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J Thorac Cardiovasc Surg 2004;127:167-173
© 2004 The American Association for Thoracic Surgery
Surgery for acquired cardiovascular disease |
a Cardiopulmonary Research Science and Technology Institute (CRSTI), Medical City Dallas Hospital, Dallas, Tex, USA
b Lenox Hill Hospital, New York, NY, USA
c Cardiac Surgical Associates, P.A., Minneapolis, Minn, USA
d Washington Hospital Center, Washington, DC, USA
Received for publication February 14, 2003; revisions received June 11, 2003; accepted for publication August 18, 2003.
* Address for reprints: Michael J. Mack, MD, 7777 Forest Lane, Suite A323, Dallas, TX 75230, USA
mjmack{at}earthlink.net
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Abstract |
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METHODS: In 4 centers with off-pump coronary surgery experience, a retrospective analysis of all coronary artery bypass grafting in a 3-year period was performed. Groups were compared to determine selection criteria, mortality, and morbidity, then computer-matched by propensity score to control for selection bias. Multivariate logistic regression identified risk factors predictive of mortality. Specific subgroups most likely to benefit were identified.
RESULTS: In all, 17,401 isolated coronary artery bypass grafts were performed, 7283 (41.9%) off-pump coronary artery bypass grafts and 10,118 (58.1%) conventional coronary artery bypass with cardiopulmonary bypass. Factors determining selection of patients for off-pump coronary artery bypass grafting included female gender (55.5% vs 44.5%), preexisting renal failure (57.0% vs 43.0%), and reoperations (52.6% vs 47.4%). Operative mortality was 2.8%; off-pump coronary artery bypass grafting versus conventional coronary artery bypass with cardiopulmonary bypass (1.9% vs 3.5%, P < .001) had the same predicted risk. Of the patients with multivessel disease, 11,548 were matched by propensity scoring. Mortality was significantly less in the off-pump coronary artery bypass grafting group (2.8% vs 3.7%, P < .001). By multivariate logistic regression analysis of the matched sample, predictors for mortality were female gender (odds ratio 1.83, confidence interval 1.37-2.44), preexisting renal failure (odds ratio 2.85, confidence interval 2.64-4.95), history of stroke (odds ratio 1.74, confidence interval 1.08-2.80), previous coronary artery bypass grafting surgery (odds ratio 4.22, confidence interval 2.92-6.09), use of cardiopulmonary bypass (odds ratio 2.08, confidence interval 1.52-2.83), and recent myocardial infarction (odds ratio 2.31, confidence interval 1.68-3.22). Cardiopulmonary bypass was predictive of mortality in reoperations, female patients, and patients aged 
75 years. Off-pump coronary artery bypass grafting was associated with less morbidity, including reductions in blood transfusion (32.6% vs 40.6%, P < .001), stroke (1.4% vs 2.1%, P = .002), renal failure (2.6% vs 5.2%, P < .001), pulmonary complications (4.1% vs 9.5%, P < .001), reoperation (1.7% vs 3.2%, P < .001), atrial fibrillation (21.1% vs 24.99%, P < .001), and gastrointestinal complications (3.6% vs 4.8%, P = .02).
CONCLUSION: In 4 centers with beating-heart operation experience, there is an overall early benefit in off-pump surgery, especially in patients traditionally considered at high risk for coronary artery bypass grafting.
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Methods |
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In the 4 study centers, data were prospectively collected on all patients who had cardiac surgery; date were subsequently entered into a relational database. Three of the 4 centers used standardized criteria and definitions set forth by the Society of Thoracic Surgeons (STS) for measuring the study variables.5 The fourth institution collected data as mandated by the New York State Department of Health. Data from this institution were recoded as necessary to match the STS definitions. STS data elements that were not present in the New York State data set were eliminated. Data elements for which 10% or more data was missing were also excluded.
The resulting database consisted of 20 preoperative risk factors (Table 1). The Parsonnet risk stratification model was used to broadly define differences between treatment groups that might influence selection and patient outcomes.11 The Parsonnet risk stratification is a logistic regression model in which 47 potential risk factors are considered to determine risk preoperatively. This model is a tool to compare expected mortality rates with observed mortality. Perioperative data collected and compared between groups are listed in Table 2. Complete data were available for 17,401 patients during the study period, including 7283 (41.9%) patients who underwent OPCAB and 10,118 (58.1%) patients who underwent CABG-CPB. The total number of cases for each study center as well as the number and percent performed by each technique are listed in Table 3.
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Demographic characteristics and preoperative risk factors of the off-pump and on-pump groups were compared using chi-squared and Pearson t test. Results indicated the groups were statistically different on several important characteristics (Table 4). Because the decision to perform CABG on- or off-pump at all 4 centers was at the surgeons' discretion, propensity score matching was conducted for treatment and control groups to control for selection bias.12,13
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Results |
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Overall mortality for the study population was 2.8%. Parsonnet risk scores indicated no significant difference between the 2 groups (OPCAB vs CABG-CPB, 5.8 vs 6.6). Although Parsonnet risk scores predicted equal expected mortality in each group, observed mortality for the patients who underwent CABG-CPB compared with OPCAB was 3.5% versus 1.9% (P < .001). Ten percent of the CABG procedures were single-vessel bypasses with 23.3% off-pump and with a mortality of 0.9% compared with single-vessel operations performed with cardiopulmonary bypass with a mortality of 3.3% (P < .001). However, this higher mortality of single-vessel disease operated with cardiopulmonary bypass is explained by a higher portion of emergency/salvage patients in that group.
