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J Thorac Cardiovasc Surg 2003;125:797-808
© 2003 The American Association for Thoracic Surgery
Surgery for Acquired Cardiovascular Disease |
From the Divisions of Cardiothoracic Surgery,a Cardiology,b and Cardiac Anesthesiology,c Emory University School of Medicine, and the Emory Center for Outcomes Research,d Atlanta, Ga.
Supported by grants from Medtronic, Inc, Minneapolis, Minn, and The Carlyle Fraser Heart Center Foundation, Atlanta, Ga.
Read at the Eighty-second Annual Meeting of The American Association for Thoracic Surgery, Washington, DC, May 5-8, 2002.
Received for publication May 5, 2002. Revisions requested June 14, 2002; revisions received June 27, 2002. Accepted for publication July 8, 2002. Address for reprints: John D. Puskas, MD, MSc, FACS, Associate Professor of Surgery (Cardiothoracic), Emory University School of Medicine, Crawford Long Hospital, Sixth Floor, Medical Office Tower, 550 Peachtree St, NE, Atlanta, GA 30308 (E-mail: john_puskas{at}emoryhealthcare.org).
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Abstract |
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Introduction |
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For more than 3 decades, surgical coronary revascularization has been accomplished with the use of cardiopulmonary bypass (CPB) in the vast majority of cases. However, off-pump coronary artery bypass grafting (OPCAB) has enjoyed a recent resurgence in interest and popularity, first in South America
1,2 and then worldwide. 3,4 This has been fueled by a growing awareness of the morbidity attributable to CPB and by refinement of instrumentation to facilitate beating heart surgery.
The diffuse inflammatory response elicited by CPB has been invoked to explain the small but finite incidence of serious injury in multiple organ systems, causing numerous postoperative complications and prolonging hospital length of stay. CPB, aortic crossclamping, and cardioplegia entail global myocardial ischemia during the crossclamp interval. Although effective cardioplegia reduces this risk, myocardial injury may still occur. This is manifested by release of myocardial enzymes and a small but finite incidence of Q-wave myocardial infarction. Hemodilution and hemolysis are direct consequences of CPB and may produce significant coagulopathy, necessitating transfusion of blood products.
Although OPCAB now accounts for approximately 18% of coronary artery bypass grafting (CABG) procedures performed in the United States, as reported by the National Society of Thoracic Surgeons Adult Cardiac Database Fall 2001 report (data reported through June 2001, http://www.sts.org), concern has been raised about the difficulty of this rediscovered operation and the possibility that incomplete revascularization may compromise patient outcomes. 5,6 Those retrospective comparisons favoring OPCAB over CABG with CPB 7 have been appropriately criticized for bias inherent in patient selection for OPCAB and for "fast-track" management selectively applied to patients undergoing OPCAB. Thus previous retrospective and nonrandomized comparisons of OPCAB versus CABG with CPB may have used inadequately matched control groups. Even retrospective studies applying sophisticated statistical analyses to very large databases 8,9 and reporting significant benefits of OPCAB over CABG with CPB for mortality, stroke, and many other complications are subject to this criticism. Moreover, most previous studies have failed to adequately address legitimate concerns about the completeness of revascularization provided or to document the quality of anastomoses constructed during OPCAB by directly assessing graft patency. Indeed, all but a very few 10 of the dozens of nonrandomized comparisons published since 1998 reported statistically fewer grafts performed in the OPCAB group. Of course, this does not necessarily indicate incomplete revascularization in the OPCAB group, because surgeons during the "learning curve" may have correctly and consciously excluded some patients from OPCAB on the basis of coronary anatomy. In particular, multiple grafts to the lateral wall may be challenging without considerable OPCAB experience. Nonetheless, this observed difference in number of grafts between groups does at least indicate an imperfect composition of the chosen "matched" control groups.
