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Ann Thorac Surg 2006;82:608-614
© 2006 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Propensity Case-Matched Analysis of Off-Pump Versus On-Pump Coronary Artery Bypass Grafting in Patients With Atheromatous Aorta

Departments of Cardiothoracic Surgery and Cardiac Anesthesiology, Escorts Heart Institute and Research Centre, New Delhi, India

Accepted for publication March 24, 2006.

^_* Address correspondence to Dr Mishra, Department of Cardiac Anesthesiology, Escorts Heart Institute & Research Centre, Okhla Rd, New Delhi 110025, India (Email: manishamishra{at}yahoo.com).

	Abstract

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
BACKGROUND: Patients with severe atheromatous disease of the aorta who underwent coronary artery bypass grafting have an increased risk of stroke and death. We hypothesize that in these high-risk patients off-pump coronary artery bypass grafting is associated with lower rates of stroke and mortality.

METHODS: From January 1995 through June 2004, a total of 24,107 patients underwent coronary artery bypass grafting. Routine intraoperative transesophageal echocardiography was performed in 18,501, of which 6,991 (29.0%) were found to have severe atheromatous disease in the ascending aorta or aortic arch. Propensity matched-pairs analysis was used to match patients undergoing off-pump coronary artery bypass grafting (n = 3,000) with 3,000 patients undergoing conventional coronary artery bypass grafting by age, sex, ejection fraction, diabetes, preoperative intraaortic balloon pump, congestive heart failure, chronic obstructive pulmonary disease, acute myocardial infarction, peripheral vascular disease, history of stroke or cerebrovascular disease, renal disease, carotid artery disease, atrial fibrillation, emergency surgery, or previous cardiac surgery.

RESULTS: Univariate analysis revealed decreased hospital mortality (1.4% versus 3.3%; p < 0.001) and stroke prevalence (0.50% versus 0.97%; p = 0.05) in off-pump coronary artery bypass grafting compared with conventional coronary artery bypass grafting. Multivariate analysis revealed that increased mortality was associated with conventional coronary artery bypass grafting (odds ratio, 2.6; p = 0.001), age (odds ratio, 2.1; p = 0.003), acute myocardial infarction (odds ratio, 1.8; p = 0.03), history of stroke or cerebrovascular disease (odds ratio, 1.6; p = 0.04), congestive heart failure (odds ratio, 2.1; p = 0.04), and diabetes mellitus (odds ratio, 1.9; p = 0.03). Multivariate analysis showed that off-pump coronary artery bypass grafting technique was the only independent predictor of decreased stroke rate (odds ratio, 1.4; p = 0.05).

CONCLUSIONS: Off-pump coronary artery bypass grafting surgery in patients with atheromatous disease of the aorta is associated with lower risk of stroke and death. Routine intraoperative evaluation of the aorta is helpful in identifying the disease and directs the appropriate surgical technique.

	Introduction

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
Neurologic complications constitute a major cause of morbidity and mortality after coronary artery bypass grafting (CABG) [1, 2]. Severe atheromatous disease of the aorta has been identified as an independent risk factor for both stroke and mortality [3–6]. Risk of stroke attributable to aortic atheromatous disease is an important consideration in the referral of patients with coronary artery disease for surgical myocardial revascularization. Several surgical techniques have been suggested to minimize the risk of stroke in these patients [7–9]. The significance of the aorta as a source of atheroembolism and hence as a risk factor for perioperative stroke was highlighted by Katz and colleagues [4], who found a stroke incidence of 25% among patients with mobile plaques of the aortic arch. Several subsequent studies have identified severe atheromatous disease of the aorta as an independent risk factor for both mortality and stroke in patients undergoing coronary revascularization [10–13].

Off-pump coronary artery bypass (OPCAB) grafting has proven to be a feasible and safe alternative to conventional myocardial revascularization, as it obviates most of the perioperative and postoperative morbidity related to on-pump CABG [14, 15]. The safety and efficacy of OPCAB are well established [16, 17]. Decreased morbidity, shorter length of hospital stay, and reduced cost are often cited as some of the advantages of OPCAB [18–20]. Nonrandomized comparative studies have shown significant reduction in both mortality and major neurologic events with OPCAB technique [13, 17, 21, 22]. A prior unmatched comparative study also demonstrated an association between the OPCAB technique and improved outcomes [5].

The routine use of intraoperative transesophageal echocardiography (TEE) to evaluate all patients undergoing coronary revascularization for the presence of atheromatous aortic disease has helped in identifying, grading, and localizing the atheromatous lesions that are associated with a high risk of perioperative neurologic risk [23–25]. We have been routinely using intraoperative TEE at our institution for all cardiac surgical procedures for the last 15 years. Each patient is evaluated and graded for presence of atheromatous disease in the ascending aorta, aortic arch, and descending aorta [2, 10, 11].

