operating on the beating heart utilizing a mechanical coronary stabilizer. Avoiding bypass may lessen the risk of cerebral complications. These techniques allow earlier postoperative mobilization and discharge. They are more technically demanding, usually not suitable for more than two grafts, and do not have established durability.
The operative mortality rate is increased in patients with poor left ventricular function (left ventricular ejection fraction < 35%) or those requiring additional procedures (valve replacement or ventricular aneurysmectomy). Patients over 70 years of age, patients undergoing repeat procedures or those with important noncardiac disease (especially renal insufficiency and diabetes) or poor general health also have higher operative mortality and morbidity rates, and full recovery is slow. Thus, CABG should be reserved for more severely symptomatic patients in this group. Early (1-6 months) graft patency rates average 85-90% (higher for internal mammary grafts), and subsequent graft closure rates are about 4% annually. Early graft failure is common in vessels with poor distal flow, while late closure is more frequent in patients who continue smoking and those with untreated hyperlipidemia. Antiplatelet therapy with aspirin improves graft patency rates. Smoking cessation and vigorous treatment of blood lipid abnormalities are necessary, with a goal for LDL cholesterol of = 100 mg/dL and of HDL cholesterol = 45 mg/dL. Repeat revascularization (see below) is often necessitated by progressive native vessel disease and graft occlusions. Reoperation is technically demanding and less often fully successful than the initial operation.
B. Percutaneous Transluminal Coronary Angioplasty (PTCA) and Stenting
Coronary artery stenoses can be effectively dilated by inflation of a balloon under high pressure. This procedure is performed in the cardiac catheterization laboratory under local anesthesia either at the same time as diagnostic coronary arteriography or at a later time. The mechanism of dilation involves both rupture of the atheromatous plaque and remodeling of the vessel.
This procedure was at one time reserved for proximal single-vessel disease, but now it is widely employed in multivessel disease with multiple lesions, though only rarely in left main disease. PTCA is possible but often less successful in bypass graft stenoses. Bypass graft patients with multivessel disease have lower mortality rates and fewer nonfatal myocardial infarctions with surgery than with percutaneous interventions. Optimal lesions for PTCA are relatively proximal, noneccentric, free of significant calcification or plaque dissection, and removed from the origin of large branches. With improved catheter systems, experienced operators are able to successfully dilate 90% of lesions attempted. The major early complication is intimal dissection with vessel occlusion. This can usually be treated by repeat PTCA or by deployment of an intracoronary stent. The use of platelet glycoprotein IIb/IIIa inhibitors (abciximab, tirofiban) has markedly reduced the rate of acute vessel closure. Placement of intracoronary stents improves initial results, especially with complex and long lesions. Although the early experience with stents was complicated by an unacceptable rate of acute thrombosis, this problem has largely been prevented by aggressive antithrombotic therapy (chronic aspirin plus clopidogrel for 30 days, with acute use of platelet glycoprotein IIb/IIIa inhibitors in high-risk patients). Restenosis rates have fallen with the use of stents. Stents are now used in the majority of patients undergoing percutaneous revascularization.
The major limitation with PTCA has been restenosis, which occurs in the first 6 months in 30-40% of vessels dilated, though it can often be treated successfully by repeat PTCA. Factors associated with higher restenosis rates include diabetes, small luminal diameter, longer and more complex lesions, and lesions at coronary ostia or in the left anterior descending coronary artery. The use of stents has reduced the restenosis rate by half. Coated stents that elute antiproliferative agents such as sirolimus or paclitaxel have restenosis rates of < 10%. These devices are likely to have significant impact on clinical practice, leading to increased use of percutaneous interventions in patients with multiple and less favorable lesions. However, caution is required in adopting new practices, since follow-up with coated stents is relatively limited and the incidence of late stent thrombosis seems to be increasing. Currently, recurrent in-stent restenosis is treated with brachytherapy, but restenting with coated stents is also being employed.
The number of PTCA and stenting procedures now exceeds the number of CABG operations, but the justification for many of the procedures performed in patients with stable angina is unclear. Several studies have shown PTCA to be superior to medical therapy for symptom relief but not in preventing infarction or death. In patients with no or only mild symptoms, aggressive lipid-lowering and antianginal therapy may be preferable to PTCA.
Several studies of PTCA versus CABG in patients with multivessel disease have been reported. The consistent finding has been comparable mortality and infarction rates over follow-up periods of 1-3 years but a high rate (approximately 40%) of repeat procedures following PTCA. As a result, the choice of revascularization procedure is often a matter of patient preference. However, it should be noted that less than 20% of patients with multivessel disease met the entry criteria, so these results cannot be generalized to all multivessel disease patients. Outcomes with percutaneous revascularization in diabetics have been inferior to those with CABG. However, these trials preceded the widespread use of stenting.
C. Experimental Approaches
Several experimental approaches have been studied in patients with refractory angina who are not candidates for percutaneous or surgical revascularization procedures. Lasers have been used either from the epicardial surface of the left ventricle during surgery or from the ventricular cavity by catheter-based techniques. Several studies have reported an improvement in symptoms, but this has not been associated with objective evidence of improved perfusion, raising the possibility of a placebo effect. Other studies have evaluated intracoronary injections of growth factors such as vascular endothelial growth factor (VEGF) or fibroblast growth factors, but thus far there is little evidence of benefit. Lastly, mechanical extracorporeal counterpulsation, which entails repetitive inflation of a high-pressure chamber surrounding the lower half of the body during the diastolic phase of the cardiac cycle for daily 1-hour sessions over a period of 7 weeks, has been advocated. Although modest increases in exercise tolerance have been reported, the response has been variable, often time-limited, and not shown to be associated with improved myocardial perfusion - so again, a placebo effect may be responsible.
Summary of Results of Treatment
Several randomized trials have shown that over follow-up periods of several years, the mortality and infarction rates with percutaneous revascularization and CABG are generally comparable. An exception may be diabetic patients, who have had better outcomes with CABG. Recovery after PTCA is obviously faster, but the intermediate-term success rate of CABG is higher both because of the high restenosis rate with PTCA and, less importantly, with stenting. The increasing popularity of PTCA and stenting primarily reflects the lower cost and shorter hospitalization, the perception that CABG is best done only once and can be reserved for later, and the preference of patients for less invasive treatment. These arguments make PTCA the procedure of choice for revascularization of single-vessel disease, though this is not usually indicated except when symptoms are refractory. The situation is less clear with multivessel disease. It should also be noted that the excellent outcome of patients treated medically has made it difficult to show an advantage with either revascularization approach except in patients who remain symptom-limited or have left main lesions or three-vessel disease and left ventricular dysfunction. The expected availability of drug-eluting stents may shift the balance toward percutaneous revascularization in the future.
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Revision date: July 4, 2011
Last revised: by Andrew G. Epstein, M.D.