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Ann Thorac Surg 2004;78:2005-2014
© 2004 The Society of Thoracic Surgeons

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Original Article: Cardiovascular

The Effect of Bilateral Internal Thoracic Artery Grafting on Survival During 20 Postoperative Years

^a Departments of Thoracic and Cardiovascular Surgery, Cleveland, OH, USA
^b Biostatistics and Epidemiology, The Cleveland Clinic Foundation, Cleveland, Ohio, USA

Accepted for publication May 17, 2004.

^* Address reprint requests to Dr Lytle, Department of Thoracic and Cardiovascular Surgery, The Cleveland Clinic Foundation, 9500 Euclid Ave #F25, Cleveland, OH 44195 (E-mail: lytleb{at}ccf.org).

Presented at the Fortieth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 26–28, 2004.

	Abstract

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

 
BACKGROUND: To compare survival of patients receiving bilateral internal thoracic artery grafts and single internal thoracic artery grafts more than 20 postoperative years, assess magnitude of benefit, and identify predictors of benefit.

METHODS: From cohorts of 8123 patients receiving single internal thoracic artery grafts and 2001 receiving bilateral internal thoracic artery grafts during primary isolated bypass operations for multivessel coronary disease between 1971 and 1989, we identified 1152 propensity-matched pairs. Mean follow-up of survivors was 16.5 years, with 51 patients followed for 20 years or more. Hazard function methodology was used to identify risk factors for mortality, compare survival, and assess magnitude of benefit.

RESULTS: Comparison of the matched pairs showed survival of the bilateral internal thoracic artery and single internal thoracic artery groups at 7, 10, 15, and 20 years was 89% versus 87%, 81% versus 78%, 67% versus 58%, and 50% versus 37%, respectively (p < 0.0001). Divergence of bilateral internal thoracic artery and single internal thoracic artery hazard function curves continued to widen through 20 postoperative years. At 20 years, bilateral internal thoracic artery grafting was predicted to produce worse survival in 2.8% of patients, a survival advantage of less than 5% in 12.9%, greater than 10% in 52%, and greater than 15% in 7.6%. Combinations of cardiac and noncardiac descriptors were used to define higher and lower risk patient subsets. Advanced age, abnormal left ventricular function and noncardiac risk factors decreased overall survival but the incremental benefit of bilateral internal thoracic artery grafting persisted.

CONCLUSIONS: Bilateral internal thoracic artery grafting produces improved survival compared with single internal thoracic artery grafting during the second postoperative decade, and the magnitude of that benefit increases through 20 postoperative years.

	Introduction

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

 
The use of both internal thoracic arteries (ITA) as bypass grafts during operations for coronary revascularization is a strategy that has been used for more than 30 years and studied extensively. In 1999 we published the results of an observational study comparing the outcomes for patients who received single ITA grafts (SITA group) with those for patients who received bilateral ITA grafts (BITA group) with or without accompanying vein grafts for both groups [1]. Multiple statistical methods were used to address issues of patient selection and heterogeneity and those analyses consistently showed that the BITA group experienced better survival, fewer reoperations, and fewer interventions when compared to the SITA group at a mean postoperative interval of 10 years. Some other institutions have noted similar observations, [2, 3] although survival benefit has not been demonstrated in all studies [4].

However, the use of the BITA strategy has not become widespread. The Society of Thoracic Surgeons registry (data from the Society of Thoracic Surgeons Adult Cardiac National Databases Thirteenth Year – Fall 2003) has noted countrywide BITA utilization between 3% and 4% with little upward trend. Bilateral ITA grafting is time consuming, increases technical difficulty of the operation, and in some patient subsets has been associated with an increased risk of serious sternal wound complications. Sparse utilization of the BITA strategy appears to be based on concern about its short-term disadvantages combined with lack of conviction concerning any long-term benefit. To provide further data regarding differences in survival based on BITA versus SITA grafting, we re-examined the same patient subset studied primarily in an attempt to achieve the following goals:

1 To establish whether or not the incremental benefit of BITA grafting changes during longer follow-up.

2 To adjust for bias in patient selection by pair matching based on propensity score.

3 To identify patient-related variables that predict either greater or lesser benefit from BITA grafting.

4 To quantify magnitude of benefit associated with BITA grafting.

	Patients and Methods

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

 
Patients from which the study sample is drawn came from 1000 patient-per-year cohorts of nonforeign, nonemergent patients undergoing primary isolated myocardial revascularization operations at the Cleveland Clinic Foundation during the years 1971 through 1989. These patients have been followed by questionnaire every 5 years. Of that group, 2001 patients underwent BITA grafting and 8123 had SITA grafting with or without accompanying vein grafts for treatment of multi-system coronary artery disease. Twenty-six BITA patients who also received non-ITA arterial grafts were excluded.