In the present study, we hypothesized that in these high-risk patients, OPCAB is associated with lower rates of stroke and mortality. We have used propensity case matching to explore the relationship between surgical revascularization technique and outcomes in the subset of patients with aortic atheromatous disease.

	Patients and Methods

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
From January 1995 through June 2004, a total of 24,107 patients underwent isolated CABG. Patients with associated other cardiac procedures (ie, valve surgery, left ventricular aneurysm repair, ascending aorta replacement, carotid endarterectomy) were excluded from the study. We screened 18,501 patients intraoperatively by TEE to identify aortic atheromatous disease, and 6,991 (29.0%) patients were found to have severe atheromatous disease in the ascending aorta or aortic arch. The study was approved by the institutional review board. Because this is a retrospective study with a large number of patients whose data had been collected over a long duration, the institutional review board waived the requirement for informed consent on the condition that the subjects' identities were hidden before any matching or analytical procedures were performed.

The grading of atheromatous lesions of the aorta on TEE were done according to our previously established institutional criteria as follows [2]:

Grade I: Simple, smooth-surfaced plaques, focal increase in echocardiographic density, and thickening of intima extending less than 5 mm into the aortic lumen.

Grade II: Marked irregularity of intimal surface, focal increase in echocardiographic density, and thickening of adjoining intima with overlying shaggy echogenic material extending more than 5 mm into the aortic lumen.

Grade III: Plaques with a mobile element.

If atheromatous lesions of any grade were demonstrated in the ascending aorta on TEE, then epiaortic scan was routinely performed for better localization of the lesions. Severe atheromatous disease of the aorta is defined as grade II and III atheromatous disease of the ascending aorta or the aortic arch, and the technique of CABG was individualized to suit each patient for avoidance of atheroembolism. Most surgical techniques that we currently use concentrate on minimizing the direct handling of the diseased aorta [8, 10, 11]. In the presence of extensive atherosclerosis of the ascending aorta, the technique of CABG was individualized to suit each patient for avoidance of atheroembolism. In the conventional coronary artery bypass grafting (CCAB) group, if the disease was present in the proximal portion of ascending aorta, then a high aortic cannulation was done or a long aortic cannula was passed beyond the left subclavian artery, along with relocation of vein grafts and cardioplegia needle to another site. Greater use was made of arterial conduits, and a side-biting clamp was avoided. If fewer proximal anastomoses were possible on the diseased aorta, then more sequential arterial grafts were done or conduits were taken piggyback on the internal mammary arteries. If the disease was present in the distal part of the ascending aorta or aortic arch precluding aortic cannulation, cardiopulmonary bypass (CPB) was established by femoral artery cannulation and fibrillatory arrest without cross-clamping the aorta.

In the OPCAB group, we performed more total arterial grafts with more sequential anastomoses or conduits were taken piggyback on the internal mammary arteries. When proximal anstomoses had to be performed on the aorta, we used the side-biting clamp only once to minimize the handling of the diseased aorta. We used special proximal anastomotic devices to avoid application of the side-biting clamp all together, ie, the Enclose device from Novare Surgicals (Novare Surgical Systems Inc, Cupertino, CA) was most commonly used. Hybrid procedures were also performed in which CABG was combined with percutaneous transluminal coronary angioplasty. The other technique used was transmyocardial laser revascularization combined with OPCAB depending on the coronary anatomy and location of the atheromatous disease.

Propensity matched-pairs analysis was used to match patients undergoing OPCAB with patients undergoing CABG with CPB. The propensity matched-pairs analysis is a balancing score method that attempts to correct bias in patient selection by creating equivalent risk groups for analysis. The propensity score is the predicted probability of the dependent variable for each observation in the data set. The single score (between 0 and 1) then represents the relationship between multiple characteristics and dependent variables as a single characteristic. The propensity score also provides the probability that patients received a particular treatment, in this case OPCAB; patients from off-pump and on-pump groups were matched by using this propensity score so that the treatment outcomes could be compared.

We determined the independent factors associated with group membership in OPCAB versus CCAB by use of multivariate logistic regression. These factors included age, acute myocardial infarction, history of stroke or cerebrovascular disease, congestive heart failure, hypertension, and diabetes mellitus. After this parsimonious model was created, we established a saturated model by adding other important clinical variables. These factors included sex, renal disease, carotid artery disease, atrial fibrillation, peripheral vascular disease, chronic obstructive pulmonary disease, left ventricular ejection fraction less than 0.30, urgent or emergency operation, previous cardiac surgery, and patients with preoperative intraaortic balloon pump for hemodynamic instability. Finally patients in the on-pump group were matched to patients in the OPCAB group by use of greedy matching strategy. The cohorts of 3,000 patients, who underwent OPCAB, were matched with 3,000 CCAB patients. Patient data was prospectively collected. Stroke was defined as an episode of new focal or global loss of cerebral function with symptoms lasting more than 24 hours, confirmed on computed tomography or magnetic resonance imaging.

