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Ann Thorac Surg 2004;77:1245-1249
© 2004 The Society of Thoracic Surgeons

Original article: cardiovascular

Coronary surgery in patients with peripheral vascular disease: effect of avoiding cardiopulmonary bypass

Shishir Karthik, FRCSa, Ghassan Musleh, FRCSc, Antony D. Grayson, BSb, Daniel J. M. Keenan, FRCSc, D. Mark Pullan, FRCS(C-Th)a, Walid C. Dihmis, FRCS(C-Th)a, Ragheb Hasan, FRCS(C-Th)c, Brian M. Fabri, FRCS(Ed)a*

a Department of Cardiothoracic Surgery, The Cardiothoracic Centre, Liverpool, United Kingdom
b Department of Research and Development, The Cardiothoracic Centre, Liverpool, United Kingdom
c Department of Cardiothoracic Surgery, Manchester Royal Infirmary, Manchester, United Kingdom

Accepted for publication September 10, 2003.

* Address reprint requests to Dr Fabri, The Cardiothoracic Centre-Liverpool, Thomas Dr, Liverpool, L14 3PE UK
e-mail: brian.fabri{at}ctc.nhs.uk


   Abstract
Top
 Abstract
Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
 References
 
BACKGROUND: An increasing number of patients with peripheral vascular disease are undergoing coronary artery bypass grafting. Such patients have an increased risk of adverse outcomes. Our aim was to quantify the effect of avoiding cardiopulmonary bypass in this group of patients.

METHODS: Between April 1997 and March 2002, 3,771 consecutive patients underwent coronary artery bypass grafting performed by five surgeons. Four hundred and twenty-two (11.2%) had peripheral vascular disease and of these, 211 (50%) received off-pump surgery. We used multivariate logistic regression analysis to assess the effect of off-pump surgery on in-hospital mortality and morbidity, while adjusting for treatment selection bias. Treatment selection bias was controlled for by constructing a propensity score, which was the probability of receiving off-pump surgery and included core patient characteristics. The C statistic for this model was 0.8.

RESULTS: Off-pump patients were more likely to have preoperative renal dysfunction, previous gastrointestinal surgery, and less extensive disease. The left internal mammary artery was used more in off-pump compared to on-pump cases (90.1% vs 82.9%; p = 0.033). In the univariate analyses, off-pump patients were less likely to have a postoperative stroke (p = 0.007), and had shorter postoperative hospital stays (p < 0.001). However, the incidence of new atrial arrhythmia was higher (p = 0.028). After adjustment for differences in case-mix (propensity score), avoidance of cardiopulmonary bypass was still associated with a significant reduction in postoperative stroke (adjusted odds ratio 0.09 [95% confidence interval 0.02 to 0.50]; p = 0.005), and shorter postoperative hospital stay (p = 0.001).

CONCLUSIONS: Off-pump coronary surgery is safe in patients with peripheral vascular disease, with acceptable results. The incidence of postoperative stroke is substantially reduced when avoiding cardiopulmonary bypass in patients with peripheral vascular disease.


   Introduction
Top
 Abstract
 Introduction
Material and methods
 Results
 Comment
 Acknowledgments
 References
 
Due to the improvement in outcomes following coronary artery bypass grafting (CABG) that have been achieved over the last two decades, a greater number of patients are being referred for CABG. Also, the patients undergoing CABG are more likely to be older, with a higher surgical risk [1, 2]. Patients with peripheral vascular disease (PVD) have a significantly higher risk of early postoperative adverse events following CABG [3–5]. Many studies have also shown that this subgroup of patients have poorer long-term survival after CABG [6–9].

In view of the significant early and long term mortality and morbidity risk in these patients undergoing CABG, and as they represent a significant percentage of patients (between 10% and 30% of most CABG populations) [7–9], strategies that may lead to a reduction in adverse outcomes in these patients will have a significant impact on resources and outcomes. This group of patients may benefit from off-pump CABG (OPCAB) [5]. We therefore analyzed our results of CABG to see if patients with PVD derived any benefit from OPCAB, while adjusting for differences in patient characteristics.


