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Ann Thorac Surg 2006;81:807-814
© 2006 The Society of Thoracic Surgeons
Department of Cardiovascular Surgery and Cardiology, National Cardiovascular Center, Osaka, Japan
Accepted for publication September 9, 2005.
* Address correspondence to Dr Nakajima, Department of Cardiovascular Surgery, National Cardiovascular Center, 5-7-1 Fujishirodai, Suita, Osaka 565-8565, Japan (Email: hnakajim{at}hsp.ncvc.go.jp).
| Abstract |
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METHODS: The postoperative angiograms of 458 patients after total arterial revascularization with an off-pump, no aortic manipulation technique beginning in December 2000 were reviewed. Reverse flow was defined as the lack of opacification of a distal anastomotic site during graft angiography, but clear retrograde graft opacification during native coronary angiography. We analyzed characteristics of the target coronary branches, and bypass conduits. The potential interactions between coronary branches and sequential anastomoses were categorized as those with two 75% stenotic branches (situation 1); one 75% stenotic branch at the end of the graft and a 99% to 100% stenotic branch at the middle of the graft (situation 2); and a composite Y (or K) graft with one end to a 75% stenotic branch and the other to a 99% to 100% stenotic branch (situation 3).
RESULTS: A total of 18 bypasses (1.1%) were occluded while reverse flow was found in 4.5% (74 of 1,627). In a multivariate analysis of the 521 bypass conduits having more than two distal anastomoses, the predictors of reverse flow or occlusion were a right coronary artery target with 75% or less stenosis (p = 0.006), more than three distal anastomoses with a conduit (p = 0.005), situation 1 (p = 0.04), situation 2 (p<0.0001), and situation 3 (p < 0.0001).
CONCLUSIONS: Interactions between coronary branches and graft arrangement play important roles in flow distribution. Graft arrangement should be adjusted for each patient to minimize reverse flow.
| Introduction |
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In these procedures, sequential anastomoses or composite grafts are necessary for multivessel revascularization. However, when two or more distal anastomoses share a single in-situ graft for inflow, there are concerns over the risk of reduced antegrade flow in the in-situ graft and the potential for segmental flow reversal in sequential grafts. Because bypass grafts with reverse flow do not contribute to coronary perfusion, these grafts may be counterproductive. In addition, reduced blood flow in some segments may cause narrowing or occlusion of arterial grafts, because of the tendency for arteries to adapt to flows in their run-off territory [4, 5]. These potential disadvantages to sequential arterial grafting may outweigh the possible reduction in complications for some patients.
The direction of flow in a graft is dependent upon the differential pressures in the bypass conduit and the coronary branches, rather than the graft type per se. In a bypass conduit with two or more distal anastomoses, such as sequential or composite grafts, the determinants of flow distribution are more complex than that in an individual graft. For individual bypasses, postoperative evaluations have assessed the graft type, target coronary characteristics, and degrees of angiographic patency. This type of analysis, however, is not sufficient for various configurations of bypass conduits to several coronary branches [6]. In the present study, we made the assumption that the optimal graft arrangement would maximize antegrade bypass flow. Thus, our angiographic evaluation was aimed at determining the dominant flow direction in each segment of a bypass graft, as well as the anatomic patency of the graft overall. Multivariate analyses were performed, including as variables the possible configurations of both the conduits and the target coronary branches.
| Patients and Methods |
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A patent graft was defined as one with complete continuity of the lumen for its entire length from the origin of the in-situ graft to the anastomosis with a coronary branch, irrespective of the flow direction. When the continuity of a graft lumen was interrupted at any level, or when a bypass graft was not visualized by either native coronary angiography or graft angiography, that was defined as an occlusion, which was regarded as a no-flow situation with closure of the lumen. Reverse flow was defined as a situation in which at least one of the distal anastomotic sites was not opacified from the graft injection, but did fill clearly by retrograde flow from the native coronary injection. Any bypass graft graded as occluded or having reverse flow was considered not functioning, because it did not contribute to coronary perfusion and relief of ischemia in the target region. A patent bypass without reverse flow was graded as functioning, and the rate of functioning grafts in a given patient was defined as the proportion of functioning bypasses to the total number of bypassed vessels. The functioning rate for bypass conduits was defined as the proportion of entirely patent conduits without reverse flow to the total number of bypass conduits.
