Abstract
Coronary artery bypass grafting has been performed predominantly with the use of cardiopulmonary bypass and cardioplegic arrest, which allows optimization of the surgical field and consistent placement of grafts. However, the use of cardiopulmonary bypass is also associated with numerous complications. A surgical technique avoiding cardiopulmonary bypass should, in theory, reduce the incidence of such complications and lead to improved patient outcomes. This assumption has rekindled interest in performing off-pump coronary artery bypass surgery, which is currently the focus of scientific scrutiny. The existing world medical literature contains a staggering amount of research related to this technique. Although the available evidence from a large number of randomized clinical trials, nonrandomized clinical trials, propensity-matched analyses, and experimental data suggests that outcomes are better after off-pump than after on-pump coronary artery bypass surgery, skepticism still exists about the safety and efficacy of the off-pump technique. In the present era of evidence-based medicine, results from randomized clinical trials are given the highest recognition. This review attempts to evaluate the best currently available evidence from clinical trials about the safety and efficacy of off-pump coronary artery bypass surgery.
Key words: Coronary artery bypass, coronary artery bypass, off-pump, randomized controlledtrials/methods/mortality/statistics & numerical data/trends
Conventional coronary artery bypass grafting (CABG) using cardiopulmonary bypass (CPB) and cardioplegic arrest has for many years represented the gold standard of coronary revascularization.1 However, the price of a still and bloodless field is ultimately paid by the patient in the form of negative sequelae of CPB, including blood trauma, activation of a series of inflammatory responses, nonpulsatile flow, and possible embolization of air or débris—most particularly embolization of atherosclerotic débris from the aorta.2 The objective of avoiding these deleterious effects of CPB led to the rediscovery of off-pump coronary artery bypass (OPCAB) surgery. Interestingly, since its rediscovery in the late 1990s, OPCAB has been in search of an identity.2 At present, abundant evidence suggests that excellent results can be achieved when cardiopulmonary bypass is avoided.3 However, for nearly a decade, skeptics have regarded OPCAB as a technique associated with intraoperative myocardial ischemia, suboptimal anastomoses, and a protracted learning curve.3
In the current era of evidence-based medicine, the best strategy for countering skepticism and validating the safety and efficacy of a therapeutic method is to analyze the best available evidence.4 Results from double-blind, randomized controlled trials (RCTs) are seen as most persuasive and are regarded as the best approach to ascertain the value of a particular therapy.5 This review article evaluates the best currently available evidence from RCTs in an attempt to validate the safety and efficacy of OPCAB.
Search Methodology
MEDLINE, EMBASE, Cochrane Controlled Trials Register (CCTR), Cochrane Database of Systematic Reviews (CDSR), Database of Abstracts of Reviews of Effects (DARE), Science Citation Index (SCI), Current Contents, NHS Economic Evaluation Database (NEED), and International Network of Agencies for Health Technology Assessment (INAHTA) databases were searched from the date of their inception to the end of December 2004, using the search terms off-pump, minimally invasive, beating heart, coronary artery bypass, and their variants. The search was undertaken in accordance with Cochrane Collaboration recommendations6 and was aimed at finding all published reports of randomized trials comparing OPCAB with conventional CABG. Articles written in English and in languages other than English were included. Tangential electronic exploration of related articles and hand searches of bibliographies, scientific meeting abstracts, and related journals were also performed.
Inclusion Criteria
All blinded or unblinded RCTs, whether comparing OPCAB or minimally invasive direct coronary artery bypass (MIDCAB) on the beating heart with conventional CABG using CPB and cardioplegia, were included if the trial recruited adult patients undergoing single- or multiple-vessel bypass and reported at least 1 pertinent clinical or economic outcome. Studies evaluating the outcomes of hybrid procedures (that is, OPCAB plus balloon angioplasty) and those studies using robotically assisted surgery or circulatory assist devices were excluded.