   Material and methods
Top
 Abstract
 Introduction
 Material and methods
Results
 Comment
 Acknowledgments
 References
 
Patient population and data
Between 1 April 1997 and 31 March 2002, 3,771 consecutive patients underwent isolated coronary artery bypass surgery performed at two institutions (Cardiothoracic Center-Liverpool, and Manchester Royal Infirmary). Of these, 48% (n = 1813) underwent OPCAB and 52% (n = 1958) had on-pump CABG (ONCAB). Patients undergoing CABG that was incidental to heart valve repair or replacement, resection of a ventricular aneurysm, or other surgical procedure were not included. These patients represented the entire coronary revascularisation practice of five surgeons (DJMK, DMP, WCD, RH, and BMF).

A total of 448 (11.9%) patients had documented PVD. The presence of peripheral vascular disease was defined in accord with The Society of Cardiothoracic Surgeons of Great Britain and Ireland minimum dataset [10]. This includes patients with a history of evidence of aneurysm or occlusive peripheral vascular disease, intermittent claudication, all patients who had been investigated and diagnosed to have PVD (including aortic and carotid artery disease), and all patients who had undergone intervention in the past (including aortic and carotid surgery). We have excluded 26 patients undergoing concomitant CABG and vascular surgery. Hence, our study group consisted of 422 patients with PVD.

Details of the different surgical techniques used in these patients, OPCAB and ONCAB, have been published previously [11]. Overall, 211 (50.0%) patients received off-pump coronary surgery (94 without aortic manipulation, 117 with aortic manipulation), while 211 (50.0%) patients underwent ONCAB.

Definitions and data collection methods have been previously published [12]. Data were collected prospectively during the patient's admission as part of routine clinical practice and entered into our cardiac surgery registry on the variables listed in Table 1 and Table 2.


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  		Table 1. Patient Characteristics Based on Procedure Performed

 

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  		Table 2. Crude Postoperative Data Based on Procedure Performed

 
In-hospital mortality was defined as death within the same hospital admission regardless of cause. All patients transferred from the base hospital to another hospital were followed up to confirm their status at discharge. Reexploration for bleeding was defined as bleeding that required surgical reoperation after initial departure from the operating theater. Postoperative atrial arrhythmia was defined as the occurrence of new atrial arrhythmia in the absence of preoperative persistent or paroxysmal atrial arrhythmias. Postoperative stroke was defined as a new focal neurologic deficit and comatose states occurring postoperatively that persisted for more than 24 hours after its onset and was noted before discharge. We excluded confused states, transient events, and intellectual impairment from our study to avoid any subjective bias. Renal failure was defined as patients with a postoperative creatinine level greater than 200 µmol/L or patients requiring dialysis. Postoperative myocardial infarction was defined as a new Q-wave postoperatively in two or more contiguous leads on an electrocardiogram, or significant rise in postoperative cardiac enzymes (creatine kinase-MB) combined with hemodynamic and echocardiographic signs of myocardial infarction. Deep sternal wound infection was defined in accord with the published evidence-based guidelines by the Centers for Disease Control and Prevention [13]. Gastrointestinal complications, which included gastrointestinal bleeding, pancreatitis, ischemic bowel, and perforation, were in line with the definitions of The Society of Cardiothoracic Surgeons of Great Britain and Ireland minimum dataset [10].

