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a Joseph B. Whitehead Department of Surgery, Division of Cardiothoracic Surgery, Carlyle Fraser Heart Center, Cardiothoracic Surgery Clinical Research Unit, Atlanta, Georgia
b Department of Biostatistics, Rollins School of Public Health, Emory University School of Medicine, Atlanta, Georgia
Accepted for publication April 17, 2008.
* Address correspondence to Dr Lattouf, Division of Cardiothoracic Surgery, Emory University School of Medicine, 550 Peachtree St NE, Atlanta, GA 30308 (Email: omar.lattouf{at}emoryhealthcare.org).
Presented at the Poster Session of the Forty-fourth Annual Meeting of The Society of Thoracic Surgeons, Fort Lauderdale, FL, Jan 28–30, 2008.
| Dr Lattouf discloses that he has a financial relationship with Medtronic Inc, and Estech Corporation; Drs Thourani, Puskas, and Guyton with Medtronic Inc and Boston Scientific/Maquet.
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Abstract |
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 Top Abstract IntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Methods: From 1997 to 2006, 12,812 consecutive patients underwent isolated CABG at a single academic center. Ten-year survival data were obtained by cross-referencing patients with the national Social Security Death Index. A propensity score analysis of 46 preoperative characteristics balanced risk factors between surgical groups. A proportional hazards regression analysis modeled the hazard of death as a function of surgery type (on versus off), distal group (1 to 3 versus 4 to 7 vessels), ICOR, and propensity score.
Results: Proportional hazards regression analysis showed no significant influence of surgery type or number of grafts on long-term survival within the four groups: OPCABG 1 to 3 grafts (n = 3,946; ICOR 1.11), OPCABG 4 to 7 grafts (n = 1,721; ICOR 1.56), on-pump CABG 1 to 3 grafts (n = 3,380; ICOR 1.21), and on-pump CABG 4 to 7 grafts (n = 3,765; ICOR 1.64). Irrespective of technique of revascularization, there was a survival advantage for patients with higher ICOR.
Conclusions: Long-term survival was similar for patients receiving 1 to 3 or 4 to 7 grafts by either on-pump or off-pump techniques. However, higher ICOR was associated with improved long- term survival within all groups.
The last decade's increasing interest in lessening the dependence on cardiopulmonary bypass by an increasing number of surgeons adopting use of off-pump coronary artery bypass graft surgery (OPCABG) was met by concerns that such a shift could be fraught with hazards [1]. Such concerns have been amply refuted by numerous recent reports on the short-term safety, improved benefits, and the decreased morbidity and mortality associated with beating-heart surgical revascularization techniques [2–8]. Off-pump technique benefits have been affirmed for men, women, patients with compromised ejection fractions, and elderly patients as well as for patients with other comorbid factors such as diabetes mellitus, renal insufficiency, chronic obstructive pulmonary disease, and history of cerebrovascular diseases [9–11]. The growing confidence in the short-term benefits of OPCABG revascularization due to increasingly established safety and short-term efficacy has led to the supportable conclusion that OPCABG surgery should remain a therapeutic option in modern cardiac surgery; however, it was advised that long-term clinical results are still warranted. Thus, a natural follow-up question was raised on the impact of such OPCABG techniques on intermediate- and long-term patient benefits and on event-free survival [12, 13] and whether such short-term benefits are maintained on long-term follow-up.
The concern that OPCABG techniques may be safe for single- or double-vessel revascularization but not for extended multiple-vessel revascularization was addressed in a large series of patients presented at the forty-third annual meeting at The Society of Thoracic Surgeons (STS). In this study, OPCABG conferred a greater safety margin for patients requiring extensive revascularization than did on-pump coronary artery bypass graft surgery (ONCABG) [14]. Furthermore, analysis of the STS National Database conducted and reported by Dr Fred Edwards at the aforementioned meeting has confirmed the 30-day morbidity and mortality reduction in OPCABG cohorts as compared with the patient groups treated on cardiopulmonary bypass. Thus, the growing body of evidence affirms the safety and efficacy of OPCABG in coronary artery revascularization. The increasing reports in support of OPCABG application, the continued technical advancements and new positioning and stabilization devices being introduced, and the improved monitoring and hemodynamic support that have significantly facilitated the performance of, and broadened the application of, beating-heart surgery, OPCABG as a safe, reliable, and cost-effective method for surgical coronary revascularization has been broadly adopted [2–8].
