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Ann Thorac Surg 2005;79:544-551
© 2005 The Society of Thoracic Surgeons
a Department of Thoracic and Cardiovascular Surgery
b Department of Biostatistics and Epidemiology, The Cleveland Clinic Foundation, Cleveland, Ohio
Accepted for publication July 19, 2004.
* Address reprint requests to Dr Sabik, Department of Thoracic and Cardiovascular Surgery, The Cleveland Clinic Foundation, 9500 Euclid Ave, Desk F24, Cleveland, OH 44195 (E-mail: sabikj{at}ccf.org).
Presented at the Fortieth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 26–28, 2004.
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Abstract |
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 Top Abstract IntroductionPatients and MethodsResultsCommentSummaryDISCUSSIONReferences |
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METHODS: From 1972 to 1999, 50,278 patients underwent primary coronary surgery; subsequently, 4,333 had angiography of 2,121 internal thoracic artery and 8,733 saphenous vein grafts. Longitudinal analysis was used to model graft occlusion and identify risk factors. Using the model, patency was calculated twice for each graft and compared first as if an internal thoracic artery, and second as if a saphenous vein, were used.
RESULTS: Unadjusted 1-, 5-, and 10-year patency was 93%, 88%, and 90% for internal thoracic arteries and 78%, 65%, and 57% for saphenous veins. At 10 years, internal thoracic arteries were more likely than saphenous veins to be patent to left anterior descending in 99.1% of cases, to diagonals in 98.3%, to circumflex in 98.3%, to posterior descending artery in 98.5%, and to right coronary arteries in 82.5%. For right coronary arteries, saphenous vein patency was equivalent to or better than internal thoracic artery patency early after surgery. However, by 10 years, internal thoracic artery patency was better in right coronary arteries with 70% stenosis or greater. At all times after surgery and all levels of clinically important coronary stenosis, internal thoracic artery patency surpassed saphenous vein patency in grafts to the left anterior descending, diagonal, circumflex, and posterior descending arteries.
CONCLUSIONS: Internal thoracic arteries demonstrate better patency than saphenous veins except when grafting moderately stenosed right coronary arteries. When bypassing right coronary arteries with less than 70% stenosis, saphenous veins may be a better choice.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsCommentSummaryDISCUSSIONReferences |
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Patients and Methods |
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TopAbstractIntroductionPatients and MethodsResultsCommentSummaryDISCUSSIONReferences |
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Between 1972 and 2001, 6,193 postoperative angiograms, with 15,386 individual graft observations, were performed (Table 2); 2,928 patients (68%) had one postoperative angiogram, 1,048 (24%) had two, 282 (6%) had three, and 75 (2%) had four or more. Angiography was performed as early as the same day as CABG to as late as 25 years postoperatively (Fig 1).
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Data Analysis
Risk Factors For Graft Occlusion
Longitudinal (mixed model) analysis was used to model time-related graft occlusion and identify associated risk factors (SAS PROC GENMOD, SAS Institute Inc, Cary, NC), accounting for both repeated catheterization and multiple grafts within a patient [12]. Factors considered in the multivariable analyses are listed in the Appendix. Variables were first screened by multivariable logistic regression using liberal inclusion criteria, and the resulting candidate variables were analyzed simultaneously, eliminating those not significant at p less than 0.05. Investigation of modulation of temporal rates was achieved by forming interactions of all variables with interval between CABG and angiography. Initially, data were modeled for each graft anastomotic site separately; these results were then combined to formulate the overall model. Transformations of scale were investigated for ordinal and continuous variables to calibrate risk factors with outcome.
Simulation
After the model for graft occlusion was derived, patency for each graft was calculated from it twice and the results compared. Graft patency was calculated first as if an ITA graft were used, and second as if an SVG were used.
Presentation
Graphic presentation of results is for mean effects, after considering the two sources of within-patient variability. Graphic solutions of the longitudinal equation are risk adjusted by holding values for variables in the model constant, as described in the figure legends.
