Biomechanics of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a condition whereby the terminal aorta permanently dilates to dangerous proportions, risking rupture. The biomechanics of AAA has been studied with great interest since aneurysm rupture is a mechanical failure of the degenerated aortic wall and is a significant cause of death in developed countries. In this review article, the importance of considering the biomechanics of AAA is discussed, and then the history and the state-of-the-art of this field is reviewed--including investigations into the biomechanical behavior of AAA tissues, modeling AAA wall stress and factors which influence it, and the potential clinical utility of these estimates in predicting AAA rupture.     

The ruptured abdominal aortic aneurysm—a diagnostic problem

The ruptured abdominal aortic aneurysm continues to be a diagnostic problem. Review of 187 cases admitted to the Vancouver General Hospital showed that 92 cases were operated on. Of this surgical group, the diagnosis was correct in 61 and missed in 31 (34%). In the group which did not come to operation the condition was diagnosed correctly in 38, while in 57 (60%) it was completely unsuspected. The effects of early and late diagnosis and misdiagnosis were reflected in the increasing mortality rate of 46, 55 and 100% respectively. Means of improving the accuracy of diagnosis in this condition are discussed.     

Hemodynamics of the mouse abdominal aortic aneurysm

The abdominal aortic aneurysm (AAA) is a significant cause of death and disability in the Western world and is the subject of many clinical and pathological studies. One of the most commonly used surrogates of the human AAA is the angiotensin II (Ang II) induced model used in mice. Despite the widespread use of this model, there is a lack of knowledge concerning its hemodynamics; this study was motivated by the desire to understand the fluid dynamic environment of the mouse AAA. Numerical simulations were performed using three subject-specific mouse models in flow conditions typical of the mouse. The numerical results from one model showed a shed vortex that correlated with measurements observed in vivo by Doppler ultrasound. The other models had smaller aneurysmal volumes and did not show vortex shedding, although a recirculation zone was formed in the aneurysm, in which a vortex could be observed, that elongated and remained attached to the wall throughout the systolic portion of the cardiac cycle. To link the hemodynamics with aneurysm progression, the remodeling that occurred between week one and week two of the Ang II infusion was quantified and compared with the hemodynamic wall parameters. The strongest correlation was found between the remodeled distance and the oscillatory shear index, which had a correlation coefficient greater than 0.7 for all three models. These results demonstrate that the hemodynamics of the mouse AAA are driven by a strong shear layer, which causes the formation of a recirculation zone in the aneurysm cavity during the systolic portion of the cardiac waveform. The recirculation zone results in areas of quiescent flow, which are correlated with the locations of the aneurysm remodeling.     

Evolving mechanical properties of a model of abdominal aortic aneurysm

The novel three-dimensional (3D) mathematical model for the development of abdominal aortic aneurysm (AAA) of Watton et al. Biomech Model Mechanobiol 3(2): 98–113, (2004) describes how changes in the micro-structure of the arterial wall lead to the development of AAA, during which collagen remodels to compensate for loss of elastin. In this paper, we examine the influence of several of the model’s material and remodelling parameters on growth rates of the AAA and compare with clinical data. Furthermore, we calculate the dynamic properties of the AAA at different stages in its development and examine the evolution of clinically measurable mechanical properties. The model predicts that the maximum diameter of the aneurysm increases exponentially and that the ratio of systolic to diastolic diameter decreases from 1.13 to 1.02 as the aneurysm develops; these predictions are consistent with physiological observations of Vardulaki et al. Br J Surg 85:1674–1680 (1998) and Lanne et al. Eur J Vasc Surg 6:178–184 (1992), respectively. We conclude that mathematical models of aneurysm growth have the potential to be useful, noninvasive diagnostic tools and thus merit further development.     

Stereoscopically observed deformations of a compliant abdominal aortic aneurysm model

A new experimental setup has been implemented to precisely measure the deformations of an entire model abdominal aortic aneurysm (AAA). This setup addresses a gap between the computational and experimental models of AAA that have aimed at improving the limited understanding of aneurysm development and rupture. The experimental validation of the deformations from computational approaches has been limited by a lack of consideration of the large and varied deformations that AAAs undergo in response to physiologic flow and pressure. To address the issue of experimentally validating these calculated deformations, a stereoscopic imaging system utilizing two cameras was constructed to measure model aneurysm displacement in response to pressurization. The three model shapes, consisting of a healthy aorta, an AAA with bifurcation, and an AAA without bifurcation, were also evaluated with computational solid mechanical modeling using finite elements to assess the impact of differences between material properties and for comparison against the experimental inflations. The device demonstrated adequate accuracy (surface points were located to within 0.07?mm) for capturing local variation while allowing the full length of the aneurysm sac to be observed at once. The experimental model AAA demonstrated realistic aneurysm behavior by having cyclic strains consistent with reported clinical observations between pressures 80 and 120?mm Hg. These strains are 1-2%, and the local spatial variations in experimental strain were less than predicted by the computational models. The three different models demonstrated that the asymmetric bifurcation creates displacement differences but not cyclic strain differences within the aneurysm sac. The technique and device captured regional variations of strain that are unobservable with diameter measures alone. It also allowed the calculation of local strain and removed rigid body motion effects on the strain calculation. The results of the computations show that an asymmetric aortic bifurcation created displacement differences but not cyclic strain differences within the aneurysm sac.     

