A numerical investigation on the steady and pulsatile flow characteristics in axi-symmetric abdominal aortic aneurysm models with some experimental evaluation

Steady and pulsatile flow characteristics in rigid abdominal aortic aneurysm (AAA) models were investigated computationally (using Fluent v. 4.3) over a range of Reynolds number (from 200 to 1600) and Womersley number (from 17 to 22). Some comparisons with measurements obtained by particle image velocimetry under the pulsatile flow conditions are also included. A sinusoidal inlet flow waveform 1 + sin omega t with thin inlet boundary layers was used to produce the required pulsatile flow conditions. The bulk features of the mean flow as well as some detailed features, such as wall shear stress distributions, are the foci of the present investigation. Recirculating vortices appeared at different phases of a flow cycle causing significant spatial and temporal variations in wall shear stresses and static pressure distributions. A high level of shear stresses usually appeared at the upstream and downstream ends of the bulge. Effects of pressure rise caused by the increase in cross-sectional area were transmitted into the downstream tube. Further simulation studies were conducted using simulated physiological waveforms under resting and exercise conditions so as to determine the possible implication of vortex dynamics inside the AAA model.     

Experimental investigation of steady flow in rigid models of abdominal aortic aneurysms

Abdominal aortic aneurysms occur in as much as 2–3% of the population, and their rupture produces a mortality rate of 78–94% (1), causing 15,000 deaths per year in the U.S. alone. As an investigation into the mechanical factors that lead to aneurysm rupture, flow field measurements are presented for steady flow through a range of aneurysm sizes and Reynolds numbers. Seven rigid symmetric models of aneurysms were constructed with uniform lengths of 4d and diameters that ranged from 1.4 to 3.3d, whered is the inner diameter of the undilated entrance tube. Color Doppler flow imaging was used to visualize the flow fields, while laser Doppler velocimetry was used to quantify the flow field velocities and to determine critical Reynolds numbers for the onset of, and complete transition to, turbulent flow. Estimates of mean and peak wall shear stresses were derived from velocity measurements. Flow in these models varied from fully laminar to fully turbulent over the range of Reynolds numbers corresponding toin vivo flows. There was a large range over which the flow was intermittently turbulent. High wall shear occurred in the models when the flow was turbulent, suggesting that turbulence inin vivo aneurysms may contribute significantly to their risk of rupture.     

A steady flow analysis on the stented and non-stented sidewall aneurysm models.

As part of a general investigation on the effects of blood flow patterns in sidewall aneurysm, in vitro steady flow studies on rigid aneurysm models have been conducted using Particle Image Velocimetry over a range of Reynolds number from 200 to 1600. Above Reynolds number 700, one large recirculating vortex would be formed, occupying the entire aneurysmal pouch. The centre of the vortex is located at region near to the distal neck. A pair of counter rotating vortices would however be formed at Reynolds numbers below 700. For all the aneurysm models considered, the vortex strength, in general, is stronger at higher Reynolds numbers but lower at larger aneurysm size. Maximum strength of the vortex is about 15% of the bulk mean velocity in the upstream parent tube. Estimates of the wall shear stresses are derived from the near wall velocity measurements. Highest level of wall shear stresses always appears at the distal neck of the aneurysmal pouch. Stents and springs of different porosity have been used to dampen the flow movement inside the aneurysm so as to induce the possible formation of thrombosis. It is found that the flow movement inside the aneurysmal pouch can be suppressed to less than 5% of the bulk mean velocity by both devices. Furthermore, regions of high wall shear stresses at the distal neck could also be suppressed by almost 90%. The present results would be useful for further improvements in stent (or spring) technology.     