Statistical methods
Continuous variables are shown as median with 25th and 75th percentiles and categorical variables are shown as a percentage with 95% confidence intervals (CI). Comparisons were made with Wilcoxon rank sum tests and {chi}2 tests as appropriate. Standard statistical tests were used to calculate odds ratios (OR) with 95% CI. The European System for Cardiac Operative Risk Evaluation (EuroSCORE) was derived to assess differences in patient case mix between OPCAB and ONCAB patients [14]. Logistic regression was used to adjust in-hospital outcomes for differences in patient and disease characteristics (treatment selection bias) [15]. Treatment selection bias was controlled for by constructing a propensity score. The propensity score was the probability that a patient would receive OPCAB, and was constructed from the variables listed in Table 1 (C statistic = 0.8) [17]. Once the propensity score is constructed for each patient, there are three ways of using the score for comparisons: matching, stratification, and multivariable adjustment. Due to the small sample size available to us for this study, we have decided to use multivariable adjustment, because matching would have reduced the study size even further and stratification can be difficult to interpret. The propensity score is then included along with the comparison variable (OPCAB or ONCAB) in multivariable analyses of outcome producing adjusted odds ratios, as shown in Table 3. The propensity score adjusts for the treatment selection bias, which is evident in Table 1, between one group and the other (eg, extent of disease, number of distal anastomoses, and hypertension). Using a propensity score as the sole means for adjusting outcomes was preferable, due to the low number of events in our study, and provides better adjustment for those factors driving treatment selection; the overall effect is more complete risk adjustment [16]. In all cases a p value less than 0.05 was considered significant. All statistical analyses were performed retrospectively with SAS for Windows Version 8.2.


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  		Table 3. Postoperative Data Based on Procedure Performed Adjusted for the Propensity Score

 

   Results
Top
 Abstract
 Introduction
 Material and methods
 Results
Comment
 Acknowledgments
 References
 
Table 1 lists patient and disease characteristics based on the procedure performed. In OPCAB patients, 13.3% (95% CI 9.1 to 18.8) had carotid disease compared to 8.1% (95% CI 4.9 to 12.8) in ONCAB patients (p = 0.083). The incidence of aortic disease was also similar in the two groups. Patients receiving OPCAB were more likely to have hypertension (p = 0.022), renal dysfunction (p = 0.043), prior gastrointestinal surgery (p = 0.011), and receive the left internal mammary artery (p = 0.033). However, OPCAB patients had less extensive disease (p < 0.001) and received fewer distal anastomoses (p < 0.001). There was no significant difference between the groups with regard to the EuroSCORE risk profile (Table 1).

Off-pump CABG patients were more likely to have a shorter postoperative length of stay (median, 7 days [25th and 75th percentiles: 6 to 10]) compared to ONCAB patients (median, 8 days [25th and 75th percentiles: 7 to 11]; p < 0.001).

In-hospital outcomes by procedure performed are shown in Table 2 (crude) and Table 3 (adjusted for the propensity score). The propensity score included extent of disease, the number of distal anastomoses, use of the left internal mammary artery, cerebrovascular disease, renal dysfunction, and prior gastrointestinal surgery, which were identified as independent predictors of off-pump group membership (C statistic = 0.8).

There was no association between avoiding cardiopulmonary bypass and in-hospital mortality, reexploration for bleeding, sternal wound infection, renal failure, gastrointestinal complications, and perioperative myocardial infarction in either univariate or multivariate analyses.

Off-pump CABG was associated with a significantly higher incidence of new atrial arrhythmias in the univariate analysis. However, after adjusting for the propensity score in the multivariate analysis this difference disappeared.

The incidence of postoperative stroke was significantly lower in patients receiving OPCAB in both the univariate and multivariate analyses (adjusted OR 0.09, p = 0.005). Fewer OPCAB patients had lengths of postoperative stay more than 7 days (adjusted OR 0.46, p = 0.001).


   Comment
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
Acknowledgments
 References
 
One of the major changes that have occurred in the last 15 years is a rekindling of interest in OPCAB, as many believe that avoiding cardiopulmonary bypass could possibly lead to a further reduction in mortality and morbidity rates. The benefits of OPCAB are more likely to be seen in the high-risk cases, in which a significant part of the risk is believed to be associated with the need for cardiopulmonary bypass [18–20].

Patients with PVD are a major subgroup of patients undergoing CABG as these patients have a higher incidence of coronary artery disease. In fact, coronary artery disease is the leading cause of both early and late mortality following peripheral vascular revascularization [21, 22]. In our study, 448 patients out of a total of 3,771 (11.9%) had evidence of PVD. This compares well with several previous studies in which the prevalence of PVD in patients undergoing coronary artery revascularization has been between 10% and 30% [3, 8, 9].