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Results |
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Comparison of SVGs and ITA Grafts
ITA grafts are more likely to be patent than SVGs when used to bypass all left-sided coronary arteries and the PDA at all times after CABG and at all degrees of proximal coronary artery stenosis (Fig 6). Early (< 5 years) after operation, SVGs have equivalent or better patency than ITA grafts when bypassing the main RCA; however, at 10 years postoperatively, ITA grafts are more likely to be patent in RCAs with 70% stenosis or greater (see Fig 6).
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Comment |
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TopAbstractIntroductionPatients and MethodsResultsCommentSummaryDISCUSSIONReferences |
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Unlike ITA grafts, SVGs are susceptible to intimal hyperplasia, arteriosclerosis, progressive stenosis, and occlusion. At 1 year after operation, because of thrombosis and intimal hyperplasia, only 80% to 90% of SVGs are patent. From 1 to 5 years postoperatively, SVG occlusion occurs at a rate of 1% to 2% per year, and from 6 to 10 years after surgery at 4% to 5% per year. By 10 years, only 60% of SVGs are patent, and of those, only half are free of stenosis [13, 14].
Do SVGs have better patency than ITA grafts in circumstances of important competitive flow? Although there are many studies of graft patency, there has been no direct comparison of SVG and ITA patency with respect to degree of proximal coronary artery stenosis. Therefore, we identified factors associated with graft patency and evaluated whether there may be situations of native coronary artery competitive flow that result in superior patency for SVGs.
Principal Findings
Internal Thoracic Artery Grafts
As we have previously reported, ITA graft patency decreases as native coronary artery stenosis decreases [12]. This observation is consistent with the physiology of arterial grafts [6–11], which autoregulate blood flow in response to metabolic demand. As proximal coronary artery stenosis decreases, competitive flow increases, and demand for ITA blood flow decreases. This results in arterial graft constriction and, over time, possibly graft atrophy and occlusion.
Magnitude of the effect of proximal coronary artery stenosis on ITA graft patency depends, however, on the coronary artery. Decreasing proximal coronary artery stenosis only mildly decreases patency of ITA grafts to the LAD, but strongly decreases patency to the RCA.
Why do lesser degrees of proximal coronary artery stenosis have a greater effect on ITA graft patency in the right coronary arteries than the left? One explanation might be the different diameters of the coronary arteries. The main RCA is relatively large, and a moderate stenosis results in a larger residual lumen than does a moderate stenosis of the smaller-caliber LAD. Native competitive flow in a moderately stenosed main RCA will be much greater than in the smaller-lumen LAD and thus is more likely to lead to ITA spasm and eventual occlusion.
Like others, we found that ITA graft patency was best when the LAD was bypassed [12]. It was lower when ITAs were used to bypass diagonals, LCx, and PDA, and worse when used to bypass the main RCA. There are probably many reasons why graft patency is best when the ITA is used to bypass the LAD, but two likely explanations are (1) technical ease of grafting anterior coronary arteries and (2) greater outflow of the LAD compared with other coronary arteries. Internal thoracic artery graft patency decreases only slightly with increasing time from CABG. This is most likely due to lack of arteriosclerosis.
Saphenous Vein Grafts
Patency of SVGs, in contrast to ITA grafts, is unaffected by native coronary artery competitive flow; that is, over a wide range of proximal coronary artery stenosis, SVGs have equivalent patency. This is probably because, unlike arteries, saphenous veins are not muscular and therefore cannot autoregulate their diameter and blood flow in response to metabolic need. They therefore cannot constrict and occlude in response to important competitive flow.
Similar to ITAs, SVGs have the best patency when used to graft the LAD, next when used to graft diagonals, LCx, and PDAs, and worse when used to graft the main RCA. Unlike ITA grafts, increasing time from CABG strongly decreases saphenous vein patency. This is due to development of arteriosclerosis leading to progressive stenosis and occlusion. In a recent study, Shah and colleagues [15] reported similar findings.
Comparison of Internal Thoracic Artery and Saphenous Vein Grafts
For all degrees of proximal coronary artery stenosis and at all times after CABG, ITA grafts have better patency than SVGs when used to bypass the LAD, diagonals, LCx, and PDA. The only situation we identified in which SVGs were superior is early (< 5 years) after operation when grafting a RCA with moderate stenosis. Early after surgery, SVG and ITA patency appear similar to RCA with stenosis of 70% or greater. However, because of more rapid decline in SVG patency with time, ITAs are more likely to be patent late postoperatively (10 years) when used to bypass RCAs with 70% or greater stenosis.
