|Home | About | Journals | Submit | Contact Us | Français|
Although coronary artery bypass grafting (CABG) remains the treatment of choice for certain types of coronary artery disease (CAD), percutaneous coronary intervention (PCI)—particularly coronary angioplasty with stenting—has become the most popular nonmedical treatment approach to CAD. Some have speculated that, with the advent of drug-eluting stents (DESs), PCI will replace CABG entirely. However, the complete disappearance of CABG is both unlikely and unwarranted, for several reasons. Published randomized trials of CABG, PCI, and medical approaches to CAD compared only highly selected subgroups of patients because of strict exclusion criteria that often favored the PCI cohorts. Therefore, their results do not constitute sufficient evidence for the superiority of PCI over CABG in all CAD patients requiring revascularization. As PCI indications broaden to include more complex lesions and more high-risk patients, outcomes will not remain as favorable. In addition, although PCI is less invasive than surgery, CABG offers more complete revascularization and better freedom from repeat revascularization. Furthermore, no long-term patency data on DESs yet exist, whereas excellent 10- and 20-year patency rates have been reported for the left internal mammary artery-to-left anterior descending artery graft used in most CABG procedures. While PCI has been changing, CABG has not been stagnant; recently, advances in many aspects of the CABG procedure have improved short- and long-term outcomes in CABG patients. Both CABG and PCI technologies will continue to advance, not necessarily exclusive of one another, but no data yet exist to suggest that DESs will render CABG obsolete any time soon.
For several decades, coronary artery bypass grafting (CABG) has been the treatment of choice for certain types of patients with coronary artery disease (CAD). However, percutaneous coronary intervention (PCI) technology continues to advance, and the use of coronary artery stents has expanded rapidly. The growth of the PCI industry and the consequent decline in the number of patients referred for CABG1 has produced much speculation about the future role of each type of intervention. Because the new drug-eluting stents allow PCI to be performed with lower rates of early restenosis than do bare-metal stents or percutaneous transluminal coronary angioplasty (PTCA) alone,2–8 some have predicted that surgical revascularization will soon be obsolete.
Randomized clinical trials performed during the 1970s and early 1980s clearly established the advantages of CABG over medical therapy in patients with triple-vessel CAD, left main coronary artery stenosis, double-vessel CAD with proximal left anterior descending (LAD) coronary artery stenosis, or left ventricular dysfunction.9–19 When treated with CABG, these subgroups of patients had longer survival rates and better quality of life, including greater freedom from angina, than did patients who received medical therapy alone.18,19
The treatment of CAD in the decades that followed was substantially affected by these trials, despite the well-documented flaws in their methods. For example, many of the trials had stringent exclusion criteria that eliminated a large percentage of potential participants. Furthermore, many of these exclusion criteria, such as left main CAD and an ejection fraction of less than 0.40, eliminated patients for whom CABG would have been beneficial. In addition, because of the high rate of crossover from the medical to the surgical groups,13 the use of intent-to-treat analysis, although appropriate for randomized trials, may have biased the results in favor of medical treatment.
The numerous technical and technological advances made since these trials were completed limit the degree to which their results resemble those of the CAD treatments used today. For example, the maximal medical therapy used during the inclusion dates of these trials did not routinely include lipid-lowering agents, β-blockers, angiotensin-converting enzyme (ACE) inhibitors, clopidogrel, or some of the other drugs cur-rently used for CAD. Similarly, the CABG groups did not benefit from advances that were subsequently made in preoperative imaging, perfusion and myocardial protection, anesthesia, and perioperative and intensive care practices. Further, in several of the studies, CABG did not routinely include the use of left internal mammary artery (LIMA) grafts, much less other arterial conduits.20 Finally, PCIs, including balloon angioplasty and stenting, were not included in these trials.
Subsequent randomized trials comparing balloon PTCA with medical therapy showed better outcomes for PTCA.9,21–25 However, randomized trials comparing PTCA with CABG revealed dramatically higher re-intervention rates in the PTCA groups and better angina relief in the CABG groups, although there were no significant differences in death or myocardial infarction rates.26,27 The Duke database study9 showed better survival rates with PTCA than with CABG in patients with single-vessel CAD, whereas CABG produced better survival than did PTCA in patients with severe, triple-vessel CAD.
These results are not necessarily representative of the results obtainable today with PTCA and CABG, for several reasons. First, stents were not used in the PTCA patients in these trials. Second, operative mortality rates for the CABG groups were higher than the rates currently found in the Society of Thoracic Surgeons (STS) database.28 Third, the inclusion/exclusion criteria of these studies eliminated a high percentage of those patients who might have benefited more from CABG than from PTCA, instead including only patients for whom PTCA was deemed technically feasible.