Data Extraction and Validation of the Studies
The published reports found by the search strategy were then evaluated. Each paper was subjected to a structured analysis using critical appraisal checklists.7 Such checklists are widely available in several formats and are helpful in assessing the methodological and analytical soundness of a trial and in uncovering any serious methodological flaws.7 The following information was extracted from each published report: 1st author, year of publication, characteristics of the study population, inclusion and exclusion criteria, number of patients who underwent surgery with each technique, and key outcomes. Finally, for each outcome or aspect of OPCAB evaluated, a conclusion was formulated on the basis of the validity of the studies identified, taking into consideration the source and the strength of the evidence by using the grading system proposed by the Oxford Centre for Evidence-Based Medicine (Table I).8
Results
Of a total of 3,237 citations screened, 67 published reports of RCTs9–73 that fulfilled the inclusion criteria were retrieved for evaluation. One report35 pooled the analyses of 2 trials. Only the trial by Carrier and colleagues29 recruited high-risk patients; most of the other RCTs excluded high-risk patients. Evidence from these RCTs was evaluated to determine the impact of OPCAB on 12 outcomes: systemic inflammation, blood loss and blood transfusion requirements, acute renal failure, myocardial dysfunction, atrial fibrillation, cerebral dysfunction, pulmonary dysfunction, gastrointestinal complications, length of hospital stay, mortality, cost, graft patency, and quality of life.
Systemic Inflammation
Cardiopulmonary bypass is associated with an intense inflammatory response because of conversion to laminar flow, blood contact with the artificial bypass surface, cold cardiac ischemia, and hypothermia.74 This inflammatory response, which is characterized by the activation of 5 plasma protein systems and 5 types of blood cells, results in the production of an acute, massive defense reaction that produces a consumptive coagulopathy; circulates more than 70 hormones, cytokines, chemokines, vasoactive substances, cytotoxins, reactive oxygen species, and proteases of the coagulation and fibrinolytic systems; induces mild-to-huge interstitial fluid shifts; generates a host of microemboli (<500 μm); and causes temporary dysfunction of nearly every organ.75
Evidence from a large number of RCTs suggests that, despite comparable surgical trauma, the systemic inflammatory response produced by OPCAB and its associated sequelae are much less severe than those produced by conventional CABG using CPB (Table II). The beneficial effect of OPCAB in blunting this systemic inflammatory response may be attributed to the omission of CPB and the avoidance of global myocardial ischemia.22
Blood Loss and Transfusion Requirements
Excessive bleeding is an important cause of morbidity and mortality after CPB.76 Bleeding after cardiac surgery has several causes.77,78 The exposure of blood to synthetic, nonendothelial surfaces causes severe hemostatic defects that inhibit or alter many components of the thrombotic and fibrinolytic systems.79 Because of the associated decrease in hemostatic function and the resultant blood loss, patients undergoing CPB are often given allogeneic blood transfusions.80 Transfusion of allogeneic blood products exposes the patient to additional risks (transfusion reactions, viral transmission, immunosuppression, and increased mortality rates) and increases the cost of the procedure.79 As many as 3% to 5% of patients require surgical re-exploration of the mediastinum because of postoperative hemorrhage.79 Morbidity and mortality rates and hospital costs increase substantially when such re-exploration is necessary.81
A large number of RCTs26–35,62 have shown that OPCAB results in a clinically important decrease in perioperative blood loss. The prospective RCT conducted by Lo and associates24 provided evidence that, compared to OPCAB, the addition of CPB is associated with a substantially more pronounced activation of coagulation and fibrinolysis during the immediate postoperative period. Ascione and coworkers,25 in an RCT specifically designed to investigate the effect of coronary bypass with or without CPB on postoperative blood loss and transfusion requirements, found that the average postoperative blood loss was 1.6 times higher for patients undergoing on-pump CABG than for those undergoing OPCAB. Furthermore, the requirements for fresh frozen plasma and platelets (both P <0.05) and the mean transfusion cost per patient (P <0.01) were significantly lower for the OPCAB patients than for the on-pump patients.25 Avoiding CPB leads to better preservation of hemostasis and less need for allogeneic transfusion.
