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Title: The relationship between aortic aneurysm wall distensibility and aneurysm growth and rupture
Author: Wilson, Katherine Ann
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2002
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Introduction: Community mortality for ruptured abdominal aortic aneurysm (AAA) exceeds 80% and the operative mortality is 30-70%. Elective repair of AAA virtually eliminates the risk of rupture and is associated with an operative mortality of 5-10%. The decision to operate on an asymptomatic AAA, therefore, involves weighing the risk of rupture against that of surgical intervention. Rupture is related to maximum diameter, growth rate and possibly blood pressure, but no size of AAA is entirely free of rupture risk. A variable that provides a more precise quantification of rupture risk on an individual patient basis is required to improve the clinical and cost-effectiveness of surgery. AAA wall distensibility, which is measurable non-invasively, may be related to aortic wall structure and thus aneurysm growth and rupture. Aims: The primary aim was to determine the relationships between AAA wall distensibility, diameter, expansion and risk of rupture. Secondary aims were to evaluate the variability of the technique and to assess the error caused by use of brachial, as opposed to central, blood pressure. Methods: Distensibility [pressure-strain elastic modulus (Ep) and stiffness (P)] was measured using a real time B-mode ultrasound scanner with echo-tracking software (Diamove). Brachial pressure was measured using automated sphygmomanometry (Omron, Japan). Central aortic pressure was derived using pulse wave analysis (Sphygmocor). Follow-up was 6-monthly for a median (IQR) period of 19.7 (9.2- 29.9) months. Outcome measures included ruptured AAA and surgical repair of intact AAA. Death certificate information was collected on those who died before the end of the study. Results: 210 patients (163 males and 47 females) were recruited. Median (IQR) age was 72 (68-77) years, AAA diameter 48 (41-54) mm, BP 140/80 (128-160/ 72-90) mmHg, Ep 2.91 (1.99-4.37) 10⁵Nm⁻², and β 19.4 (14.4-29.4). Intra- and inter-observer CV_ME for directly measured variables were low (≤10%) while CV_ME for the derived variables were higher (≤35%). The CV_ME is a parametric test; however, when these skewed data were logged to normality intra-observer CV_ME for β was ≤10%. Bland-Altman plots showed that Ep and β calculated using brachial, as opposed to derived central pressure, were systematically over-estimated by 11% (p=0.001) and 5%. (p=0.040) respectively. At baseline, AAA in the rupture group tended towards being more distensible than the intact AAA but this did not attain statistical significance. At last follow-up, the rate and direction of change in distensibility were not related to diameter or expansion. Cox proportional hazard model showed that, after adjusting for age and sex, female gender, larger AAA diameter, higher diastolic pressure and a larger proportional increase in distensibility were related to a shorter time to rupture (all p≤0.01). Conclusions: The relationships between AAA distensibility and rupture are complex and depend upon AAA diameter, gender and/or outcome group. Change in distensibility over time appears to be related to rupture risk. An increase in distensibility in conjunction with increasing diameter may indicate an increased risk of rupture.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available