Severe stenosis of carotid artery was defined as narrowing of more than 70% of vessel cross-sectional area. Patients with symptomatic or asymptomatic disease of more than 70% confirmed on carotid angiography were treated with simultaneous carotid endarterectomy along with CABG. These patients are not included in the study.

Statistical analysis of categorical variables was carried out using cross-tables with the Pearson ² test. If the expected values were small, Fisher's exact test (two-sided) was used. In all statistical tests, a probability value of less than 0.05 was considered to be significant. Values are expressed as the mean ± standard deviation unless otherwise indicated. Multivariate logistic regression analysis was performed on all independent variables found significant by univariate analysis, including CPB, for predicting the outcome. Stepwise regression analysis was done separately for hospital mortality and neurologic injury with background elimination method. A variable is entered into the model if the probability is less than 0.05, and is removed if the probability is greater than 0.1. Statistical analysis was performed with the statistical software SPSS 13.0 (SPSS, Inc, Chicago, IL).

	Results

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
Of the 6,991 (29.0%) with severe atheromatous disease of the aorta, 3,105 underwent OPCAB, whereas the remaining 3,886 had CCAB surgery. The cohorts of 3,000 patients who underwent OPCAB were matched with 3,000 CCAB patients.

The demographic profile of the patients and the risk factors were fully matched and balanced, with a propensity score difference of 0.005 between the two groups. There were almost 87% males, one third of the patients had diabetes mellitus, poor left ventricular ejection fraction (<0.30) was present in 28.4% in the OPCAB versus 29.7% in the CCAB group, and 12.6% were older than 70 years of age. Previous history of stroke or cerebrovascular disease was present in 1.4% in the OPCAB group and 1.7% in the CCAB group, whereas 354 in the OPCAB and 375 in the CCAB group underwent an urgent or emergent surgery. Carotid artery disease was present in nearly 7.7% of patients in both groups (Table 1).

Multivariate analysis showed that OPCAB technique was the only independent predictor of decreased stroke rate (odds ratio, 1.4; p = 0.05; Table 5).

	Comment

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

Magee and colleagues [14] reviewed two large databases from two institutions to demonstrate that elimination of CPB improves early survival in multivessel CABG. They had 6,466 patients who underwent CCAB and 1,983 had OPCAB; the mortality was 3.5% versus 1.8% in the two groups, respectively. Another study compared the two groups, off-pump and on-pump CABG surgery, by univariate analysis for risk factors and postoperative complications, and predicted risk was determined by The Society of Thoracic Surgeons risk algorithm. There was a significant difference in the observed mortality in the OPCAB and CCAB groups, 1.9% versus 3.5% [30]. The results of our study are comparable for both groups, with a mortality of 1.4% in our off-pump group versus 3.3% in the on-pump group.

A published meta-analysis of randomized trials comparing OPCAB and CCAB showed a trend toward a reduction in the risk of composite end points (death, stroke, and myocardial infarction) for patients who had OPCAB (odds ratio, 0.48; 95%, confidence interval, 0.21 to 1.09; p = 0.08). There was no heterogeneity with respect to the primary end points among all trials [31]. Puskas and associates [32] in a recent preoperative randomized comparison of 200 unselected patients undergoing OPCAB versus CCAB found the 30-day mortality and stroke rate to be similar in both groups.

A study by Cleveland and colleagues [18] suggests OPCAB reduces risk-adjusted postoperative mortality compared with CCAB across all risk groups. They studied a total of 126 experienced centers, which performed 118,140 total CABG procedures. The use of OPCAB was associated with a decrease in risk-adjusted operative mortality from 2.9% with CCAB to 2.3% with OPCAB (p < 0.001). The use of an off-pump procedure decreased the risk-adjusted major complication rate from 14.15% with conventional CABG to 10.62% in the off-pump group (p < 0.001). Patients receiving OPCAB were less likely to die (adjusted odds ratio, 0.81; 95% confidence interval, 0.70 to 0.91) and less likely to have major complications (adjusted odds ratio, 0.77; 95% confidence interval, 0.72 to 0.82) [18].

