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Ann Thorac Surg 2007;84:1206-1213
© 2007 The Society of Thoracic Surgeons
a Department of Cardiothoracic Surgery, Mount Sinai School of Medicine, New York, New York
b Department of Anesthesiology, Mount Sinai School of Medicine, New York, New York
Accepted for publication May 1, 2007.
* Address correspondence to Dr Etz, Department of Cardiothoracic Surgery, Mount Sinai School of Medicine, One Gustave L. Levy Place, New York, NY 10029 (Email: christian.etz{at}mountsinai.org).
Presented at the Forty-third Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 29–31, 2007.
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Abstract |
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Methods: From 1986 to 2005, two or more computed tomography studies were obtained as part of routine postoperative surveillance in patients with Dacron grafts implanted to replace diseased aortic segments. Scans were digitized to evaluate the entire thoracic aorta. The median diameters of 547 grafts (18 to 34 mm) in the ascending (349) and descending (198) aorta were calculated from more than 2,000 postoperative computed tomography scans.
Results: In scans obtained 7 or fewer days after implantation, the median graft diameters increased from the manufacturer’s measurement by 17% in the ascending aorta (n = 169; interquartile range, 11% to 21%; p < 0.0001) and 21% in the descending aorta (n = 63; interquartile range, 12% to 25%; p < 0.0001). From an initial scan within 30 days to at least one other within 18 months after implantation, ascending aorta grafts dilated further, at a median rate of 2.8% per year (n = 143; interquartile range, –2.2% to +6.9%; p = 0.0001). Descending grafts dilated less markedly: 1.1% per year (n = 80; interquartile range, –4.0% to +6.1%; p = 0.14). After 18 months, median graft expansion gradually diminished to less than 1% per year.
Conclusions: Significant initial expansion and early growth of woven vascular Dacron grafts occurs; it is slightly different in the ascending and descending aorta. Graft expansion should be anticipated when selecting grafts for aortic valve–sparing procedures to prevent development of regurgitation, and, for endoluminal repair of thoracoabdominal aneurysms, to prevent development of type III endoleaks in the projected landing zone.
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Introduction |
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The tendency of Dacron vascular grafts (Medi-Tech, Boston Scientific, Natick, MA) to expand after placement in the ascending and descending thoracic aorta has previously been noted in a case report [1]. Graft dilatation has also been studied in several small series of patients, primarily in the infrarenal aorta. From these studies, a fairly dramatic initial expansion of the graft, followed by much slower growth thereafter, has emerged. It has also become apparent that knitted grafts tend to expand more than woven ones. Currently, for thoracic aortic replacement, most aortic centers use a woven Dacron prosthesis with collagen or albumin impregnation [2].
Growth rates for woven Dacron grafts (Medi-Tech) in the thoracic aorta have not previously been described in detail. In the abdominal aorta, Blumberg and associates [3] did not find an association between graft dilation and complications. Vascular Dacron graft failure is rare [4], although a few cases of late graft rupture have been reported [5–7]. But recent developments in aortic surgery rely on the long-term dimensional stability of the Dacron prosthesis, and therefore mandate a closer look at its long-term behavior in the clinical setting.
In the ascending aorta, for example, valve-preserving aortic replacement has become an accepted treatment for aneurysm or dissection of the aortic root with morphologically intact leaflets, using various different configurations of Dacron grafts [8–10]. However, both the reimplantation and the remodeling procedures have a relatively high incidence of postoperative aortic regurgitation, to which postoperative dilatation of the Dacron graft could contribute.
In the descending aorta, thoracic endoluminal grafting of thoracic aortic aneurysms is progressively being promoted as a safe and feasible alternative to open graft repair [11–14]. In addition, hybrid techniques—combining simultaneous open graft replacement and endoluminal stent grafting of large thoracic aortic aneurysms or dissections—are being used to reduce the magnitude of surgery in high-risk patients [15, 16]. Such hybrid repairs rely on stable diameters of the Dacron graft to prevent development of type III endoleaks in the projected landing zone.