Patients
The coronary angiograms of 458 patients who underwent off-pump CABG using only arterial grafts with avoidance of aortic manipulation between December 2000 and March 2004 were reviewed. There were 380 men and 78 women with a mean age of 65.6 ± 9.3 years (Table 1). Patients who failed to complete postoperative coronary angiography, had individual coronary grafts only, or had one or more saphenous vein grafts were excluded. Coronary and graft angiography was performed at a median of 14 days after surgery. The angiograms were independently evaluated by cardiologists. The severity of stenosis was determined in each coronary branch. Stenoses were grouped as 51% to 75%, 76% to 90%, and 91% to 100% by a precise measurement of the luminal diameter and labeled as 75%, 90%, and 99% to 100%, respectively, for purposes of statistical analysis. The grade of maximal stenosis was recorded for each target coronary branch.
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Graft Selection and Strategy
The choice of bilateral or unilateral ITA was based on a consideration of the potential operative risks. Bilateral ITAs were preferentially used for patients less than 75 years of age who maintained an active lifestyle and did not have either severe chronic obstructive pulmonary disease or diabetes mellitus requiring insulin therapy. This choice was based on studies showing that bilateral ITAs provide more abundant flow than a single ITA [8], and improve late outcomes after surgery [9]. For elderly patients, we used at least one ITA to bypass the left anterior descending artery (LAD), and the radial artery was our first choice for a free graft. We harvested only a single radial artery from the nondominant forearm. One radial artery was divided into two pieces when necessary. The gastroepiploic artery was harvested in patients who had significant cardiomegaly, a subclavian artery stenosis, or inadequate ulnar collateral flow.
We utilized the various bypass conduits as summarized in Table 2. The arrangement of the grafts was determined primarily by the spatial relationships of the target coronary arteries.
Analysis 1: Bypass Grafting to 1,627 Coronary Branches
We first conducted a univariate analysis of the data for bypass conduits and target coronaries for all 1,627 anastomoses to determine those factors that predicted a functioning graft at 2 weeks postoperatively. Variables in the univariate analysis included the territory of the target coronary distribution (divided into LAD, left circumflex [LCX], or right coronary artery [RCA]); the severity of the native coronary stenosis (75%, or 90% or greater); the diameter of the target coronary (1.0 mm, 1.25 mm, 1.5 mm, or 2.0 mm as determined by the shunt used); the type of bypass graft (ITA, or other); the number of branches for the bypass conduit (composite Y graft = 1, composite K graft = 2, or straight conduit = 0); the number of distal anastomoses for the bypass conduit (more than 3, or 3 or fewer); and the type of anastomosis (end to side, or side to side).
Analysis 2: 521 Conduits With Two or More Distal Anastomoses
Data for the 521 bypass conduits with two or more distal anastomoses were evaluated separately. Variables assessed by univariate analysis included the number of distal anastomoses (three or fewer, or more than three); the number of revascularized territories (one, two, or three territories), the number of branches for the bypass conduit (composite Y graft = 1, composite K graft = 2, or straight conduit = 0); and the presence (or absence) of a coronary branch with 75% stenosis in the LAD, LCX, or RCA territory.