A prospective, randomized study 11 among 80 selected patients heightened concerns about completeness of revascularization in OPCAB when it reported significantly fewer grafts in the OPCAB group than in the CABG with CPB group (2.6 ± 0.5 vs 3.1 ± 0.8, P = .04). Two other groups have published results of randomized comparisons of OPCAB versus CABG with CPB with substantial statistical power. Van Dijk and colleagues 12 randomly assigned 281 selected patients and reported equivalent completeness of revascularization but only 2.4 ± 1.0 grafts in the OPCAB group versus 2.6 ± 1.1 grafts in the CABG with CPB group (P = .05). Fewer than 25% of patients enrolled in this study had three-vessel disease; 78% had normal left ventricular function. Ascione and associates 13 randomly assigned 200 selected patients and performed 2.23 ± 0.83 grafts in the OPCAB group and 2.31 ± 0.86 grafts in the CABG with CPB group (difference not statistically significant). None of these patients required grafting of the distal circumflex territory, and 80% had left ventricular ejection fractions greater than 50%. There have been no published reports comparing OPCAB versus CABG with CPB among randomly assigned patients unselected for coronary anatomy, ventricular function, or comorbidities.
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Methods |
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Patients were randomly assigned by means of a computer-generated random number table. This was done for each patient after the operating surgeon had documented the intended revascularization to be performed, which was based on review of the preoperative coronary arteriogram. Random assignment was stratified by sex and diabetic status to insure roughly equal numbers of patients within strata defined by these prognostically relevant characteristics. Random assignment was carried out separately within each stratum with randomly permuted blocks of size 4 and 6 to preserve approximate balance and prevent correct prediction of the next assignment by the clinical recruiting staff.
Crossing over between groups and blinding to group assignment
Patients were allowed to cross over between groups whenever the treating surgeon considered that this would be unequivocally in the patient's best interest. All patients, their families, referring cardiologists, and nonoperative care providers were blinded as to group assignment. This blinding will be maintained through the 1-year follow-up period.
Patient management protocols
Patients participating in the SMART Study were identified simply as "SMART Study patients," and their care was managed according to strict, unbiased, criteria-driven, printed protocols. There protocols were developed specifically to standardize management of study patients and were applied identically to both groups. Protocols controlled preoperative care, anesthesia and analgesia (before, during, and after the operation), extubation, intensive care unit management and transfer, ambulation, diet, medications, transfusion, all laboratory and radiologic examinations, and discharge planning. Each of these protocols was translated to patient order forms and placed on the chart of each patient enrolled. Thus, for instance, each study patient received identical anesthetic premedication and identical anesthesia in the operating room. A separate, defined protocol determined whether an individual patient was extubated in the operating room or in the intensive care unit. Each patient was required to meet specific criteria to be extubated, and each patient was extubated as soon as these criteria were met. Thus the opportunity for biased or selective "fast-tracking" of patients undergoing OPCAB was minimized or eliminated.
Data acquisition and management
Preoperative data acquisition included a complete assessment of patient demographics, preoperative risk factors, and preoperative medications. Preoperative neurologic and quality of life assessments included administration of the National Institutes of Health Stroke Scale by a trained physician assistant and the Euroquol and SF-36 self-reporting questionnaires. Table 1 reports the timing of various investigations.
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Power calculations and statistical analysis
The sample size was chosen for the ability to detect a difference of 5% between treatment groups in the primary end point, graft patency. Assuming a conservative estimate of an average of 3 grafts performed per patient, and making the further assumption that patency outcomes of grafts within a patient are independent, 100 patients in each treatment arm would yield 78% power to detect a difference in patency rate of 5%, assuming a type 1 error rate of .05. Eighty percent power would be achieved with an average of 3.08 grafts per patient. Although an overall higher number of grafts per patient was anticipated, the nonindependence of patency outcomes within patients, which are taken into account in the analysis, would effectively lower the power to a small extent. Overall, 100 patients in each treatment group should conservatively yield 80% power to detect the 5% difference in patency rates that is considered clinically important.