This study describes the short-term, medium-term, and long-term follow-up of vascular Dacron grafts in more than 500 patients with ascending and descending aortic replacement. The results of this study should be useful in the evolution of both valve-sparing procedures and hybrid operations.
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Material and Methods |
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The scans were obtained as part of routine postoperative surveillance. The first scan, which was used for determination of initial expansion, was taken within a week after graft implantation, intended as a baseline measurement. Subsequent scans were usually obtained annually, principally for evaluation of possible aneurysm formation in the remaining unoperated on aorta. The periods for evaluation of later graft dilatation were determined by the availability of multiple scans at least 6 months apart during follow-up.
This study was approved by the Mount Sinai Institutional Review Board and did not require individual patient consent.
Vascular Dacron Grafts
Different types of woven vascular Dacron grafts (n = 547) from a number of manufacturers were implanted in an end-to-end fashion. Most of the grafts (n = 505) were Hemashield platinum (Boston Scientific, Natick, MA), but 42 were various other types of grafts: Gelweave (Vascutek, a Terumo Company, Renfrewshire, Scotland), Microvel (Boston Scientific/Medi-Tech, Natick, MA), Lopor, or Tascon (Tascon Medical Technologies Inc, Irvine, CA, a division of Medtronic Inc, Minneapolis, MN). The sizes implanted in the ascending and descending aorta are shown in Figure 1. No knitted grafts were used during the study period at our institution. All were straight tubular grafts.
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Size changes were analyzed in three periods, determined in part by the availability of the CT studies that had been obtained as part of routine patient follow-up. Initial expansion was determined from the difference between graft implant size and its dimensions on a CT scan obtained within 7 days of implantation. Growth after initial expansion—during the first 18 months—was calculated from the difference between the initial postoperative scan and one obtained at least 6 months later—often at 1 year. Later growth was determined by subsequent scans at least 6 months apart.
Statistical Methods
For analyzing changes as a function of time, graft sizes were characterized as a percent of the implant size. Any study within the first 7 days was used to determine initial expansion, which was defined as the difference between the size on the first CT study and the manufacturer’s size. For grafts with an initial scan within 7 days of the implant, paired Student’s t tests were used to test the significance of the mean change of diameter from its size at implantation to the mean size ascertained from analysis of the initial CT scan. Analysis of variance or linear regression was used to study the influence of factors on the changes in size.
The growth rates of the serial CT scans of patients contributing two or more scans are presented in two ways. First, all of the observed measurements of the serial scans are shown in spaghetti plots of each graft. Second, growth rates were divided into three periods, as described above: initial expansion, describing growth within the first week after implantation; early growth, describing further expansion within the first 18 months; and late growth, describing expansion beyond 18 months.
Grafts were included in the statistical analysis of changes from the first scan to the last one within 18 months of the implant if they had at least two CT scans within 18 months of the implant: the first was within 30 days of the implant, and at least 6 months had to have elapsed between the first and the last in that period. Similarly, grafts were included in the analysis of changes after 18 months of the implant if they had at least two CT scans beyond 18 months, and at least 6 months had elapsed between the first and the last in that interval. For both of these periods, hierarchical models for repeated measures were used to estimate and test for the significance of change with time, and to test whether the rates of change were significantly influenced by specific other factors.
Statistical analyses were performed with SAS software (version 9.1; SAS Institute Inc, Cary, NC). Characteristics are described as percentages or as mean and standard deviation.
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Results |
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In vascular grafts used for ascending aortic replacement, the proportional increase in grafts with smaller initial implant size was significantly greater than in those with larger initial implant diameters (p < 0.0005).
Initial Expansion of Descending Aortic Grafts
In the descending aorta, vascular Dacron grafts with manufacturer’s initial sizing of 18 to 22 mm (n = 13) experienced a median increase in diameter of 25% (interquartile range, 16% to 34%). Grafts with an initial implant size of 24 to 26 mm (n = 38) showed a median increase in diameter of 20% (interquartile range, 12% to 26%; Fig 3). Vascular grafts with an initial implant diameter greater than 26 mm showed a median increase in diameter of 15% (interquartile range, 11% to 21%; Fig 3). In vascular grafts in the descending aorta, the proportional increase in grafts with a smaller initial implant size was marginally higher than in those with larger initial implant diameters (p = 0.096).
