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Myocardial Revascularization with Arterial Conduits

Piet W. Boonstra MD, PhD

Thoraxcenter, University Hospital Groningen
the Netherlands

Introduction

Grafting strategy with arterial conduits

Specific considerations

Experience at the Thoraxcenter, Groningen
University Hospital
Results
Discussion
Conclusion
References

Introduction

From the early 80s the long-term patency of internal mammary artery (IMA) grafts has been clearly demonstrated to be superior to that of safenous vein grafts. In particular, clinical studies have demonstrated improved long-term survival, decreased incidence of cardiac events and fewer repeated procedures of revascularization in patients receiving one IMA and namely the left IMA (LIMA) (usually) anastomised to the left descending coronary artery (LAD) 1,2,3. Therefore it seemed logic to add the right IMA (RIMA) as a bypass graft in order to further improve long-term outcomes. However, although the bilateral IMA grafting strategy dates back to the same period of single LIMA grafting, it has been difficult to show a clinical advantage for the use of two IMA despite a number of investigations4,5,6,7,8. Probably the non-homogeneity in the selection of patients receiving bilateral versus single IMA grafts and the need of at least a 10 years. long follow-up to show comparable results were the reasons that rendered difficult to demonstrate a clinical benefit using two IMA grafts versus LIMA alone.
Recently the Cleveland Clinic group shed a new light on this issue clearly demonstrating the advantage of bilateral IMA grafting in terms both for late survival and recurrence of cardiac events; in that paper they demonstrate that bilateral IMA grafting, compared with single IMA grafting, was an independent predictor of lower rates of angina recurrence, late myocardial infarction and any cardiac event.
As a logical consequence of these results, as well as the assumption that the long-term patency of arterial grafts will avoid the necessity of repeated operation, there has been an increasing interest to complete arterial revascularisation. This interest promoted several surgical approaches using IMAs, alone or in conjunction to other arterial conduits like the right gastroepiploic artery (GEA), the radial artery (RA) or the inferior epigastric artery (IEA). (slide 2)

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Grafting strategy with arterial conduits

Internal mammary arteries

Several techniques have been developed both for in situ and free mammary grafting. Sequential grafting, construction of diamond shaped anastomosis, possibility to route the RIMA through the transverse sinus allow to revascularise a number of coronary vessels with in situ IMAs grafting. A different approach popularised by Tector10 consists in using the RIMA anastomised to the LIMA in a T-shaped anastomosis. This allows the IMAs to reach the posterior branches of the circumflex coronary artery (Cx) and the right coronary artery (RCA) in nearly every patient. (slide3)

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The common IMAs grafting strategy is to anastomose the grafts to "the best myocardium"11 using the LIMA for the left anterior descending (LAD) and diagonal artery area and the RIMA for the remaining most dominant graftable vessel. Cx branches should be preferred for RIMA graft due to the reasons that the patency rates of RIMA anastomised to the distal RCA and its branches are not totally satisfying12. (slide4) Moreover the left-sided bilateral IMAs showed better results than RIMA to RCA13 (because the bilateral IMA can revascularise the coronary arteries that supply more left ventricular muscle). With in situ grafts this is best accomplished by routing RIMA through the transverse sinus.

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Right gastroepiploic artery (slide 5-6)

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GEA has been used in clinical practice since the late 1980s14 and satisfactory results both in mid than long-term are reported. In our institution GEA combined with the IMAs are the arterial conduit of choice to achieve total arterial revascularisation as demonstrated by our results15,16. The advantages of GEA are that it is an in situ graft similar to AMI with respect to size, flow, length, comparable pharmacologic responses and low susceptibility to atherosclerosis 17,18,19. The target vessels for GEA are the RCA and its distal branches; theoretically due to its length it is possible to reach the LAD and the CX. However considering that the most distal part of the GEA could be very thin we prefer to reduce properly its length and anastomose it to the RCA or its distal branches in an end-to-side way

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Radial artery (slide 7)

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The RA was clinically employed very early20 in the era of coronary artery surgery by Carpentier’s group but it was abandoned after early reports of poor patency rate21. In the last few years the interest in this graft has been renewed after the finding of patent RA graft in some patients of the early Carpentier’s experience after more than 15-years22. In addition, other authors reported satisfactory results with this graft23,24. Main advantages of the RA are its diameter, which is slightly wider than IMAs and the possibility to harvest this graft at the same time of LIMA harvesting. The average usable length of the RA is 18-20 cm. Initially RA was used as free graft with a proximal anastomosis to the ascending aorta; now RA is mainly used anastomised to the LIMA and its target vessels are Cx and its branches as well as the RCA and its distal branches.

