Ann Thorac Surg 2003;75:62-67
© 2003 The Society of Thoracic Surgeons
Original article: cardiovascular
On-pump coronary artery surgery versus off-pump exclusive arterial coronary grafting: a matched cohort comparison
Michael Haase, MDa,
Anamika Sharma, BMedScia,
Anja Fielitz, BPharma,
Shigehiko Uchino, MDa,
Jens Rocktaeschel, MDa,
Rinaldo Bellomo, FRACPa*,
Laurie Doolan, FANZCAa,
George Matalanis, FRACSb,
Alexander Rosalion, FRACSa,
Brian F. Buxton, FRACSb,
Jai S. Raman, FRACSb
a Department of Intensive Care Medicine (University of Melbourne), Victoria, Australia
b Department of Cardiac Surgery, Austin and Repatriation Medical Centre, Heidelberg, Victoria, Australia
Accepted for publication July 26, 2002.
* Address reprint requests to Dr Bellomo, Department of Intensive Care, Austin and Repatriation Medical Centre, Heidelberg 3084, Victoria, Australia.
e-mail: rinaldo.bellomo{at}armc.org.au
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Abstract
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BACKGROUND: It is unknown whether coronary artery bypass grafting without cardiopulmonary bypass and with exclusive use of arterial grafts (arterial off-pump CABG) offers any significant short-term advantages over standard CABG with cardiopulmonary bypass. Accordingly, we performed a comparison of the short-term outcomes of arterial off-pump and standard CABG patients matched for preoperative risk and number of grafts.
METHODS: We studied 90 consecutive arterial off-pump CABG patients during a 2-year period, obtained demographic and clinical features and surgical characteristics, and calculated their predicted surgical risk (EuroSCORE). Using a database of 750 contemporaneous patients treated with standard CABG, we created a matched cohort of 90 patients using an iterative process prioritizing number of grafts, target vessels, EuroSCORE, age, and sex. We compared the two groups for baseline features and short-term clinical outcomes.
RESULTS: There were no differences in age (65.9 versus 64.7 years), sex, EuroSCORE (3.3 versus 3. 6), number of grafts (2.1 versus 2.1), and preoperative left ventricular function. Arterial off-pump CABG, however, was associated with decreased duration of operation (213 versus 252 minutes; p < 0.0013), decreased peak postoperative troponin I levels (mean, 10.8 versus 29.1 ng/mL; p < 0.0001), decreased peak norepinephrine dose (2.3 versus 4.1 µg/min; p < 0.0082), and decreased likelihood of receiving red blood cell transfusion (17.8% versus 40%; p = 0.0016). There were no differences in duration of intensive care unit or hospital stay, incidence of atrial fibrillation, or other clinical complications. There was one death in each group.
CONCLUSIONS: After matching for number of grafts and other important preoperative risk markers, arterial off-pump CABG still decreases the need for red blood cell transfusion and offers other moderate clinical advantages compared with standard on-pump CABG.
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Introduction
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Coronary artery bypass grafting (CABG) is one of the most common operations worldwide, and its use is expected to increase [1]. Accordingly, any improvements in its efficacy and safety or any decrease in its costs would have significant public health repercussions. One potential improvement would be related to the ability to perform such an operation without the application of cardiopulmonary bypass. Such so-called off-pump coronary artery bypass grafting (OPCAB) could increase efficacy and safety by decreasing the cardiopulmonary bypass-associated morbidity such as artificial membrane exposure, aortic cannulation, cardioplegia, and full heparinization [2–7]. It could also decrease costs [8]. Another way in which CABG can be further improved in terms of long-term graft patency might be through exclusive arterial revascularization [9–19]. If OPCAB and exclusive arterial revascularization were combined (arterial OPCAB), the patient might benefit from short-term and long-term decreases in morbidity. The short-term efficacy and safety of arterial OPCAB, however, remain unknown. Accordingly, we have performed a case-control comparison of arterial OPCAB and standard CABG using a recently validated preoperative risk assessment method for case matching: the number of grafts, the target vessels, age, left ventricular function, and sex. We now report our findings.
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Material and methods
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This investigation was classified as an anonymous and confidential audit of current practice by the Institutional Ethics Committee, and the need for informed consent was waived.
