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Ann Thorac Surg 1999;67:1968-1970
© 1999 The Society of Thoracic Surgeons
a Cardiothoracic Surgical Unit, Queen Elizabeth Hospital, Birmingham, United Kingdom
b Department of Radiology, Queen Elizabeth Hospital, Birmingham, United Kingdom
c School of Mathematics and Statistics, University of Birmingham, Birmingham, United Kingdom
Address reprint requests to Mr Bonser, Cardiothoracic Surgical Unit, Queen Elizabeth Hospital, Edgbaston, Birmingham B15 2TH, UK;
e-mail: address:r.s.bonser{at}bham.ac.uk
Presented at the Aortic Surgery Symposium VI, April 30–May 1, 1998, New York, NY.
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Abstract |
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Methods. We studied 88 consecutive patients undergoing serial computed tomographic or magnetic resonance imaging scanning to monitor thoracic aortic aneurysm progression. In interval scans of at least 6 months, we measured minimum coronal aortic diameter at seven set levels and maximal diameter, yielding 780 segment-intervals.
Results. The linear expansion rate (mean 2.6 mm/year) increased with incremental aortic diameter (aortic diameter <40 mm: 2.0; 40–49 mm: 2.3; 50–59 mm: 3.6; 
60 mm: 5.6 mm/year; p < 0.01). Regression analysis showed close correlation between predicted and sample data, but there were significant differences between observed and expected measurements. The Yale formula underestimated growth by 0.8 mm, while Mt. Sinai and Osaka formulae overestimated actual change by 1.5 and 0.2 mm, respectively. The expansion rate derived from our population was: last diameter = initial diameter x e(0.00367 x time) (r = 0.617).
Conclusions. Although formulae derived from one thoracic aortic aneurysm sample population may not extrapolate exactly to others, there is close concordance of results for patient populations in three different continents.
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The aim of this study was to validate expansion rate equations in our local population of TAA patients, examining growth in both the maximally dilated area and in predetermined aortic segments, and to calculate an ER formula that describes this sample population.
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Material and methods |
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35 mm. All examinations were retrospectively analyzed by one observer. AD was measured in the area of maximal dilatation and at seven predetermined segment levels. To assess growth, measurements were undertaken in matched segments between initial and subsequent scans. Segment matching on separate scans was undertaken by identification of anatomical landmarks. If matching was not possible, that segment-interval was withdrawn. If patients had more than three consecutive scans, each adjacent combination of two consecutive scans was used as an independent comparison.
Statistical analysis
In a pilot study, analysis of repeated measurements of the minimum AD showed an intraobserver correlation coefficient of -0.999 and a coefficient of repeatability of 2.25. From the measurements obtained, a linear expansion rate (ER) was calculated: ER = (last diameter - initial diameter) [mm]/interval [yr]. For each size range, data are expressed for convenience as mean ± standard error, although data were skewed in each range towards higher ERs. Therefore, to analyze the effect of size, a Kruskal-Wallis test was used with p < 0.05 indicating significance. The published equations were applied to the sample data, and observed diameter and predicted diameter were compared using regression analysis, Wilcoxon’s signed rank test, and scattergram plots. Using the method of Hirose and colleagues we derived a further equation that best described the data of this sample population [13].
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Results |
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35 mm or that were dissected were available for 88 TAA patients (52 men, 36 women) yielding 780 segment-intervals. There were 52 patients with nondissecting aneurysms, 30 patients with chronically dissected segments alone, and 6 patients with mixed pathology. ER increased exponentially with incremental initial diameter (p < 0.001; Fig 1). The ER for segments with an initial diameter 60 mm was 5.6 ± 0.81 mm/year, and for segments initially < 50 mm, ER was 2.1 ± 0.18 mm/year.
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This study, in agreement with others, demonstrates that TAAs enlarge exponentially [11–13]. The ER of the maximally aneurysmal segment (2.6 ± 0.27 mm/year) is in broad agreement with other studies: Masuda and colleagues [14] reported an ER of 1.3 mm/year, Coady and colleagues 1.2 mm/year [12], Cambria and colleagues 2.0 mm/year [15], Hirose and colleagues 4.2 mm/year [10], and Dapunt and colleagues 4.3 mm/year [11]. Any differences between series would appear to be related to differing initial size composition of the aneurysm population studied. The ER for aneurysms of initial diameter 60 mm (5.6 ± 0.81 mm/year) was also similar [11] to previously published results.
In our attempts to predict ER in this sample population using formulae derived from other sample population data, we found that aneurysms in these geographically and ethnically different populations behaved similarly in exhibiting an exponential pattern of growth, but also that such formulae may under- or over-estimate mean growth. The marked scatter of actual ERs and low r value (0.617) seen in this study clearly indicate that although aneurysm behavior can be described in the population, the ER of any individual aneurysm remains unpredictable.
When comparing the ER of one TAA population against another, several factors need to be considered. The referral-based nature of any series introduces several areas of potential bias. Although specialist centers may be referred relatively higher risk patients, the aggressiveness of surgical approach within an institution will clearly affect which patients are included in follow-up surveillance studies, and will therefore determine the character of the follow-up population. The time interval between serial images will also affect the ER of the population studied: the longer the interval, the more patients are removed from the study because of death, rupture, or operation. In contrast, the shorter the interval, the greater the effect of measurement error on ER.
Despite small differences, the ERs of aneurysmal segments in this study showed remarkable concordance with those predicted by formulae derived from other centers despite ethnic disparities. Within our sample population, however, there were several instances of zero expansion rate, making it impossible to derive an equation comparable to the Mt. Sinai format, which mathematically requires a positive growth rate [11].
The possibility of errors in measurements is an important concern in all such studies. To minimize this possibility, a standardized measurement technique was validated and carried out by a single observer. However, scans from diverse referral sources were compared with scans from within the institution, and no protocol existed to determine the number of cross-sectional levels for any particular scan. Moreover, the accuracy of the measurement tool within each scan was not determined, and segment matching was dependent upon identification of other anatomical structures. Another limitation of this and other referral-based studies is that it is not known whether the findings can be extrapolated to clinically silent aneurysms.
Only one of the aneurysms in this study ruptured (just before a scheduled admission for surgery), and knowledge of expansion rates alone are insufficient data on which to base intervention decisions. Initial AD is an important factor determining rupture risk, however, and when a large initial AD is accompanied by other factors such as advanced age, pain, and the presence of chronic obstructive pulmonary disease, the risk of rupture is significant [16].
In conclusion, this study confirms a similar pattern of TAA expansion in different sample populations. Further studies are required to establish which clinical factors may affect the natural history of thoracic aortic aneurysms.
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This article has been cited by other articles:
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