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Ann Thorac Surg 1998;66:1289-1294
© 1998 The Society of Thoracic Surgeons
a Department of Clinical Radiology, Kuopio University Hospital, Kuopio, Finland
b Department of Surgery, Kuopio University Hospital, Kuopio, Finland
c Department of Medicine, Kuopio University Hospital, Kuopio, Finland
Accepted for publication May 5, 1998.
Address reprint requests to Dr Manninen, Department of Clinical Radiology, Kuopio University Hospital, SF-70210 Kuopio, Finland
| Abstract |
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Methods. Baseline and follow-up coronary angiograms at a mean of 2 years after operation in 91 patients with 194 arterial and 204 venous graft anastomoses were analyzed.
Results. Ninety-two percent of the arterial and 87% of the venous graft anastomoses were patent at follow-up angiography (p = 0.05, odds ratio = 2.63). Unlike that of arterial grafts, the patency rate of venous graft anastomoses correlated negatively with decreasing severity of the bypassed lesion. In contrast to venous grafts, in which angiographic graft function was basically dichotomous (fully patent or occluded), compromised flow of the arterial graft anastomoses was registered in 12%. Progression of the disease was more common in segments bypassed with venous grafts than with arterial grafts (p = 0.001, odds ratio = 2.03).
Conclusions. Angiographic determinants of functional graft patency and progression of occlusive changes in the bypassed artery segments are different for arterial and venous grafts.
| Introduction |
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| Material and methods |
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Coronary operations
The percentages of arterial grafts anastomosed to various vascular areas, eg, left anterior descending, circumflex, and right coronary artery, were 65%, 29%, and 6%, respectively, whereas the corresponding percentages for venous grafts were 15%, 38%, and 47%. The total number of arterial grafts was 144; the left ITA was used in 90 patients, the right ITA in 43 patients, and the right gastroepiploic artery in 9 patients. Forty-nine (34%) of the arterial grafts were sequential. Seventy-two patients received a total of 111 saphenous vein grafts, 75 (68%) of which were sequential. The number of arterial graft anastomoses was 194 and of venous graft anastomoses 204, ie, a mean of 4.37 anastomoses per patient (range, 2 to 7). All analyses of the present study were performed per graft anastomosis, unless otherwise mentioned.
Coronary angiography
In both preoperative and follow-up angiography the native coronary arteries were selectively imaged using biplane cineangiography in eight to ten identical projections.
Analysis of the angiograms
Preoperative and follow-up arteriograms were analyzed independently by two experienced angiographers (H.I.M., M.S.) on two side-by-side cinefilm projectors (Tagarno 35 AX Xenon; Tagarno AS, Horsons, Denmark). The coronary vasculature was subdivided into 27 numbered anatomic segments using the standardized nomenclature of the Coronary Artery Surgery Study [1]. In the follow-up angiograms the vessel segments were further classified as (1) bypassed (segments immediately proximal to the graft anastomosis), (2) segments situated more proximally in the bypassed vessel, (3) nonbypassed arteries, and (4) segments situated distal to the graft anastomosis.
Stenosis determination
The degree of the most severe stenosis within each segment was visually estimated as a percent diameter stenosis in relation to an appropriate adjacent reference diameter. The Pearson correlation coefficient between the two readers interpretations was 0.90 for the preoperative and 0.88 for the follow-up angiography for all segments with registered stenoses. The mean difference between the readers in the percent diameter stenoses of all 1,555 registered lesions (775 on preoperative and 780 on postoperative films) was 0.9% (SD, 20.9%; p = 0.10 by t test for paired observations). For the statistical analyses, the means of the percent diameter stenoses from the two readers were used. The stenosis type was graded using a four-point scale: 1 = short concentric; 2 = short eccentric; 3 = long tubular; and 4 = diffusely diseased segment.
Disease progression
Using side-by-side comparison of the cinefilms, the readers judged the individual stenoses as being (1) unchanged, (2) slightly progressed, (3) clearly progressed, or (4) totally occluded since the baseline angiogram. The kappa statistic for interobserver consistency of interpretation of lesion progression was 0.80. In the statistical comparisons, progression was coded only when both of the observers registered progression. Lesion regression was registered in only five stenoses, and no further analyses of lesion regression were undertaken.
Graft patency
The angiographic patency of the distal anastomoses was graded as (1) fully patent with good antegrade flow, (2) patent but with compromised flow, or (3) occluded. Anastomotic stenoses were registered. The "slender sign," defined as flow dysfunction of the graft because of superior flow through the native artery [3] was also noted. The slender sign manifests as rapid dilution of the contrast media on entering the native coronary artery from the graft, together with considerable retrograde filling of the graft during contrast agent injection into the native coronary artery. In the statistical analyses compromised flow and the slender sign as well as anastomotic stenosis of the graft were coded only when both readers agreed.
