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Eur J Cardiothorac Surg 2004;26:549-553
© 2004 Elsevier Science NL

Operative mortality after conventional versus coronary revascularization without cardiopulmonary bypass

^a Section of Cardiac Surgery, Washington Hospital Center, 106 Irving Street NW, Suite 316, South Tower, Washington, DC 20010, USA
^b Department of Statistics, MedStar Research Institute, Washington, DC, USA

Received 29 November 2003; received in revised form 10 May 2004; accepted 19 May 2004.

^* Corresponding author. 1201 South Eads Street, Apt 1909, Arlington, VA 22202, USA. Tel.: +1-202-361-2377; fax: +1-703-521-1715
e-mail: cvsisfun{at}hotmail.com
e-mail: paul.j.corso{at}medstar.net

	Abstract

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion References

 
Objective: Off-pump coronary artery bypass (CABG) is a safe revascularization option with comparable or superior results to the conventional on-pump CABG. However, comparative analysis of the type of surgical approach on the mortality rate is largely unknown. This study sought to investigate whether CABG without cardiopulmonary bypass (off-pump CABG) is associated with lower operative mortality than the conventional on-cardiopulmonary bypass (on-pump) approach. Methods: From October 1998 to June 2001, off-pump CABG was performed on 2477 patients and on-pump CABG was performed on 3077 patients. The patients undergoing off-pump CABG were randomly matched to on-pump patients via propensity score. Seventy-four percent of the off-pump CABG patients were matched with on-pump patients via propensity scores. A logistic regression model was used to test the difference in the postoperative mortality rate between off-pump CABG and on-pump CABG, controlling the correlation between matched sets. A multiple logistic regression model predicting the risk of mortality adjusted by risk factors of mortality and operation type was computed. Results: Results from the general estimating equation showed that patients who had on-pump CABG were 1.6 (95% confidence intervals (CI)=1.2–2.0, P<0.01) times more likely to die during the first 30 days after surgery than patients who had off-pump CABG. Independent predictors of 30-day mortality identified from the multiple logistic model included on-pump CABG (versus off-pump CABG), advanced age, female gender, carotid artery disease, chronic renal failure, depressed ejection fraction, reoperative CABG, preoperative intraaortic balloon counterpulsation, and recent myocardial infarction. Conclusion: Excellent clinical results and a lower operative mortality rate can be achieved with the off-pump CABG technique compared with the conventional on-pump approach.

Key Words: Coronary artery bypass • Surgical procedures • Minimally invasive • Cardiopulmonary bypass

Abbreviations: CABG, coronary artery bypass graft • On-pump, CABG, CABG with cardiopulmonary bypass • Off-pump, CABG, CABG without cardiopulmonary bypass • CHF, congestive heart failure • COPD, chronic obstructive pulmonary disease • CVA, cerebrovascular accident • MI, myocardial infarction • IABP, intraaortic balloon counterpulsation • OR, odds ratio • CI, confidence intervals

	1. Introduction

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion References

 
The use of minimally invasive techniques for coronary revascularization is based on the premise that patient morbidity and mortality can ultimately be reduced without compromising the results of conventional coronary revascularization. Extracorporeal circulation elicits a series of physiologic derangements, including activation of a systemic inflammatory response and hematologic effects that hinder normal hemostasis [1–3]. Compared with on-pump techniques, coronary artery bypass grafting (CABG) without cardiopulmonary bypass (off-pump CABG) has been associated with lower stroke rates and improved perioperative outcomes [4–7].

The current study was conducted in a large clinical setting to systematically investigate whether off-pump CABG is associated with lower operative mortality rates than the conventional on-pump CABG.

	2. Methods

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion References

 
2.1. Patients
The computerized database of the Division of Cardiac Surgery was used to identify all patients who underwent CABG at Washington Hospital Center between October 1998 and June 2001 (n=5554); 3077 (55%) had on-pump CABG and 2477 (45%) had off-pump CABG. For on-pump CABG, standard anesthesia, surgical techniques, extracorporeal circulation, and myocardial protection methods (blood cardioplegia was used in all patients) were used. All procedures were performed by the same group of cardiac surgeons who were equally skilled and experienced in the technique of off-pump surgery. Surgical techniques for off-pump CABG have been described previously [8].