The index of completeness of revascularization (ICOR, number of grafts performed/number of grafts intended) was compared between treatment arms with the Wilcoxon rank sum test. Dichotomous morbidity and mortality outcomes were analyzed with the Fisher exact test. All data analysis was according to intent to treat. All 4 patients who crossed over between groups were also analyzed according to intent to treat. Analysis of covariance was used to compare postoperative laboratory values between treatment groups after adjustment for preoperative levels. Discrete data are presented as percentages; continuous data are presented as mean ± SD.
Follow-up
All 192 eligible patients completed personal or telephone interviews approximately 30 days after surgery. These interviews assessed adverse events, complications, readmissions, and reinterventions.
Surgical technique
General
Surgical access to the heart was through a standard median sternotomy in all cases. All incisions and closure techniques were the same for both groups, limiting variability and maintaining blinding of group assignment for patients, families, and referring cardiologists. A cell saver reservoir (COBE Cardiovascular, Inc, Arvada, Colo) was set up for all patients in both groups. Blood was spun down and returned to all patients when the quantity was sufficient. All patients had epiaortic ultrasonography performed immediately after pericardiotomy. Patients discovered to have a heavily calcified or severely atheromatous ascending aorta (grade 
3) that precluded safe cannulation or crossclamping were treated with an aortic "no-touch" technique. This required that such patients randomly assigned to OPCAB had internal thoracic arteries used for sole coronary inflow and those randomly assigned to CABG with CPB were converted to the OPCAB group. Patients randomly assigned to the off-pump technique for whom complete revascularization was not technically feasible without the use of CPB were allowed to cross over to the on-pump group. Thus enrollment in this study did not compromise the standard of care provided to any patient. Fine monofilament suture (8-0 Surgipro; United States Surgical Corporation, Norwalk, Conn) was used for all distal anastomoses, with the exception of heavily calcified, large-caliber coronaries that could not be penetrated with an 8-0 needle. A humidified carbon dioxide blower (Medtronic DLP, Minneapolis, Minn) was used to disperse blood from the anastomotic site during construction of distal anastomoses in both groups. Proximal anastomoses were performed with 5-0 or 6-0 Surgipro suture for venous anastomoses and 6-0 or 7-0 Surgipro suture for arterial anastomoses to the aorta.
OPCAB technique
OPCAB was performed with the Medtronic Octopus II (March 2000-May 2000) and Octopus III (May 2000-August 2001) stabilizing devices for coronary stabilization and deep pericardial traction sutures for cardiac displacement and presentation. This was accomplished according to techniques previously described elsewhere, with great care taken to maintain hemodynamic stability (mean arterial blood pressures 65 mm Hg) throughout the operation. 14 Perfusion-assisted direct coronary artery bypass with the Quest Medical myocardial protection system (Quest Medical, Dallas, Tex) was used at the discretion of the surgeon in 1 patient in the OPCAB group with critical left main coronary artery stenosis and profoundly depressed left ventricular ejection fraction. 15
CABG with CPB technique
Conventional CABG with CPB was accomplished with every effort made to minimize the impact of CPB. Patients without diabetes received 250 mg methylprednisolone sodium succinate (Solu-Medrol) intravenously before induction of anesthesia in the CABG with CPB group only. Cannulation and single aortic clamping were guided by the results of individual epiaortic ultrasonographic scanning of the ascending aorta. The SARNS "soft-flow" aortic cannula, a two-stage venous cannula, and retrograde blood cardioplegia catheter were used for all patients in the CABG with CPB group (3M Health Care, Ann Arbor, Mich). A phrenic nerve pad was used to thermally insulate the heart and to protect the phrenic nerve. The patients were routinely cooled to 34°C for operations when 3 or fewer grafts were anticipated. When 4 or more grafts were anticipated, the patient was cooled to 32°C. The COBE disposable arterial-venous circuit was used with a 40-µm arterial line filter and 0.5-inch thick-walled pump head boot. The COBE CML Duo oxygenator and the COBE heart-lung machine were used. Hemoconcentration was used for patients in congestive heart failure or renal failure and for those whose venous reservoir accumulated more than 2 L of volume. The Quest Medical Myocardial Protection System was used to deliver cold (5°C-8°C), intermittent, high-potassium antegrade and retrograde 4:1 blood cardioplegia in all on-pump cases.
