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Ann Thorac Surg 2005;80:2121-2125
© 2005 The Society of Thoracic Surgeons
a Department of Cardiac Surgery, Centro Cardiologico Monzino IRCCS, University of Milan, Milan, Italy
b Biostatistics Unit, Centro Cardiologico Monzino IRCCS, University of Milan, Milan, Italy
Accepted for publication May 9, 2005.
* Address correspondence to Dr Parolari, Department of Cardiac Surgery, University of Milan, Centro Cardiologico Monzino IRCCS, Via Parea, 4, 20138, Milan, Italy (Email: aparolari{at}cardiologicomonzino.it).
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Abstract |
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METHODS: A literature search for the period beginning January 1990 until December 2004 supplemented with manual bibliographic review was performed for all peer-reviewed English-language publications. A systematic overview (meta-analysis) of randomized trials was conducted to assess differences between OPCAB and coronary artery bypass grafting in graft occlusion rates.
RESULTS: Literature search yielded five comparable randomized studies, for a total of 872 and 998 grafts performed during OPCAB and coronary artery bypass grafting procedures, respectively. Meta-analysis of these studies showed an increased risk of graft occlusion in the OPCAB group of patients, both when all the studies were analyzed together (odds ratio, 1.51; 95% confidence intervals, 1.15 to 1.99; p = 0.003), and when low-quality (odds ratio, 1.46; 95% confidence intervals, 1.05 to 2.03; p = 0.02) and high-quality (odds ratio, 1.65; 95% confidence intervals, 0.99 to 2.75; p = 0.05) studies were analyzed separately.
CONCLUSIONS: Cumulative analysis of the few prospective randomized studies currently available in the literature documents a reduction in postoperative patency of coronary artery bypass grafts performed during OPCAB procedures. The risk of reduced graft patency needs to be considered when choosing OPCAB as tailored strategy for selected patients.
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Introduction |
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Even if many studies comparing OPCAB with CABG are now available, no clear-cut conclusion on the clinical benefit of off-pump surgery has been reached. The majority of trials are retrospective or observational. Moreover, nonrandomized comparisons of single-center data may increase the risk of biases in favor of either technique. The few available randomized trials, indeed, do not have enough power in terms of clinically relevant outcomes. Recently, to detect differences in clinical outcomes, the published randomized data have been aggregated through meta-analysis, showing potential advantages of the off-pump technique [2–5].
An important question, still unanswered, is whether or not the patency of grafts is affected by the technique used. Again, evidence coming from randomized studies, in terms of number of patients enrolled and number of patients undergoing postoperative graft angiography, is still limited. Thus, in this study we have analyzed graft patency in off-pump and on-pump procedures by meta-analysis of comparable published randomized trials comparing OPCAB and CABG.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentReferences |
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All peer-reviewed studies published in English that reported postoperative graft patency data in OPCAB versus CABG were identified. To assess the quality of the search strategy, we sampled three studies that we already had collected and considered relevant for that topic; the search strategy was able to identify these articles. All titles and abstracts of the identified articles were reviewed by two investigators (A.P., F.A.) to determine potential eligibility for subsequent analysis.
The first step of the analysis was to collect all the trials conforming to the following criteria: prospective randomized studies comparing CABG and OPCAB; data about graft patency assessed after the discharge of the patient from the hospital reported in the study; graft patency assessed with coronary angiography.
Authors of all included trials were also asked (e-mail and express mail, sent on January 16, 2005) to provide additional data about their study by answering a simple questionnaire to obtain additional information about the location of occluded grafts and the type of conduit used, and to classify the occluded grafts following the intention-to-treat principle or based on the real operative technique (OPCAB versus CABG) used. In case of no response, available data published in each study were independently abstracted onto study data forms by two authors (A.P., F.A.), and disagreements were solved by consensus.