Despite the early observed morbidity and mortality benefits of OPCABG, concerns remain about long-term graft patency [15], thus raising the question of long-term benefits of OPCABG, and hence suggesting that specific consents should be obtained when performing OPCABG [16]. Heretofore, it was not known whether the number of grafts or the method of revascularization impacted the long-term, risk-adjusted outcomes for OPCABG versus ONCABG patients. The present study was designed to assess the long-term outcomes, to determine whether multiple-vessel OPCABG is safe, and to compare the results to a similar cohort of patients who underwent ONCABG. Additionally, we wanted to examine the impact of the index of completeness of revascularization on long-term survival for patients in each subgroup.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Demographic and Preoperative Data
Before analysis, preoperative risk factors for the outcomes of interest were identified and harvested from the STS database (Table 1). Standard STS definitions of each risk factor and outcome were used. The timing of each patient's most recent cerebrovascular accident and myocardial infarction (if any) were recoded as separate nominal variables as were surgeon identity and diabetes mellitus control method (diet, oral, insulin, none). Race was dichotomized as Caucasian or non-Caucasian. The family history of cardiac diseases, smoking history, systemic arterial hypertension, congestive heart failure, number of diseased vessels, ejection fractions, STS mortality risks, renal function status, preoperative use of cardiac, antiplatelet, anticoagulant, and anti-inflammatory drugs (steroids) were each coded also as separate nominal values. The index of completeness of revascularization (ICOR) was computed by dividing the total number of distal vessels bypassed by the number of diseased vessels.
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Interventions, Surgeons, and Surgical Technique
Each patient underwent one surgical session consisting of OPCABG or ONCABG, performed at the discretion of any of 17 faculty surgeons, who varied in their adoption of off-pump surgery. Institutional adoption of OPCABG over time is displayed in Figure 1. Off-pump CABG was performed with one of several commercially available cardiac positioning and coronary artery stabilizing devices, using techniques that have been previously described [6]. On-pump CABG was performed with standard techniques, utilizing roller head pumps, membrane oxygenators, cardiotomy suction, arterial filters, cold antegrade and retrograde blood cardioplegia, and moderate systemic hypothermia (30° to 34°C). Patients who had to be converted intraoperatively from OPCABG to ONCABG were entered into the database and analyzed according to the operation they ultimately received. Intraoperative conversion began to be measured and routinely recorded as part of the institutional database in 2004.
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Long-Term Follow-Up
The Social Security Death Index (SSDI) is a database of death records extracted from the United States Social Security Administration's death master file extract. Persons who have died since 1963 and who had a social security number and whose death has been reported to the Social Security Administration will be listed in the SSDI.
To assess the mortality status of individual study patients, the institutional data manager first referenced the social security number of all patients in the study cohort. The SSDI may be accessed free through the Internet by entering one social security number at a time; however, arrangements were made with the SSDI whereby individual death records for all patients in the study cohort were purchased in bulk. Thus, for each patient who died before the cutoff date of March 31, 2007, a mortality date was provided. The number of days between the date of surgery and the mortality date is the primary variable of interest in the long-term portion of the study. For patients still alive at the study cutoff date, no mortality date was given, and these patients were considered to be censored for the purposes of the survival analysis. Importantly, the cause of death was not considered, nor available, in this study; thus, the study seeks to compare all-cause mortality between the surgery types.