In addition, we examined the effect of potential interactions between target coronary branches that were connected to each other by a composite or sequential graft. Based on these preliminary analyses, we hypothesized three situations at high risk for occlusion or reversal flow states as follows (Fig 1). The first is a graft with sequential anastomoses to more than two coronary branches each with 75% or less stenosis (situation 1). The second was a graft with a sequential anastomosis to one coronary branch with 75% or less stenosis located at the distal end of the graft and a more proximal anastomosis to a coronary branch with 99% to 100% stenosis (situation 2). The third was when one end of a composite Y (or K) graft was connected to a 75% stenotic branch and the other end to a 99% to 100% stenotic branch (situation 3). The presence or absence of these situations in each bypass conduit was entered into the analysis.
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| Results |
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| Comment |
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The functional adequacy of a bypass conduit is determined by two factors: its flow capacity, and its pressure capacity. In previous reports, construction of a composite Y graft has been shown to increase free flow in a single ITA pedicle by 75% and leave a sufficient flow reserve for the graft after it is connected to its target distribution [12, 13]. In addition, follow-up studies have shown rapid growth of the in-situ ITA composite Y graft to produce increases in the amount of graft flow [14].
For predicting the dominant direction of blood flow in a given segment of a graft, the pressure differential between the graft segment and the native coronary will outweigh the overall flow capacity of the graft. Interestingly, these two factors (pressure and flow) do not always vary proportionally. Diastolic graft pressure is regarded as a significant predictor of bypass flow, because the left ventricular myocardium is perfused exclusively in diastole [15, 16]. In spite of similar flow capacities, diastolic pressures in ITA grafts have been found to be significantly higher than in gastroepiploic artery grafts. This finding has been implicated as a major cause of insufficient antegrade flow in gastroepiploic artery grafts [17, 18].
Royse and colleagues [19] have demonstrated that the cut-off for a stenosis in the target coronary that will leave graft occlusion was higher for a composite Y graft than for a noncomposite in-situ graft. Thus, despite an increased flow capacity, composite Y and K grafts have lower pressure capacities than straight conduits. Pressure capacity of a given conduit may be determined by certain anatomic characteristics of the conduit, such as its shape and length. We hypothesize that the number of branches created in a conduit should correlate with the pressure capacity of the graft, because the Y and K graft has a following relative stenosis. The proximal part of a composite Y graft consists of an in-situ ITA and the distal part is either an ITA or a free radial artery.
In previous studies, the measured diameter of the ITA was reported to be smaller than that of the radial artery: 2.13 mm (1.8 to 2.6 mm) versus 2.75 mm (2.5 to 3.0 mm) [4, 2022]. Using these size estimates, the ratio of the cross-sectional area of an average radial artery to an ITA would calculate to be 1.67. The ratio of the cross-sectional area of the entire outflow of a composite Y graft to its inflow would be 2.67 (= 1 + 1.67). Thus, at its creation there is a relative stenosis proximal to the radial artery portion of a composite Y graft of 63% (= 1 1 / 2.67). Since the outflow of a composite K graft consists of an ITA and two segments of radial artery, a K graft would have an 77% (= 1 1 / [1 + 1.67 + 1.67]) stenosis. On the other hand, linear extension of an in-situ ITA with a radial artery would produce only about 40% (= 1 1 / 1.67) stenosis, which is not significant. The length of the conduit (ie, the distance from the proximal origin of the ITA to the target coronary anastomosis) may correlate with pressure capacity. It is also true that the middle portion of the conduit has a higher pressure potential than the end of the conduit. In our multivariate analysis, the location of the target branch (ie, RCA territory) and an anastomosis at the end of the conduit were significant predictors of reverse flow or occlusion, whereas creation of branches on the in-situ ITA did not correlate significantly with graft function.
The results of this study suggest some strategies for graft arrangement when multiple distal anastomoses are planned with one graft. For the LAD territory, the isolated in-situ ITA to LAD has been widely accepted as the "gold standard" bypass graft, providing both long-term durability and improved survival. For management of the diagonal branch, it is necessary to construct grafts so as not to interfere with the ITA to LAD bypass. The functioning rate for sequential anastomoses with an in-situ ITA to both the diagonal and LAD was satisfactory, even when both branches had only 75% stenoses, and comparable with that of a composite Y graft to the diagonal and LAD. Therefore, either of these bypasses are equivalent options and the choice should depend upon the relative positions of the planned anastomotic sites to prevent graft kinking beyond the diagonal. Either configuration can help avoid having more than four anastomoses arising from the opposite ITA.