Acute Renal Failure
Postoperative acute renal failure is a serious complication of CABG and is associated with substantial increases in morbidity and mortality rates.82,83 Depending on the population studied and the definitions used, the reported rates of occurrence of acute renal failure range from 1% to 30%, and the reported risks of death range from 7% to 38%.84 No single mechanism explains the association of renal failure with cardiac surgery; instead, this sequel is caused by a complex interplay of related factors.85 Of these factors, however, renal hypoperfusion and inflammatory damage resulting from CPB are widely regarded as the most important causes of acute renal failure.86,87
To date, 3 small RCTs36–38 have specifically studied the impact of OPCAB on renal function. Loef and associates36 recruited 22 low-risk patients (10 of whom underwent OPCAB and 12, conventional CABG) to an RCT studying the progress and extent of renal damage after bypass surgery. The results showed that OPCAB induced significantly fewer changes in microalbuminuria (P <0.01), free sodium clearance (P <0.05), free water clearance (P <0.01), N-acetyl glucosaminidase activity (P <0.01), and free hemoglobin (P <0.01) than did bypass procedures using CPB. Similar findings were reported by Ascione and coworkers.37 These findings suggested that, for 1st-time coronary bypass patients, OPCAB offers better renal protection than conventional coronary revascularization with CPB and cardioplegic arrest. These findings were contradicted, however, by the results of an RCT conducted by Tang and colleagues,38 who recruited 40 low-risk patients with normal preoperative cardiac and renal function who were awaiting elective CABG. Those researchers found that, although no renal complications or serum markers of kidney dysfunction were present, sensitive indicators detected substantial and similar injury to renal tubules and glomeruli after both conventional CABG and OPCAB. These findings suggest that OPCAB offers no added protection to low-risk individuals against perioperative renal dysfunction; however, the findings may be different for high-risk patients, in whom the avoidance of CPB, thereby eliminating renal hypoperfusion and blunting of the systemic inflammatory response, might prevent progressive renal impairment.88
Myocardial Dysfunction
During CPB, before and after cardioplegia or fibrillatory arrest, the heart, like all other organs and tissues, is subject to microemboli, protease and chemical cytotoxins, activated neutrophils and monocytes, and regional hypoperfusion.89 Both myocardial edema and distention of the flaccid cardioplegic heart during aortic cross-clamping reduce myocardial contractility.90 Furthermore, if myocardial contractility is weak, excessive preloading or high afterloading during weaning from CPB increases ventricular end-diastolic volume, myocardial wall stress, and oxygen consumption.89 All of these factors, coupled with reperfusion injury after ischemia, result in postoperative cardiac dysfunction.
Several RCTs (Table III) have found that the incidence of postoperative myocardial infarction is comparable after OPCAB and CABG with CPB but that OPCAB is associated with a more rapid recovery of myocardial oxidative metabolism, better myocardial function, and a lower requirement for inotropic agents. In a recently published study, Selvanayagam and coworkers39 reported the results of an RCT using preoperative and early postoperative contrast-enhanced cardiovascular magnetic resonance imaging (MRI) for the assessment of irreversible myocardial injury and cine MRI for the assessment of global left ventricular function. The results showed that OPCAB is associated with significantly better left ventricular function early after surgery than is CABG with CPB (P = 0.04) but does not reduce the incidence or extent of irreversible myocardial injury. Avoiding aortic cross-clamping and CPB during OPCAB minimizes the risks of global myocardial ischemia and myocardial stunning, as well as the potentially damaging effects of cardioplegia.