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Results |
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Discussion |
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8,9 have reported significant reductions in risk-adjusted mortality, stroke, acute renal failure, prolonged ventilator dependence, reexploration for bleeding, and other complications with OPCAB. However, legitimate criticism of these conclusions has centered on the nonrandomized nature of the studies and the inherent bias of patient selection for OPCAB. Although statistical methods of risk adjustment and modeling have been applied in earnest, they are imperfect techniques. It is important to note that in Cleveland and coworkers' rigorous analysis of the Society of Thoracic Surgeons database, 8 the CABG with CPB group had greater proportions of patients with three-vessel disease and of urgent and emergency cases than did the OPCAB group, illustrating the role of patient selection in determining the composition of these groups.
Randomized comparisons of OPCAB and CABG with CPB are therefore necessary to obviate these concerns. However, even the larger randomized studies to date, 12,17 including this report, have limited statistical power to detect differences between groups in end points that occur infrequently. No difference was found in mortality, stroke, perioperative myocardial infarction, or acute renal failure in either this study or previous randomized studies.
This study was designed to evaluate the crucial end points of completeness of revascularization and graft patency. Although 1-year graft patency data will not be available until after August 2002, results presented here demonstrate comparable completeness of revascularization in the two treatment groups, despite the fact that there was no exclusion of patients from this study on the basis of coronary anatomy, left ventricular function, or other comorbidities. All patients referred for elective, primary, isolated CABG were considered eligible. The documentation of the intended revascularization before random assignment of patients minimized bias in the evaluation of completeness of revascularization performed. Although many authors have reported difficulty grafting the lateral wall of the left ventricle without CPB in their early experience, 12,16 this study was undertaken after the operating surgeon had accumulated a personal experience in excess of 350 OPCAB cases. Thus the achievement of complete revascularization of the lateral wall in unselected patients by an experienced OPCAB surgeon has been demonstrated. Although it would be inappropriate to generalize these results to surgeons early in their experience with OPCAB, it is reasonable to believe that similarly complete revascularization would be achieved by other surgeons who have developed extensive experience with OPCAB techniques. Thus broad application of OPCAB techniques to patients with multivessel disease need not lead to incomplete revascularization, with its documented negative consequences. 17
Previous publications have documented the strong association between serum evidence of myocardial necrosis and future adverse cardiac events in patients undergoing percutaneous interventions 18 and CABG with CPB. 19 Although it is possible that some of the myocardial enzyme release in the CPB group was caused by cannulation of the right atrium, it is likely that the large majority derived from ventricular myocardium. Thus the finding that serum levels of myocardial enzymes were lower in the OPCAB group than in the CABG with CPB group at all time points measured is of particular interest. This result is consistent with those of previous randomized 12,20 and nonrandomized 21 comparisons. Planned longitudinal follow-up of these two randomized groups of patients will allow documentation of longer-term outcomes correlated with perioperative elevation in serum levels of myocardial enzymes.
Among the various reported benefits of OPCAB relative to CABG with CPB, reduction in transfusion requirement has been remarkably consistent across multiple studies. 7,12,22 This study found that multiple indices of coagulopathy were significantly less deranged in OPCAB than in CABG with CPB and that patients undergoing OPCAB received fewer units of blood, were more likely to avoid transfusion altogether, and had a higher hematocrit at the time of hospital discharge. Interestingly, cumulative chest tube output was not significantly different between groups during the first 12 hours, despite the demonstrated difference in coagulopathy and the fact that most of the difference in transfusion requirement was seen during the day of surgery. Thus it seems likely that the cardiopulmonary circuit itself either consumes or damages red blood cells to some degree, causing removal of these cells by the reticuloendothelial system in the first hours after surgery. A cell saver device was used in every case, to equalize efforts between groups to preserve red cell mass.
