Changes in the Hemodynamic Stresses Occurring During the Enlargement ofAbdominal Aortic Aneurysms

This research seeks to quantify the role that mechanical stimuli play during
the growth of abdominal aortic aneurysms (AAA). The formation of disturbed
patterns of wall shear stresses (WSS) and gradients of wall shear stresses
(GWSS) has been shown to affect the wall structural integrity, through
their effect on the morphology and functions of the endothelial cells (EC)
and circulating blood cells.
Particle Image Velocimetry measurements of the pulsatile blood flow have
been performed in aneurysm models, while changing systematically their
geometric parameters. The parametric study shows that the flow separates
from the wall even at early stages of the disease (dilatation < 50%).
A large vortex ring forms in symmetric AAA, leading to the formation of two
distinct region: a region of flow detachment, with low oscillatory WSS, and
a downstream region of flow reattachment, with large negative WSS and
sustained GWSS.
The loss of symmetry in the geometry engenders a helical flow. The
dominant vortex, whose strength increases with the asymmetry parameter,
is shed from
the most bulged wall (anterior). It results in the formation of a large
recirculating region, where ECs are subjected to quasi-steady reversed
WSS of low magnitude, while the posterior wall is exposed to positive WSS.
Lagrangian tracking of blood cells inside the aneurysm models shows a
dramatic increase in the cell residence time as the aneurysm grows. While
recirculating, cells experience higher shear stress, which may lead to
their activation. The flow vortical structure convects the cells towards
the wall, increasing the probability for cell deposition and ipso facto for
an intraluminal thrombus to form.