Factors Associated With Initial Graft Expansion
The factors examined were as follows: sex (63% male), age, history of arterial hypertension (66%), history of smoking (41%), concomitant coronary artery bypass grafting (16%), diabetes (5%), Marfan’s syndrome (4%),and infective endocarditis (4%). Among these factors, only arterial hypertension was associated with significantly higher absolute (p = 0.026) and proportional (p = 0.013) growth rates during the first 7 days, and only in grafts in the descending aorta.
There is no difference in the degree to which grafts of the same initial diameter in the ascending and descending aorta expand during the first several days after implantation. Smaller diameter grafts have proportionally greater increases, so the overall differences reflect the differences in the proportion of smaller grafts in the ascending and descending aorta: the ascending aorta has a higher proportion of larger grafts, and therefore a smaller expansion rate during the first several days after implantation.
Early and Late Growth
The spaghetti plots in Figures 4 and 5
show the pattern of growth of all the grafts with at least 2 scans measured in this study. The average annual rates of growth were estimated by separate statistical models for ascending and descending aorta grafts during the two periods of follow-up. During the early period, from an initial scan within 30 days to at least one other within 18 months after implantation, ascending grafts dilated further, at an average rate of 2.8% each year (p < 0.0001). Descending grafts during this period increased at an average annual rate of 2.3% (p = 0.03).
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Factors Associated With Increasing Rate of Change
During the first 7 days, Dacron prostheses with smaller implant diameters were associated with an increased rate of change in both the ascending (p < 0.0001) and the descending aorta (p = 0.02). The rate of increase remained higher in smaller prostheses only in the ascending aorta (
18 month,: p = 0.03; >18 months, p = 0.02).
Possible risk factors such as age, sex, or a history of smoking and hypertension were not associated with an increased growth rate in ascending aorta grafts. In contrast, a history of hypertension was associated with higher growth rates in descending aortic grafts within the first 7 days (p = 0.02) and after 18 months (p = 0.02).
Calculation of Expected Graft Diameter During Early and Late Follow-Up
From the extensive measurements made, formulas were generated that allow the calculation of expected numbers for graft size in the ascending and in the descending aorta at various intervals after implantation of Dacron grafts. The numbers are calculated from the mixed random effects models for the periods within 18 months of implant and after 18 months, respectively, with factors that were statistically significantly associated with rates of change. In each of the formulas below, S0 is the size of the graft as a percentage of the implant size: HxHtn = 1 if the patient had a history of hypertension; otherwise = 0. Expected graft size = (S0 x implant size)/100.
Ascending grafts within 18 months of implantation:
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Ascending grafts beyond 18 months of implantation:
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Descending grafts within 18 months of implantation:
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Descending grafts beyond 18 months of implantation:
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Analysis of Grafts Exceeding Expected Growth
The distribution of the differences between calculated and observed graft sizes in the ascending and descending aorta during follow-up is plotted in Figure 6. Three hundred thirty-four patients contributed scans to one or two of the four groups for which slopes were estimated (ascending or descending and 18 months or >18 months).
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To answer the question of whether excessive graft growth may be a marker for graft failure, the last scan contributed for each of the patients was identified. Deaths from all causes were ascertained, and the last CT scan during the previous year for each patient who died was then reviewed to reveal whether there had been excessive graft growth. As can be seen in Figure 6, most actual measurements cluster around the predicted size, including those in patients who died during follow-up. This analysis showed no excessive mortality in patients who had increases in graft dilatation up to 40% higher than those predicted by the formulas.
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Comment |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionReferences |
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Alimi and associates [18], in 1994, reported on woven and knitted aortic prosthetic grafts evaluated by CT scanning in 58 patients after infrarenal aortic reconstruction, and found significant early increases in woven (12.6%) and knitted (28%) prosthetic graft diameters, as compared with values provided by the manufacturer. In that study, dilatation continued to progress significantly for a mean interval of 19 months: 2.2% for woven, and 6.2% for knitted grafts [18].