In vitro velocity measurements down strem from the lonescu-Shiley aortic bioprosthesis in steady and pulsatile flow

Velocity measurements were made in vitro using laser Doppler anemometry (LDA) downstream from an lonescu-Shiley (IS) bioprosthetic aortic heart valve. Velocity measurements were made in both steady and pulsatile flow. A systematic, flow mapping approach to the measurement methodology showed that the IS valve generated a large jetlike flow constriction. The acceleration ratio, defined as the maximum mean velocity for the IS valve divided by that for no valve obstructing the flow, was as high as 2·4 for steady flow and 2·6 for pulsatile flow. It was concluded that the IS valve generated a flow quite unlike that observed by other in vestigators for the natural human aortic valve, after which the leaflet design of the IS valve was modelled. In addition, a comparative analysis of steady and pulsatile results was undertaken. It was found that the pulsatile flow results for the systolic ejection interval could be divided into three phases, denoted early, mid, and late systole, as defined by the flow structure at the data plane location. Only during midsystole were the pulsatile flow results approximated by the steady flow results. Also, it was found that the magnitude of the flow disturbance measured in steady flow tended to be an upper bound on that measured for pulsatile flow.     

Blood flow and structure interactions in a stented abdominal aortic aneurysm model

Since the introduction of endovascular techniques in the early 1990s for the treatment of abdominal aortic aneurysms (AAAs), the insertion of an endovascular graft (EVG) into the affected artery segment has been greatly successful for a certain group of AAA patients and is continuously evolving. However, although minimally invasive endovascular aneurysm repair (EVAR) is very attractive, post-operative complications may occur. Typically, they are the result of excessive fluid-structure interaction dynamics, possibly leading to EVG migration. Considering a 3D stented AAA, a coupled fluid flow and solid mechanics solver was employed to simulate and analyze the interactive dynamics, i.e., pulsatile blood flow in the EVG lumen, pressure levels in the stagnant blood filling the AAA cavity, as well as stresses and displacements in the EVG and AAA walls. The validated numerical results show that a securely placed EVG shields the diseased AAA wall from the pulsatile blood pressure and hence keeps the maximum wall stress 20 times below the wall stress value in the non-stented AAA. The sac pressure is reduced significantly but remains non-zero and transient, caused by the complex fluid-structure interactions between luminal blood flow, EVG wall, stagnant sac blood, and aneurysm wall. The time-varying drag force on the EVG exerted by physiological blood flow is unavoidable, where for patients with severe hypertension the risk of EVG migration is very high.     

A numerical investigation of the dependence of NMR signal from pulsatile blood flow in CINE pulse sequences

The Bloch equations have been solved using numerical techniques for a uniform fluid undergoing periodic pulsatile flow in an NMR imaging experiment. The magnetization and NMR signal have been calculated for experimental parameters appropriate for a CINE sequence (TR = 40 ms, (TE = 14 ms) applied to the study of pulsatile aortic or other arterial flows. The flow velocity profile is obtained by Fourier superposition of different harmonics and it is shown that the steady-state NMR signal has reduced high-frequency components. There is also a time delay between peak signal intensity and flow because the backflow effects that can be as much as 100 ms. The apparent pulsatility depends on the NMR sequence parameters. Some limitations of the phase contrast flow-imaging method are also discussed for nonuniform flow.     

Turbulence downstream from the Ionescu-Shiley bioprosthesis in steady and pulsatile flow

The turbulence generated downstream from an aortic Ionescu-Shiley bioprosthesis has been investigated in vitro with both steady and pulsatile flow; Instantaneous point velocities were measured using laser-Doppler anemometry (LDA) at numerous preselected locations in the flow. The mean and RMS velocities from these data at each location were then used to estimate the laminar and turbulent shear in the bulk flow as a function of radial position on a cross-section of the flow system downstream from the mounted prosthesis. Estimated total shear stresses were found in the bulk flow that were of sufficient magnitude to possibly cause haemolysis and initiate platelet chemical-release reactions. For steady flow and at peak pulsatile flow, maximum total shear stresses were estimated to be 120 N m⁻² and 100 N m⁻², respectively, over more than 5 per cent of the flow cross-section. The spatial distribution of the elevated shear stresses correlates well with the valve superstructure. It is concluded that these elevated stresses are a direct consequence of the notable flow constriction generated by the valve’s fully opened leaflets. deceased     

