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Ann Thorac Surg 2001;71:170-175
© 2001 The Society of Thoracic Surgeons
a Division of Cardiothoracic Surgery, Department of Surgery, University of Utah Health Sciences Center, Salt Lake City, Utah, USA
Address reprint requests to Dr Bull, Division of Cardiothoracic Surgery, Department of Surgery, University of Utah Health Sciences Center, 50 N. Medical Dr, Salt Lake City, UT 84132
e-mail: david.bull{at}hsc.utah.edu
Presented at the Thirty-sixth Annual Meeting of The Society of Thoracic Surgeons, Fort Lauderdale, FL, January 31–Feb 2, 2000.
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Abstract |
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Methods. We prospectively followed 80 patients undergoing CABG: 40 patients undergoing OPCAB and 40 patients undergoing CABG with CPB. OPCAB patients underwent angiography within 48 hours of surgery to determine early graft patency. Incidence of complications, length of stay, and costs were recorded for each patient. The influence of the number of vessels bypassed was analyzed.
Results. OPCAB patients (n = 40) underwent grafting of 2.7 ± 0.7 vessels per patient compared with 3.6 ± 0.8 vessels per patient in the CABG with CPB group (n = 40) (p < 0.0001). Angiography demonstrated 105 of 108 (97%) of grafts were patent in the OPCAB group. Incidence of complications, length of stay, and costs did not differ between the OPCAB and CABG with CPB groups. Number of vessels grafted showed a positive correlation to total costs in both groups.
Conclusions. While OPCAB provided satisfactory early graft patency, there was no significant difference between OPCAB and CABG with CPB with regard to cost, length of stay, or incidence of complications. In this study, eliminating CPB did not reduce morbidity or cost after CABG.
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Introduction |
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TopAbstractIntroductionMaterial and methodsResultsCommentDiscussionReferences |
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Material and methods |
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TopAbstractIntroductionMaterial and methodsResultsCommentDiscussionReferences |
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Sample size calculations were performed before the initiation of the study. Using data from the literature, we assumed a total cost of $20,000 ± 4,000 and calculated the sample size needed to detect a $4,000 difference between groups (Sample Power Version 1.2; SPSS, Inc, Chicago, IL). These calculations revealed a power level of 87% for accrual of 40 patients (20 per group) and a power level of 99% for accrual of 80 patients (40 per group).
All patient data were collected according to the Society of Thoracic Surgeons national cardiac surgery database (STS.NCSD). Data regarding patient age, gender, and incidence of preoperative risk factors including diabetes, history of myocardial infarction (MI), stroke (CVA), chronic obstructive pulmonary disease (COPD), and renal insufficiency were collected for both groups. Perioperatively, number of vessels grafted, procedure time, time to extubation, chest tube output, and transfusion of blood products were recorded for both groups. Postoperative data regarding need for reoperation for bleeding and incidence of complications including myocardial infarction, stroke, renal failure, mediastinitis, atrial fibrillation, and death were recorded for both groups. Length of stay in the intensive care unit (ICU) and to discharge was noted. Hospital costs in surgery, the ICU, and inclusive from the time of surgery to discharge were recorded. Costs incurred during the postoperative coronary angiograms or catheter-based interventions were not included in the analysis of the OPCAB group, as angiography was utilized for the study to determine graft patency.
OPCAB was performed through a sternotomy incision. Conduits for CABG including the left internal mammary artery, radial artery, and saphenous vein were harvested in the standard fashion. Deep pericardial traction sutures were placed to facilitate elevation of the apex of the heart and exposure of the lateral wall of the myocardium. The right pleural space was opened when required to allow displacement of the heart to facilitate exposure of the circumflex coronary vessels. Revascularization of the left anterior descending coronary with the left internal mammary artery was typically performed first, followed by revascularization of the right coronary artery and the circumflex coronary artery distributions. After completion of the distal anastomoses, the proximal anastomoses were performed with the assistance of a partial occlusion aortic clamp. Stabilization of the coronary arteries was accomplished using a commercially available stabilization system.