Data abstracted included the incidence of postoperative (after discharge from the hospital) occlusion of coronary artery bypass grafts, and were analyzed by means of RevMan 4.2.7 (RevMan 4.2.7, The Cochrane Review Manager, 2004; The Cochrane Collaboration, http://www.cochrane.org, Oxford, UK) using both the fixed-effect and random-effect models. In a fixed-effect model, it is assumed that the effect of treatment in each study is the same, whereas in a random-effect model, it is assumed that the studies are not all estimating the same treatment effect (eg, there is variation among studies). Heterogeneity was assessed with the Q test.
Two authors (A.P., Y.B.C.) also autonomously assigned each trial a Jadad quality score that assesses randomization, blinding, and completeness of follow-up [6], and disagreements were solved by consensus. High-quality studies were defined as follows: (1) studies with a Jadad score 
3; and (2) data analyzed and reported in the paper according to the intention-to-treat principle, or, in case of patency data reported not in accordance to the intention-to-treat principle, if the authors completed the questionnaire providing patency data following the intention-to-treat principle. Based on study quality, separate analyses were performed for studies with low or high quality, as well as a cumulative analysis of all studies.
Results are reported as odds ratio (OR) with 95% confidence intervals (95% CI). An OR of 1.0 suggests that there were no differences in postoperative graft patency between OPCAB and CABG, whereas an OR less than 1.0 indicates that the occurrence of postoperative graft occlusion was lower in the OPCAB group; on the contrary, an OR greater than 1.0 indicates the occurrence of postoperative graft occlusion was higher in the OPCAB group.
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Results |
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Of these, five matched selection criteria [7–11], for a total of 872 grafts performed during OPCAB and 998 grafts done during CABG procedures; in high-quality studies (n = 3), 459 and 512 grafts were studied in OPCAB and CABG groups, respectively, whereas in low-quality studies (n = 2), 413 and 486 grafts were studied in OPCAB and CABG groups, respectively. The clinical features of included trials are reported in Table 1; patency rates were assessed between 3 and 12 postoperative months.
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All the authors of the five selected studies were mailed a letter and a questionnaire (see methods section). The authors of one of the studies [11] refused to provide additional data, whereas the authors of the other studies [7–10] did not answer to either mail.
Meta-Analysis
Evaluation of each selected study showed that only one of the five detected a statistically significant difference between OPCAB and CABG in the incidence of postoperative graft occlusion [7]. When taking all these studies into account, the incidence of postoperative graft occlusion was 16% (140 of 872) in OPCAB and 12% (122 of 998) in CABG patients.
No statistically significant heterogeneity was found for the cumulative analysis of all the studies, or for the analysis of high-quality studies; on the other hand, statistically significant heterogeneity was found for the analysis of low-quality studies. For this reason both fixed-effect and random-effect models were tested.
Meta-analysis using the fixed-effect model showed an increased risk of graft occlusion in the OPCAB group of patients both when all the studies were analyzed together (OR, 1.51; 95% CI, 1.15 to 1.99; p = 0.003), and when low-quality (OR, 1.46; 95% CI, 1.05 to 2.03; p = 0.02) and high-quality (OR, 1.65; 95% CI, 0.99 to 2.75; p = 0.05) studies were analyzed separately (Fig 1). Using the random-effect model for all the studies the OR was 1.65 (95% CI, 1.08 to 2.53; p = 0.02). Funnel plot showed no clear evidence of publication bias.
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Comment |
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On the other hand, owing to the obvious difficulties in conducting prospective randomized trials in this field and to the consequent low number of patients that can be enrolled, the statistical power of randomized studies has been low, especially because the outcomes of major interest (mortality, stroke, myocardial infarction) have a low incidence in the average patient population undergoing coronary artery bypass grafting. In fact, randomized controlled trials tend to exclude higher risk patients, the most likely to benefit from the OPCAB approach. Thus, to provide additional statistical power to compare outcomes in OPCAB and CABG [2–5], some authors have carried out meta-analysis of available data. Even if a statistically insignificant trend favoring the off-pump approach emerges from these studies, the evaluation of the hard end points, eg, mortality, stroke, myocardial infarction, and renal failure, however, does not provide a clear-cut, statistically significant advantage to either surgical approach. Interestingly, off-pump procedures seem to be associated with a reduced incidence of postoperative atrial fibrillation rates and donor blood transfusions, and with a shorter hospital stay [3–5].