In the current study, there was immediate expansion by 17% in the ascending and 21% in the descending aorta in woven—predominantly Hemashield—grafts. Initial expansion appears to be less marked in the ascending aorta only because it is more dramatic in smaller graft sizes, and the relative prevalence of small grafts in the ascending aorta is much less than in the descending aorta.
We found that ascending grafts continued to dilate—after their initial expansion within the first 7 days—at a median rate of 2.8% per year, and that descending aortic grafts dilated less markedly, at a rate of 2.3% per year. The earlier studies cited relied on single slice measurements cut perpendicular to the aorta with a more-or-less circular cross section, whereas in the current study, the diameter of each graft was more accurately determined using multiple calculated truly perpendicular cross sections.
This study confirms that significant expansion of woven Dacron prostheses in the ascending and descending thoracic aorta occurs early, most likely soon after releasing the aortic cross-clamp for the graft, as demonstrated in this study by analysis of scans from 1 to 7 days after graft implantation, and confirmed by recent measurements in the operating room. Our finding of an association between the extent of early expansion and a patient’s history of hypertension supports the theory that the pronounced initial increase in graft diameter probably occurs as a result of the acute pressurization of the graft.
On the whole, Dacron prostheses have proven exceptional durable, and the current report does not call into question their long-term suitability as large vascular prostheses. Although ruptures of knitted and woven vascular prostheses have been reported [4, 6], they are rare, and have recently been related to degeneration of the round black yarn used as the guideline in those grafts [7]. Nucho and Gryboski [4] and others have described the occurrence of dilatation or aneurysmal change and rupture of double-velour knitted Dacron grafts during a long follow-up period, and have reported that aneurysms of double-velour knitted Dacron grafts result from degeneration of the graft fiber. Kawamura and coworkers [5] also reported on longitudinal late-stage, nonanastomotic rupture of a double-velour Dacron graft (20-mm Cooley, Meadox Medicals, Inc, Oakland, NJ) in the descending aorta after marked enlargement (from 39 to 50 mm) in diameter during a 16-month period, 23 years after graft placement. Most cases of graft rupture have occurred more than 20 years after implantation [5, 7].
Our current observations suggest that expansion of woven Dacron grafts shortly after implantation should be anticipated, and that the extent of initial expansion depends somewhat on the graft diameter, but is usually approximately 20% of implant size. Further expansion after the first month is variable, and is usually much more modest: it occurs at a rate of 2% to 3% per year for the first 2 years. Thereafter, there seems to be even slower dilatation, with enlargement of about 1% per year after 18 months. A number of case reports have suggested that elongation of grafts may also occur, but this has not been examined in detail.
It is reassuring that growth in diameter of up to more than 40% of expected values does not appear to be associated with imminent graft failure. As we did not observe growth greater than 40% of what the formulas predicted in this study, however, and because there are anecdotal reports of graft rupture associated with excessive growth, the finding of a very markedly enlarged graft may still be cause for concern. Rupture of woven Dacron grafts occurs very rarely, but the literature suggests that in some grafts that have been in place for more than 20 years, graft failure may be heralded by more dramatic increases in size before rupture.
This study reveals that woven aortic vascular Dacron graft expansion is typically rapid initially, and thereafter slow and asymptotic. Time-related Dacron graft expansion appears to differ somewhat in the ascending and descending aorta. Graft expansion should be anticipated when selecting grafts for aortic valve–sparing procedures to prevent development of aortic regurgitation, and when selecting grafts for endoluminal repair of thoracoabdominal aneurysms—particularly if the endoluminal procedure is planned shortly after or simultaneous with the open procedure—to prevent development of type III endoleaks in the projected landing zone. Graft expansion to 40% more than the predicted increase in diameter does not appear to be associated with increased mortality.
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Discussion |
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DR ETZ: Thank you, Dr Robicsek. We had some discussion of what to call the increase in the size of the graft, and ended up saying that we will term the initial change expansion, and thereafter, growth.
We haven’t looked into longitudinal graft growth yet, but hope to analyze that in the future. It is a little bit more complicated because you have to determine the exact location of the start and end point of each graft, and with 2,000 scans, this takes some time.