The introduction of coronary artery stenting resulted in better outcomes than those produced by balloon angioplasty alone29–33 or by other adjuncts, including rotational atherectomy, brachytherapy, and laser angioplasty.34–36 Since then, stent designs and delivery techniques have advanced considerably. The use of coronary stents has greatly decreased the necessity of emergent CABG for technical failure of PCI and for dissection or rupture of coronary arteries during PCI.37 Another major advance in the application of PCI is the use of the antiplatelet agent clopidogrel in addition to aspirin after PCI, as well as the use of glycoprotein (GP) IIb/IIIa receptor inhibitors during the procedure. These adjuncts have significantly reduced the incidence of acute and subacute thrombosis after PTCA with stenting.38–42
Randomized trials comparing PTCA plus stenting with PTCA alone have shown that stenting significantly reduces rates of restenosis and re-intervention, as well as the frequency of emergent CABG.29–32,43 On the other hand, randomized trials of stenting versus surgery have produced less conclusive results regarding the mid-term survival and freedom from adverse events.44–46 For example, the Stent or Surgery (SOS) trial reported a greater need for repeat revascularization in the stent group (21%) than in the CABG group (6%) and a survival advantage in the CABG group (hazard ratio, 2.91; 95% CI, 1.29–6.53; P = 0.01) during the 3-year follow-up period. Additionally, angina and the use of anti-angina medications were less common in the CABG group at 1-year follow-up.44 The ARTS45 and ERACI II47 trials also reported an increased need for revascularization in the stent groups but did not show a survival advantage in the CABG groups. This was due in part to a higher operative mortality rate in the CABG group than reported in the STS database.28 In the ERACI II trial, for example, the operative mortality rate in the CABG group was 5.7%, as opposed to 0.9% in the angioplasty with stenting group.47 However, like the PCI versus CABG trials mentioned previously, these randomized trials involved a select group of patients with relatively low expected mortality rates and relatively high expected technical success with PCI.
Observational data on patient outcomes and practice patterns collected in retrospective analyses of large patient databases comparing CABG with PCI plus stenting can offer useful information to the clinician. These data show that, because of the greater invasiveness of surgical revascularization, CABG produces greater operative mortality than does PCI. However, in patients with multivessel CAD, the risk-adjusted survival rates at 2.5 years of follow-up are no better for PCI than for CABG.48 Additionally, 3 recent risk-adjusted observational studies showed that the CABG patients had a significant survival advantage at 3- to 8-year follow-up.49–51 Risk adjustment in these studies was crucial, because the CABG patients had significantly more preoperative risk factors than did the PCI patients in each study. For example, before matching or adjustment, the CABG groups in each study included significantly more patients with triple-vessel disease and fewer patients with double-vessel disease than did the PCI groups. However, the currently published studies with mid- to long-term follow-up results do not include the most recent advances in surgical or PCI technology.
The Achilles' heel of PCI is restenosis and the need for repeat revascularization. Stents have decreased the rate of acute and subacute periprocedural thrombosis, but in-stent restenosis remains a problem, particularly with bare-metal stents. The newer, drug-eluting stents (DESs) have improved in-stent restenosis rates, especially in the carefully selected patient populations studied in the early DES trials.2–8 Indeed, in the RAVEL trial,52 the early reports of zero in-stent restenosis compared favorably with the 27% in-stent restenosis rates in the bare-metal stent control group at 6-month follow-up. However, the RAVEL trial excluded patients with lesions longer than 18 mm, ostial targets, calcified or thrombosed targets, or target arteries less than 2.5 mm in diameter.52
The media frenzy that followed the release of these findings created a public demand for these new “miracle” stents that apparently did not re-occlude. Stories of CAD patients refusing conventional PCI and CABG —instead, adding their names to the list of patients waiting for U.S. Food and Drug Administration (FDA) approval of DESs—appeared to change the practice patterns of cardiologists and cardiac surgeons overnight. After the FDA approved the Cordis Cypher™ DES (Cordis Corporation, a Johnson & Johnson company; Miami Lakes, Fla), a few reports of subacute thrombosis and hypersensitivity reactions prompted the FDA to release a public health notification on 29 October 2003.53 Soon, there were calls for class-action lawsuits and the recall of various DES models. Nonetheless, the Boston Scientific Corporation (Natick, Mass) reported, on 30 September 2004, that it had “sold $8.3 million worth of its Taxus drug-coated cardiac stents per day” during that month, according to a report in the Boston Globe.54
The SIRIUS trial had slightly less strict exclusion criteria than did the RAVEL trial, admitting patients with target lesions 2.5 to 3.5 mm in diameter and 15 to 30 mm long, as well as patients with diabetes mellitus (who constituted 26% of the total group).5 The SIRIUS trial also differed from the RAVEL trial in that the reported end-point was in-segment restenosis, rather than in-stent restenosis. The results showed a significant advantage of DESs over bare-metal stents for preventing in-segment restenosis (9.2% vs 32.3%) and target failures (10.5% vs 19.5%), but major adverse cardiac events were more frequent in the DES group than in the bare-metal stent group (3.7% vs 1.0%). Interestingly, the 6-month restenosis rates of the bare-metal stents in the RAVEL and SIRIUS control groups were much higher than the 19% 12-month restenosis rate associated with bare-metal stents in an earlier study comparing bare-metal stents with PTCA.31 In fact, the restenosis rates in the RAVEL and SIRIUS control groups more closely resembled the 40% restenosis rate reported for the PTCA control group in the earlier study.