2.2. Selection criteria
Indications for off-pump CABG included patients who were considered high risk for on-pump CABG because of medical comorbidities such as renal failure, diffuse cerebrovascular and peripheral vascular disease, aortic atherosclerosis, chronic obstructive pulmonary disease, and religious convictions that precluded blood transfusions [8].

2.3. Definitions
Previous stroke was defined as a history of a central neurologic deficit persisting for more than 72 h. Chronic renal failure was defined as a serum creatinine value of 2.0 mg/dl. Diabetes was defined as a history of diabetes mellitus, regardless of duration of the disease or need for oral agents or insulin. Recent myocardial infarction was defined as a myocardial infarction occurring within 24 h before CABG. Prolonged ventilation was defined as the need for respiratory support for more than 24 h.

Clinical events were source-documented and adjudicated. Baseline demographics, procedural data, and perioperative outcomes were recorded and entered prospectively in a prespecified database by a dedicated data-coordinating center. Emergent CABG patients were reviewed and added to the database retrospectively.

2.4. Data analysis
2.4.1. Univariate analysis
Univariate comparisons of operative and postoperative characteristics were performed between the on-pump CABG and off-pump CABG groups using the Mantel–Haenszel ²-test of general association. However, Fisher's exact test was used when sample sizes were small, and the Cochran–Armitage test for trends was used for ordered data. Continuous data were compared using the Wilcoxon rank sum test. All tests were two-sided, and P-values of 0.05 or less were considered significant.

2.4.2. Multivariate analysis
Preoperative risk factors for mortality were identified from Parsonnet's risk stratification system [9]. The risk factors were compared between on-pump CABG and off-pump CABG in a univariate analysis. ² analysis was used primarily. Variables that met a significance level of 0.15 in the univariate analysis were included in a multiple logistic regression analysis testing for the likelihood of selection for off-pump CABG. An analysis testing for strong linear dependencies among the explanatory variables was done using tolerance and the variance inflation factor [10] prior to running the logistic regression. Model fit was assessed through residual analysis (deviance and dfBetas) and the Hosmer and Lemeshow goodness-of-fit statistic. The profile likelihood confidence intervals (CI) for adjusted odds ratios were reported.

2.4.3. Matching on propensity score
To compare 30-day (operative) mortality rates between the on-pump group and the off-pump CABG group, the groups had to be equal in the distribution of preoperative risk factors for mortality. One way to eliminate the imbalance is through matching. A propensity score, or the predicted probability of being selected for off-pump CABG, was calculated from the logistic regression equation described above. The off-pump CABG patients were matched to the on-pump patients, using the logit of this score and time of surgery. The result of this pairing is that a person who has a high probability of being selected for off-pump CABG is matched with a person who has a low probability of being selected for on-pump CABG. Previous studies have shown that matching on the propensity score can control selection bias [5,11,12]. A general estimating equation that adjusts for the correlation in the matched pairs was run to predict the probability of the patients dying within 30 days after surgery [13].

	3. Results

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion References

 
3.1. Unmatched sample
Univariate comparisons between type of operation and risk factors for mortality are presented in Table 1. Multivariate comparisons of the preoperative risk factors between on-pump CABG and off-pump CABG are shown in Table 2. Patients who had off-pump CABG tended to be older, females, have a higher ejection fraction, and a higher rate of cardiogenic shock than patients who had on-pump CABG. Patients who had on-pump CABG had higher rates of diabetes, hypertension, depressed ejection fraction and a higher number of vessels grafted. These variables, including the number of grafts were entered in the multivariate analysis model to adjust for the imbalances between the two groups. No statistical differences were found in Parsonnet's predicted mortality rates between the on-pump and off-pump CABG patients.

View larger version (13K): [in this window] [in a new window]   Fig. 1. Causes of death after on-pump and off-pump CABG.

To summarize the analysis, the unadjusted odds ratio shows that patients undergoing on-pump CABG are 1.51 times (95% CI=1.05–2.16) more likely to die than off-pump CABG patients. Adjusting for the differences in preoperative risk factors and number of grafts, through propensity score matching, raises the odds ratio to 1.6 (95% CI=1.2–2.0). Adjustment through matching achieves balance and assures that the two groups will be comparable with respect to the preoperative risk factors, including the number of grafts used in the creation of the propensity score.

