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Vascular Haemodynamics

The tension acting on the AAA wall is of obvious importance when predicting vessel rupture, but the way in which the blood is flowing through a vessel and the shear stress induced by the blood flow on the inner surface is of also of immense clinical-relevance. The videos above show the distribution of particles (e.g. monocytes) flowing through a diseased aortic bifurcation (left) and the development/breakdown of vortical structures (right) during the cardiac cycle.  

Wall shear stress (WSS) is several orders of magnitude smaller than wall tissue stress and has major implications in cardiovascular disease (CVD). It is widely regarded that atherosclerosis develops in regions experiencing low WSS, but evidence also indicates that oscillatory shear stress may be a better marker.

Some of our work in this area has focused on modelling the haemodynamics in AAA, over time. It is not particularly common for a patient to have multiple CT scans during the course of treatment, so those that do, represent a good opportunity for us to model the evolving haemodynamics and geometry.

We use a combination of patient-specific 3D reconstructions from CT and computational fluid dynamics (CFD) to compute the flow and resulting WSS.

The image on the right shows velocity-coloured streamlines of a patient-specific AAA from baseline (left) to 2.5 years (right).

Does an AAA expand in regions of low WSS?

We found that in a stable case (which did not expand by much over 5 years, but was repaired nonetheless) the time averaged WSS (TAWSS) was quite uniform along the vessel and did not drop below a critical threshold of WSS (<0.4 Pa). Whereas, in the ruptured case, the TAWSS showed a significant patch of low TAWSS in the upper region – which is exactly where the AAA locally expanded over the 2.5 years of CT imaging.

Interestingly, as the vascular surgeons in Limerick now sketch the location of rupture whenever possible, we also saw that the region that actually burst appears to be in an area of very low TAWSS. The image on the right shows the intra-operative sketch together with the TAWSS.

We sometimes use a critical threshold of approximately 0.4 Pa to define very low WSS. Early studies (see the work of Lawrence et al., 1987 & 1995, and Worthen et al., 1987) examined the behaviour of certain monocytes under different flow conditions and found that no free-flowing leukocytes could attach above 0.4 Pa. Our paper here describes a method to model the attachment of monocytes to the vessel wall using this threshold.

Where does thrombus develop?

Intraluminal thrombus (ILT) is found in the vast majority of AAAs and is a complex structure that varies from patient to patient. At VascLab we investigate the biomechanics of this soft tissue and also examine the relationship between haemodynamics and ILT development. Again, by using CFD we try can the answer the question of whether or not ILT forms in regions of low velocity flow. We found that in the case of rapid expansion, the ILT only formed in the upper part of the aneurysm, whereas in the very slowly expanding case, the ILT only formed in the lower region. Our paper can be found here.

Site-specificity of aneurysmal disease

Aneurysms tend to develop in specific anatomical locations. The more common locations being the aorta, the cerebral, carotid, femoral and popliteal arteries. However, there are also some regions that do not develop aneurysms (or at least very rarely). Aneurysms often progress into the common and internal iliac arteries, yet rarely into the external iliac artery, even though they are all in very similar anatomical locations and in regions of birfurcation. Paul Norman has investigated this for over 10 years. We now use CFD to explore this phenomenon.

By modelling the haemodynamics in in the iliac arteries in the case of health and disease, we can investigate the WSS distributions in the regions and determine if certain areas may be more susceptible to disease based on the WSS. The image below shows the oscillatory shear index (OSI) calculated in the iliac region of a case with AAA (left) and a healthy case (right).