A numerical study on steady flow in helically sinuous vascular prostheses

The objective of the present study is to investigate flows in helically sinuous tubes with a small amplitude of helicity for physiological and clinical applications. Three-dimensional computations of steady flows in helically sinuous tubes are carried out by using a Navier-Stokes solver based on the spectral/hp element method for high accuracy. Results show that the flow fields are affected by the curvature and the torsion of the helically sinuous tube geometry in terms of axial velocity, axial vorticity and wall shear stress. The position of the maximum axial velocity is influenced more by the curvature than by the torsion. Most importantly, the torsion remarkably changes the Dean vortices produced by the curvature to a predominantly single vortex. Accordingly, it is proposed that the fluid dynamics knowledge gained from the present investigation can be utilized for the design of innovative prosthetic grafts that can control the biological reactivity of coagulant interacting with the prosthetic vascular surface or wall.     

In vitro evaluation of the effects of intraluminal thrombus on abdominal aortic aneurysm wall dynamics

The optimum time to treat abdominal aortic aneurysms (AAAs) still remains an uncertain issue. The decision to intervene does not take in account the effects that wall curvature, intraluminal thrombus (ILT) properties and thickness have on rupture. The role of ILT in aneurysm dynamics and rupture has been controversial. In vitro testing of four silicone AAA models incorporating the ILT and aortic bifurcation was studied under physiological conditions. Pressures (P) and diameters (D) were analysed for models with and without ILT at different locations. The diametral strain, compliance and P/D curves were influenced by the presence, elastic stiffness and thickness of the ILT. In this case, the inclusion of ILT reduced the lumen area by 77% that resulted in a 0.5–81% reduction in compliance depending on ILT properties. With an increase in ILT stiffness from 0.05 to 0.2 MPa, the compliance was reduced by 81%. In the region of maximum diameter, there was a reduction of diametral strain and compliance except for the softer ILT which was more compliant throughout the proximal region. The shifting of the maximum diametral strain and compliance to the proximal neck was pronounced by an increase in ILT stiffness, thus creating a possible rupture site.     

Histochemical and ultrastructural characteristics of leg muscle fibres in patients with repairative abdominal aortic aneurysm (AAA)

Tibialis anterior (ta) muscle biopsies before and after elective abdominal aortic aneurysm (AAA) repair operation were obtained, in order to observe possible changes after the aortic declamping reperfusion. Open muscle biopsies were taken from each of eight patients (60-75 years old) which were processed for enzyme histochemistry, and for transmission electron microscopy (EM). Morphometric analysis was applied to estimate the number and the area of muscle fibres of each fibre type. Rectus abdominis muscle biopsies were served as controls. Before the operation the predominant elements found were the presence of atrophic muscle fibres, fibre size diversity, localised cellular reactions, increased extent of connective tissue, disappearance, in many cases, of the mosaic pattern, predominance of type I and oxidative fibres, and existence of fibres with core-like structures in the sarcoplasm. Type I fibres consisted of 66.95 +/- 9% of all muscle fibres, the mean cross sectional area of which was 3,372.8 +/- 1,016 microm(2) and of type II fibres was 3,786.5 +/- 6,046 microm(2). After the aortic clamping was performed mitochondrial swelling was found, as well as disorganisation of sarcomeres. After declamping of the aorta, there were also severe edema, local fibre necrosis, and adhesion of leucocytes, whereas muscle fibre areas became 3,935.18 micro 531 microm(2) for type I and 5,804 +/- 1,075 microm(2) for type II. The short ischemic period during aortic clamping and the subsequent reperfusion resulted mainly in ultrastructural changes.     