As economic considerations influence medical decision making to an increasing degree worldwide, the lengths of stay associated with alternative methods of surgical coronary revascularization are increasingly relevant. Consistent with previous randomized comparisons of selected patients, 12,17 this study of unselected patients with multivessel disease demonstrated that OPCAB shortened postoperative length of stay by 1 full day. With all other outcome measures either equivalent or superior for the OPCAB group, the decrease in length of stay may prove to be an important influence on third-party payers and institutional administrators.
Conclusion
Relative to CABG with CPB in this prospective, randomized study of 200 unselected patients undergoing elective primary CABG, OPCAB achieved comparable completeness of revascularization and similar in-hospital and 30-day outcomes, shorter length of stay, reduced coagulopathy and transfusion requirement, and less myocardial injury.
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Appendix: Discussion |
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My first question is, are the differences that you noted in enzyme elevation clinically important? It seems to me that those differences, although statistically significant, are not vast. Do you plan to establish the importance of those differences with long-term follow-up?
My second question is, when you talk about arterial grafts, there are a lot of different kinds of arterial grafts. My sense is that this is sort of a left internal thoracic artery to anterior descending and then radial to something else kind of study, which is a standard technique but not the same as complex internal thoracic artery grafting. Is that the case?
My third and final question is, you used the perfusion-assisted direct coronary artery bypass (PADCAB) type strategy, which I know that you have advocated in other work, for only 1 patient in this group to keep things standard, and with the good results of your standard off-pump technique, do you think that the other strategy is no longer necessary? What are your feelings about that?
Dr Puskas. Thank you, Dr Lytle. We have all benefited from your leadership in the field of CABG and your careful examination of outcomes. Your first question was in regard to the difference in enzyme release and whether that is a clinically important difference. The Q-wave infarct rate was not statistically different between groups. The enzyme release clearly was statistically different between groups. It may be that the prolonged length of stay, 1 more day, in that group had something to do with enzyme release or myocardial injury, or it may be that we need longer longitudinal follow-up of these groups to discover adverse cardiac events that may occur later. Certainly in the immediate in-hospital period, the difference in enzyme release was not correlated with dramatically different cardiac outcomes; the infarct rate was not significantly different, and the time spent in the intensive care unit was not significantly different. The difference in length of stay was after intensive care unit transfer for the most part. So we do intend to follow up these patients through time. We will be doing annual follow-ups for the foreseeable future and trying to correlate adverse cardiac events with preoperative and postoperative events and markers from the hospital stay.
Your second question related to arterial grafting. You are correct in assuming that with 41% of all grafts being arterial conduits, the left internal thoracic artery was essentially a uniform graft for all patients. Left radial artery grafting was also very common. We did, however, have several bilateral thoracic arteries, both in situ or as composite grafts, and in several cases composite grafts were taken off the internal thoracic pedicles and anastomosed to the lateral wall in either a surgical Y or sequential fashion to avoid manipulation of the ascending aorta. So there were a handful, or a couple of handfuls, of what you might consider complex arterial grafts with composite right off left internal thoracic artery anastomoses to multiple lateral wall vessels. As you know, that is not often an absolute necessity, and our standard grafting procedure was left internal thoracic artery to the left anterior descending coronary artery, left radial artery to the biggest lateral wall graft and veins to the right side.
Your third question related to PADCAB and our use of it. We did use the PADCAB for 1 procedure in the OPCAB group, a complex five-vessel CABG on a morbidly obese man with a profoundly depressed left ventricular ejection fraction and preoperative myocardial infarction. We considered the PADCAB to be helpful in keeping that man in stable condition during the operation. I think that it is also helpful during one's learning curve and for treating similar patients after one's learning curve. I think it does play a role. But this study was not directed to studying the benefit of PADCAB, so we tried to limit its application in this study to avoid confounding influences.