CABG with CPB was performed through a sternotomy incision. Conduits for CABG including the left internal mammary artery, radial artery, and saphenous vein were harvested in the standard fashion. CPB was established by cannulating the ascending aorta and the right atrium. Both antegrade and retrograde blood cardioplegia were used for myocardial protection during the period of cardiac arrest.
After surgery, both groups of patients were admitted to the ICU. The patients were extubated as soon as clinically indicated. An early extubation protocol was employed for both groups of patients. The patients were transferred from the ICU and discharged to home when clinically indicated by the attending surgeon.
The data are expressed as a mean ± standard deviation. Data were analyzed using the intention to treat principle. Analysis of variance was used to analyze continuous variables, while 
2 was used to analyze nominal variables. A p value less than 0.05 was considered statistically significant. Regression analysis was used to analyze multiple variables between groups (Statview; Abacus Concepts, Berkeley, CA). Fisher’s r to z test was used to determine the p value for the regression analysis.
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Results |
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As illustrated in Table 1, there was no difference between the OPCAB group and the CABG with CPB group with regard to age, gender, incidence of diabetes, previous myocardial infarction, stroke, chronic obstructive pulmonary disease, or renal insufficiency.
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Comment |
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We found no differences in the incidence of postoperative complications between the OPCABG and CABG with CPB groups. Based on the patient profiles in our study, one might predict a low incidence of complications for both the OPCAB and CABG with CPB groups. In our experience, low-risk patients undergoing CABG with CPB already have low morbidity and mortality rates and tend to leave the ICU rapidly. As patients still have to recover from coronary revascularization with a sternotomy, elimination of CPB for CABG does not significantly impact morbidity in these patients. Additionally, because the rates of many complications are very low (in the range of 1% to 5%), to show a significant difference between groups would require a sample size of well over 1,000 patients.
We also found no meaningful difference between the two groups with regard to overall length of stay or costs. Recent studies, both retrospective and prospective, suggest that OPCAB can reduce morbidity and cost when compared with CABG with CPB [3, 4]. This contradicts our present findings. The resolution of this incongruence lies in a closer examination of these recent studies. Several of these studies suffer from methodologic limitations in the OPCAB group, such as exclusion of patients requiring grafts to the distal circumflex [4] and two or fewer grafts per patient [5]. Some use historical controls for the CABG with CPB group [5]. Each of these factors leads to comparison of disparate patient populations. For instance, more than 60% of patients undergoing CABG nationally have three or more grafts placed, while only 16% have two or fewer grafts placed [6]. Regression analysis in our study demonstrated a positive correlation between the number of vessels grafted per patient and total costs for patients after OPCAB or CABG with CPB. Comparing OPCAB patients with a database of CABG with CPB patients would statistically weight the CABG with CPB group with patients who underwent more grafts. Patients undergoing grafting of two or fewer vessels with OPCAB would therefore be expected to have comparatively decreased costs. Interestingly, the correlation between number of vessels grafted and cost in our study does not manifest as a lower total cost in the OPCAB group, likely because the majority of patients undergoing grafting of less than three vessels with OPCAB still had a hospital stay of at least 3 days.
Morbidity and cost after CABG appear to be driven by factors beyond the use of CPB: the preoperative risk profile of the patient and decisions made by the surgeon in the operating room. Importantly, while OPCAB patients had shorter procedure times and lower chest tube outputs compared with CABG with CPB patients, they also had fewer grafts per patient. This suggests that intraoperative considerations may cause a surgeon to perform fewer bypasses per patient compared with patients undergoing CABG with CPB. Whether this will result in increased future reinterventions or cardiac events for OPCAB patients remains to be determined.
A limitation of our study is that the patients were not randomized. The attending surgeon determined which procedure was performed on a particular patient. This introduces the possibility of selection bias. The preoperative coronary artery pathology appears to have played a role in patient selection: patients requiring four or more grafts were more likely to undergo CABG with CPB. Notably, 90% of CABG with CPB patients had circumflex grafts placed, while only 70% of OPCAB patients had circumflex grafts placed. Because costs in the OPCAB group were not significantly lower, the ratio of costs to number of vessels grafted per patient may actually be higher in the OPCAB group. The resolution of these issues will require a prospective randomized trial to definitively compare the two techniques.