Ninety consecutive patients undergoing OPCAB with exclusive arterial revascularization were identified during a 2-year period using the combined database from two intensive care units and one department of cardiac surgery serving two hospitals. These two institutions are geographically located in close proximity, and all cardiac surgery is performed by the same team of surgeons with the assistance of the same team of anesthesiologists and perfusionists. Postoperative care is conducted in both institutions by the same team of intensivists according to the same protocols, as previously described [20, 21].
Surgical philosophy and technique
Arterial OPCAB operation was first used to revascularize the left anterior descending coronary artery and its branches. The learning curve for most surgeons extended into 1997 and early 1998. During this period, procedures were performed on the basis of specific indications such as significant aortic atheroma or aortic calcification. As confidence and familiarity with the technique grew, the frequency of off-pump procedures increased from 12 arterial OPCAB procedures in 1999 to 47 in 2000 and 31 in 2001. However, the predominant indication to perform an off-pump operation during the study period remained the presence of aortic atheroma and calcification (visualized on transesophageal echocardiography). As a result, the majority of patients had pedicled arterial grafts (see Results). If required, free radial or right internal mammary artery grafts were used to complement the grafting with the in situ internal mammary arteries acting as inflows. This philosophy on the use of conduits and avoidance of grafting from the aorta applied to on-pump surgical procedures as well.
Patient selection
For all patients information necessary for the calculation of the EuroSCORE [22] was obtained. Information was also obtained on the number of bypass grafts, the target vessels, age, sex, and degree of left ventricular dysfunction. Using a patient list from the same databases, we identified 750 patients who had had a standard CABG operation during the same period. We used this latter database to create a matched cohort of patients. Matching was by iterative selection prioritizing, in a sequence: number of grafts, target vessels, EuroSCORE, age, left ventricular function, and sex. Thus for a patient who had grafting of three vessels with a particular set of target vessels by arterial OPCAB, a sequential group of patients from the 750-patient database was culled. Next the EuroSCORE was calculated. The database was scrolled until several EuroSCORE matches were found. Then age was used to further select the standard CABG patient with the closest age to the index arterial OPCAB. If several choices still remained, left ventricular function was used to decrease potential matches further and then sex, until the closest match could be selected. This process was repeated for each arterial OPCAB patient. During the selection process, the selectors were blinded to patient outcome.
For all the arterial OPCAB and their standard CABG case matches, several baseline variables were obtained in addition to those needed for the above selection. This information included presence of diabetes, lipid disorders, body weight, history of smoking, baseline serum creatinine and urea, presence of chronic atrial fibrillation, and preoperative troponin I concentration.
After selection and collection of baseline data, patient information was entered in a new database and outcome information was then sought.
Outcome measures
Outcome information was divided into intraoperative and postoperative results. Intraoperative information included use of epicardial wires, use of the Octopus device, use of the CPS/CTS stabilizer, aortic clamp time and cardiopulmonary bypass time (for standard CABG patients), and transfusion of red blood cells, platelets, and fresh-frozen plasma.
Postoperative information included the following variables: highest postoperative creatinine and urea levels, highest troponin I, highest lactate, highest norepinephrine dose, highest milrinone dose, transfusion of red blood cells, platelets, and fresh-frozen plasma, occurrence of postoperative stroke, tamponade, need for return to the operating room, atrial fibrillation, ventricular tachycardia or ventricular fibrillation, bradycardia, length of intensive care unit stay, length of ventilation, length of hospital stay, and survival to hospital discharge.
Statistical analysis
Data were entered into a commercially available statistical program (Statview, Abacus Inc, Berkeley, CA). We applied nonparametric statistical analysis to all numerical data and compared groups using the Mann-Whitney U test. Dichotomous variables in the two groups were compared using the 
2 test or Fisher’s exact test for situations of low event frequency. All results are presented as mean (± standard deviation) or percentage. A p less than 0.05 was considered statistically significant.
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Results
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Matching and preoperative features
The iterative matching process was successful in creating two groups, which were well matched for most preoperative demographic, clinical, biochemical, and prognostic features (Table 1).
However, despite such matching some differences were still detectable. For example, arterial OPCAB patients were more likely to suffer from dyslipidemia and were also somewhat more likely to have a history of recent smoking, whereas patients having standard CABG had a significantly higher mean serum creatinine (Table 1).