Atrial Fibrillation
Atrial fibrillation (AF), the most common complication of CABG, occurs in 20% to 40% of patients.91 Although the cause of postoperative AF is multifactorial, and the exact pathogenesis of postoperative AF among patients undergoing CABG is still not understood, a large prospective RCT by Ascione and colleagues50 showed that CPB with cardioplegia is the main independent predictor of postoperative AF for patients undergoing coronary revascularization. The “post-pump syndrome” associated with CPB is thought to precipitate AF in patients who have an identifiable electrophysiologic substrate.92
By avoiding the atrial myocyte alterations that result from aortic cross-clamping and avoiding the systemic inflammatory response to CPB, OPCAB should decrease the incidence of postoperative AF. To date, 4 meta-analyses have been performed to evaluate the impact of OPCAB on the incidence of atrial fibrillation,93–96 and all found that the incidence of postoperative AF is substantially lower after OPCAB. It is, however, important to point out that only the meta-analysis by Raja and co-authors93 included solely RCTs. Angelini and colleagues35 performed a pooled analysis of the Beating Heart Against Cardioplegic Arrest Studies (BHACAS 1 and 2) and found that the incidence of AF decreased from 37% to 13% with OPCAB, which was a highly significant finding.
Cerebral Dysfunction
Cerebral injury, an important cause of morbidity and mortality after cardiac surgery, occurs in 2 distinct forms.97 Overt injury, usually a stroke, occurs in 3% of patients undergoing CABG. Injury leading to cognitive impairment, which is detected only by detailed neuropsychologic testing, occurs in as many as 80% of patients soon after surgery and persists in one fourth of these patients 6 months after surgery.97–99 Early postoperative cognitive impairment correlates with a later progression of cognitive decline and impaired quality of life.100 Although the pathogenesis of cerebral injury and cognitive dysfunction after cardiac surgery is multifactorial, there is increasing evidence that multiple microemboli arising from the ascending aorta, the heart chambers, or the bypass circuit are the primary pathophysiologic mechanisms producing diffuse ischemic cerebral injury.101
One can hypothesize that results should be improved by eliminating the source of gaseous and particulate emboli, namely the extracorporeal circuit. In fact, cerebral microembolization is markedly reduced when CPB is avoided.51,102,103 However, reducing cerebral microembolization by avoiding CPB has not resulted in a statistically significant reduction in the incidence of stroke and postoperative neurocognitive dysfunction (Table IV). Available evidence from the RCTs does not unequivocally show that OPCAB is better than conventional CABG with respect to occurrence of stroke and postoperative neurocognitive dysfunction,26,28–35,44,52,54,56,58,59,62 although OPCAB substantially reduces the release of s100 protein and neuron-specific enolase (NSE).53–55,57 The comparable incidence of postoperative stroke after OPCAB may be attributable to aortic manipulation related to the use of a side-biting clamp that is applied during the construction of the proximal anastomoses.104 The adoption of the aortic “no touch” technique, which avoids intraoperative atheromatous embolization from the atherosclerotic aorta into the cerebral circulation, may improve neurologic outcomes after OPCAB.105
Pulmonary Dysfunction
Postoperative pulmonary dysfunction is an important cause of morbidity after surgery with CPB.106 Although severe postoperative acute respiratory distress syndrome is now rare, substantial impairment of pulmonary function does occur and leads to derangements of gas exchange and prolonged extubation times.107
The occurrence of pulmonary impairment after cardiac surgery is believed to be multifactorial. Some of the contributors to the development of lung dysfunction include atelectasis, increased intrapulmonary shunting, and alterations in lung and chest wall mechanics.108 Surgical factors secondary to sternotomy, pleurotomy, and harvesting of the internal mammary artery have also been implicated.107 In addition, it is well accepted that CPB induces an inflammatory response that produces increased pulmonary endothelial permeability, parenchymal damage, and changes in the composition of alveolar surfactant.109,110