Recommendations
Because ITAs have better patency when used to graft left-sided coronary arteries, we believe they should be used for these before grafting the main RCA. Our preference when using both ITAs is to use one for the LAD and the second for the next most important left-sided coronary artery. If there is not another left-sided coronary artery requiring bypass and the RCA needs to be grafted, we suggest grafting the PDA instead of the main RCA.
Secondary Findings
Graft patency was better earlier in the study period. This may be because early in the 1970s, patients underwent angiography for routine monitoring of bypass graft patency [1]; later, angiography was performed mostly for recurrence of symptoms. Previous studies have demonstrated better graft patency in asymptomatic patients [16, 17].
Other factors associated with lower graft patency were (1) younger age, (2) diabetes, and (3) female gender. Young patients presenting for CABG tend to have diffuse and aggressive arteriosclerosis. Similar to young patients, patients with diabetes often have diffusely diseased, small coronary arteries. Because of their smaller size, women have smaller coronary arteries than men. A recent report has shown lower patency in bypass grafts to smaller coronary arteries. This may be because of technical difficulties in grafting small coronary arteries and their poor outflow [15].
Limitations
Right ITA Grafts
There were 24 ITA grafts performed to main RCAs. Although this number is small, the ITA grafts were well distributed among all degrees of clinically important stenosis of the RCA, and follow-up angiograms of these grafts were well-distributed across time. This allowed us to model and predict ITA graft patency to the RCA up to 10 years postoperatively, accompanied by confidence limits that reflect the small number of grafts.
Angiographic Follow-Up
Patients in our series underwent repeat angiography mostly for recurrence of anginal symptoms. Although we expect absolute graft patency in our study to be lower than if patients were prospectively studied irrespective of symptoms, we do not believe this would affect the comparison of graft patency. All estimates of graft patency should be reduced equally, and therefore comparisons of patency should be reliable. The best method of comparing graft patency would be to study prospectively many patients at multiple times over many years. For a number of reasons, including practicality, risk, and cost, such a study awaits advances in noninvasive high-resolution coronary and bypass graft imaging.
Competitive Flow
We used degree of proximal coronary artery stenosis as a surrogate for competitive flow. Although other factors, such as coronary artery size, outflow, and sequential stenoses, affect competitive flow, degree of proximal coronary artery stenoses was the only variable we had available.
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Summary |
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TopAbstractIntroductionPatients and MethodsResultsCommentSummaryDISCUSSIONReferences |
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Appendix |
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Symptoms
New York Heart Association functional class (highest angina or functional limitation, I-IV), emergent operation.
Left Ventricular Function
Left ventricular dysfunction grade (none, mild, moderate, severely impaired), prior myocardial infarction.
Cardiac Comorbidity
Atrial fibrillation.
Noncardiac Comorbidity
Smoking history, carotid artery disease, peripheral vascular disease, hypertension, diabetes (insulin-treated, oral hypoglycemic), renal disease, chronic obstructive pulmonary disease, family history of coronary artery disease.
Coronary Disease
Preoperative maximal degree of proximal coronary artery stenosis between aorta and five general sites of internal thoracic artery graft anastomoses: left anterior descending, diagonal, circumflex marginal, right coronary artery, posterior descending artery.
Preoperative Laboratory Data
Hematocrit (%), creatinine (mg/dL), cholesterol (mg/dL), triglycerides (mg/dL), blood urea nitrogen (mg/dL), bilirubin (mg/dL).
Operation
Year of operation, emergent/nonemergent, free internal thoracic artery used, right internal thoracic artery used.
Site of Graft
Left anterior descending, diagonals, right coronary artery, circumflex marginals, posterior descending artery.
Angiography
Time from coronary artery bypass grafting to angiography.