Changes of flow characteristics by stenting in aneurysm models: influence of aneurysm geometry and stent porosity

An endovascular technique using a stent has been developed and successfully applied in the treatment of wide neck aneurysms. A stent can facilitate thrombosis in the aneurysm pouch while maintaining biocompatible passage of the parent artery. Insertion of the stent changes the flow characteristics inside the aneurysm pouch, which can affect the intra-aneurysmal embolization process. The purpose of this study is to clarify the velocity and wall shear stress changes that are caused by stenting in fusiform and lateral aneurysm models. We used a flow visualization technique that incorporated a photochromic dye in order to observe the flow fields and measure the wall shear rates. The intra-aneurysmal flow motion was significantly reduced in the stented aneurysm models. Coherent inflow along the distal wall of the aneurysm was diminished and inflow was distributed along the pores of the stent wall in the stented models. Also, sluggish intra-aneurysmal vortex motion was well maintained in the stented aneurysm models during the deceleration phase. A less porous stent generally reduced the intraneurysmal fluid motion further, but the porosity effect was not significant. The magnitude and pulsatility of the wall shear rate were reduced by stenting, and the reductions were more significant in the lateral aneurysm models compared to the fusiform aneurysm models. The hemodynamic changes that were observed in our study can help explain the efficacy of in vivo thrombus formation caused by stenting. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8780-y, 8719Xx     

An experimental evaluation of the use of an ensemble average for the calculation of turbulence in pulsatile flow

A comparison was made between turbulence calculated by subtracting an ensemble average from the instantaneous velocity and calculations made with a high pass digital filter. Velocity was measured with a laser Doppler anemometer in vitro in the region of a normal porcine aortic valve and in patients with a hot film anemometer in the region of normal aortic valves. From the velocity obtained in patients, the absolute turbulence intensity calculated using an ensemble average of 50 beats was nearly twice the turbulence intensity calculated using a digital filter. Individual beats sometimes showed differences of 150% compared to calculations based upon the use of a digital filter. Inspection showed that the ensemble average varied widely from the actual nonfluctuating velocity. Studies in vitro showed less beat to beat variation than occurred in patients. The absolute turbulence intensity measured in vitro, when calculated using an ensemble average, was only 20% greater than calculations using a digital filter. The differences were due primarily to beat-to-beat variations of the nonfluctuating velocity, but these beat-to-beat variations were less prominent than occurred in patients. These observations suggest that ensemble averaging may not be appropriate for the calculation of turbulence, particularly in patients.     

Multislice CT angiography in abdominal aortic aneurysm treated with endovascular stent grafts: evaluation of 2D and 3D visualisations

Endovascular repair of abdominal aortic aneurysms has been introduced into the clinical practice for more than a decade and has been confirmed to be an effective alternative to conventional open surgery, especially in patients with co-morbid medical conditions. Helical CT angiography is the preferred imaging method in the follow-up of endovascular repair. Recent introduction of multislice CT scanners has augmented its diagnostic role in this area. Diagnostic value of multlislice CT has been complemented by a series of 3D post-processings, which assist vascular surgeons in accurately assessing the effect of endovascular repair by providing additional information when compared to conventional 2D axial images. These reconstructions include multiplanar reformation, curved multiplanar reconstruction, shaded surface display, maximum intensity projection, volume rendering and virtual endoscopy. This article aims to demonstrate the generation of these 2D/3D reconstructions based on multislice CT data acquired from a group of patients with abdominal aortic aneurysm following endovascular repair. A brief introduction of generating each reconstruction was provided; potential clinical applications of each reconstruction were briefly discussed. Images were presented in a dynamic format with the aim of allowing the reader to easily understand the post-processing of these reconstructions.     

A population-based study of polymorphisms in genes related to sex hormones and abdominal aortic aneurysm