It can be hypothesized that avoiding CPB should lead to less postoperative pulmonary dysfunction, because the CPB-induced systemic inflammatory response will be absent. However, the results of several RCTs (Table V) have produced conflicting evidence in this regard. Most RCTs that have recruited low-risk patients have shown that OPCAB definitely reduces the duration of ventilation and allows earlier extubation of OPCAB patients, yet its effects on respiratory mechanics, oxygenation, and effort of breathing are similar to those in patients undergoing CABG with CPB. On the other hand, the benefits of OPCAB are more pronounced in patients with preexisting pulmonary disease. In these patients, adverse effects on the alveolar stability by activation of the complement cascade, sequestration of the neutrophil in the pulmonary microvascular bed, release of oxygen-derived free radicals, and changes in the composition of alveolar surfactant are not seen, because CPB is avoided.65,66
Gastrointestinal Complications
Gastrointestinal (GI) complications occur in approximately 2.5% of patients undergoing cardiac surgery, are associated with a high mortality rate (about 33%), and account for approximately 15% of all postoperative deaths.111 Most of the GI complications that occur after CABG have been attributed to low cardiac output and visceral hypoperfusion, which result in mucosal ischemia and necrosis. Cardiopulmonary bypass reduces mucosal blood flow, leads to mesenteric sequestration of neutrophils, and stimulates the systemic inflammatory response.112 Severe intestinal mucosal ischemia can occur during CPB even when the indices of global perfusion remain normal. Factors released during CPB, such as vasopressin, catecholamines, and thromboxane A2 and B2, lead to redistribution of blood flow away from the mucosa because of regional vasoconstriction, and may contribute to mucosal ischemia.113
Velissaris and colleagues69 recruited low-risk patients for an RCT evaluating global oxygen flux and gastric mucosal oxygenation during CABG with and without CPB. Their findings suggested that both procedures are associated with a similar degree of gastric mucosal hypoxia but that the postoperative trends are worse for patients who undergo OPCAB. However, their study did not investigate whether these find-ings relate to the incidence of GI complications after OPCAB or after surgery with CPB; nor did the researchers evaluate whether these findings are applicable to high-risk patients.
To date, only 1 RCT has specifically investigated the incidence of GI complications in similar cohorts of patients: 150 patients (115 men; mean age, 64; range, 45–75 years) undergoing CABG with CPB and cardioplegia, and 150 patients (114 men; mean age, 64; range, 38–66 years) undergoing OPCAB.70 None of the 300 patients had previously undergone abdominal surgery. The overall incidence of GI complications was 4% (11 patients in the on-pump group and 1 patient in the OPCAB group; P <0.0001). Nine (6%) patients in the on-pump group and none in the OPCAB group died. The on-pump patients who experienced GI complications had undergone significantly prolonged periods of both CPB (192.4 ± 83.2 min for patients with GI complications and 112.1 ± 49.6 minutes for patients with no such complications; P <0.0001) and aortic cross clamping (105.3 ± 45.6 min for patients with GI complications and 68.2 ± 35.6 min for patients with no such complications; P <0.0001). The lower incidence of GI complications associated with OPCAB may have occurred because this procedure avoids nonocclusive mesenteric ischemia due to a low cardiac output state or long pump duration, coupled with a reduced systemic inflammatory response.
Length of Hospital Stay and Mortality Rates
The length of hospital stay and the incidence of death after CABG is a function of multiple variables primarily related to a patient's premorbid condition and the morbidity associated with the procedure. Three large meta-analyses96,114,115 have reported that the mortality rate after OPCAB is comparable with that seen after CABG and CPB but that the duration of hospital stay is substantially reduced in patients who undergo OPCAB. Two of these meta-analyses114,115 pooled the results of RCTs only. Similar results were later found by several RCTs.30,32,62,73 By reducing the morbidity rates associated with the procedure and decreasing the incidence of complications, the use of OPCAB has decreased the length of hospital stay.