Effects of major endoleaks on a stented abdominal aortic aneurysm

Insertion of a stent-graft into an aneurysm, especially abdominal aortic aneurysms (AAAs), is a very attractive surgical intervention; however, it is not without major postoperative complications, such as endoleaks. An endoleak is the transient accumulation of blood in the AAA cavity, which is formed by the stent-graft and AAA walls. Of the four blood pathways, a type I endoleak constitutes the major one. Thus, focusing on both proximal and distal type I endoleaks, i.e., the minute net influx of blood past the attachment points of a stent-graft into the AAA cavity, the transient three-dimensional interactions between luminal blood flow stent-graft wall, leakage flow, and AAA wall are computationally simulated. For different type I endoleak scenarios and inlet pressure wave forms, the impact of type I endoleaks on cavity pressure, wall stress, and stent-graft migration force is analyzed. The results indicate that both proximal type I-a and distal type I-b endoleaks may cause cavity pressures close to a patient's systemic pressure; however, with reduced pulsatility. As a result, the AAA-wall stress is elevated up to the level of a nonstented AAA and, hence, such endoleaks render the implant useless in protecting the AAA from possible rupture. Interestingly enough, the net downward force acting on the implant is significantly reduced; thus, in the presence of endoleaks, the risk of stent-graft migration may be mitigated.     

Preoperative evaluation of a patient for abdominal aortic aneurysm repair

Coexistent cardiovascular disease is common in patients presenting for repair of aortic aneurysms. However, preoperative cardiac evaluation prior to abdominal aortic aneurysm (AAA) surgery remains contentious with significant variations in practice between countries, institutions and individual anesthetists. The following case report raises some everyday issues confronting clinical anesthetists.     

Inflammatory markers associated with abdominal aortic aneurysm

Purpose

Inflammation with leukocytic infiltration, degradation of extracellular matrix (ECM), and depletion of vascular smooth muscle cells (VSMC) are pathological hallmarks of abdominal aortic aneurysm (AAA). The aim of this study was to further evaluate relationships betweenAAAand inflammatory biomarkers, interleukin- 6 (IL-6), tumour necrosis factor-α (TNF-α), endothelin-1 (ET-1) and soluble urokinase-type plasminogen activator receptor (suPAR), by comparing levels in 65-year-old men with and without AAA at ultrasound screening.We also evaluated whether any biomarker can independently predict AAA at screening, and clarified potential correlations between aortic diameter and blood levels of these biomarkers.

Results

There were significant (p ? 0.05) differences between subjects with and without AAA for the following variables: p-leukocyte count (TLC) (p<0.001), p-homocysteine (p<0.001), p-TNF-α (p = 0.023), p-IL-6 (p<0.001), p-ET-1 (p = 0.002), p-suPAR (p<0.001), ankle brachial index (ABI) (p<0.001), plasma (p)-creatinine (p = 0.049), p-total cholesterol (p<0.001), p-high density lipoprotein (HDL) (p<0.001) and low density lipoprotein (LDL) cholesterol (p = 0.001), smoking habits (p<0.001), and use of antihypertensive (p<0.001) and lipid-lowering (p = 0.001) drugs. When the above variables were stepwise excluded in a logistic regression model, only p-IL-6 (p = 0.002), p-homocysteine (p = 0.015), p-HDL (p = 0.004), ABI in the right (p = 0.005) and left (p = 0.094) leg, smoking habits (p = 0.003), and antihypertensive drug use (p = 0.045), differed between groups. Significant correlations with aortic diameter existed for p-TNF-α (p = 0.028), p-IL-6 (p<0.001), p-ET-1 (p = 0.002) and p-suPAR (p<0.001) in the entire study population, and for p-TNF-α (p = 0.023), p-ET-1 (p = 0.009) and p-suPAR (p = 0.001) among men with AAA.