The finding that OPCAB does not demonstrably lower morbidity and cost for low-risk patients undergoing CABG does not negate the potential benefits of OPCAB. OPCAB may lower morbidity for particular subgroups of patients after CABG [7, 8]. Before the emergence of alternative revascularization techniques, the use of CPB had become ubiquitous for CABG. OPCAB represents an alternative platform to safely and effectively accomplish revascularization, allowing the surgeon to consider whether CABG might be accomplished more safety in a particular patient without the use of CPB. Indeed, the greatest application of OPCAB may be in extending CABG to patients who would otherwise not be candidates because of an increased operative risk with exposure to CPB [7].
Questions not addressed by our study include whether OPCAB procedures can be confirmed prospectively to benefit selected subgroups of patients requiring CABG: the elderly, the calcified ascending aorta, renal insufficiency, or patients requiring reoperative bypass grafting. Another important question is whether OPCAB can reduce cognitive dysfunction after bypass grafting compared with CABG with CPB [9, 10]. Finally, long-term follow-up is necessary to determine whether the need for reinterventions and subsequent cardiac events will differ between OPCAB and CABG with CPB patients. The study of the appropriate application of OPCAB procedures to patients undergoing CABG should be a productive area of study for many years.
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Discussion |
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This is a study of 80 patients prospectively identified over 12 months. To ensure equivalency of the groups, all patients had to be candidates for either on-pump or off-pump surgery. However, having been thus identified, patients were not randomized to either group, but the surgeon assigned them on a case-by-case basis, thus opening the door to possible selection bias and perhaps rendering the groups not equivalent. Indeed, there is a statistically significant difference in number of vessels grafted per patient and perhaps in other areas. Though the data were collected in accordance with the STS National Database, the predicted risk of mortality for the two groups was not reported, and thus, we do not know if it is similar.
There was a 5% conversion rate from the off-pump group to the on-pump surgery. In groups active in off-pump surgery, conversion rates ultimately range from 0% to 2.5%. Noting that conversion rates diminish in time, we would wonder if the early portion of the learning curve may not have been included in this study and, thus, if the study may not be a bit premature. Further, it is noted that under the intention to treat format, patients who were converted to on-pump surgery were retained in the off-pump group for purposes of analysis of the data.
Over 12 months, 80 patients were identified who were candidates for this study. It is not stated what the selection criteria were, or from how many patients these 80 came. In a preliminary analysis of our own database of on-pump and off-pump patients, we agree with you that differences in morbidity are only noted in the highest risk patient populations. This observation is in concurrence with your manuscript statement that perhaps your patient selection criteria were too narrow to yield significant differences.
Finally, perhaps the end points measured were not adequately sensitive to differentiate differences in the populations. No significant differences in stroke rate were noted, but as you have mentioned, the more subtle changes in the neurocognitive dysfunction were not measured. In closing, I would like to ask Dr Bull several questions.
From how may patients were your 80 patients chosen? Did the selection criteria eliminate the high-risk patients who would most likely to show different outcomes? What were the preoperative predicted mortality of the two groups and were they comparable? Would the data have been different if the patients who were converted to cardiopulmonary bypass were not retained in the off-pump group? And based on this analysis, going forward, what patients will you and your group select for the off-pump procedure?
Again, I congratulate you on this excellent work, your high patency levels, and thank you for the privilege to discuss this paper.
DR VALAVANUR A. SUBRAMANIAN (New York, NY): I rise to congratulate Dr Bull and his colleagues for a provocative paper indicating equivalency of OPCAB versus conventional coronary bypass in an extremely low-risk group of patients. Six years ago, when we started minimally invasive cardiac surgery, we felt the benefit of this approach would most probably be demonstrated in high-risk group of patients undergoing CABG.
I was impressed by the early graft patency data, which you have presented here. Can you tell us the graft patency only in the posterior circumflex vessels and do you have quantitative coronary angiography data in these grafts, not merely the qualitative appearance of the anastomosis, because from our experience, the quantitative data correlate well with late reintervention of the target vessels grafted?
