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Ann Thorac Surg 2003;76:1510-1515
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Meta-analysis of short-term and mid-term outcomes following off-pump coronary artery bypass grafting

^a Department of Health Technology Assessment, ECRI, Plymouth Meeting, Pennsylvania, USA

Accepted for publication June 30, 2003.

^* Address reprint requests to Dr Turkelson, 5200 Butler Pike, Plymouth Meeting, PA 19462-1298, USA.
e-mail: cturkelson{at}ecri.org

Abstract

BACKGROUND: Uncertainty continues to surround the relative benefits and harms of conventional coronary artery bypass grafting (CABG) and off-pump coronary artery bypass grafting (OPCABG). Possible reasons are that high-quality studies have not comprehensively examined relevant patient outcomes and have enrolled a limited range of patients. Some studies may have been too small to detect clinically important differences in patient outcomes. The present study addresses these issues using meta-analysis.

METHODS: We comprehensively retrieved randomized and nonrandomized controlled studies according to predetermined criteria. We performed meta-analyses for each outcome and empirically determined whether potential biases that might result from differences in study design or patient characteristics actually biased a study's results. We also conducted sensitivity analyses and tested for publication bias.

RESULTS: Rates of perioperative myocardial infarction, stroke, reoperation for bleeding, renal failure, and mortality were lower after OPCABG than after CABG. Reductions in length of hospital stay, atrial fibrillation, and wound infection were also associated with OPCABG, but statistically significant differences among study results for these outcomes could not be explained by available information. Midterm (3 to 25 months) angina recurrence did not appear to differ between treatments; a trend was noticed toward lower reintervention rates with CABG, and a trend toward lower overall mortality with OPCABG, at least when performed at experienced centers. These midterm outcome results require confirmation.

CONCLUSIONS: Off-pump coronary artery bypass grafting appears to reduce length of hospital stay, operative morbidity, and operative mortality relative to on-pump CABG. More studies are required before firm conclusions can be drawn concerning the effect of OPCABG on midterm mortality, angina recurrence, and repeat intervention.

Off-pump coronary artery bypass grafting (OPCABG) is increasingly being used as an alternative to conventional CABG with cardiopulmonary bypass. Despite this practice and the fact that a few randomized controlled trials (RCTs) have compared these procedures, uncertainty remains concerning their relative benefits and harms [1–3]. Possible reasons for this uncertainty are that existing RCTs have not comprehensively studied all relevant patient outcomes, have enrolled a limited range of patients, and some may have been too small to detect clinically important differences. The potential for publication bias (nonpublication of studies that find no statistically significant difference between OPCABG and CABG), and the fact that most of the published data are from retrospective studies, further compound the difficulties in comparing these two procedures.

We used a series of meta-analyses to address two main issues. First, meta-analysis provides additional statistical power to overcome the problem that most published studies may have been too small to find statistically significant differences for some outcomes, particularly those that are relatively uncommon (eg, stroke). We also used meta-analysis to determine empirically whether differences in study design or quality may have resulted from biases in studies of less rigorous design. If we found evidence for bias due to study design, we based our results only on the studies of "superior" design (eg, randomized or prospective trials). If no evidence of bias was found, we included all studies in the meta-analysis.

Although some investigators include all off-pump procedures under the term OPCABG, in this report we consider OPCABG to include only those off-pump procedures performed through a full median sternotomy. We did not evaluate minimally invasive direct off-pump coronary artery bypass grafting performed through a thoracotomy or alternative small incisions (commonly referred to as MIDCABG).

Material and methods

Study selection
We included studies in our analysis only if they met certain a priori inclusion criteria. They had to be controlled studies that compared OPCABG and CABG; they had to report results obtained from patients receiving OPCABG through a sternotomy separately from results of patients receiving related procedures (eg, MIDCABG); and they had to include at least 15 patients in each treatment arm (we adopted this criterion because almost all of the morbidity outcomes we evaluated have occurrence rates below 10%). Studies that used pharmacologic stabilization of the heart were excluded. Studies had to report patient-oriented outcomes (such as mortality or various types of morbidity). Studies had to report whether their study population consisted of patients with single-vessel disease, multivessel disease, or a mix of both. When different studies reported results for the same group of patients, only the most recent and most comprehensive publication was included to avoid double counting of patients.

Trial identification
We searched PubMed and EMBASE (through January 2003) for identification of relevant studies, and 19 other databases were searched for additional information [1]. In addition to searching Current Contents–Clinical Medicine on a weekly basis, we routinely reviewed more than 1,000 journals and supplements maintained in ECRI's collections. We reviewed 180 published studies; of these, 40 lacked a conventional CABG control group. Of the remaining 140 studies, 87 did not meet our other inclusion criteria or were earlier versions of studies that we included. Fifty-three studies met our criteria for inclusion in this report.