Cost Savings
The current emphasis on cost containment in health care has stimulated interest in the economics of surgical procedures. Resource utilization is an even more important issue when a new technology is introduced. To date, 5 RCTs have compared the economic outcomes of OPCAB with those of conventional CABG and CPB.32,34,61,71,73 All of these trials have found that OPCAB is less expensive than conventional CABG because of lower complication rates, shorter intubation times, and shorter stays in the ICU and hospital for OPCAB patients.
Graft Patency and Quality of Anastomosis
Reliable graft patency has been the foundation of surgical therapy for coronary artery disease. The goal of off-pump bypass grafting is to minimize procedural complications and patient trauma without compromising the outcomes traditionally achieved by surgical revascularization. One of the main concerns about OPCAB is that performing vascular anastomoses on the small arteries of a beating heart compromises graft patency and results in incomplete myocardial revascularization.3 Four RCTs have evaluated graft patency after OPCAB.27,31,61,73 The 3 larger trials31,61,73 showed no significant differences in patency at various time points. Khan and associates27 found decreased patency 3 months after OPCAB and thus revived the debate about the impact of OPCAB on early and long-term graft patency. However, a closer analysis of this study27 shows that the limited experience of the surgeons, coupled with the intraoperative administration of a relatively low dose of heparin, the absence of aggressive postoperative antiplatelet therapy with clopidogrel, and the failure to use new suction devices to optimize exposure, may have confounded the analyses of graft patency among patients who underwent off-pump procedures.116,117 Furthermore, the design of the study by Khan's group27 did not confer sufficient statistical power to detect differences in graft patency. The study can also be criticized for the nature of its statistical analysis, which appears to be post hoc and parsimonious.1
Quality of Life
Measures of quality of life (QOL) reveal the patient's perception of the burden of disease.118 Because patients are interested not only in survival and relief of angina but also in the resumption of their daily activities, a comprehensive assessment of these aspects of QOL is important. A number of studies have reported improvements in both the physical and mental functional health status of patients after coronary revascularization.119–121 Four RCTs60,61,72,73 have compared patients' reports of QOL after OPCAB and after conventional CABG. Puskas and colleagues73 administered questionnaires to patients, both the EuroQOL-6 and the Short Form-36, 30 days and 1 year postoperatively. That group found that the patients' perception of their QOL improved similarly in the OPCAB group and in the conventional CABG group over time. Van Dijk and coworkers60 and Nathoe and associates61 administered the EuroQOL questionnaire (original version) at baseline and at 1, 3, 6, and 12 months postoperatively and found similar perceived improvements in QOL between the 2 groups over time. In their reports of the Beating Heart Against Cardioplegic Arrest Studies 1 and 2, Ascione and co-authors72 stated that the scores of both groups of patients, calculated from the EuroQOL-5 and Short Form-36 at a median follow-up of 3 years, were very similar and did not differ significantly in any of the dimensions evaluated.
Conclusion
There has been a resurgence of interest in OPCAB during the last decade, and the advantages associated with the use of this technique have been repeatedly emphasized. However, controversies and skepticism still exist concerning the benefits of OPCAB in comparison to those of conventional CABG. An analysis of the best available existing evidence from RCTs clearly suggests that, compared to conventional CABG, OPCAB is associated with lower morbidity rates, similar graft patency, and substantially lower costs (Table VI). In the current era of evidence-based medicine, the rapid acceleration of catheter-based procedures and closer scrutiny of the morbidity associated with CPB appear to threaten the future of coronary artery surgery. The future of such procedures is extremely dependent on surgeons' ability to adapt quickly to change and innovation. Because the safety and efficacy of OPCAB have now been validated, the time has come for its universal adoption as a primary technique for surgical myocardial revascularization.
Footnotes
Address for reprints: Shahzad G. Raja, MRCS, Department of Cardiac Surgery, Royal Hospital for Sick Children, Yorkhill NHS Trust, Dalnair Street, Glasgow G3 8SJ, United Kingdom
E-mail: drrajashahzad@hotmail.com
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