	Methods

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

 
Using the preoperative variables listed in Table 1, a propensity score was created to quantify the likelihood that a given patient would have received BITA grafts [5]. By matching propensity scores, 1142 BITA patients could be excellently pair-wise propensity matched to an SITA patient. The BITA patients who could not be matched were analyzed (Table 2) and were younger, taller, heavier, more likely to be male, and less likely to have abnormal left ventricular function, congestive heart failure, or diabetes. In other words, they were good risk patients who were highly likely to receive BITA grafts and good matches from the SITA group were not available.

	Results

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

 
Overall survival of the BITA versus SITA groups (Fig 1) was 89% versus 87%, 81% versus 78%, 67% versus 58%, and 50% versus 37%, at 7, 10, 15, and 20 postoperative years, respectively (p < 0.0001). The death hazard function curves for both groups showed that the time-related risk of death continued to widen in favor of the BITA group through 20 postoperative years (Fig 2). Survival of unmatched BITA patients (75% at 15 years, Table 2) was superior to that of the matched BITA group (p < 0.001).

View larger version (13K): [in this window] [in a new window]   Fig 1. Comparison of matched pairs of patients receiving BITA and SITA grafts. The numbers of patients surviving at selected follow-up intervals are listed (p < 0.001). Each symbol represents a death, and vertical bars depict the 68% confidence limits (equivalent to one standard error) of Kaplan-Meier estimates. Solid lines, enclosed within 68% confidence limits, are parametric estimates. (BITA = bilateral internal thoracic artery; SITA = single internal thoracic artery.)

View larger version (12K): [in this window] [in a new window]   Fig 2. Hazard function curves demonstrate the increased risk of death associated with SITA grafting with increasing follow-up interval. Dashed lines are 68% confidence bands. (BITA = bilateral internal thoracic artery; SITA = single internal thoracic artery.)

View larger version (23K): [in this window] [in a new window]   Fig 3. Survival of matched pairs stratified by BITA and SITA according to normal or mildly impaired LVF (LVF normal/mild) or moderately or severely impaired left ventricular dysfunction (moderate/severe LVF). (BITA = bilateral internal thoracic artery; LVF = left ventricular function; SITA = single internal thoracic artery.)

View larger version (11K): [in this window] [in a new window]   Fig 4. Cumulative distribution of differences in survival between BITA and SITA for each patient in the study. The nonproportional hazard equations were solved twice for each patient, once as if the patient had SITA grafting and once as if the patient had BITA grafting. (BITA = bilateral internal thoracic artery; SITA = single internal thoracic artery.)

View larger version (24K): [in this window] [in a new window]   Fig 5. Predicted outcomes for patient subsets based on age and the presence or absence of LV dysfunction and noncardiac risk factors. (A) Patients with neither LV dysfunction nor risk factors had extremely good survival at 20 years and incrementally better survival with BITA grafting, although the differences were small. (B) Patients with LV dysfunction derived more benefit from BITA grafting, although overall survival was impaired. (C) Noncardiac risk factors influenced both groups negatively. (D) Combining LV dysfunction and noncardiac risk factors predicted extremely poor outcomes but still with some benefit from BITA grafting. (BITA = bilateral internal thoracic artery; LV = left ventricular; SITA = single internal thoracic artery.)

View larger version (53K): [in this window] [in a new window]   Fig 6. Three-dimensional depiction of risk factor profile, age, and duration of follow-up on predicted survival difference between BITA and SITA. (A) "Ideal" patient profile (red grid, and see Fig 5A) and combined cardiac and noncardiac morbidity profile (blue grid, and see Fig 5). (B) Cardiac morbidity patient profile (blue grid and see Fig 5B) and noncardiac morbidity profile (red grid and see Fig 5C). (BITA = bilateral internal thoracic artery; SITA = single internal thoracic artery.)