Conclusions

Several inflammatory biomarkers were significantly elevated and correlated with aortic diameter among 65-year old men with AAA at ultrasound screening. IL-6, homocysteine and use of antihypertensive medication remained elevated in the logistic regression model, together with known risk markers for AAA such as smoking and signs of atherosclerosis.

     

Non-linear viscoelastic behavior of abdominal aortic aneurysm thrombus

The objective of this work was to determine the linear and non-linear viscoelastic behavior of abdominal aortic aneurysm thrombus and to study the changes in mechanical properties throughout the thickness of the thrombus. Samples are gathered from thrombi of seven patients. Linear viscoelastic data from oscillatory shear experiments show that the change of properties throughout the thrombus is different for each thrombus. Furthermore the variations found within one thrombus are of the same order of magnitude as the variation between patients. To study the non-linear regime, stress relaxation experiments are performed. To describe the phenomena observed experimentally, a non-linear multimode model is presented. The parameters for this model are obtained by fitting this model successfully to the experiments. The model cannot only describe the average stress response for all thrombus samples but also the highest and lowest stress responses. To determine the influence on the wall stress of the behavior observed the model proposed needs to implemented in the finite element wall stress analysis.     

On the inheritance of abdominal aortic aneurysm.

To determine the mode of inheritance of abdominal aortic aneurysm, data on first-degree relatives of 91 probands were collected. Results of segregation analysis performed on these data are reported. Many models, including nongenetic and genetic models, were compared using likelihood methods. The nongenetic model was rejected; statistically significant evidence in favor of a genetic model was found. Among the many genetic models compared, the most parsimonious genetic model was that susceptibility to abdominal aortic aneurysm is determined by a recessive gene at an autosomal diallelic major locus. A multifactorial component in addition to the major locus does not increase the likelihood of the data significantly.     

Long term relative survival after surgery for abdominal aortic aneurysm in Western Australia: population based study

Objective: To determine the long term relative survival of all patients who had surgery for abdominal aortic aneurysm in Western Australia during 1985-94. Design: Population based study. Setting: Western Australia. Subjects: All patients who had had surgery for abdominal aortic aneurysm in Western Australia during 1985-94. Main outcome measures: Morbidity and mortality data of patients admitted and surgically treated for abdominal aortic aneurysm in Western Australia during 1985-94. Elective, ruptured, and acute non-ruptured cases were analysed separately. Independent analyses for sex and patients aged 80 years or more were also undertaken. Postoperative (>30 days) relative survival was assessed against age and sex matched controls. Results: Overall, 1475 (1257 men, 218 women) cases were identified. The crude five year survival after elective surgery, including deaths within 30 days of surgery, was 79% for both men and women. When compared with a matched population the five year relative survival after elective surgery was 94.9% (95% confidence interval 89.9% to 99.9%) for men but only 88.0% (76.3% to 99.7%) for women. The five year relative survival of those aged 80 years and over was good: 116.6% (89.1% to 144.0%) compared with 92.4% (87.7% to 97.0%) for those under 80 years of age (men and women combined). Cardiovascular disease caused 57.8% of the 341 deaths after 30 days. Conclusion: In a condition such as abdominal aortic aneurysm, which occurs in elderly patients, relative survival is more clinically meaningful than crude survival. The five year relative survival in cases of elective and ruptured abdominal aortic aneurysm was better in men than in women. This is probably because of greater comorbidity in women with abdominal aortic aneurysm and this deserves more attention in the future. The long term survival outcome in octogenarians supports surgery in selected cases.

Key messages

• Background mortality for conditions such as abdominal aortic aneurysm in elderly patients needs to be taken into account when assessing long term survival after surgery
• Relative survival methodology can correct for background mortality
• The five year relative survival for patients surviving beyond 30 days of elective surgery for abdominal aortic aneurysm was 95% for men and 88% for women
• For octogenarians, five year survival after elective surgery was greater than that expected of an age matched population
• Age over 80 years should not preclude consideration for elective surgery for abdominal aortic aneurysm

     

Survivors of ruptured abdominal aortic aneurysm: the iceberg's tip.

In four and a half years 25 patients in one community suffered a ruptured abdominal aortic aneurysm. Eleven died at home, nine died without operation in hospital, and only five had the aneurysm removed. There were four survivors. A further seven patients might have lived had they had a prompt operation. The average operative mortality for ruptured aneurysms among series reported in British journals is 53%, but the survivors are a small minority of the total number of people in the community whose aneurysms rupture. No basis could be found for the view that replacing an aortic aneurysm with a straight graft (while leaving behind aneurysmal common iliac arteries) lowers the operative mortality. On the contrary, oversimplifying the operation may be hazardous.