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Table 1. Preoperative Characteristics of Patients for Arterial Off-Pump Coronary Artery Bypass Grafting and Standard Coronary Artery Bypass Grafting
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Surgical and intraoperative issues
Conduits during arterial off-pump coronary artery bypass grafting
Surgeons performing arterial OPCAB used a variety of conduits and anastomotic connections. Importantly, grafting from the aorta was only used in four cases with two radial grafts to the posterior descending artery and two to the obtuse marginal artery. In the control group, such direct aortic grafting was also rare (one graft to the left anterior descending artery and one to the obtuse marginal artery). During arterial OPCAB, the left internal mammary artery was used alone in 25 cases, in conjunction with one or more radial arteries in 35, in conjunction with the right internal mammary artery in 15, and in conjunction with both right internal mammary artery and radial or gastroepiploic in one case each. The right internal mammary artery was used alone in two cases, and radial arteries alone were used in four. The right internal mammary artery and radial artery combination was used in one case only. There were 18 sequential (skip) grafts and 34 Y-grafts.
Target vessels during arterial off-pump coronary artery bypass grafting
The most-common target vessel was the left anterior descending coronary artery (n = 80 cases). The left anterior descending coronary artery was followed in frequency by the diagonal or circumflex arteries (n = 29), the posterior descending artery (n = 26), the obtuse marginal artery (n = 21), and the right coronary artery (n = 8). Other smaller coronary vessels were the target in six cases.
Features and outcomes
The intraoperative features and outcomes are summarized in Table 2. The findings confirmed the effectiveness of the matching in terms of number of grafts and target vessels. The most important differences were in the decreased operation time and a dramatic decrease in the need for epicardial pacing wires (Table 2).
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Table 2. Intraoperative Features and Outcome for Arterial Off-Pump Coronary Artery Bypass Grafting and Standard Coronary Artery Bypass Grafting Patients
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Postoperative outcomes
The postoperative outcomes are summarized in Table 3.
They show several differences between the two groups. These differences appear to broadly favor arterial OPCAB with a significant decrease in postoperative troponin I values, decreased use of norepinephrine, decreased use of milrinone, decreased need for red blood cell transfusion, decreased incidence of bradycardia, and a trend toward decreased duration of mechanical ventilation and need for operative reexploration
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Table 3. Postoperative Outcome of Patients With Arterial Off-Pump Coronary Artery Bypass Grafting and Standard Coronary Artery Bypass Grafting
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Comment
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The use of OPCAB remains controversial [1] because of the lack of level I evidence and of long-term follow-up of graft patency. Similar uncertainty surrounds the use of complete arterial revascularization [13, 14]. There is no information on the early or late outcome of patients receiving the combination of complete arterial OPCAB. Accordingly, we conducted this study to compare the early outcomes of standard CABG and arterial OPCAB.
Our investigation revealed several important findings with regard to immediate intraoperative outcomes. First, it is possible to perform complete arterial revascularization during OPCAB. Second, standard CABG results in a longer operation compared with arterial OPCAB, despite the similarity in the number of grafts and target vessels, with an average decrease of a little over half an hour. This finding is consistent with expectations, and the difference might translate into a degree of cost advantage for arterial OPCAB. Third, arterial OPCAB markedly decreased the need for epicardial pacing wire insertion. This difference might be the consequence of removing the deleterious effect of cardioplegia on myocardial function and should necessarily translate into a decrease in the small but known morbidity associated with such wires (infection, bleeding, malfunction). It is also likely to further decrease costs.
Several important differences emerged for postoperative outcomes as well. The postoperative release of troponin I was greater during standard CABG. Troponin I is a more sensitive marker of myocardial injury than other tests (electrocardiogram, creatine kinase or creatine kinase-MB), and this finding suggests a decrease in myocardial cell injury during arterial OPCAB. It is also consistent with the findings of other investigations studying the effect of OPCAB in general [23, 24]. To a degree, it allays some concerns associated with the fear of radial artery vasospasm [6]. This decrease in myocardial injury might be clinically important as greater degrees of myocardial necrosis after revascularization predict worse clinical outcomes [6]. Our study, however, was not sufficiently powered to detect such potential clinical differences. However, in keeping with the avoidance of cardioplegia and decreased myocardial injury, there was a significant decrease in the amount of milrinone required in the postoperative period. The avoidance of cardiopulmonary bypass, its associated inflammatory reaction, and the decreased used of milrinone are also likely to have contributed to decreased postoperative vasodilation, which was manifested clinically with decreased requirements for norepinephrine. These changes in the need for hemodynamic support were associated with a lower blood lactate concentration, which closely approximated statistical significance. This type of clinical outcome difference has not been previously reported. Arterial OPCAB also markedly decreased the requirements for postoperative blood transfusion. This finding is consistent with expectations and with previous reports [23]. It constitutes a clinically important advantage of OPCAB in general, which appears to apply to arterial OPCAB as well. This decreased incidence of bleeding might also lead to a reduction in the need for immediate postoperative reexploration. In our study the need for reexploration was greater with standard CABG and close to the preset values for statistical significance.