	Comment

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

These patients were not randomized to BITA or SITA grafting. Therefore, the first and most important problem with this type of analysis is the possibility that surgeons may have selected particularly good risk patients for BITA grafting and that those preoperative patient-related differences might account for that group's improved survival. To approach that issue we created a propensity score using the variables listed in Table 1. A propensity score is a numerical expression of the probability that a patient with such characteristics would have received BITA grafting. We then matched pairs of patients according to that score. This type of matching does not create pairs matched exactly for every variable in Table 1 but it does create very closely matched patient groups as demonstrated in Table 2. Not all the patients in the BITA group from our first study could be matched with patients in the SITA group. This occurred because during the years of the study it was our opinion that young patients, usually male with favorable risk factors, should receive BITA grafting and such patients were so likely to receive BITA grafting that there were not enough patients in the SITA group that could be matched with these very good-risk BITA patients. The effect of this matching, therefore, was to exclude some very good-risk BITA patients from the analyses of this current study. This is evidenced by the fact that the survival of the unmatched BITA group was superior to that of the matched BITA patients.

The survival experience of the patients included in the matched pairs was then analyzed using three different actual and constructed patient populations. First, simple comparisons of the actual matched pairs were made for the overall groups (Fig 1) and for subgroups separated according to left ventricular function (Fig 3). Second, to allow calculation of the predicted benefit of BITA grafting for each patient, risk factor analysis was used to derive coefficients pertaining to each variable and the entire patient population was examined changing only one variable, whether or not the patient received BITA grafting (Fig 4). Third, to allow identification of patient subsets defined by multiple variables of patients receiving greater or lesser benefit from BITA grafting, patient profiles were constructed using multiple coefficients (Figs 5 and 6).

Simple comparison of the BITA and SITA matched pairs (Fig 1) demonstrated a superiority in survival for the BITA group. The survival difference continued to widen out to 20 postoperative years and hazard function analysis found that the time-related risk of death increased for both groups with increasing follow-up, but increased more for the patients in the SITA group. A risk factor analysis (Table 3) of the patients in the matched pair population also identified SITA grafting as a risk factor for death in the constant and late phase although not in the early phase.

The observation that the entire group had a statistically significantly improved survival over the entire follow-up period does not necessarily mean that all patients would benefit in a clinically significant way. The analyses depicted in Figure 4, however, indicated that the incremental survival benefit at 20 post operative years would be 10% or greater for the majority of the study patients. Construction of patient profiles with coefficients of multiple variables identified elderly patients with a small body surface area as the only subset likely to have worse outcomes with BITA grafting. This decreased survival was not large and occurred early in the follow-up period (Fig 6).

All other patient subsets were predicted to have better survival with BITA grafts, although different subsets were affected differently and at different points during the follow-up. Young patients without risk factors were at a low risk of death regardless of surgical strategy and the benefit of BITA grafting occurred late in the follow-up period although the incremental benefit increases with time (Fig 5A). For patients without either left ventricular dysfunction or noncardiac risk factors, the 20-year outcomes are important even in older age groups because substantial numbers of patients survive to that point. For patients with left ventricular dysfunction alone there appears to be a substantial benefit of BITA grafting (> 10%) for most age groups although for patients greater than 70 years old the benefit decreases during the last 5 years of follow-up because of the poor survival of both the BITA and the SITA group at that time. Noncardiac morbidity substantially reduces life expectancy regardless of age. However, even for patients who are substantially impacted by noncardiac morbidity, BITA grafting appears to improve survival, although that improvement occurs early in the follow-up period before the majority of the patients in both groups have succumbed. Death ends the benefit of BITA grafting.

In our last study that compared the SITA and BITA strategies we examined the end points of reoperation and percutaneous revascularization in addition to death [1]. In that study the benefit of BITA grafts in protecting against reoperation was stronger than the benefit in protecting against death. Reoperation or percutaneous intervention may be a more accurate measure of the failure of revascularization as virtually all such procedures are performed for patients who have recurrent myocardial ischemia whereas many deaths occur for reasons unrelated to ischemia. However, we focused solely on survival in this study for multiple reasons, the most important being that demonstration of incremental survival benefit is the most compelling reason for use of the BITA strategy.

