Revascularization
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.