We have several recommendations for sequential grafting of the LCX and RCA territories. First, it is recommended that the number of distal anastomoses for each conduit be kept to less than three, especially when a branch with stenosis of 75% or less is present. As the number of the coronary branch connections increase, the total amount of cross-sectional area in the distal graft increases, and the pressure potential per target vessel decreases. As shown in Table 3, when a conduit had four or more distal anastomoses, even branches with 90% stenoses were associated with reverse flow. Second, end-to-side anastomoses should be minimized. Utilization of straight composite grafts is preferable, because they have only one end whereas composite Y and K grafts have two and three ends, respectively. Third, we have to pay special attention to the management of target branches with 99% to 100% stenoses. As shown in situations 2 and 3, when the middle portion of a graft is connected to a severely stenotic branch, reverse flow can be provoked more distally in the end of the graft. The overall graft arrangement for a given patient should be designed to create a favorable pressure slope from proximal to distal. That can be accomplished by selecting an appropriate orientation (clockwise or counterclockwise), or harvesting another in-situ graft to provide a separate inflow for additional bypass grafts.
A sequential bypass to two coronary branches having moderate stenosis (situation 1) was also associated with reverse flow or occlusion. In our experience, the incidence of reverse flow and occlusion can be reduced by the use of bilateral ITAs. However, harvesting more in-situ grafts to separate inflow sources and avoid multiple sequential anastomoses, or utilizing an aortic connection (which should provide the highest driving pressure), may be reasonable options. However, high pressures can promote atherosclerotic graft disease, and spoil the long-term durability of the graft [23].
A moderately stenotic branch of the RCA as a target vessel was one of the most significant predictors of reverse flow. There is an ongoing controversy regarding the management of a moderately stenotic branch in the RCA territory [5, 19, 24]. In the present study, bypass grafting with a composite arterial graft to an RCA branch in a side-to-side fashion provided a satisfactory patency rate, even when the native coronary stenosis was moderate. These results suggest that arterial composite grafting might be an effective solution for this situation.
The purpose of this study was not to prove the superiority of totally arterial off-pump CABG without aortic manipulation, but to examine strategies for optimal sequential graft arrangement and minimizing the potential disadvantages of this technique. In the present study, occlusion and reverse flow were assessed as a composite outcome variable. The reasons for this were the following: (1) both occlusion and reverse flow are unfavorable results with at least segments of nonfunctioning grafts; (2) it is impossible to distinguish occlusion from reverse flow when flow velocity is extremely low; and (3) as reverse flow mainly affects coronary branches with moderate stenoses, this analysis would not be applicable to bypass grafts to severely stenotic coronaries. The flow demands of the myocardium, the vascular resistance in the supplied coronary bed, and the effect of a phasic delay in flow from the in-situ grafts to composite Y or K-grafts may each play a role in determining the direction of flow through a composite graft. However, we do not have reliable methods for quantifying each of these factors. An additional factor may be the luminal size at the anastomotic site itself. This factor is not precisely measurable, especially when a side-to-side anastomosis is performed at a near 90-degree angle to the coronary (diamond shape) or the anastomotic site is not clearly visualized because of mixed blood flow from the native coronary and the bypass graft. These factors are limitations to this study.
In conclusion, off-pump CABG with all arterial grafts and no aortic manipulation provided satisfactory graft patency in our hands, but the incidence of nonfunctioning grafts was not negligible. Since interactions between the target coronary branches played a distinct role in the occurrence of reverse flow, the arrangement of sequential grafts needs to be adjusted according to each patient's anatomy in order to minimize the occurrence of reverse flow and occlusion.
| Acknowledgments |
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| References |
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