For most patients BITA grafting is a more complex operation when compared to the SITA-vein graft strategy and many arguments have been advanced to justify avoiding BITA grafts. One is that the patient population today differs from the patients in this study. Overall, that is true. However, examination of patient subsets that are comparable to today's still show a benefit of BITA grafting except for elderly patients with a small body surface area. Another argument is that the benefit of BITA grafting occurs so long into the follow-up period that it is irrelevant. However, even for the patients in the study who are more than 70 at operation, substantial numbers are still alive 15 years after surgery and are still receiving incremental benefit. For patients less than 50 years old at the time of their surgery, approximately 80% are still alive 20 years after surgery, a point where the benefit of BITA grafting is still increasing. Today we have available pharmacologic agents such as platelet inhibitors and statin type drugs that may further prolong overall survival and make the incremental benefit of arterial grafting even more important [10–12].

The incremental survival benefit of BITA grafting over SITA grafting may not be as large as the benefit of LITA to LAD grafts when compared to a vein graft only strategy [13, 14]. Not all patients benefit from BITA grafting equally and that benefit occurs at different points in the follow-up period for different patient subsets. However, the idea that once a LITA to LAD graft has been done, that no other operative strategy will further improve the long-term survival is not correct. This study compares a good operation (SITA grafting) to a better one (BITA grafting). It has taken some time to demonstrate the incremental survival benefit of BITA grafts, but benefit there is. The longer the follow-up, the more effective the BITA strategy appears.

	Discussion

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DR JOHN D. PUSKAS (Atlanta, GA): President Guyton, Secretary Murray, members, and guests. Doctor Lytle has presented a fundamentally important third installment in his epic trilogy of landmark papers in coronary surgery from the Cleveland Clinic. This trilogy began in 1986 with the first conclusive demonstration that the use of a single internal thoracic artery (ITA) improved 10-year survival and that patients who received only vein grafts had increased risk for late MI, cardiac events and reoperation.

The second installment came in 1999 when Dr Lytle demonstrated that two ITA grafts are better than one. This retrospective review of some 2000 bilateral and more than 8000 single ITA cases featured multivariate and propensity score matching by Dr Blackstone and revealed that survival at 5, 10, and 15 years was superior for patients who received bilateral ITAs. During mean follow-up of 10 years, death, reoperation, and percutaneous transluminal coronary atery (PTCA) were significantly more frequent for patients who received single ITA grafting, however, the survival benefit of bilateral ITA grafting was relatively small.

Now, in this third landmark paper, Drs Lytle and Blackstone have carefully studied 1152 propensity-matched pairs of patients who had bilateral or single ITA grafting between 1971 and 1989. They report a single end point: survival more than 20 years follow-up. Bilateral ITA patients had significantly better survival, and these hazard function curves continued to diverge at 20 years. Age, left ventricular dysfunction, and noncardiac risk factors decreased overall survival, but importantly, incremental benefit of bilateral ITA grafting persisted despite these preoperative risk factors.

This is a fundamentally important landmark paper that draws on several tremendous strengths. Doctor Blackstone performed a very rigorous statistical analysis, creating a propensity-matching model that included 78 patient factors and a sophisticated predictive statistical model that quantified expected benefit of bilateral ITA grafting in various patient subsets. Moreover, the sample size is very large, 1152 pair-matched groups. These were followed for a very long mean period of 16.2 years to monitor a single, unequivocal, hard end point: death.

For the sake of discussion I point out that this is necessarily a retrospective study and that it is conceivable that patient selection bias could relate to factors not quantifiable or not included in the propensity model. Moreover, after reading this paper, I am hungry for more information about these two groups.

Again, for the sake of discussion, it must be mentioned that not all studies of this question have reached the same conclusion. Doctor Hartzell Schaff and his group at the Mayo Clinic reported in 1997 on 160 bilateral ITA grafts versus 161 single ITA grafts. Importantly, the single ITA patients had more diabetes, and during 10-year follow-up, multivariate analysis showed that the number of ITA grafts was not an independent predictor of death. Diabetes in the single ITA group seemed to account for most of the survival advantage in the bilateral ITA group. However, it is important to note that in the present study Drs Lytle and Blackstone performed, the two groups were very carefully matched to have identical incidence of diabetes.