Some randomized studies have recently addressed the question of whether or not a more demanding surgical technique (OPCAB) can provide similar graft patency rates with respect to standard surgical therapy (CABG). As previously mentioned for hard outcomes assessment, the number of patients enrolled, as well as the number of anastomoses performed and subsequently restudied with angiography, did not reach sufficient statistical power to detect possible differences in patency among these groups. Also, recent evidence coming from a nonrandomized retrospective risk-adjusted study suggests that OPCAB might be associated with increased need of revascularization procedures, percutaneous and surgical, at follow-up [27].
The meta-analysis here reported shows that patients undergoing OPCAB have a definite risk of increased rates of postoperative graft occlusion and questions the durability of revascularization of this technique: although the absolute risk of graft occlusion within the first year was moderately low in both CABG and OPCAB (12% versus 16% of the studied grafts were occluded at angiographic follow-up), in relative terms the off-pump approach significantly increased this risk, with the odds of graft occlusion 51% higher (95% confidence limits, 15% to 99%).
There are several reasons for the increased likelihood of graft occlusion in the OPCAB group: OPCAB is more technically demanding than CABG and this may have contributed to lower patency rates; OPCAB surgery may have altered the location of distal anastomoses in less favorable coronary portions with respect to CABG; or other technologies more commonly used in OPCAB (intraluminal shunts, proximal anastomoses connectors) may have adversely influenced graft patency.
In addition to these technical reasons, it has been shown that the early months after CABG procedures is when patients are at the highest risk for bypass occlusions, the occurrence of which can reach 30% a few months after surgery [28]. As this parallels a prothrombotic state that lasts up to 2 months after CABG surgery [29], it is possible that differences in this state might affect differently the graft patency in OPCAB and CABG [30]. Although a recent study from our group did not show an increased prothrombotic status in off-pump compared with on-pump procedures, as reflected by thrombin generation markers and fibrinogen [31], this hypothesis cannot be completely excluded a priori.
In conclusion, meta-analysis of available data speaks in favor of reduced patency rates in OPCAB. It should be mentioned, however, that evidence exists in the literature that indicates that the off-pump procedure is associated with a lower incidence of selected early minor outcomes [3–5]. In consideration of the fact that off-pump and on-pump surgery procedures may show specific advantages and disadvantages in specific patient subsets, the risks and benefits of either approach need to be considered when choosing a tailored strategy for a single patient, to maximize long-term benefit and minimize short-term risks.
Although these data underscore an important message, we acknowledge some limitations of our statistical analysis. First, this meta-analysis was based on a few, small, randomized trials, which were all the available evidence at the time of analysis, and thus must be treated with caution. Second, confounding variables add biases to the interpretation of data, complicating the ascertainment of graft patency differences between OPCAB and CABG. In fact, the techniques and strategies of grafting are widely different between surgical groups, as well as equipment and anesthesia techniques; all of these may increase the clinical heterogeneity in patient groups. Third, even if randomization does the best possible job of neutralizing preoperative variables between CABG and OPCAB, it cannot obviously neutralize intraoperative variables such as the learning curve and comfort levels of the surgeons who operated on the patients of the included studies, and this might increase bias in data analysis. Fourth, it is important to recognize that one study selected for meta-analysis (the only one to document, among all selected studies, a statistically significant difference in graft patency between CABG and OPCAB) caused some degree of heterogeneity in the analysis [7], as it lay outside the 95% confidence lines of the funnel plot (data not shown). However, even when this study was excluded from the analysis, the difference in graft patency between OPCAB and CABG was statistically significant, and, notably, both fixed-effect and random-effect models yielded the same result (OR, 1.37; 95% CI, 1.03 to 1.82; p = 0.03).
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