	4. Discussion

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion References

 
Off-pump CABG has been used with increasing frequency during the past decade. Compared with on-pump CABG, off-pump CABG has been associated with reduced rate of atrial fibrillation [4], lower stroke rates [5,11] and improved perioperative outcomes [1,2,4–6]. The decreased organ dysfunction obtained with off-pump CABG has been largely ascribed to avoidance of the systemic inflammatory response elicited by the cardiopulmonary bypass circuit.

Previous researchers have focused on the lesser degree of myocardial injury after off-pump CABG versus on-pump CABG [14]. Specifically, troponine T, troponine I, and creatine kinase MB (CK-MB) isoenzyme are lower in off-pump CABG patients, suggesting that off-pump CABG may cause less myocardial damage than on-pump CABG. Khan et al. in a prospective randomized study demonstrated that off-pump CABG was associated with lower myocardial damage as compared to on-pump CABG (the area under the curve of troponin T levels was higher during the first 72 h in the on-pump group than in the off-pump group, P=0.02) [15]. However, the graft-patency rate was lower at three months in the off-pump group than in the on-pump group, and the authors raised concerns with respect to the long-term outcome of off-pump CABG [15]. On-pump CABG carries a potential risk of renal dysfunction related to the systemic inflammatory response, hypoperfusion, and loss of pulsatile perfusion of cardiopulmonary bypass [16]. Off-pump CABG also has been shown to reduce the need for blood transfusions [17].

Cerebral microemboli generated during on-pump CABG with cardiopulmonary bypass may be implicated in postoperative neurologic impairment [18]. BhaskerRao et al. [18] documented in a prospective study that cerebral function was better after off-pump CABG compared with on-pump CABG. Pathologic examination of the brain by Moody et al. [19] after conventional CABG also revealed the presence of multiple emboli lodged in brain vessels.

4.1. 30-day mortality
The overall operative (30-day) mortality rate in both groups demonstrates that off-pump CABG can be carried out safely, effectively with lower 30-day mortality than with on-pump CABG. The two groups were matched with a computerized method to eliminate imbalances in preoperative characteristics. In the multivariate analysis of this matched population, on-pump CABG emerged as an independent predictor of 30-day mortality. Although previous reports [1,2] failed to demonstrate any difference in mortality between the two approaches, small sample size significantly limited the power of these studies (type II error). The better outcomes of off-pump CABG surgery are more evident in high-risk patients, as previously reported [20]. Specifically, Arom et al. [20] has demonstrated significantly lower mortality rates for high-risk patients after off-pump CABG (7.7%) versus on-pump CABG (28.8%, P=0.008), whereas no significant differences were found in medium- and low-risk patients.

4.2. Clinical implications
The current study has demonstrated improved clinical outcome and lower mortality after off-pump CABG versus on-pump CABG. The heightened mortality rate documented after on-pump CABG is related to the postoperative organ dysfunction triggered by cardiopulmonary bypass and the systemic inflammatory response it elicits (‘post-pump syndrome’) [20,21]. Avoidance of cardiopulmonary bypass is associated with significantly lower in-hospital mortality rates, emphasizing the deleterious effects of on-pump cardiopulmonary bypass on early patient survival.

Technical improvements and better stabilization also have facilitated an increase in the rate of revascularization procedures performed on a beating heart. In the current study, off-pump CABG became more routine over time (in 1994 only 2% of coronary procedures were done on a beating heart, whereas the respective value for 2001 was 68%).

4.3. Limitations
The limitations of our study include the retrospective methodology and the lack of angiographic follow-up. However, all the data elements were prospectively recorded according to prespecified definitions. The long-term efficacy and durability of grafting performed with minimally invasive surgery remain to be ascertained.

	Footnotes

 
¹ Address reprint requests to : Paul J. Corso, MD, Chief, Section of Cardiac Surgery, Washington Hospital Center, 106 Irving Street NW, Suite 316, South Tower, Washington, DC 20010, USA. Tel.: +1-202-291-1430; fax: +1-202-291-1436.

	References

Top Abstract 1. Introduction 2. Methods 3. Results 4. Discussion References

 

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