Interestingly, not every paper Dr Blackstone has authored reports a survival benefit of bilateral ITA grafting. In 1998, he and Dr Paul Sergeant published a retrospective study of 9,600 patients operated between 1971 and 1992 in Belgium. A multivariable hazard function analyzed the impact of arterial grafting on freedom from infarction. Arterial grafting lowered rates of infarction and improved 10-year survival, however, no additional benefit could be demonstrated for more than one arterial graft, even though there were more than 1700 patients in that subset.

Doctor Lytle, let me congratulate you on another superb contribution to the field of coronary surgery. We are all indebted to you for your leadership.

I have two sets of questions that I hope will be of interest to our membership. The first set is very hands-on and technical, the second more analytic and philosophical. Which vessels do you graft with your bilateral ITAs? In particular, where do you put the right ITA? What, therefore, is your most common configuration of conduits for an isolated three, four, or five vessel bypass? What, if any, is the added benefit of a third arterial conduit after bilateral grafting to the left system? Will skeletonized harvest of the ITA allow us to safely apply bilateral ITA grafting to broader patient groups and should we expect the same benefit?

Why have some comparisons of bilateral versus single ITA grafting not shown an independent benefit? Your matched patients were generally young, male, thin and nondiabetic, but we presently face an epidemic of obesity and diabetes in America and increasingly we are referred many patients with severe LV dysfunction. Does this new reality temper your enthusiasm for bilateral ITA grafting? In what percentage of your own cases do you perform bilateral ITA grafting. Who are these patients, and who does not get bilateral ITAs? Specifically, who is too old or too thin or too sick to benefit? Will your study increase your own use of bilateral ITA grafts? Are multiple arterial grafts the future of coronary bypass surgery? Is this how we counter the threat of drug-coated stents?

And, in your opinion, what should be the frequency of bilateral ITA grafting in the STS database, and how do we as a profession get there? Thank you.

DR LYTLE: Thank you, Dr Puskas. Selection bias is always an issue; there is no statistical method that makes a nonrandomized trial a randomized trial. On the other hand, we have been very careful to try to exclude those patients from analysis that were not comparable. When we reviewed our first study and considered the statistical methods that we used, we felt that the pair-matching strategy used in this current study was better. The reason it was better was because we did exclude the very, very good risk patient where we were very likely to do bilateral internal thoracic artery grafting from comparative analysis by virtue of the fact that they were not suitable matches in the single internal thoracic artery group.

I think that the fact that the selective factor that we can't account for very well in this database is coronary anatomy. We did examine issues like LAD stenosis, circumflex stenosis, right stenosis, and so on, but those variables don't tell you when the right is a huge vessel and the LAD is small, and I'm not aware of any database that really does account for that very well. Certainly, those factors do influence what we as surgeons would do at the time of an operation.

Today the most common grafting configuration that I use is to use the left internal mammary artery as an in situ graft and bring the right internal mammary artery off the left internal mammary artery as a free graft. Dr Tector has advocated that strategy for many years. There are a bunch of advantages of this approach. Most importantly, you can use the right ITA for a lot of anastomoses on the left side of the heart because you have a lot of length to work with. Sometimes it is possible to use it as a graft to the right coronary artery as well but I don't do that routinely. My colleague, Dr Joe Sabik, is going to present a paper later in this meeting that is going to show that patency of IMA grafts to the right coronary system is not quite as good as those grafted to the left coronary system. Although I sometimes used 3rd and 4th arterial conduits, I really don't have long-term follow-up information about that.

We do skeletonized internal thoracic arteries when possible and that strategy has extended the extent to which I use them for diabetic patients. I do not use bilateral internal thoracic artery grafting in obese, elderly, female, insulin-taking diabetics. I do use this strategy for a lot of other diabetics who are younger and not obese. I am aware that other studies have not shown comparable benefit in terms of survival. One of the things that we learned from this study is that all patient subsets do not have the same type of survival improvement at the same point of time during the follow-up period. Particularly for reasonably good risk patients, those patients have to be followed for a long period of time and there have to be a lot of them. A well-done study by Dr Paul Sergeant did not show survival benefit from BITA grafting. However, in that study the numbers of patients receiving BITA grafts were smaller and they were followed for less than 10 postoperative years. As we have seen in this study, good risk patients do benefit but only during the second follow-up decade.