Key outcomes
We evaluated short-term (30 days or less) and midterm (3 to 25 months) outcomes in this report. Short-term outcomes included length of hospital stay, operative mortality, and the following operative morbidities: myocardial infarction (MI), stroke, reoperation for bleeding, atrial fibrillation (AF), renal failure, and wound infection. Midterm outcomes included need for reintervention with percutaneous transluminal coronary angioplasty (PTCA) or CABG, angina recurrence, and overall mortality.

Statistical analysis
We performed fixed effects meta-analyses using odds ratios for dichotomous outcomes and weighted mean difference for the one continuous outcome (length of stay) that we examined [4]. If a study reported multiple key outcomes it was included in multiple meta-analyses. When possible, we calculated odds ratios using exact methods in the software package StatXact [5, 6]. To determine whether potential moderator variables might have affected the results of our meta-analyses, we tested for heterogeneity using either Zelen's exact test of homogeneity of odds ratios or the Q statistic. Where the dataset was too large for the Zelen test, a Monte Carlo estimate of the exact p value was obtained [5]. In the present analysis, a statistically significant p value from these tests suggests that variations in study quality or differences in patient characteristics may have affected the results of some studies in our analysis. Because the Q statistic is conservative [7], we adopted a p value of 0.10 as the critical value for statistical significance when using this test.

We explored sources of heterogeneity using meta-regression. Due to incomplete reporting in published articles, we could not test whether certain patient characteristics were correlated with differences in study results. However, we were able to test whether such differences were correlated with study design. Thus, the variables that we tested include whether a study was randomized or prospective, whether all patients had multivessel disease, and whether between-group differences in patient risk factors might favor better results for OPCABG or CABG. Two analysts (blinded from each other) reviewed each study and separately determined whether a study might favor OPCABG or CABG; disagreements were resolved in conference. If differences in risk factors "explained" why the results of some studies differed from the results of others, then studies that favored one treatment were removed and only the remaining studies were included in the meta-analysis.

We used the Q_E statistic to determine whether a statistically significant amount of variance was explained by our meta-regressions. The Q_E test of meta-regressions is equivalent to the Q test for meta-analyses, so we used a p value of 0.10 as the critical value for this statistic.

To test the robustness of meta-analytic results, we performed sensitivity analyses on meta-analyses by removing studies in which the largest (or smallest) effect was found; the study that enrolled the most patients; studies that observed no events (and studies of similar or smaller size); and studies that potentially favored one treatment or another (based on between-group differences in patient risk factors). Additionally, we performed a random-effects meta-analysis. We also used random-effects models when statistically significant heterogeneity was not "explained" by meta-regression.

To address the possibility that some studies with statistically nonsignificant results were not published, we tested for publication bias using the "trim and fill" method of Duval and Tweedie [8]. This method estimates the number of unpublished studies and the magnitudes of their effects based on the degree of asymmetry in a funnel plot. After adding "missing" studies to the meta-analysis, the overall effect size is reestimated to determine whether the results of the initial meta-analysis were replicated. Because publication bias tests are appropriate only when the results of a meta-analysis are not heterogeneous, we used the trim and fill method only under this circumstance.

Results

We identified 53 studies that met the inclusion criteria [1, 9–60]. They comprise 10 RCTs, five prospective controlled studies, and 38 retrospective controlled studies. Eighteen studies were from the United States and 35 were from non-U.S. centers. Collectively, these trials enrolled 46,621 patients who received OPCABG.

Patient selection criteria differed somewhat among the individual studies. Most studies (55%) included patients with single and multivessel disease, whereas the remaining 45% included only patients with multivessel disease. Five studies (9%) included only patients receiving elective surgery. Patient exclusion criteria were reported in 36 studies (68%). The most frequently reported exclusion criteria were repeat operation (26%), renal dysfunction (21%), emergency operations (15%), low ventricular ejection fraction (13%), and prior stroke or ischemic attack (11%).

The results of meta-analyses of short-term and midterm outcomes of studies that compared OPCABG to conventional CABG are summarized in Table 1. Among short-term outcomes, OPCABG showed a statistically significant benefit compared with conventional CABG in reduced rates for MI, reoperation for bleeding, stroke, renal failure, and operative mortality (results of the meta-analyses of MI and reoperation for bleeding were adjusted to homogeneity by removing studies that favored OPCABG). Neither our sensitivity analyses nor the test for publication bias overturned the results of these meta-analyses.

Among midterm outcomes (3 to 25 months after surgery), reintervention with PTCA was lower among patients receiving CABG (Table 1). Sensitivity analysis reduced this effect to statistical nonsignificance. The initial meta-analysis of angina recurrence found statistically significant heterogeneity, subsequent meta-regression analysis did not explain it, and the results of a random-effects model were nonsignificant (Table 1). The initial meta-analysis of overall mortality showed a statistically significant difference favoring OPCABG (Table 1). Sensitivity analysis rendered this effect nonsignificant. No results from (homogeneous) meta-analyses were overturned by the test for publication bias.