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DISCUSSION |
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TopAbstractIntroductionPatients and MethodsResultsCommentSummaryDISCUSSIONReferences |
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First, before we condemn the saphenous vein for its long-term patency or decreased patency, I think we should realize that this is an observational study, and less than 10% of the patients in this 50,000 series were studied; and even though I know Cleveland was studying some at one year in the early days, after 1972, most of these patients came back for their studies because they had symptoms. So I think this is a weighted study. Also, the manuscript states this study is a comparison of internal mammaries to saphenous veins, I think the saphenous vein occlusion will be higher in this series than occlusion of the internal mammary. We know from the Vineberg experience that the internal mammary will stay open even if it is plugged into the myocardium, while it supplies very little blood flow.
(Slide) This is an illustration of an internal mammary that is 31 years old, and if one looks at patency, it looks good, but it is occluded at the distal end, just as it was 12 years ago. So if you are assessing only internal mammary patency, this is a patent internal mammary. However, the patient does have objective evidence of ischemia in the anterior lateral portion of the heart. A vein in a similar situation would have occluded.
Before we throw away the saphenous vein, I recall an old Texas saying that some of you may have heard, "you dance with the one who brung you," and in one sense, the saphenous vein brung us to this point, especially with multivessel disease.
The second comment: I think there is great variability in the techniques of harvesting the saphenous vein. In some places you have the most inexperienced person taking the vein out. Also, with endoscopic removal, there is a disruption in the side branches many times. I think a meticulous, gentle technique, almost no touch, in removal and treatment of the saphenous vein is likely the greatest determinant of long-term patency. And when you look back at some of these veins that are starting to occlude, you see segmental stenosis. Portions of it look fine, and you will see a vein to a right or an LAD or something that looks very, very good, but there is one vein that is segmentally stenotic, and if you use silver clips, which I don't, it is usually where those silver clips are. I think anywhere you disrupt the edge of the saphenous vein, or the branch, that is a site for platelet deposition. It may not occlude then, but those things become like unstable plaques in a coronary, and over time they are going to close off. A recent study from Sweden, reported almost a year ago from Souza, revealed a patency rate of 95% with the saphenous vein and 93% with the internal mammary.
(Slide) This is a saphenous vein from a fairly famous patient to a diagonal. It is part of five grafts. It is 17 years old. I don't think anybody would go in there and replace that.
I have three questions. First, do you use both left and right internal mammary arteries in patients with multivessel disease, excluding the right RCA that you mentioned, 100% of the time, especially in older women, and if not, why? Second, do you use the radial arteries routinely? If not, why? And, third, what techniques do you now use in excising the saphenous vein, endoscopic, no touch, et cetera? Again, I would like to thank the authors for their excellent paper and the Association for the privilege to discuss it.
DR SABIK: I would like to thank Dr Isom for his comments. His first two questions dealt with conduit choices, and I believe the best way to answer them is to give our approach to arterial grafting.
If we have a non-diabetic, healthy patient under the age of 70, we will use both internal thoracic arteries. We preferentially use the left internal thoracic artery to graft the LAD and the right internal thoracic artery to graft the next most important left-sided coronary artery. If there are arteries remaining on the left side that need to be grafted, we will use the radial artery. For a moderately obstructed right coronary, we will use a vein graft. However, if there is a high-grade obstruction or occlusion of the right coronary artery, we may use the radial or gastroepiploic artery.
In patients with diabetes, conduit selection is more individualized. In an obese patient with diabetes, we usually do not use bilateral ITAs. In younger, non-obese, relatively healthy patients with non–insulin-dependent diabetes, we may use two internal thoracic arteries. Otherwise, we will use the left internal thoracic artery to bypass the LAD, and radial, gastroepiploic, and saphenous vein grafts to bypass the other coronary arteries.
Our present technique for harvesting saphenous vein is endoscopic. Our assistants have learned how to do this atraumatically, and we use this technique routinely.
DR LAURENS R. PICKARD (Houston, TX): Has there been any study of use of statin drugs and other methods for lipid control that might reduce the progression of saphenous vein graft disease and might this be broken down in your patency evaluation? The other question is, I see on the graph lines of patency going below 60% proximal stenosis. Do you advocate grafting of vessels at less than 60% stenosis?
DR SABIK: There have been studies showing the benefits of statin drugs in patients who have had previous bypass surgery, and our feeling is that statins may improve saphenous vein graft patency. We bypass coronary arteries with 50% or greater stenosis.
Thank you.
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References |
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