The practical advantages of DESs over bare-metal stents are evident; nonetheless, we still do not have sufficient mid-term or long-term clinical data to argue that PTCA with DESs is preferable to CABG in “real-world” patients who require revascularization. Although DESs will likely provide better outcomes than bare-metal stents for many patients for whom stenting is indicated, a general extrapolation of existing data to justify the use of DESs in patients for whom CABG is currently indicated is an unsubstantiated leap of faith, because the lesion and patient characteristics that lead to the failure of PCI are multifactorial.
Patients who have lesions with unfavorable characteristics (including long, totally occlusive, branch, small-diameter, calcified, multiple, left main, ostial, and diffuse lesions) are being treated with PCI more often. Patients with diabetes mellitus, multiple lesions, and multiple comorbidities are also undergoing PCI. However, in these types of patients, PCI cannot be expected to routinely produce results as good as those reported in the highly selective clinical trials—especially when PCI is performed by less experienced operators.
Over the last 4 decades, surgical coronary artery revascularization techniques and technology have advanced significantly. As a result, despite an increasingly older and sicker patient population, CABG outcomes continue to improve. For example, the predicted mortality of CABG patients has increased steadily over the past decade, yet observed operative mortality rates have decreased.28 This is partly because advances in preoperative evaluation, including more precise coronary artery and myocardial imaging and diagnostic techniques, have allowed more appropriate patient selection and surgical planning. In addition, preoperative, intraoperative, and postoperative monitoring and therapeutic interventions have made CABG safer, even for critically ill and high-risk patients. Improvements in cardiopulmonary perfusion and careful myocardial protection, as well as the use of off-pump and on-pump beating-heart techniques in selected patients, have also decreased perioperative morbidity and mortality rates.1,55,56
The long-term benefits of CABG with regard to survival and quality of life are dependent on prolonged graft patency. The LIMA-to-LAD bypass, which is now performed in more than 90% of CABG procedures, shows excellent patency in 10- to 20-year angiographic follow-up studies,57–61 setting the gold standard with which other revascularization strategies should be compared. Tatoulis and colleagues60 reported that LIMA-to-LAD grafts had a 97.1% patency rate in patients who underwent angiography for cardiac symptoms. Those authors also found high patency rates at 5-year (98%), 10-year (95%), and 15-year (88%) follow-up. However, there are not yet long-term data on bare-metal stents or DESs, and by the time 10- or 20-year data are available, DESs probably will have been replaced by a newer, more advanced technology.
Because of the reported success of the LIMA-to-LAD bypass, other types of arterial conduits are also being used much more frequently. Conduit selection has become an area of great interest to cardiac surgeons, and conduit studies are expanding our understanding of the mechanisms of graft failure and ways to improve bypass graft patency. For example, studies have shown that patients who undergo CABG with both LIMA and right internal mammary artery (RIMA) conduits have better results than those who undergo CABG with one IMA and one or more saphenous vein grafts.62 The patency rates of RIMA grafts are not equal to those of LIMA-to-LAD grafts,60 probably because of the advantageous outflow characteristics of the LAD. The use of the radial artery is gaining in frequency as an arterial conduit, but its reported patency rates are more variable than those of other conduits, and concerns about potential vasospasm have kept this conduit from universal use.60,63–72 Calcium channel blocking agents may improve the patency of radial artery grafts, but this notion is controversial.72,73
To maximize the odds of long-term graft patency, surgeons carefully harvest the graft as a pedicled or skeletonized conduit using “no touch” techniques. It is important to avoid mechanical or thermal injury that can cause spasm, intraluminal instillation of normal (acidic) saline or overdistention in an attempt to protect the endothelium, and kinking of or tension in the graft or anastomosis. Furthermore, using careful anastomotic technique to avoid excessive turbulence at the anastomosis site will prolong graft patency. The quality of the conduit is crucial; excessively narrow or dilated saphenous vein graft segments and significantly atherosclerotic arterial conduits should be avoided.