Randomized controlled trials are commonly assumed to yield less biased results than nonrandomized studies. Whether this assumption is correct depends on whether the results of RCTs and non-RCTs differ. If no such difference is observed, then claiming that a presumed bias has influenced the results of any given study is difficult.

No statistically significant differences were observed between the results of RCTs and nonrandomized studies for nine of 10 outcomes evaluated in this report (only one RCT reported information on renal failure). Furthermore, RCTs showed a larger effect (favoring OPCABG) than non-RCTs for seven of 10 outcomes, including one outcome (AF) that was significantly different between study designs (p = 0.009). Again, one would assume that if the non-RCTs were biased in favor of OPCABG, they would report larger effects than RCTs, yet this was not the case in the present dataset. This lack of a consistent bias in non-RCTs agrees with the findings of other studies [61, 62].

Comment

The present study suggests that OPCABG reduces length of hospital stay, operative morbidity, and operative mortality as compared with on-pump CABG, although these findings may not be generalizable beyond experienced centers. The evidence does not suggest that results of our analyses were biased by including nonrandomized studies. More studies are required before firm conclusions can be drawn concerning the effect of OPCABG on midterm mortality, angina recurrence, and repeat intervention.

Unlike the present report, a recent multicenter RCT (the Octopus trial) did not find a statistically significant difference between OPCABG and CABG for short-term morbidity and mortality [2]. This trial did not meet our inclusion criteria because 8% of patients in the OPCABG group received a MIDCABG procedure. However, the Octopus trial lacked the necessary statistical power to detect a difference in outcomes with low event rates (including mortality and most types of morbidity). It also enrolled a relatively low-risk patient group; most had one- or two-vessel disease and patients with poor left ventricular function were excluded. Most studies included in our assessment did not exclude such patients, and about half did not include patients with single-vessel disease. Higher-risk patients may be more likely to benefit from OPCABG. Three large retrospective studies that did not meet our inclusion criteria (all included patients who received thoracotomies) found results similar to ours. All found reduced morbidity with OPCABG and two also found reduced mortality [63–65].

One ongoing concern is the learning curve for surgeons associated with performing OPCABG. However, recent reports have shown that a switch to OPCABG is not automatically associated with higher adverse event rates during the learning curve [37], and that off-pump procedures can be safely taught to supervised surgical trainees [66].

Due to the technical difficulties of graft harvesting and anastomosis construction on a beating heart, a concern with OPCABG involves long-term graft patency. A 10-year graft patency rate of 95% or higher associated with left internal mammary artery to left anterior descending coronary artery grafts performed with conventional CABG was reported in one study [67]. However, this rate has not been substantiated by other studies and is lower for saphenous vein grafts. Some have recommended that angiography be used during or immediately after beating-heart surgery to verify that the graft is providing normal blood flow, so problems can be promptly detected and fixed [68]. An early postoperative graft patency rate of 98.8% for OPCABG procedures was reported by one retrospective study evaluated in this report [50]. Midterm graft patency was evaluated in only a small percentage of patients in one retrospective study evaluated in this report [13].

An indirect indication of graft patency is need for revascularization with PTCA or CABG. Evidence from our meta-analysis of seven studies (three RCTs, four retrospective studies) suggested that patients who received OPCABG may be more likely to need reintervention at 6 to 25 months after surgery, although more studies are needed for confirmation. This outcome may reflect the learning curve associated with beating-heart surgery. This need for reintervention, if confirmed, could partially offset the early in-hospital cost savings attributed to OPCABG. However, as a surgeon's experience increases the need for reintervention is expected to decrease.

Several newer technologies may reduce the technical difficulty of beating-heart surgery. These tools include mechanical couplers, vascular clips or staplers, robot-assisted suturing techniques, and biological glues. Robotic surgical systems provide an expanded surgical field of view, allowing easier surgery through small incisions [69].

Off-pump coronary artery bypass grafting is becoming a competing technology for conventional CABG with cardiopulmonary bypass. Of all CABG procedures performed during the year 2000, 25% to 30% were beating-heart procedures; most of these were OPCABG procedures [70]. However, certain limitations of OPCABG still leave conventional CABG the preferred treatment for certain patients, such as those with diffusely diseased, calcified, or buried coronary vessels. Despite such limitations, continued technological improvements and new procedural modifications will likely increase the percentage of bypass procedures performed by the off-pump method.

Acknowledgments

This study was internally funded by ECRI, a nonprofit, independent healthcare research organization and a collaborating center of the World Health Organization. We thank Dr Jonathan Treadwell for helpful comments during the course of this project.

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