Lachlan J. Kelsey

Haemodynamics and stresses in abdominal aortic aneurysms: A fluid-structure interaction study into the effect of proximal neck and iliac bifurcation angle

Our knowledge of how geometry influences abdominal aortic aneurysm (AAA) biomechanics is still developing. Both iliac bifurcation angle and proximal neck angle could impact the haemodynamics and stresses within AAA. Recent comparisons of the morphology of ruptured and intact AAA show that cases with large iliac bifurcation angles are less likely to rupture than those with smaller angles. We aimed to perform fluid-structure interaction (FSI) simulations on a range of idealised AAA geometries to conclusively determine the influence of proximal neck and iliac bifurcation angle on AAA wall stress and haemodynamics. Peak wall shear stress (WSS) and time-averaged WSS (TAWSS) in the AAA sac region only increased when the proximal neck angle exceeded 30°. Both peak WSS (p<0.0001) and peak von Mises wall stress (p=0.027) increased with iliac bifurcation angle, whereas endothelial cell activation potential (ECAP) decreased with iliac bifurcation angle (p<0.001) and increased with increasing neck angle. These observations may be important as AAAs have been shown to expand, develop thrombus and rupture in areas of low WSS. Here we show that AAAs with larger iliac bifurcation angles have higher WSS, potentially reducing the likelihood of rupture. Furthermore, ECAP was lower in AAA geometries with larger iliac bifurcation angles, implying less likelihood of thrombus development and wall degeneration. Therefore our findings could help explain the clinical observation of lower rupture rates associated with AAAs with large iliac bifurcation angles.

Viscosity and haemodynamics in a late gestation rat feto-placental arterial network

The placenta is a transient organ which develops during pregnancy to provide haemotrophic support for healthy fetal growth and development. Fundamental to its function is the healthy development of vascular trees in the feto-placental arterial network. Despite the strong association of haemodynamics with vascular remodelling mechanisms, there is a lack of computational haemodynamic data that may improve our understanding of feto-placental physiology. The aim of this work was to create a comprehensive 3D computational fluid dynamics model of a substructure of the rat feto-placental arterial network and investigate the influence of viscosity on wall shear stress (WSS). Late gestation rat feto-placental arteries were perfused with radiopaque Microfil and scanned via micro-computed tomography to capture the feto-placental arterial geometry in 3D. A detailed description of rat fetal blood viscosity parameters was developed, and three different approaches to feto-placental haemodynamics were simulated in 3D using the finite volume method: Newtonian model, non-Newtonian Carreau–Yasuda model and Fåhræus–Lindqvist effect model. Significant variability in WSS was observed between different viscosity models. The physiologically-realistic simulations using the Fåhræus–Lindqvist effect and rat fetal blood estimates of viscosity revealed detailed patterns of WSS throughout the arterial network. We found WSS gradients at bifurcation regions, which may contribute to vessel enlargement, and sprouting and pruning during angiogenesis. This simulation of feto-placental haemodynamics shows the heterogeneous WSS distribution throughout the network and demonstrates the ability to determine physiologically-relevant WSS magnitudes, patterns and gradients. This model will help advance our understanding of vascular physiology and remodelling in the feto-placental network.

The mechanistic causes of peripheral intravenous catheter failure based on a parametric computational study

Peripheral intravenous catheters (PIVCs) are the most commonly used invasive medical device, yet up to 50% fail. Many pathways to failure are mechanistic and related to fluid mechanics, thus can be investigated using computational fluid dynamics (CFD). Here we used CFD to investigate typical PIVC parameters (infusion rate, catheter size, insertion angle and tip position) and report the hemodynamic environment (wall shear stress (WSS), blood damage, particle residence time and venous stasis volumes) within the vein and catheter, and show the effect of each PIVC parameter on each hemodynamic measure. Catheter infusion rate has the greatest impact on our measures, with catheter orientation also playing a significant role. In some PIVC configurations WSS was 3254 times higher than the patent vein, and blood damage was 512 times greater, when compared to control conditions. Residence time is geometry-dependent and decreases exponentially with increasing insertion angle. Stasis volume decreased with increasing infusion rate and, to a lesser degree, insertion angle. Even without infusion, the presence of the catheter changes the flow field, causing low velocity recirculation at the catheter tip. This research demonstrates how several controllable factors impact important mechanisms of PIVC failure. These data, the first of their kind, suggest limiting excessive infusion rates in PIVC.