Data Management and Statistical Analysis
All data for consecutive patients were entered into a computerized cardiac surgical database, utilizing the data fields and definitions of the STS national adult cardiac database. Checks for data quality are employed both at the institutional level and before final entry into the STS national adult cardiac database. Patients were primarily classified according to number of grafts (1 to 3 or 4 to 7) and the surgery type (OPCABG or CABG on cardiopulmonary bypass) they received. To control for potential selection bias, propensity scores, described by Blackstone [20] and D'Agostino [21] were calculated for each patient based on 40 risk factors (including surgeon identity, year of surgery, and six indicators of missingness) available preoperatively. For the propensity score calculation, a multiple logistic regression model was used nonparsimoniously to model OPCABG (yes or no) as a function of all 40 risk factors (Table 1). The resulting conditional probability of a patient receiving OPCABG is the propensity score. Importantly, because patient age is the single most reliable risk factor for long-term survival, it was left out of the propensity score calculation so that its effect was directly adjusted for in the final survival regression modeling. To account for 17 different surgeons who performed coronary artery bypass surgery during the study period, surgeon identity was included in the propensity score.
Long-term survival estimates were made at 1-, 3-, 5-, and 10-year intervals using Kaplan-Meier product-limit methods. Life-table curves were plotted for each graft/surgery type combination. After verifying the proportional hazards assumptions through Schoenfeld residual analysis, long-term survival comparisons were made using Cox proportional hazards regression models. The proportional hazards regression modeled the instantaneous hazard of death as a function of grafts (1 to 3 or 4 to 7), surgery type (OPCABG or CABG on cardiopulmonary bypass) and their interaction, adjusted for the propensity score, ICOR, and patient age. Hazard ratios were generated for grafts (1 to 3 or 4 to 7) and surgery type, along with 95% confidence intervals. The data were managed and analyzed using SAS version 9.1 (SAS Institute, Cary, North Carolina) and STATA 9.0 (Stata Corp, College Station, Texas). All statistical tests were two-sided, using an 
= 0.05 level of significance.
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Results |
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TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Table 2 shows vessel groups (1 to 3 grafts versus 4 to 7 grafts) with respect to on-pump versus off-pump and long-term survival estimates at 1, 3, 5, and 10 years of follow-up. The survival estimates at those yearly intervals reveal no discernable differences between the respective groups. An apparent sharp drop in survival at year 10 (38.1%, 95% confidence interval: 0.017 to 0.796) in the OPCABG 4 to 7 group is based on small sample sizes (n = 3) and is not statistically different from the other groups at 10 years.
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Comment |
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TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Our increasing experience with coronary stabilization devices and our improved understanding of hemodynamic management during off-pump coronary revascularization have led to a level of skill maturation that has enabled us to expand OPCABG techniques to patients needing complex multivessel revascularization. The increased confidence of the surgeon and anesthesiologist in the management and control of hemodynamic parameters has led to a substantial shift in our overall approach in coronary revascularization in favor of OPCABG. Encouraged by our institutional retrospective observations as well as by prospective randomized clinical trials [10], we have continued to expand the utilization of off-pump techniques to the point that in 2006, 75% of patients receiving 4 to 7 vessel grafts and 87% of patients receiving 1 to 3 vessel grafts were revascularized without the use of cardiopulmonary support (Fig 1). In this study, we conducted a retrospective review of all consecutive patients who underwent isolated primary coronary artery revascularization. The objective of this study was to investigate the long-term outcome of the OPCABG patients requiring limited (1 to 3 vessels) or extensive (4 to 7 vessel) bypass operations as compared with outcomes of patients undergoing similar grafting requirements utilizing cardiopulmonary bypass.
Few and multiple-vessel OPCABG patients were similar in age and had STS predicted risk of mortality equivalent to that of the ONCABG cohorts. The number of diseased vessels in the OPCABG 1 to 3 group was less than that of the ONCABG 1 to 3 group (2.31 versus 2.45, p < 0.001); whereas it was slightly higher for the OPCABG 4 to 7 groups as compared with the ONCABG 4 to 7 group (2.82 versus 2.80, p = 0.23; see Table 1). Although there was a lower degree of ICOR in the OPCABG 1 to 3 vessel group (n = 3,946; ICOR 1.11) than in the ONCABG 1 to 3 group (n = 3,380; ICOR 1.21, p < 0.001), and the same applied when comparing the ICOR between the OPCABG 4 to 7 grafts group (n = 1,721; ICOR 1.56) and the ONCABG 4 to 7 grafts cohort (n = 3,765; ICOR 1.64, p < 0.001), the predicted risk of mortality was slightly higher in the 1 to 3 vessel OPCABG group compared with 1 to 3 vessel ONCABG group (0.024 versus 0.022, p = 0.022), and was also higher in the 4 to 7 vessel OPCABG as compared with the ONCABG cohort (0.021 versus 0.020, p = 0.56). Of great interest for the purpose of this paper is the finding that the observed long-term product-limit survival estimate was equivalent among all surgery type and vessel groups (Table 2, Fig 2). Table 3 shows that the hazard ratio for death analysis reveals that age and ICOR significantly disadvantaged long-term survival while the method or extent of revascularization did not. Additionally, life-table survival curves by ICOR ranges revealed gradual worsening of long-term survival by lower rates of ICOR (see Fig 3).