The importance of the aorta as a source of emboli has become apparent only since the advent of TEE. This technique has made possible high-resolution imaging of the atherosclerotic aortic wall in great detail. We have developed elaborate screening techniques to detect preoperatively and intraoperatively some of the known lesions and factors that may cause perioperative stroke. We have demonstrated in our previous studies a drastic reduction in stroke rate with the routine use of intraoperative TEE and with appropriate modification of surgical technique [2, 8].

Although the proportion of strokes caused by aortic atheroemboli rather than concomitant cerebral atherosclerotic disease has not been clearly defined, it has been demonstrated in a small number of patients with severe aortic atherosclerosis that a decrease in perioperative stroke incidence can be affected by modifying cross-clamping, cannulation, and graft anastomosis techniques that specifically respect the embolic potential of aortic plaques [10]. In a recent collective review of more than 35,000 patients, the stroke rate ranged from 0.9% to 3.9% after isolated CABG, with a mean stroke rate of 2%. The mortality from stroke in the review was 13% to 25% [33].

Stamou and coworkers [34] studied the stroke rate in propensity-matched groups of off-pump and on-pump patients undergoing CABG. Patients undergoing CCAB were 1.8 (95% confidence interval, 1.0 to 3.1; p = 0.3) times more likely to suffer a stroke postoperatively than OPCAB patients after controlling for preoperative risk factors through matching.

Sharony and associates [12] demonstrated outcomes of OPCAB and CCAB in patients with severe atheromatous aortic disease by propensity case-match methods. The study demonstrated a significantly lower prevalence of hospital mortality, perioperative stroke, and overall complications in the OPCAB group. Multivariable analysis of preoperative risk factors showed that increased hospital mortality was associated with CCAB (odds ratio, 2.7; p = 0.01), fewer grafts (p = 0.05), acute myocardial infarction (odds ratio, 11.5; p < 0.001), chronic obstructive pulmonary disease (odds ratio, 2.4; p = 0.03), previous cardiac surgery (odds ratio, 10.2; p = 0.05), and peripheral vascular disease (odds ratio, 2.1; p = 0.05). Cardiopulmonary bypass was the only independent risk factor for stroke (odds ratio, 3.6; p = 0.03). In our study, the multivariate analysis of risk factors showed that increased mortality was associated with conventional on-pump CABG, age of 70 years or older, acute myocardial infarction, previous history of stroke or cerebrovascular disease, peripheral vascular disease, congestive heart failure, and diabetes, whereas conventional on-pump CABG performed on CPB was the only predictor of increased stroke rate in the present study.

It is not clear whether the avoidance of CPB has an independent advantage in reducing neurologic events, whether it is more important just to avoid aortic manipulation and proximal anastomosis, or whether elimination of CPB combined with complete avoidance of aortic manipulation may further reduce the incidence of perioperative stroke [7, 9, 25]. The current study shows that avoidance of CPB is independently associated with a lower stroke rate when there is significant atheromatous disease in the ascending aorta or arch. The separate effect of a no-touch technique for the diseased ascending aorta could not be ascertained from our data.

A recently published scientific statement from the American Heart Association has compared the two surgical procedures, OPCAB and CCAB, after reviewing many clinical trials, including several large retrospective analyses, meta-analyses, and randomized trials that address the different aspects of both procedures. Although it was difficult to reach a definitive conclusion, it showed that patients may achieve an excellent outcome with either type of procedure, and that individual outcomes are likely to depend more on factors other than whether patients underwent OPCAB or CCAB. Nonetheless, it showed a trend in which length of hospital stay, mortality rate, and long-term neurologic function and cardiac outcome appear to be similar in the two groups [35].

An important limitation of the technique is the nonrandomization of patients into the two groups, although the sample size is large. However, by using the propensity score matching analysis, we attempted to minimize bias between the two groups. Indeed, the CCAB and OPCAB groups had an approximately equal distribution of all significant preoperative variables [36]. Another limitation of the study is that the OPCAB and CCAB groups are not distributed uniformly on a yearwise basis, as OPCAB surgery was performed more frequently in the later years. Although we made every attempt to acknowledge all clinically significant variables, this time period also encompasses our transition from CCAB to OPCAB along with the learning curve, and the surgical procedures were performed by a group of surgeons.

We conclude that the off-pump technique of myocardial revascularization in patients with atheromatous disease of the aorta is associated with a lower risk of stroke and death. The routine use of intraoperative TEE in evaluation of the aorta is helpful in identifying the disease in patients who are at a higher risk for neurologic events after CABG.

	Acknowledgments

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 
We thank Sudhir Shekhawat for providing assistance with statistical analysis.

	References

Top Abstract Introduction Patients and Methods Results Comment Acknowledgments References

 

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