Inferior epigastric artery (slide8)

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The IEA was initially introduced into surgical practice25,26 , in the 1980s; it was initially used as a free-graft anastomised to the ascending aorta. The reported results were non totally satisfying27 . The distal size of the IEA is quite small, about 1-1.2 mm; moreover the proximal anastomosis on the ascending aorta could be technically demanding because of the small diameter of the graft and the thickness of the aortic wall. Different surgical approaches are reported using this arterial conduit with improving results; Calafiore promoted the use of the IEA mainly as lengthening of other in situ arterial conduits and avoiding, whenever possible, the ascending aorta for proximal anastomosis 23. Other authors prefer to use the IEA for the RCA system and to anastomose it proximally to the ascending aorta 28. Surgical indications for this graft are limited to cases in which others conduits are not available or technically not feasible.

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Specific considerations

Right coronary artery system (slide 9)

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As before mentioned the results of grafting the RCA system are less satisfying than to the left coronary system. Anatomical and technical problems can explain these results. The anatomical configurations of the RCA system is, unlike the LAD system, inconsistent and this fact makes difficult to compare these two coronary systems. The atherosclerotic process is very often significant in the region of the origin of the posterior descending coronary artery and there is a propensity of the disease to increase over the time in the proximal RCA and in the proximal part of the posterior descending branch. The segment before the takeoff of the posterior descending coronary artery, is theoretically the most appropriate anatomical site to anastomise a graft conduit, especially the in situ RIMA. Furthermore the RCA diameter could be much larger than a graft conduit. Due to these reasons a graft conduit anastomised in this site could have a significant risk. On the other hand grafting more distally the diameter of the distal part of the posterior descending artery can be excessively small; furthermore, if the in situ RIMA has been chosen as conduit, it may be stretched29 adding an adjunctive technical risk.

Intrinsic properties of the arterial conduits

Unlike the IMA, the right gastroepiploic artery as well as the radial artery and the inferior epigastric artery are prevalently muscular arteries according to the morphology of the media (elastic, elastomuscular, or muscular)30. This fact leads to important technical considerations. The flow adaptation is immediate so if the grafted vessel has only a moderate stenosis and/or high coronary resistance (other distal stenosis) there is a low-flow pattern that can cause a reduction (string sign) up to total occlusion of the conduit. As a consequence these arterial conduits are preferred to be anastomised to severely stenosed or occluded coronary vessels in order to have an high-flow pattern and avoid the reduction in diameter or occlusion of the graft due to the adverse pattern of flow. (slide 10)

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It is known that functional changes like intimal hyperplasia and loss of the endotelium capability to produce substances like nitric oxide occur in the vein grafts and are at the base of the failure of such a graft conduits31 32. Furthermore nitric oxide plays an important role promoting the regulation of the vascular tone, providing a non-thrombogenic luminal surface and inhibiting vascular smooth muscle cell proliferation. These morphometric and physiologic aspects have been investigated in arterial graft conduits. IMA and GEA showed to have histological, morphometric and functional similarities33 ,19. Furthermore, IMA and GEA bypass grafts showed only minimal intimal hyperplasia two years after implantation34   although the media is different in the two conduits, being elasto-muscular in the IMA and muscular in the GEA. Differently from IMA and GEA the IEA has a mean luminal diameter significantly smaller and presents substantial intimal hyperplasia in its first segment 35. As a practical consequence the clinical utilisation of the IEA must be carefully evaluated. (slide 11)

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Free-graft and in-situ graft

The intrinsic properties of these arterial conduits are not the sole determinant of the graft patency. A very important aspect is how they are employed, if in-situ or as free graft. The patency rate of the arterial conduits utilised as a free graft is lower than the in-situ graft patency. The exposure of a free graft to the high shear stresses of the thoracic aorta may lead to rapid development of intimal hyperplasia. From a technical stand-point obviously a free-graft needs an adjunctive anastomosis that may constitute a potential procedural risk. For these reasons it is advisable use the arterial graft in-situ or, if not possible, avoid to anastomise them to the ascending aorta and prefer composite graft anastomosis. (slide 12)

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Experience at the Thoraxcenter, Groningen University Hospital

Patients and methods

At our institution we started using the GEA in combination with two IMAs in 1989 (slide 13). At present more than 550 patients underwent totally arterial myocardial revascularization with these three conduits; we present the data of the first 256 patients whose follow-up is complete at 7 years.

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Baseline characteristics of the patients are shown in table 1. Common closing date for this data was April 30, 1997.
We analysed the following clinical events: mortality from any cause including in hospital mortality, myocardial infarction including perioperative infarction, new revascularization procedures (CABG and PTCA), and the recurrence of angina pectoris. The actuarial survival was calculated according to the Kaplan-Meier method.