The major difference in our study is the postoperative stroke rate, which was significantly lower in the OPCAB group (adjusted rate 1.0% vs 5.6%, p value = 0.005). This was despite the fact that the incidence of carotid disease was higher in patients having OPCAB (13.3%) compared to those having ONCAB (8.1%). This inference is unchanged after risk adjusting with propensity score methodology [16], taking into account differences in extent of disease and other variables related to treatment selection bias. Another difference between the two groups was the significantly greater number of ONCAB patients who stayed longer than 7 days in-hospital following surgery.

Stroke represents a particularly devastating complication following CABG. While its implications, both in terms of short-term and long-term consequences, to the patients are often devastating, it also has several major implications for health care providers, as it results in longer hospital and intensive care unit stay and places a significant strain on resources. Several recent studies have shown a significant reduction in the stroke rate following CABG with OPCAB [11, 23, 24].

Patients with PVD have been shown to have a significantly higher risk of perioperative events including death, myocardial infarction, and adverse neurologic outcomes [3, 5, 25]. The cause for stroke after conventional CABG is multifactorial.

Among the various mechanisms involved are hemodynamic fluctuations, aortic manipulation, gas and particulate embolization, aortic clamping, arterial perfusion jets, and various other inflammatory and neurohumoral derangements associated with cardiopulmonary bypass [11, 18].

Patients with PVD often tend to have atheromatous involvement of all major arteries including the aorta and carotid arteries. These are likely to be associated with a higher incidence of thromboembolic episodes during cardiopulmonary bypass in these patients. Cannulation and aortic manipulation can also have disastrous consequences in these patients [25]. Off-pump CABG offers a significant chance of reducing adverse neurologic outcomes following surgery as several of the risk factors mentioned above are evaded by avoiding aortic cannulation and cardiopulmonary bypass.

There are some limitations, which may effect the conclusions drawn from this study. These include variables not measured in this study such as the quality of the coronary vessels, which is important in selecting the type of surgery and in determining the outcome, and selection bias resulting from the operating surgeon's decision to perform the procedure off-pump or on-pump. For this to effect our conclusions by a significant amount, the variables used in the propensity score would have to be uncorrelated with the variables not measured, but we do not believe that this is likely. This study also does not take into account pharmacological treatments and long-term outcomes for these patients (eg, antiplatelet drugs, graft patency, and quality of life). The additional graft received by our ONCAB patients might have a significant impact on longevity and reoperation rates.

Another important limitation is that we have not taken into account the impact of aortic manipulation, although previously we have shown that aortic manipulation did not significantly alter neurologic outcome in our OPCAB series [11]. While we do not routinely practice epiaortic echocardiography to identify ascending aortic atherosclerotic disease, we do acknowledge that this subgroup of patients might benefit from epiaortic scanning. A further limitation is that the study is spread more than 5 years and most patients in the ONCAB group are from the early part of the study period, while most belonging to the OPCAB group are from the later part of the study period. Hence, they may represent two different patient populations. Also, the OPCAB group includes patients from the period during the "learning curve" of each surgeon, maybe indicating a degree of selection bias. However, we have compared the outcomes of the ONCAB patients of the five surgeons to their own OPCAB results, thus accounting for any variation due to the surgeon factor. Also, the two patient groups are well comparable in terms of patient characteristics and preoperative variables despite the temporal differences. In addition, we have used multivariate logistic regression analysis, incorporating these patient characteristics in a propensity score to account for any significant differences between the two groups. However, it is important to note that the propensity score cannot adjust for any changes that may have occurred in policy, which may influence our outcomes.

In conclusion, we believe that this is the first study to compare the short-term outcomes of OPCAB with ONCAB in patients with PVD. While it has not revealed any significant in-hospital survival benefit, it shows a highly significant reduction in the incidence of postoperative stroke rates in this group of patients. Off-pump CABG should be considered as an alternative to ONCAB, especially in patients who are at a higher risk of suffering a stroke following CABG.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
References
 
We would like to thank Janet Deane and Suzanne Chaisty, who maintain the quality and ensure completeness of data collected in our Cardiac Surgery Registry.


   References
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
 References
 

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