To minimize the role of selection bias, 11,458 multivessel disease patients were computer-matched by propensity scoring. The mean number of grafts per patient did not differ significantly between the 2 matched groups, 2.95 in the on-pump group and 2.83 in the off-pump group. Using the variables listed in Table 5, Parsonnet risk scoring of the 2 groups indicated identical predicted risks both in the whole sample and by institution as is indicated in Table 6. Predicted risk of the propensity matched sample in each group is listed in Table 7. Despite the same predicted risk, the operative mortality was less in the OPCAB group (2.0% vs 3.7%, P < .001). Multivariate logistic regression analysis of the propensity-matched sample indicated that the 6 variables in Table 8 were independent predictors of mortality, including the use of CPB (odds ratio 2.08, confidence interval 1.52-2.83, P < .001).
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75 years, in patients undergoing reoperative CABG, and in women (Table 11).
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Discussion |
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There are many centers that have gained significant experience in off pump techniques. Consequently, their surgical expertise is relatively equal in both treatment arms (on vs off bypass). In these situations, there is an inability or unwillingness to randomize due to a perception of clinical benefit in off-pump surgery, either by the operating surgeon, the referring physician, or by the patients themselves who have frequently been referred to these institutions specifically for off-pump surgery. Hence, we hoped to gain insight into the impact of off-pump surgery on early CABG outcomes by a retrospective analysis using propensity score computer matching of cases from 4 institutions who had gained significant experience in beating-heart CABG. Although all the major risk factors for outcomes were able to be accounted for in the propensity matching, the role of the individual surgeon as a variable could not be included. Although all surgeons in each institution performed both on- and off-pump CABG, there was a predilection for some surgeons to perform a greater proportion off-pump while others performed a greater proportion on-pump. The individual surgeon is an independent variable affecting outcomes; however, in this analysis it was not possible to construct a propensity model that accounted for that variable.
The result of this analysis indicates that off-pump surgery is associated with significantly less operative mortality and morbidity than on-pump surgery and that the use of cardiopulmonary bypass is an independent risk factor for mortality in centers that have attained significant experience with beating-heart techniques.
A significant shortcoming of this study is that the format of the data collection tool does not allow for an intention-to-treat analysis. On occasion, due to intraoperative hemodynamic instability or technical issues limiting access to target coronary vessels, conversion from off-pump to on-pump is necessary to complete the procedure. Prior to the year 2000, the STS database, which was the collection tool in 3 of the 4 centers, did not collect conversion from off-pump to on-pump surgery data. Hence, in these 3 institutions in 1999, analysis of patients whose operation was initiated off-pump and converted intraoperatively to the use of cardiopulmonary bypass were included in the on-pump group, thereby conceivably adversely affecting the outcomes of the CABG-CPB cohort. Therefore, to determine how this factor might impact the data, the database was analyzed separately for the years 2000 and 2001, for which conversion data was available for all 4 centers. This allowed analysis for patients in 2000 and 2001 to be included on an intention-to-treat basis, including the OPCAB conversions with the OPCAB group. The conversion rate was 2.9% and including these 172 patients with the off-pump group rather than the on-pump group for analysis resulted in no significant differences in outcomes.
It is also important to note that these data include only early procedural outcomes and do not address longer-term follow-up. Despite early improved outcomes in operative mortality and morbidity, long-term graft patency and event-free survival remain unknown. Although early angiographic graft patency has been demonstrated to be excellent,17 an intermediate-term study is not as definitive.18 A subgroup studied at 3 years demonstrates excellent left internal thoracic artery graft patency, but saphenous vein graft patency to the posterior circulation using early generation stabilization and exposure techniques was lower than one would expect.
Certain results are worthy of note. The first is that the use of cardiopulmonary bypass is an independent risk factor for mortality. Second, there appears to be a particular benefit to avoiding CPB in those subgroups generally considered high risk for CABG surgery, including the elderly, women, and patients undergoing reoperative operations. Third, as well as a mortality benefit, the performance of beating-heart surgery is associated with a significant decrease in perioperative morbidity including the need for blood transfusions, return to the operating room for bleeding, respiratory complications, and new-onset renal failure. A further potential benefit of beating-heart surgery was hoped to be improved neurologic outcomes. Indeed, this analysis demonstrates a significant decrease in permanent neurologic deficit postoperatively in the OPCAB group (1.4 vs 2.1, P < .001).
Definitive outcomes analysis awaits a multicenter prospective randomized trial. However, due to the low incidence of major adverse outcomes (eg, mortality, stroke), such a study would require many thousands of patients to confer adequate statistical power to detect differences between on- and off-pump treatment groups. The only such study currently underway is the Veteran's Administration cooperative study but it will be many years before this study is completed and analyzed and results made available for evaluation. Even then, those results will be limited by the population studied as well as the inexperience in beating-heart techniques in many of the centers. This study represents significant experience on the part of centers who are extremely experienced and proficient in this operation. These centers have advanced past learning curves and represent a mature application of this technology. These data, despite their methodological shortcomings, support a benefit for OPCAB, which is consistent with numerous previous reports. In addition, OPCAB benefit persists despite elevated risk among those patients preferentially operated on off-pump. Ability to achieve superior outcomes in high-risk patients in terms of mortality and across a number of morbidity variables clearly speaks to the potential clinical benefit of this operation. Although not randomized, this study adds to the increasing body of patients and among sample sizes sufficient to confer adequate statistical power for detection of differences.
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Acknowledgments |
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Footnotes |
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References |
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