Side branches of the ITA grafts
The unligated side branches of the ITA grafts were registered and classified into three categories: (1) one or several tiny side branches, (2) one major side branch of at least half the diameter of the distal ITA, or (3) several patent branches of considerable size.
Diameter of the graft and native coronary artery
The diameters of the distal graft and the native vessel immediately distal to the anastomosis were measured by one of the angiographers (H.I.M.) using a digital micrometer. The measurements were calibrated by using the diameter of the contrast-filled distal portion of the diagnostic catheter. The range of variability of the measurements was defined by performing 50 randomly selected readings twice. The standard deviation of the difference of the first and the second measurement was 6.8%.
Statistical analysis
The primary aims of the statistical procedure were a proper analysis of angiographic predictors for patency and functional status of the arterial and venous grafts. The assumption that individual measurements are independent is not valid in cases like this in which vessels of the same individual tend to produce similar responses [4]. We took into account the correlation of outcomes within each patient by applying logistic regression analysis for distinguishable data (random-effects logistic regression, EGRET statistical software; Epidemiological Graphics). This method treats patency or occlusion of graft anastomoses within the same patient as dependent events. Besides statistical significance, the odds ratio and its 95% confidence limits (CL) are given.
| Results |
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Predictors of graft patency
Factors showing no statistically significant correlation with the patency of either graft type included the ejection fraction, the vessel bypassed, and the morphology of the bypassed lesion. The mean degree of stenoses bypassed with arterial grafts was 81.5% (SD, 15.7%) and of those bypassed with venous grafts 78.2% (SD, 19.7%). The patency (dichotomized as fully patent or patent with compromised flow versus totally occluded) of venous graft anastomoses showed a positive correlation with the stenosis degree of the bypassed lesion at baseline angiography (p = 0.05; odds ratio = 0.94; 95% CI, 0.89, 1.00), whereas the patency of arterial graft anastomoses was essentially independent of the degree of the stenosis bypassed (p = 0.85). This was especially evident when bypassed stenoses were grouped as moderate (ie, less than or equal to 75%) and severe (greater than 75%) (Fig 1). Up to 22.0% of venous graft anastomoses bypassing stenoses less than or equal to 75% were occluded at follow-up angiography, compared with a 7.7% occlusion rate in anastomoses bypassing narrower stenoses. For arterial conduits the proportion of totally occluded anastomoses was almost equal in both groups: 7.2% for grafts bypassing stenoses less than or equal to 75% and 6.4% for stenoses greater than 75%.
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Internal thoracic artery grafts included 20 cases with tiny side branches, 9 cases with one branch of at least half the diameter of the ITA, and 3 cases with several side branches of considerable size.
Progression of the disease
Table 1 shows progression of the disease registered in three types of vessel segments of the native coronary arteries. Because the results of the arterial and venous grafts were similar, all grafts are combined. Stenoses distal to the anastomosis were detected in only 11 patients, and this category was not included in the analyses. Progression was less commonly registered in lesions situated in nonbypassed arteries than in bypassed segments or segments situated more proximal in the grafted artery. Follow-up angiography revealed progression in 33.0% (256/775) of the coronary artery stenoses: 17.6% of the lesions showed slight progression, 4.1% of the lesions showed marked progression, and 12.1% of the stenoses had progressed to total occlusions. Progression to total occlusion was more common in bypassed segments than in nonbypassed arteries or vessel segments situated more proximally to the graft (see Table 1). Sixty-five of the 91 patients had encountered native vessel progression in at least one segment during the follow-up time.
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| Comment |
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The occlusion rate of 13% after a mean follow-up of 2 years for the venous graft anastomoses in the present study is comparable with the 13% to 15.8% rate at 1 year reported by others [5, 7]. Similarly, the occlusion rate of 8% of the arterial graft anastomoses in the present study is similar to the 5% to 7.8% occlusion rates at 12 to 24 months for ITA grafts reported by others [810], as well as the 5% to 23% occlusion rates reported for right gastroepiploic artery grafts [11].
Quantitative stenosis measurement was not used in the present study. We selected visual estimation of percent diameter stenosis as this method is generally used in clinical work as well as in angiographic follow-up studies of coronary artery disease [12]. The standard deviation of the mean variability of the visually determined percent diameter stenoses of the two readers was 20.9%, which is comparable with the 14.6% to 18% variability reported in other studies [13, 14]. Although the variability in visual stenosis evaluation is considerably greater than the 9.4% SD of the mean variability reported for quantitative stenosis determination [15], the use of two independent readers increases the confidence of stenosis evaluation [14].