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Results

No patient was lost at follow-up. Mean postoperative follow-up period was 51±15 months (up to 84 months).
Mortality. During follow-up a total of 12 patients died (4.7%). Four patients died in hospital due to in-hospital complications. In hospital mortality and morbidity are described in our previous report 36 . The other 8 patients died respectively 25, 26, 28, 38, 44, 52, 63, and 68 months after the operation. Four patients died of a non-cardiovascular cause. Four patients died of a cardiovascular cause (two of cardiac failure, one of ventricular fibrillation, one of endocarditis). Seven-year actuarial survival (including in-hospital death and death of a non-cardiovascular cause) for this group of 256 patients who received left and right ITA-grafts and GEA-graft was 91.1%. (slide 14)

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Morbidity. After discharge from hospital 2 patients suffered a myocardial infarction (one anteroseptal infarction, one inferior infarction). Seven-year myocardial infarction free survival for this group of patients (including 5 in-hospital infarctions) was 97.3%. (slide 15)

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Two patients had to undergo a redo-CABG a few hours after their primary operation and nine patients had to undergo a PTCA. Six of these nine patients underwent a PTCA because of malfunctioning of the GEA. The actuarial freedom from reintervention at 7 years after the operation was 95.4%. (slide 16)

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After leaving the hospital 28 patients experienced a return of angina pectoris. Eighteen patients were in NYHA class II, 10 patients were in NYHA class III. Angina pectoris patients from any class were included in the calculation of the seven-year-angina-free cardiac survival. Seven-year-angina-free cardiac survival was 85.4%. (slide 17)

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Graft patency. The data on patency are generated from 94 follow-up catheterizations. One catheterization was obtained in 86 patients (in 75% of the patients after giving written informed consent, in 25% because of the return of symptoms), eight patients underwent a second catheterization (4 patients after giving written informed consent, 4 patients because of the return of symptoms). A total of 306 anastomoses were examined (some of them twice). The patency rates of LITA-, RITA- and GEA-anastomoses at a mean of 24 months postoperatively (range 1 week-61 months) were 99.2% (128/129), 95.5% (85/89) and 90.9% (80/88) respectively. The patency rates of LITA-, RITA- and GEA-grafts at a mean of 24 months postoperatively (range 1 week-61 months) were 98.8% (85/86), 95.3% (82/86) and 90.6% (77/85) respectively.

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Discussion

Mortality. Actuarial 7-year-survival in our study group was 91.1% (hospital mortality included). This is a satisfactory outcome compared with studies in which vein graft, single IMA grafts or double IMA are used, especially considering that we studied only patients with 3-vessel disease and we included in-hospital mortality in the calculation of the actuarial 7-year survival curve. (slide 18)

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Morbidity. Myocardial infarction. The seven-year actuarial freedom from myocardial infarction in our study group was 97.3%. Despite the inclusion of 5 in-hospital infarctions, the percentage of patients remaining free from myocardial infarction during follow-up is higher than in comparable studies.
Reintervention. The 7-year actuarial probability of remaining free from reintervention after coronary bypass in our study (95.4%) is comparable to that in other studies. It should be noted that, in contrast to the other studies we included two in-hospital reoperations.
Angina pectoris. Actuarial freedom from angina pectoris after 7 years was 85.4%, which is considerably lower than in the studies in which vein grafts, single IMA or double IMA grafts were used.
Major limitations of this study are that we compared our group of patients with historical control subjects and that the follow-up period is still too short to draw definitive conclusions. Nevertheless, if we allow comparisons in this mid-term stage, we demonstrated that mortality, myocardial infarction and reintervention-rates in our patients with three-vessel disease are at least comparable to the results of other studies. Moreover, our results indicate a lower recurrence of angina pectoris after 7 years of follow-up

Conclusion (slide 19-20)

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It is demonstrated that the use of bilateral IMA grafting improves long term survival and decrease the recurrence of related cardiac events. As a consequence more surgeons are today more prone to achieve total arterial revascularization (but it is not known whether it will improve long term results and further studies with long follow-up period are needed to clarify this important aspects of coronary artery surgery).
Clearly many clinical aspects have to be considered approaching the single patient; very old age, diabetes, severe obesity, severe atherosclerosis, previous abdominal surgery, can be a contraindication to utilise certain arterial conduits.
The results till now obtained in our experience with total arterial revascularization are encouraging and supporting our choice to continue on this way trying to give to the patients the best chance of long term results.

References

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Co-Authors:
Marco G. Lanfranconi, MD
Jan G. Grandjean, MD,PhD
Massimo A. Mariani, MD, PhD

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Feb/10/2000