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This dataset confirmed equivalence in intermediate- and long-term safety and efficacy of OPCABG as a whole (1 to 7 grafts) compared with ONCABG. Most importantly, long-term survival outcomes for patients receiving multiple-vessel grafts were equivalent between the two groups. Previously reported short-term morbidity and mortality benefits of OPCABG techniques are maintained at the intermediate- and long-term levels for all patients undergoing off-pump CABG. From a historical perspective, it is of interest to note that the findings of Jones and associates [24, 25] reported nearly 3 decades ago revealed that completeness of revascularization was a key determinant in long-term patient survival. That stands true today despite all of the advancements in diagnostic and therapeutic interventions available in the operating rooms and the postoperative care units.
Limitations
This study has several limitations. Its retrospective nature does not permit complete accounting for all sources of bias, despite advanced statistical methodology designed to correct for both treatment selection bias and potential confounders of outcomes in preplanned analyses. Another limitation of this study is the utilization of all-cause mortality data, reliably obtained from the Social Security Death Index, rather than the more specific cardiac-related mortalities, which were not readily available for the purpose of this study. The study also did not address the relative incidence of nonfatal cardiac-related events and the requirements for repeat revascularization between the respective groups during the follow-up period. In addition, the database utilized in this study reported surgical cases according to the ultimate surgery type performed. This meant that patients whose coronary revascularization was initially attempted without cardiopulmonary bypass and who required conversion to cardiopulmonary bypass (typically owing to hemodynamic instability) were included in the ONCABG group. That may disadvantage ONCABG in the comparison of outcomes with OPCABG. Reciprocally, patients converted from ONCABG to OPCABG (usually because of intraoperative discovery of severe aortic atherosclerosis) were included from the OPCABG group. Additionally, our data analysis did not address the extent of coronary calcification and the utilization of coronary endarterectomy in each group or the incidence of conversion from OPCABG to ONCABG in the occasional cases of severe calcified coronary arteries. Such patients' potential increased incidence of complications may disadvantage one method for revascularization in comparison with the other. The database does not allow reconciliation of these data to an intention-to-treat analysis. Fortunately, intraoperative conversion is an infrequent event, affecting approximately 2% of cases [9]. Finally, although the 17 surgeons who performed coronary revascularization in this study varied greatly in their interest in OPCABG (several rarely performed OPCABG, whereas several used OPCABG in the majority of their cases), our center has maintained a strong institutional interest in OPCABG since 1997. Thus, faculty surgeons in the present study may have average experience with OPCABG that exceeds the national norm, limiting the ability to generalize these results.
In conclusion, OPCABG techniques have been shown here to provide intermediate- and long-term survival trends equivalent to those for patients treated on pump. This equivalence in survival applies for patients undergoing single-, or double-, or multiple-vessel OPCABG revascularization. The unfounded concern that the short-term benefits of OPCABG techniques may jeopardize the long-term survival of the coronary artery bypass patient can no longer be supported. Indeed, our results ascertain the intermediate- and long-term survival equivalence of off pump to on pump. For patients who need limited or extensive revascularization, cardiopulmonary bypass techniques do not contribute an intermediate- or long-term survival advantage; therefore, the perceived notion of special benefits offered by cardiopulmonary bypass cannot be supported by our data. Further studies are still required to address important unanswered questions on the relative incidence of nonfatal cardiac-related events and the requirements for repeat revascularization between the respective groups during the follow-up periods.
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Acknowledgments |
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ICOR < 0.75; solid circles = ICOR group = 0.75 