Male gender and family history are risk factors for abdominal aortic aneurysm (AAA). We hypothesized that genes involved in sex hormones might be important in AAA. We investigated the association of aortic diameter with single-nucleotide polymorphisms (SNPs) in genes determining circulating sex hormones and their action. We genotyped 74 tagging SNPs across four genes (steroid 5α reductase, subfamily A, polypeptide 1 (SRD5A1), cytochrome P450, family 19, subfamily A, polypeptide 1 (CYP19A1), androgen receptor (AR) and estrogen receptor 2 (ESR2)) related to sex hormone production and action in 1711 men, 640 of whom had an AAA. One genotype was also assessed in an independent cohort of 782 men, of whom 513 had large AAAs. Associations were assessed adjusting for other risk factors for AAA. One SNP in CYP19A1 was strongly associated with aortic diameter. Subjects who had the rare homozygote genotype (TT) for CYP19A1g.49412370C>T (SNP ID rs1961177), had an increased aortic diameter (coefficient 5.058, SE 1.394, P=0.0003, under a recessive model). This SNP was not associated with aortic diameter in an independent cohort, which included patients with larger AAAs. Our findings do not support an important role of genetic polymorphisms in genes determining sex hormones in aortic dilatation in men. The association of one SNP in CYPA9A1 with small but not large AAA may suggest differences between AAA formation and progression. This SNP warrants further investigation in another large population, including patients with small AAAs.     

Coding polymorphisms in the genes of the alternative complement pathway and abdominal aortic aneurysm

Variants in the genes of the alternative complement pathway are associated with risk of numerous inflammatory diseases. Abdominal aortic aneurysm is associated with inflammation and is a common cause of illness and death among European populations. This study tested 49 single nucleotide polymorphisms, including common putatively functional polymorphisms, in the genes of the alternative complement cascade (CFH, CFB, CFD, CFI, properdin, CR1, CR1L, CR2, CD46, vitronectin, C3, C5, C6, C7, C8A, C8B, C8G and C9). The study group were 434 cases with infra-renal aortic diameter ≥30 mm and 378 disease-free controls from two UK centres, all with self-reported European ancestry. There was no evidence for significant association with presence or size of aneurysm following correction for multiple testing. This study suggests that variation in the genes of the alternative pathway is not an important cause of abdominal aortic aneurysm development.     

Retrocaval ureter and preaortic iliac venous confluence in a patient with an abdominal aortic aneurysm

Anomalous anatomic location of a large venous system poses a potential hazard in aortic operations. We encountered a patient with an infrarenal abdominal aortic aneurysm who was also found at preoperative contrast-enhanced computed tomography to have a retrocaval right ureter and a preaortic iliac vein confluence. This combined anomaly has not previously been reported except for one postmortem case. As abdominal aortic surgery is currently performed routinely, care must be taken to avoid injury to surrounding organs due to rare anatomic anomalies.     

Leukocyte mimetic polysaccharide microparticles tracked in vivo on activated endothelium and in abdominal aortic aneurysm

We have developed injectable microparticles functionalized with fucoidan, in which sulfated groups mimic the anchor sites of P-selectin glycoprotein ligand-1 (PSGL-1), one of the principal receptors supporting leukocyte adhesion. These targeted microparticles were combined with a fluorescent dye and a T₂¹ magnetic resonance imaging (MRI) contrast agent, and then tracked in vivo with small animal imaging methods. Microparticles of 2.5 μm were obtained by a water-in-oil emulsification combined with a cross-linking process of polysaccharide dextran, fluorescein isothiocyanate dextran, pullulan and fucoidan mixed with ultrasmall superparamagnetic particles of iron oxide. Fluorescent intravital microscopy observation revealed dynamic adsorption and a leukocyte-like behaviour of fucoidan-functionalized microparticles on a calcium ionophore induced an activated endothelial layer of a mouse mesentery vessel. We observed 20 times more adherent microparticles on the activated endothelium area after the injection of functionalized microparticles compared to non-functionalized microparticles (197 ± 11 vs. 10 ± 2). This imaging tool was then applied to rats presenting an elastase perfusion model of abdominal aortic aneurysm (AAA) and 7.4 T in vivo MRI was performed. Visual analysis of T₂¹-weighted MR images showed a significant contrast enhancement on the inner wall of the aneurysm from 30 min to 2 h after the injection. Histological analysis of AAA cryosections revealed microparticles localized inside the aneurysm wall, in the same areas in which immunostaining shows P-selectin expression. The developed leukocyte mimetic imaging tool could therefore be relevant for molecular imaging of vascular diseases and for monitoring biologically active areas prone to rupture in AAA.