I actually have a slide with your paper on the young patient that shows the longitudinal growth of a crimped Dacron aortic bypass graft. We were influenced by your paper when we decided that we would call graft expansion growth.
The current study represents only woven and no knitted grafts; there is only a single case report of a woven graft rupture. We think that you are absolutely right: that woven grafts are more appropriate for ascending and descending thoracic aortic replacement.
DR GRAYSON H. WHEATLEY (Phoenix, AZ): Doctor Etz, I enjoyed your paper and I commend you on an excellent study. I have a comment first and then a question. I believe that this is a real phenomenon, and it has particular importance with regard to endovascular repair, as you have mentioned in your presentation. We have seen oftentimes people come in with a ruptured pseudoaneurysm at the descending thoracic aorta suture line where the Dacron is sewn into the aorta distally. We have gone back and collected the operative notes, and we have measured the size of the Dacron graft, and then intraoperatively we used intravascular ultrasound to get a real-time intraoperative assessment of the Dacron graft, and we found that in fact the Dacron graft has expanded. So this has, I think, significant implications when we are landing stent grafts into the Dacron material.
My question is about the techniques that you used. I saw that your study ranged anywhere from 1985 to 2005, and wondered, first of all, if you had any standardization of the computed tomography slices of the cuts, the thickness of the cuts that you had, and whether or not you think that may impact your measurements and your size determinations? And part B of that question is whether or not you have looked at any of the 3D (three-dimensional) reconstruction models such as MMS software or TeraRecon, which goes back and uses the true centerline as a transverse cut, and whether or not you think that perhaps may be a more accurate assessment?
Thank you very much.
DR ETZ: I would like to go back to one of the first slides to answer the first part of your question: it shows an example of the digitization technique we used. The technique has been pretty stable for the last 20 years. We create slices throughout the entire aorta, with a thickness that has remained pretty much the same for the 20-year period, especially in each patient followed with serial scans. In each of the digitizations, we determine the start and the end point of the graft from the OR (operating room) report, and then within each of the slices we determine the diameter in a very sophisticated way. The CT scan is put on a light table, and each slice is measured: the computer takes into account the fact that the slice may not be perfectly circular. We take the mean diameter of all the slices within the graft, measured perpendicular to the axis of the graft. It is hard to imagine that one could come up with a more precise or consistent technique to do the measurements, especially since the same person has been doing them for the past 20 years. We try to prevent any inaccuracies arising from imprecise measurements or variability in technique from scan to scan.
With regard to 3D reconstructions, we were concerned about the same question when we started this study, and therefore asked our radiologist to confirm our measurements. We took 20 patients during the years and had the radiologists analyze those grafts and let us know what they thought about the graft diameters. Their measurements were extremely close to what we found. So we consider the precision of what we do very high.
DR EDWARD Y. SAKO (San Antonio, TX): You mentioned that a history of hypertension was a factor in this study. Do you have any sense with regard to the degree of hypertension or lack of hypertensive control as far as its correlation with the amount of growth that was seen?
DR ETZ: Hypertension is hard to classify retrospectively in such a huge group of patients. Our information in that regard is quite limited. We just looked into the history of the patient to see whether the patient received any antihypertensive drugs; if so, the patient was categorized as hypertensive. It certainly would be interesting to look into this more thoroughly and see whether the severity of hypertension or its treatment affects graft growth, but we didn’t do that here.
DR MICHAEL MACK (Dallas, TX): Since the tension on the wall is related to the radius inside, did you find that larger grafts tend to expand more than smaller grafts?
DR ETZ: As can be seen from this slide, it is exactly the other way around: the smaller grafts appear to expand more. When one compares grafts with an implant size of 24 mm and 28 mm, the 24-mm graft tends to have a larger expansion within the first 7 days in the same ascending aortic position. If one looks into this statistically, the faster expansion of grafts with smaller implant sizes is highly significant within the first 7 days. The significance diminishes over time, but growth is still significantly faster in the ascending aorta with small implant sizes after 18 months, so one can say definitively that smaller size grafts expand faster than larger ones.
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