Dr Lawrence I. Bonchek (Lancaster, Pa). Completion of such a randomized study is a considerable achievement, and I congratulate Puskas and coworkers. It is interesting that another randomized study by Jansen and coworkers from The Netherlands, recently published in Circulation, found that the only major benefits of OPCAB were less release of cardiac enzymes and shorter length of stay. Neither that study nor this revealed differences where we would have hoped to see them, in the incidences of neurologic deficits, atrial fibrillation, and renal or pulmonary complications.
First, can you discuss the significance of the difference in enzyme release? Because in your study OPCAB was done with clamping of single arteries and thus only brief regional ischemia, whereas CABG with CPB was done with prolonged global ischemia and cardioplegic cell membrane paralysis, the difference in enzyme release is understandable. Of course, CABG with CPB can also be done with single-vessel occlusion without cardioplegia, as is done by our group and many others, which might well eliminate any difference in enzyme release between the two techniques, except for the unavoidable release related to cannulation of the right atrium in on-pump cases. I therefore suspect that the differences in enzyme release are unimportant.
Second, it is important to note that these excellent results were achieved by an exceptional surgeon. This does not reassure me that OPCAB is for everyone. In fact, in large groups that do a lot of OPCAB procedures, most of the cases are done by a few surgeons. Would you also comment on that observation?
And finally, because you were kind enough to discuss this protocol with me before you began the study, I know that one of your objectives was to use blinding as much as possible; however, you did not discuss that in your presentation. Certainly if the caregivers who made the decision for hospital discharge were not blinded, that is a highly subjective decision.
Dr Puskas. Indeed, Larry and I did have several conversations about the proposed strategy for designing this study about 4 years ago, and he raises some of the same concerns today. Actually, I did mention in my earlier presentation that all management was according to written, unbiased, criteria-driven protocols and that all the nonoperative care providers, patients, their families, and referring cardiologists, as well as the nursing staff, were blinded to group assignment. The patients were identified only as study patients, and whether the procedure was OPCAB or CABG with CPB was not revealed unless it was essential for appropriate clinical management.
Your first question, Dr Bonchek, related to Jansen and colleagues' report from Utrecht of a randomized study where they selected patients not requiring lateral wall grafts and also noted a decrease in creatine kinase MB release and a shorter length of stay. That study enrolled, I believe, 281 patients; ours enrolled 200. As you well know, with the statistical power of those numbers, it is unlikely and indeed unexpected that we would be able to demonstrate statistically significant changes in outcomes that occur infrequently. Specifically, mortality, stroke, and similar infrequent events are not going to be demonstrated to be different until we have a multicenter, prospective, randomized trial enrolling at least 1000 patients. Our best evidence that OPCAB does reduce those events comes from Cleveland and others, who have carefully analyzed the Society of Thoracic Surgeons database of many tens of thousands of patients and demonstrated there is a benefit of OPCAB with respect to those end points.
Dr Bonchek, you also commented that the creatine kinase MB release is understandable and therefore unimportant. I am not sure that I would agree with that logical step. I do think that it is understandable. I do think that applying a crossclamp to the aorta and inducing global myocardial ischemia is responsible for the creatine kinase MB release. I am not sure that the application of a clamp and global ischemia is therefore unimportant. The clinical significance of it may become more apparent as we more carefully examine why length of stay is shorter for OPCAB and whether longitudinal follow-up reveals these patients to have a different set of cardiac outcomes.
Your final question related to the generalizability of these results among other surgeons. It is true that OPCAB has a long learning curve; it does require a certain devotion to the field and a certain persistence in developing one's own technique. I personally enjoy the operation. I know that there are others who do not. I do think that you will find that surgeons who choose to take up this operation and perfect their own skills will be able to achieve every bit as good results as I have demonstrated today.
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Acknowledgments |
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References |
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