Computational studies of unsteady flow in arterio-venous fistulae

Abstract

Arterio-venous fistulae (AVF) are surgically created vascular connections between an artery and a vein in patients with End Stage Renal Disease, and are regarded as the 'gold standard' method of vascular access for patiets who require haemodialysis. However, up to 60% of AVF do not mature, and hence fail, as a result of various pathologies such as Intimal Hyperplasia (IH).

Highly oscillatory flow patterns are one of the factors implicated in the development of IH, and they will be studied in this thesis. Previous studies have investigated the effect of arterial curvature on blood flow in AVF using idealised (planar) AVF configurations and non-pulsatile inflow conditions. These studies are extended here to more realistic non-planar AVF configurations with pulsatile inflow conditions. Results show that, converse to previous findings, forming an AVF by connecting a vein onto the outer curvature of an arterial bend does not, necessarily, suppresses unsteady flow in the artery.

Subsequently, an optimisation process for idealised 3-dimensional geometries is introduced to identify an optimal configuration that suppress high-frequency fluctuations under steady inflow. Performance of the optimal configuration is then successfully verified with a fully pulsatile simulation. A novel medical device for maintaining AVF in the optimal shape is also proposed, and a first animal experiment is reported as a proof of principle study, which led to promising preliminary results.

Finally, realistic AVF geometries are reconstructed from 60 MRI scans. Ultrasound measurements of the flow were also collected, together with details of patient outcomes, so that computational simulations could investigate the relationships between geometrical features, flow unsteadiness and AVF maturation. Results show high variability of the geometric parameters between AVF formed in the upper arm and AVF formed in the wrist. Also, some geometric parameters in upper arm AVF resulted to be correlated with flow stabilisation and AVF maturation. However results were found to be not statistically significant.