Long-term graft patency depends not only on the conduit chosen but also on the target artery and the degree of stenosis proximal to the anastomosis.58,72,74–79 Maintaining flow patterns in the native artery, including residual flow (that is, competitive flow) and outflow, is important to avoid stasis in the graft, turbulence at the anastomosis, and vasospasm, especially in arterial conduits.75,77,80–84 Studies have shown an inverse relationship between the degree of proximal stenosis and graft patency.74,75,77 Additionally, in most studies, using the right coronary artery as a target produces the lowest patency rates, regardless of the conduit chosen.57,58,85–88 Targeting the LAD, on the other hand, produces the highest patency rates.57,58,60,85,86,88 Several characteristics of the target artery also determine graft patency, including the diameter of the target artery, the presence or absence of diffuse disease within the artery, and whether or not the artery requires endarterectomy. This growing body of knowledge about graft patency rates and the mechanisms of graft failure helps cardiac surgeons make the best use of preoperative diagnostic data and intraoperative findings to optimize the CABG procedure for each patient.
Several advances also continue to improve CABG outcomes. Surgeons can avoid atheroembolic events by handling the aorta carefully (with the help of epiaortic echocardiographic imaging) or not at all. They can also improve safety by using aggressive myocardial protection techniques; avoiding the induction of inflammatory mediators; and carefully controlling blood pressure, body temperature, and electrolyte and glucose levels. Off-pump coronary artery bypass (OPCAB) surgery has been shown to benefit certain patients, and new technology has enabled surgeons to perform OPCAB safely and reproducibly.55,89 Studies have shown that OPCAB can allow complete revascularization, and its patency rates are comparable to those of standard CABG.56
The chief disadvantage of CABG relative to PCI is that CABG is more invasive. Although there have been major innovations that have enabled surgeons to perform cardiac surgery (including CABG) less invasively, minimally invasive surgical procedures are useful only if they are at least as efficacious as conventional surgery. New technology is being developed to enhance the evolving field of minimally invasive coronary bypass surgery. For example, thoracoscopic LIMA harvesting (which may be enhanced by robotic assistance) has greatly decreased the need for exten-sive rib-spreading in minimally invasive direct coronary artery bypass (MIDCAB). In that procedure, surgeons perform the LIMA-to-LAD anastomosis directly through a 3.5- to 4-cm incision in the anterior chest without excessive rib-spreading. Totally endoscopic, robot-assisted CABG is currently being performed in some centers, albeit with limited application.90–92 Further technological advances, including newer-generation anastomotic devices, may one day enhance minimally invasive approaches for performing CABG.93,94 Although these innovative techniques are still in their early stages, the early results have been promising.
The application of CABG and PCI need not be mutually exclusive. As PCI technology improves and techniques of LIMA-to-LAD grafting become less invasive, hybrid coronary revascularization is becoming a distinct possibility. For example, a minimally invasive, off-pump, direct LIMA-to-LAD anastomosis can be combined with DES placement in a focal mid-right-coronary-artery lesion in a patient with complex proximal LAD lesions. Hybrid coronary revascularization procedures are currently being performed, with promising early results.95,96 A few centers, including the Texas Heart Institute at St. Luke's Episcopal Hospital in Houston, now have hybrid operating rooms with cardiac surgical and coronary angiographic capabilities that make it possible to perform simultaneous hybrid coronary revascularizations. Staged hybrid revascularizations are performed in standard catheterization laboratories and operating rooms.
In conclusion, DES placement will not replace CABG in the foreseeable future. We can expect continued improvement of PCI technology and a consequent improvement in outcomes. However, CABG technology and techniques also continue to develop rapidly. As PCI and CABG are refined further, surgeons and cardiologists will no doubt learn to use these improved interventional techniques and surgical procedures in a way that will optimize the treatment of each individual patient.
Stephen N. Palmer, PhD, ELS, provided editorial support.
Address for reprints: Ross M. Reul, MD, Department of Cardiovascular and Thoracic Surgery, Texas Heart Institute, MC 3-258, P.O. Box 20345, Houston, TX 77225-0345
Presented at the 14th International Meeting of the Denton A. Cooley Cardiovascular Surgical Society, 6–10 October 2004, Houston, Texas