Investigators have focused on several specific areas of early outcome. One such area is that of stroke [7]. Although we report the incidence of this complication in the two groups, our study did not have the power to detect a difference. Approximately 7,000 patients would have to be randomized to each group to have an 80% power of detecting a 20% relative reduction in the incidence of stroke with OPCAB.
Another area of great interest is that of renal dysfunction, because of the hope that avoiding cardiopulmonary bypass would protect the kidney from injury. Indeed, some investigators reported better creatinine clearance and less release of N-acetyl glucosaminidase with OPCAB [25]. These findings, however, were not confirmed by others [26]. Our study shows that there was no difference in the changes in serum creatinine from baseline to peak postoperative level during arterial OPCAB.
Some studies have suggested that OPCAB might decrease the incidence of atrial fibrillation [1]. This hypothesis, however, was not supported by a more recent and larger study [23]. Our investigation of arterial OPCAB also showed no difference in the incidence of atrial fibrillation, which approximated 30% to 35% in both groups. However, we did find that the incidence of bradycardia was halved by arterial OPCAB.
Arterial OPCAB decreased the duration of mechanical ventilation but did not have any significant impact on the duration of intensive care unit stay and the duration of hospital stay. This finding differs from that of a recent randomized study, which demonstrated a decreased duration of hospital stay [23]. However, our patients were 5 years older on average and had twice the incidence of diabetes, and some cases were repeat operations. These differences might have been responsible for the longer duration of hospital stay.
This investigation is limited in its significance by the fact that it is not a randomized study. The control population is therefore imperfectly matched, and we could not adjust for hidden selection biases. Importantly, the differences in the incidence of smoking and dyslipidemia favor CABG, whereas the preoperative values for creatinine favor arterial OPCAB. These differences might represent the consequences of selection bias: patients with marked aortic disease (smokers with high cholesterol levels) might be more likely to be selected for operation without bypass. The difference in the incidence of diabetes might explain the finding that the serum creatinine was higher in standard CABG patients and might reflect another bias toward choosing patients with better distal vessels for OPCAB. Despite such imperfections, our findings are remarkably similar in direction and magnitude to those reported by a recent randomized controlled trial [23], thus appearing to have wide applicability. Furthermore, our study offers more-detailed information on renal function, troponin I release, and inotropic and vasoactive support than available so far in the literature. We could be criticized for choosing to perform complete arterial revascularization, thus combining two inadequately tested procedures (OPCAB and complete arterial revascularization). However, high-quality evidence supporting the use of OPCAB when technically possible is rapidly emerging, and there is concern that OPCAB, when combined with saphenous vein grafts, might decrease long-term graft patency [27]. The complete use of arterial grafts might improve long-term patency. We are currently conducting a randomized controlled trial to test this hypothesis. There was one death among our arterial OPCAB patients and one among our CABG patients (1.1% overall mortality). The EuroSCORE-predicted mortality for our patients was between 2.9% and 3%.
In summary, using a retrospective case-matched cohort study design, we studied the early outcome of complete off-pump arterial revascularization. We found that complete arterial revascularization is possible and safe off-pump. On the basis of our findings, arterial OPCAB appears likely to decrease the need for blood products and epicardial wires, the degree of myocardial injury, the use of inotropic and vasopressor drugs, the need for reexploration, and the duration of mechanical ventilation. There appear to be no early outcome disadvantages with its application.
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Acknowledgments
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We thank Jenny Broadbent and the nursing staff from Warringal Private Hospital Intensive Care Unit for their assistance with the collection of data. This study was supported by the Austin Hospital Anaesthesia and Intensive Care Trust Fund.
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Abstract
Introduction