A comparison of hemodynamic metrics and intraluminal thrombus burden in a common iliac artery aneurysm

Aneurysms of the common iliac artery (CIAA) are typically found in association with an abdominal aortic aneurysm (AAA). Isolated CIAAs, in the absence of an AAA, are uncommon. Similar to AAAs, CIAA may develop intraluminal thrombus (ILT). As isolated CIAAs have a contralateral common iliac artery for comparison, they provide an opportunity to study the hemodynamic mechanisms behind ILT formation. In this study, we compared a large isolated CIAA and the contralateral iliac artery using computational fluid dynamics to determine if hemodynamic metrics correlate with the location of ILT. We performed a comprehensive computational fluid dynamics study and investigated the residence time of platelets and monocytes, velocity fields, time-averaged wall shear stress, oscillatory shear index, and endothelial cell activation potential. We then correlated these data to ILT burden determined with computed tomography. We found that high cell residence times, low time-averaged wall shear stress, high oscillatory shear index, and high endothelial cell activation potential all correlate with regions of ILT development. Our results show agreement with previous hypotheses of thrombus formation in AAA and provide insights into the computational hemodynamics of iliac artery aneurysms.

The influence of downstream branching arteries on upstream haemodynamics

The accuracy and usefulness of computed flow data in an artery is dependent on the initial geometry, which is in turn dependent on image quality. Due to the resolution of the images, smaller branching arteries are often not captured with computed tomography (CT), and thus neglected in flow simulations. Here, we used a high-quality CT dataset of an isolated common iliac aneurysm, where multiple small branches of the internal iliac artery were evident. Simulations were performed both with and without these branches. Results show that the haemodynamics in the common iliac artery were very similar for both cases, with any observable differences isolated to the regions local to the small branching arteries. Therefore, accounting for small downstream arteries may not be vital to accurate computations of upstream flow.

Fundus Image Based Blood Flow Simulation of the Retinal Arteries

Computational fluid dynamic (CFD) simulations can help to understand the hemodynamics of the retinal vascular network and the microcirculation. Systemic diseases, like hypertension and diabetes, change the geometry of the vasculature in the retina and these changes can be seen with fundus photography. Furthermore, these changes are indicators of cardiovascular diseases. The aim of this study is to create a plane 2D model of the retinal arterial network based on a high-resolution fundus photograph and to perform a CFD simulation. The blood vessels were segmented from the image with the Frangi filter method. A structural fractal tree was implemented to calculate the outflow boundary conditions representing the peripheral vascular bed. With the Frangi filter method and the high-resolution fundus image a comprehensive model of the visible retinal artery network could be achieved. The simulation results show realistic velocity and pressure distributions of the retinal blood flow in a healthy retina compared to in-vivo measurements in the literature. This work is an initial step towards creating comprehensive patient-specific models of the retinal vasculature based on readily available fundus photography.

Simulating Platelet Transport in Type-B Aortic Dissection

Aortic dissection is where the medial layer of the arterial wall is separated by a tear leading to intramural bleeding. The blood forms an alternate channel of flow known as the false lumen. Thrombosis of the false lumen (FL) is common in cases of dissection as flow conditions are typically more stagnant than in the true lumen (TL). Central to the process of thrombosis is the activation and aggregation of platelets in the blood. Therefore, the aim of this work is to simulate the transport of platelets in a case of Type-B aortic dissection in a clinically-relevant timeframe.

The Effects of Geometric Variation from OCT-Derived 3D Reconstructions on Wall Shear Stress in a Patient-Specific Coronary Artery

Computational fluid dynamics (CFD) can be used to model the blood flow in patient-specific arterial geometries and predict atherosclerotic plaque progression, as the site specificity of plaques has been shown to depend on the wall shear stress (WSS) experienced by an artery’s endothelial layer. The level of artery wall detail in CFD models is expected to underpin predictions of plaque deposition, as it influences the computation of WSS. In this study, the sensitivity of WSS computation to geometric variation in a (proximally) stented left anterior descending (LAD) coronary artery geometry is investigated. The geometry is reconstructed from intravascular optical coherence tomography (OCT) images, which are registered and merged with computed tomography (CT) to include surrounding arteries. The WSS is modelled for low-, medium- and high-resolution geometries; created by altering the point resolution of the contour algorithm used to trace the (OCT imaged) artery lumen. The results show that areas of low WSS (and thus high thrombotic susceptibility) in the stented portion of the artery depended greatly on the resolution of the geometric reconstruction. Compared with the high-resolution geometry, the low- and medium-resolutions failed to accurately determine the areas of low WSS in the stented region, as minor geometric features were inadequately represented. The sensitivity of WSS to geometric variation is also relevant to analyses using coronary artery geometries derived from other imaging modalities (particularly those of lower resolution than OCT).