Functional graft patency was evaluated in this work in two ways. First, we classified anastomotic patency. Basically, venous graft anastomoses were fully open or occluded, whereas in arterial graft anastomoses compromised flow was seen in a considerable number of grafts, especially when the bypassed stenosis was less than or equal to 75% (see Fig 1). Second, we recorded the presence of the slender sign. This phenomenon was found in a considerable number of arterial graft anastomoses (15.5%) but in only 1.0% of saphenous vein graft anastomoses. We registered the slender sign more frequently than Nakao and Kawaue [3] in right gastroepiploic artery grafts (7.8%). The slender sign was associated significantly with less severe bypassed stenoses and was more common when the ratio of the diameter of the native artery to that of the distal graft was increased. The clinical impact of this phenomenon is unclear, but it might arise when sufficient native blood flow is present in the coronary artery trunk and, consequently, the competitive graft flow is unnecessary. The slender sign is useful as it can reveal the probable reason for the graft dysfunction: the bypassed stenosis is too mild.
We registered one or more side branches of considerable size in 9% of ITA grafts, and half of these cases were associated with angiographically compromised graft flow. The clinical significance of the "steal" effect of the hypertrophied side branches is poorly understood, but Singh and Sosa [16] and Singh and Magovern [17] reported 5 symptomatic patients with unusually enlarged side branches and a case in which the lumen of the distal part of the ITA showed 50% enlargement when a prominent side branch was surgically occluded.
The occlusion rate of the saphenous vein graft anastomoses correlated significantly with the percent diameter stenosis of the bypassed lesion. Twenty-two percent of venous graft anastomoses to arteries with a stenosis of less than or equal to 75% at baseline angiogram were occluded, in comparison with a 7.7% occlusion rate for narrower lesions (see Fig 1). Also, Kroncke and associates [2] reported that 5 years after operation there was a tendency, although not statistically significant, for higher closure rates in saphenous vein grafts anastomosed to arteries with minimal disease than to arteries with severe disease at baseline. In the present study, the occlusion rate of arterial grafts showed no correlation with the stenosis severity. There are reports of the ability of ITA grafts to stay open even in no-flow situations and to hypertrophy in association with progression of the native artery disease [18, 19]. Better patency of in situ arterial grafts in mild stenoses may exist because in situ grafts function as a live conduit, with functioning smooth muscle that can adapt to the demands of flow [16, 17]. It should be pointed out, however, that frequency of the slender sign correlated with decreased stenosis severity. If these dysfunctional grafts are added to the occluded grafts, a moderate lesion in the native circulation appears to have about the same likelihood of being associated with a "bad" graft with either an arterial or a venous graft. Similarly, in the whole material, our finding about the better patency rate of the arterial grafts compared with venous grafts would disappear if grafts with compromised flow were classified as occluded.
The result of the present study that disease progression of the native coronary arteries is more common in lesions that have been bypassed than in nonbypassed lesions (39.7% versus 20.2%) is not new, and this was verified for saphenous vein grafts as early as the 1970s [20]. Data concerning the association of disease progression in the bypassed artery with patency of the graft are conflicting. Some authors reported results similar to ours for venous grafts, namely that progression is not associated with patency of the graft [20]. On the other hand, Kroncke and coworkers [2] reported a tendency for greater progression in arteries with patent venous grafts 5 years after operation, whereas Hwang and colleagues [21] found significantly more progression in arterial segments with occluded grafts compared with segments with patent grafts. An interesting result in our study was that progression of stenotic changes was more common in segments immediately proximal to the graft anastomoses than in more proximal segments of the same artery (see Table 1). This result is logical and gives support for the hypothesis of competing flows between the native and bypass conduits as an accelerating factor for vasoocclusive disease. The proportion of progression up to total occlusion was also greater in lesions of bypassed segments, a result obtained also by others [2].
We found that disease progression was more common in segments bypassed with saphenous vein than with arterial grafts (46.0% versus 32.8%). This is in accordance with the result of Cosgrove and associates [22], who found that in the first 2 years after a bypass operation, the proximal coronary arterial lesion progressed in 39% of patients with patent ITA grafts, as compared with 67% of those whose coronary arteries had been grafted with saphenous veins. It has been suggested that arterial grafts may permit more antegrade flow through the proximal stenosis than do the larger diameter saphenous vein grafts [23]. This is supported by our finding that the slender sign was seen almost exclusively in arterial grafts. Consequently, the effect of competitive graft flow to increase vasoocclusive changes in the native artery might be less with arterial grafts. The present study does not provide information about the functional patency of the grafts long term (after approximately 2 years). However, it is firmly established by several studies that between 1 and 5 years postoperatively, the rate of attrition of both ITA and saphenous vein grafts is low [24].
| Acknowledgments |
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| References |
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