In Dr Sergeant's study, their message was that many other things affect long-term survival other than surgical strategy and those other conditions overwhelm the impact of arterial grafting. It is certainly true that there are a lot of factors that influence survival that are not eliminated by arterial grafting but in our studies the benefit of BITA grafting was not overwhelmed by those other risk factors. The other risk factors are important but there is still a benefit of BITA grafting.

I am not saying what an individual surgeon should or shouldn't do in regard to the use of BITA grafts. Personally, I do things differently now than I did 10 years ago, being much more aggressive with the use of arterial grafts. The single ITA to the LAD combined with vein grafts is a good operation but BITA grafting is a better operation. The idea that has grown up that once you do an IMA to the LAD that you can't do anything else to improve the long-term outcome is not correct. BITA grafting does add incremental benefit.

DR J. SCOTT RANKIN (Nashville, TN): I rise to congratulate Dr Lytle and his colleagues on an excellent contribution and appreciate their sharing their manuscript ahead of time with me and my old friend Dr Frank Harrell, who is now Chairman of Biostatistics at Vanderbilt.

This is an outstanding study using state-of-the-art statistical methodology that demonstrates definite clinical benefits of bilateral internal thoracic artery grafting. We must remember, however, that significant selection bias existed in this series, with bilateral grafts being performed primarily in lower risk patients. While propensity matching likely compensates for this problem, the focus still is selected, lower risk patients.

(Slide) In an average 15-year follow-up of approximately 1000 patients presented at the American Heart Association Meeting last fall, we were able to minimize selection bias and base line differences by having two surgeons test these divergent grafting strategies in the same practice over the same time. Across the entire spectrum of patient risk, multiple internal mammary artery (IMA) grafting, in the top panel in red, improved 15-year survival by only 5% when compared to single IMA grafting, in green. In the bottom panel, multiple IMA grafting significantly reduced, by 9%, the 15-year composite end point of death, myocardial infarction (MI), PCI, and redo coronary bypass.

In our judgment, however, these modest benefits do not justify routine bilateral IMA harvesting across the entire spectrum of multivessel patients, the hypothesis tested in our study. But from Dr Lytle's work it is now clear that clinically significant benefits do exist in selected patients with lower risk profiles, that is, patients with prospects for longer survival. I have three questions, Bruce: How do we select patients now for bilateral internal thoracic grafts and what variables do we use? Second, what percentage of multivessel patients should have bilateral IMA grafts? And third, what impact will catheter-based interventions have in the future on the availability of these "ideal" patients for bilateral procedures?

Thank you, and I appreciate the opportunity to discuss the paper.

DR LYTLE: Thank you, Dr Rankin. First, I personally select patients for bilateral ITA grafting who do not have life-limiting, noncardiac morbidity, who are not obese diabetics, who are less than 80 years of age, and whose coronary anatomy presents two important coronary vessels with greater than 50% stenoses that can be grafted with the internal thoracic arteries. Second, the percentage of patients receiving bilateral ITA grafts will vary according to the type of patient population a physician sees. Third, and somewhat related to the second part of your question, it is clear that the advances in the safety of percutaneous procedures have encouraged patients and cardiologists to expand the types of patients now treated by intervention. Many of these patients would be ideal patients to receive complex arterial grafting. However, we must be realistic that in the past relatively few such patients actually received complex arterial grafting when they were referred for operation. Our interventional cardiologists correctly point out that when the patency rate of intracoronary stents are compared with vein grafts, they look a lot better than when they are compared with internal thoracic artery grafts. The advantages of surgery are related to long-term outcomes. Regardless of how minimally invasive we make bypass surgery, it is not going to be less invasive than a femoral artery puncture. One way to improve the long-term outcomes of bypass surgery is by the use of multiple arterial grafts. Thank you very much.

	References

Top Abstract Introduction Patients and Methods Methods Results Comment Discussion References

 

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