Coronary artery bypass grafting (CABG) and percutaneous coronary intervention (PCI) are revascularization techniques that are used to treat patients with both stable angina and acute coronary syndromes. As described below, both procedures are used in higher risk patients, with the choice of technique determined by several factors including severity of disease and the wishes of the individual. It is estimated that in 2003 CABG and PCI were carried out on approximately 270 000 and 650 000 patients in the USA, respectively.
CABG is a surgical procedure (Figure 43, right) which was introduced in the 1960s. Initially, CABG mainly involved the use of lengths of healthy superfluous blood vessels (conduits) which were removed and then attached (anastamosed) between the aorta and the coronary arteries distal to the stenosis, thus allowing a supply of blood to the heart that bypassed the obstruction. Conduits commonly used for CABG included saphenous vein segments harvested from the leg. However, these have limited long-term patency due to early postoperative thrombosis, intimal hyperplasia with smooth muscle proliferation within the first year, and the development of atherosclerosis after approximately 5–7 years. For this reason, the left internal thoracic (also termed mammary) artery (LITA) is now used for grafting much more widely than the saphenous vein. In general, the LITA is not disconnected from its parent (subclavian) artery, but is cut distally and attached to the coronary artery. Unlike the saphenous vein, 90–95% of LITA grafts remain patent after 10 years, and patients with a LITA graft to the crucial left anterior descending coronary artery have improved long-term survival compared with patients receiving saphenous vein grafts. If multivessel disease is present, the use of LITA and saphenous vein grafts can be combined. More recently, the use of both left and right internal thoracic arteries (bilateral internal thoracic artery) for grafting has become more common, especially for younger patients. For example, the right internal thoracic artery may be grafted to the left anterior descending coronary artery while the LITA is anastomosed to the circumflex system. The gastroepiploic and radial arteries can also be used for grafting.
CABG is usually perform with the patient on cardiopulmonary bypass, with the heart stopped. Blood is typically removed from the right atrium, drained into a reservoir, and then pumped through an oxygenator, then a filter and back into the aorta to perfuse the systemic circulation. The main complications of the procedure are a systemic inflammatory response, atrial fibrillation and persistent neurological abnormalities. These latter are thought to be caused by emboli, either formed in the bypass circuit or produced by disturbance of aortic plaques during cannulation, which lodge in the cerebral vasculature. These complications can be avoided by off-pump CABG, which does not involve stopping the heart. In this case, the region of the cardiac wall encompassing the target coronary segment is immobilized to allow grafting. Randomized trials show that both types of CABG offer similar outcomes. The mortality rate associated with CABG is ∼2%.
PCI, first used in 1977, is a much less invasive procedure. A guiding catheter is introduced via the femoral, brachial or radial artery, and is positioned near the target stenosis. A guiding wire is then advanced down the lumen of the coronary artery until it is positioned across the stenosis. A balloon catheter is advanced over this wire, and then inflated at the site of the stenosis to increase the luminal diameter (Figure 43, left). Emergency CABG is required in 1–2% of patients due to acute vessel closure after this procedure. PCI is judged a success if the arterial lumen at the stenosis is increased to more than 50% of the normal coronary artery diameter.
Restenosis at the site of the PCI occurs within 6 months of the procedure in 30% of patients. Restenosis can be caused by elastic recoil of the vessel or by intimal hyperplasia, a thickening of the inner layer of the artery which is initiated by endothelial denudation, and which involves proliferation of intimal smooth muscle cells and the production of connective tissue. Restenosis generally causes a return of cardiac ischaemia and angina, in which case PCI is repeated or CABG is performed.
Stents were first introduced in 1986 in an attempt to prevent elastic recoil and restenosis. Stents are cylindrical metal (e.g. stain- less steel, platinum) mesh or slotted tubes that are implanted into the artery at the site of balloon expansion following angioplasty. They are mainly used in vessels >3 mm in diameter and are designed either to be self-expanding, or to be expanded by the catheter balloon, so that they press out against the inner wall of the coronary artery, holding it open. Stenting is currently being used in ∼90% of PCI procedures as its introduction has substantially improved acute PCI success, has reduced the rate of restenosis to ∼15%, and has correspondingly decreased the need for repeat revascularizations. Various approaches are being tried to reduce this ‘in-stent’ restenosis still further. Notably, the 2002 RAVEL trial assessed the use of stents that were coated with the proliferation-inhibiting drug rapamycin (sirolimus), which gradually eluted from the stent over a month. Rapamycin caused a dramatic decrease in restenosis, and virtually abolished the need for another revascularization over the year following the procedure. Subsequent studies have shown that the use of drug-eluting stents reduces the incidence of major adverse cardiac events during the 9 months following PCI by ∼50%, so that drug-eluting stents utilizing rapamycin as well as the alternative agents paclitaxel and everolimus are now used routinely.
The main potential complication arising from stenting is thrombosis, which can be well controlled with aspirin and clopidogrel. Routine PCI bears a risk of mortality of ≤1%.
Revascularization vs medical management: which patients benefit?
In general, revascularization is preferred for patients who are at high risk of developing worsening ischaemic heart disease and/or acute coronary syndromes, or in whom pharmacological treatment is either not controlling ischaemic symptoms (e.g. angina) or is causing intolerable side effects. Particularly important indications for revascularization in stable angina include the presence of significant plaques in three coronary arteries (particularly when the left anterior descending, which perfuses the largest fraction of the myocardium, is involved) and reduced left ventricular function, which indicates the presence of chronic ongoing ischaemia.
Revascularization is also very frequently used in UA/NSTEMI (see Chapter 42), and is recommended for patients who are judged to be at moderate or high risk for death or myocardial infarction, as judged by various indices relating to the seriousness of their signs and symptoms. Revascularization is now also preferred over thrombolysis to produce immediate coronary reperfusion during acute myocardial infarction (STEMI; see Chapters 43 and 45). In heart failure, revascularization can be used to reperfuse a region of ‘hibernating myocardium’, in which cells are still alive but are contracting poorly because they are chronically ischaemic.
PCI vs CABG
PCI is preferred when one or two arteries are diseased, as long as the disease is not too diffuse and the plaques are amenable to this approach. CABG is used when all three main coronary arteries are diseased (triple vessel disease), when the left coronary mainstem has a significant stenosis, when the lesion is not amenable to PCI, and when left ventricular function is poor. CABG has been shown to reduce angina symptoms more than does PCI in the first 5 years after the procedure, but symptoms tend to return gradually over the years in either case, and eventually recur similarly after both procedures. Revascularization must be repeated much more often after PCI than CABG, although improvements in stenting will probably narrow this difference. The use of PCI is growing rapidly, while that of CABG is diminishing.
Benefits of revascularization
Compared with medical therapy, CABG improves survival in patients with severe atherosclerotic disease in all three major coronary arteries or a more than 50% stenosis of the left main coronary artery, particularly if left ventricular function is impaired. Compared with medical therapy, PCI does not improve survival. However, PCI results in a greater improvement of angina symptoms and exercise tolerance than does medical therapy, and also diminishes the need for drugs.