Did you calculate the number of days patients stayed in the hospital for postoperative angiography in the OPCAB group, and if you had subtracted that, would the length of stay have been different?
DR ROBERT A. GUYTON (Atlanta, GA): I enjoyed the paper very much. It is an excellent presentation. The point that I wish to make is that I am not sure that you can fully realize the cost benefits of off-pump coronary bypass without a concerted effort to extubate the patient either in the operating room or immediately upon arrival in the recovery room, and attempting to skip the ICU altogether to take them back to the intermediate care area on the day of surgery. This has been a focus of our work and a number of other people.
Do you have a program for extubation early and skipping the ICU, and if so, what percentage of the patients in the off-pump group skip the ICU and what percentage of the patients in the on-pump group skip the ICU?
DR HANI SHENNIB (Quebec, Canada): I enjoyed the presentation very much and I want to congratulate the author for attempting to do a prospective study looking at the difference between OPCAB and conventional bypass. My concern with the study is focused primarily on the statistical design of it.
You have calculated your power based on a very wide margin of potential difference in the cost between one group and the other, which necessitated that the numbers are much smaller than if you were to prove that there is a difference in morbidity or other outcomes between OPCAB and conventional surgery, and I caution the attempt to conclude that morbidity or others are equivalent between one and another in a very small size of the two groups that you presented.
You can only determine what you are actually looking for based on your methodology, which is a difference or not in the cost of the two operations but not the others, morbidity and so on.
DR BULL: The 80 patients reported in our study represent a subgroup of our 600 coronary revascularizations per year. Study patients had to be candidates for either OPCAB or CABG with cardiopulmonary bypass, and not have any exclusionary criteria. The predicted mortality was 2.3% for the OPCAB group and 2.7% for the CABG with cardiopulmonary bypass group. The results do not differ if the patients who converted to cardiopulmonary bypass to complete their revascularizations are analyzed with the CABG with cardiopulmonary bypass group. To demonstrate a possible difference in outcomes between OPCAB and CABG with cardiopulmonary bypass for low-risk patients will require large numbers of patients, perhaps a thousand patients per group. We are focusing our efforts, then, on selected high-risk groups of patients presenting for coronary revascularization: the elderly, patients with atherosclerotic ascending aortas, renal failure, reoperations, and patients with depressed ventricular function. OPCAB may be demonstrably superior in these subgroups of patients at increased risk of complications with the use of cardiopulmonary bypass.
Twenty-eight of the 40 OPCAB patients, or 70%, had grafts placed to the circumflex distribution. Twenty-seven of the 28 grafts were widely patient; 1 patient had a stent placed for a narrowing in the body of the graft. Unfortunately, we do not perform quantitative coronary angiography in our institution, so I am unable to provide that information. Postoperative angiography was performed through the radial artery or through the femoral artery, followed by placement of a plug device. Both techniques allowed the patients to ambulate 2 hours after angiography, so we believe the impact of postoperative angiography on length of stay in the OPCAB group was negligible.
We have an early extubation protocol for both OPCAB and CABG with cardiopulmonary bypass patients, but continue to admit both groups of patients to the ICU after surgery. We did not have the OPCAB patients on a different care track postoperatively, as we wanted to make the postoperative care plan of the two patient groups as similar as possible to isolate the contribution that cardiopulmonary bypass was making to length of stay and cost.
We performed the power analysis at the outset of the study to determine the patient sample sizes needed to detect a difference in costs between the OPCAB and CABG with cardiopulmonary bypass groups. The assumption of a $4,000 difference in costs between the two groups is based on retrospective studies in the literature. Based on other data from these retrospective analyses, we assumed that we would be able to detect a difference in morbidity as well. As our study demonstrates, once the patient groups for OPCAB and CABG with cardiopulmonary bypass become equivalent, differences in outcomes between the two procedures become more difficult to detect. We agree that to detect a possible difference in morbidity between the two procedures for low-risk patients undergoing coronary revascularization will require larger patient populations, perhaps a thousand patients per group. Until our present study, however, the fact that larger patient populations will be needed to detect possible differences in morbidity was not widely appreciated.
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